Environmental Monitoring Handbook

8/3/2019 Environmental Monitoring Handbook

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March 1979

EPA-600/4-79-019

HANDBOOK FOR ANALYTICAL QUALITY CONTROLIN WATER A ND W ASTEWATER LABORATORIES

ENVIRON MENTA L MONITORING AN D SUPPORT LABORATORYU.S . ENVIRONM ENTAL PROTECTION AGENCYO F F I C E O F R E S EA R C H A N D D E V E L O P M EN TCI NCI NNA T I , O H I O 45268


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D SCLAlM E RThe mention of trade names or commercial products in this handbook is for illustrationpurposes and does not constitute endorsement or recommendation for use by the U.S.Environmental Protection Agency.

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ABSTRACTOne of the fundamental responsibilities of water and wastewater management is theestablishment of continuing programs to insure the reliability and validity of analyticallaboratory and field data gathered in water treatment and wastewater pollution controlactivities.This han dbo ok is addressed to labor atory directors, leaders of field investigations, and o the rpersonnel who bear responsibility for water and wastewater data. Subject matter of thehandbook is concerned primarily with quality control ( Q C ) for chem ical and biological testsand measurements. Chapters are also included on QC aspects of sampling, microbiology,biology, radiochem'istry, and safety as they relate to water and wastewater pollutioncontrol. Sufficient information is offered to allow the reader to inaugurate or reinforceprograms of analytical QC that emphasize early recognition, prevention, and correction offactors leading to breakdowns in the validity of water and wastewater pollution controldata.

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ACKNOWLEDGMENTSThis handbook was prepared by the Environmental Monitoring and Support Laboratory(EMSL) of the United States Environmental Protection Agency. The contributions of thefollowing individuals in preparing the handb ook are gratefully acknowledged :

J . B. AndersonD. G. BallingerE. L. BergR. L. BoothR. H. BordnerP. W. BrittonJ . F. KoppH. L. KriegerJ. J. LichtenbergL. B. LobringJ. E. LongbottomC. I. WeberJ. A. Winter

Technical editing and preparation of the final manuscript were performed und er contract toJohn F. Holman & Co. Inc., 1 34 6 Co nne cticu t Avenu e, N.W., Wa shington, D.C. 200 36 .Inquiries regarding material contained in th e hand book should be made t o E nvironmentalProtection Agency, Environmental Research Center, Environmental Monitoring and SupportLaboratory, Cincinnati, Ohio 45 26 8.

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TABLE O F CONTENTS

Page

Chapter1

2

3

4

5

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PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . .

IMPORTANCE OF QUALITY CONTROL . . . . . . . . . . . . . . . 1- 11.2 Quality Assurance Programs . . . . . . . . . . . . . . . . . . . . 1.1.3 Analytical Methods . . . . . . . . . . . . . . . . . . . . . . . . 1-21.4 Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-41.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1- 1

LABORATORY SERVICES . . . . . . . . . . . . . . . . . . . . . . . 2-12 .1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1 2.2 Distilled Water . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1 2.3 Compressed Air . . . . . . . . . . . . . . . . . . . . . . . . . . 2-5 2.4 Vacuum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.5 Hood System . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-5 2.6 Electrical Services . . . . . . . . . . . . . . . . . . . . . . . . . 2-62.7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6

2-5

INSTRUMENT SELECTION . . . . . . . . . . . . . . . . . . . . . . 3-13.2 Analytical Balances . . . . . . . . . . . . . . . . . . . . . . . . 3-13.3 pH/Selective-Ion Meters . . . . . . . . . . . . . . . . . . . . . . 3-33.4 Condu ctivity Meters . . . . . . . . . . . . . . . . . . . . . . . . 3-63.5 Turbidimeters (Nephelometers) . . . . . . . . . . . . . . . . . . 3-73.6 Spectrometers . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-83.7 Organic Carbon A nalyzers . . . . . . . . . . . . . . . . . . . . . 3-133.8 Gas Chrom atographs . . . . . . . . . . . . . . . . . . . . . . . 3-143.9 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14

. . . . . . . . . . . . . . . . . . . . . . . . . . . ..1 Introduct ion 3-1

GLASSWARE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-14.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-14.2 Types of Glassware . . . . . . . . . . . . . . . . . . . . . . . . 4-24 .3 Volum etric Analyses . . . . . . . . . . . . . . . . . . . . . . . 4-34.4 Federal Spe cifications for Vo lume tric Glassware . . . . . . . . . 4-44.5 Cleaning of Glass and Porcelain . . . . . . . . . . . . . . . . . . 4-54 .6 Special Cleaning Requirements . . . . . . . . . . . . . . . . . . 4-64 .7 Disposable Glassware . . . . . . . . . . . . . . . . . . . . . . . 4-74.8 Specialized Glassware . . . . . . . . . . . . . . . . . . . . . . . 4-74.9 Fritted Ware . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-84 .10 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9REAGENTS. SOLVENT S. AND GASES . . . . . . . . . . . . . . . . 5-1 5.2 Reagent Quality . . . . . . . . . . . . . . . . . . . . . . . . . . 5-15.1 Introduct ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

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6

7

8

9

10

11

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5.3 Elimination of Determinate Errors . . . . . . . . . . . . . . . . 5-45.4 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4QUALITY CON TROL FOR ANALYTICAL PERFORMANCE . . . . . 6-16.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-16.2 The Industrial Approach to QC . . . . . . . . . . . . . . . . . . 6-16.3 Applying Con trol Charts in Environmental Laboratories . . . . . 6-26.4 Recommended Laboratory Quality Assurance Program . . . . . . 6-96.5 Outline of a Com prehensive Quality Assurance Program . . . . . 6-106.6 Related Top ics . . . . . . . . . . . . . . . . . . . . . . . . . . 6-136.7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13DATA HANDLING AND REP ORTING . . . . . . . . . . . . . . . . . 7-17.1 Introduct ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-17.2 Th e Analytical Value . . . . . . . . . . . . . . . . . . . . . . 7-17.3 Glossary of Statistical Term s . . . . . . . . . . . . . . . . . . . 7-37.4 Report Fo rms . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-57.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-11SPECIAL REQUIREMENTS FO R TRAC E ORGANIC ANALYSIS . . . 8-18.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-18.2 Sampling and Sample Handling . . . . . . . . . . . . . . . . . . 8-18.3 Extra ct H andling . . . . . . . . . . . . . . . . . . . . . . . . . 8-48.4 Supplies and Reagents . . . . . . . . . . . . . . . . . . . . . . . . 8-58.5 Quality Assurance . . . . . . . . . . . . . . . . . . . . . . . . . 8-78.6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-10SKILLS AND TRAINING . . . . . . . . . . . . . . . . . . . . . . . . 9-19.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-19.2 Skills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-29.3 Training . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4WATER AND WASTEWATER SAMPLING . . . . . . . . . . . . . . . 10-110.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-110.2 Areas of Sampling . . . . . . . . . . . . . . . . . . . . . . . . . 10-210.3 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10-6RADIOCHEMISTRY . . . . . . . . . . . . . . . . . . . . . . . . . . 11-111.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-111.2 Sample Collection . . . . . . . . . . . . . . . . . . . . . . . . . 11-111-3 Labo ratory Practices . . . . . . . . . . . . . . . . . . . . . . . 11-211.4 Quality Con trol . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 411.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-5MICROBIOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12-112.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12-112.2 Specific Needs in Microbiology . . . . . . . . . . . . . . . . . . 12-112.3 Intralaboratory Quality Control . . . . . . . . . . . . . . . . . . 12-212.4 Interlaboratory Quality Control . . . . . . . . . . . . . . . . . . 12-212.6 Documentation of a Quality Assurance Program . . . . . . . . . 12-312.5 Development of a F ormal Qu ality Assurance Program . . . . . . 12-3

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12.7 Chain-of-Custody Procedures fo r Microbiological Samp les . . . . 12-312.8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12-10

13 AQUATIC BIOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . 13-113.1 Sum mary of General Guidelines . . . . . . . . . . . . . . . . . . 13-113.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-213.3 Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-4

14 LABORATORY SAFETY . . . . . . . . . . . . . . . . . . . . . . . . 14-114.1 Law and Auth ority for Safety and Health . . . . . . . . . . . . . 14-114.2 EPA Policy on Laboratory Safety . . . . . . . . . . . . . . . . . 14-514.3 Laboratory Safety Practices . . . . . . . . . . . . . . . . . . . . 14-7Repo rt of Unsafe or Unhealthful Condition . . . . . . . . . . . . 14-15References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14- 1514.414.5

Append ix A-Suggested Checklist fo r th e Safety Evaluation of EPA Laboratory Areas A-1

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' ILIST O F FIGURES

Figure No . Page4-1 Example of Markings on Glassware . . . . . . . . . . . . . . . . . . . 4-56- 1 Essentials of a Control Chart . . . . . . . . . . . . . . . . . . . . . . . 6-26-3 Proce dure for Evaluating QC Data From a Monitoring Study . . . . . . 6-126-2 Shewhart Con trol Chart for Percent Recovery Data . . . . . . . . . . . 6-57-1 Exam ple of Bench Shee t . . . . . . . . . . . . . . . . . . . . . . . . . 7-87-2 Exam ple of Sum mary Data Shee t . . . . . . . . . . . . . . . . . . . . 7-97-3 Example of STO RET Repo rt Form . . . . . . . . . . . . . . . . . . . 7-107-4 Laboratory M easurements . . . . . . . . . . . . . . . . . . . . . . . 7-12Flow Chart of the Sequence of Events During a Controlled Series of

12-1 Example of Chain-of-Custody Sam ple Tag (a ) Front . (b ) Back . . . . . . 12-512-2 Exam ple of a Samp le Log Sh eet . . . . . . . . . . . . . . . . . . . . . 12-612-3 Exam ple of a Chain-of-Custody Record . . . . . . . . . . . . . . . . . 12-714-1 Hea lth and Safety Inspec tion Checklist . . . . . . . . . . . . . . . . . 14-814-2 Repo rt of Unhealthful or Unsafe Condition . . . . . . . . . . . . . . . 4-1614-3 Notice of Unhealthful or Unsafe Condition . . . . . . . . . . . . . . . 4-17

LIST O F TABLESTable N o. Page

2- 1 Water Purity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-12-2 Requ iremen ts for Reagent Water . . . . . . . . . . . . . . . . . . . . . 2-22-3 Com parison of Distillates Fr om Glass and Metal Stills . . . . . . . . . . 2-33-1 Perform ance Characteristics of Typ ical pH/Selective-Ion Meter . . . . . 3-53-2 Electrical Conductivity of Potassium Chloride Referen ce Solutions . . . 3-74-1 Tolerances for Volumetric Glassware . . . . . . . . . . . . . . . . . . . 4-44-2 Fr itted Ware Poros ity . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-86-16-2

Analyses of Total Phosphate-Phosphorus Standards. in Milligrams PerLiter of Total P04.P . . . . . . . . . . . . . . . . . . . . . . . . . .Estimates of the Range (R= IA. I) and the Industrial Statistic ( I = .B ( / ( A + B) of Three Different Parameters for Various ConcentrationRanges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7Shewhart Upper Control Limits UCL and Critical Range R , Values forthe Differences Between Duplicate Analyses Within Specific Con-centration Ranges for Three Parameters . . . . . . . . . . . . . . . . 6-86-4 Critical Range Values fo r Vary ing Co nce ntratio n Levels . . . . . . . . . 6-9

9- 1 Skill-Time Rating of Stan dard A nalytical Ope rations . . . . . . . . . . 9-3

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6-3

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10-1 Guidance for Water/Wastewater Sampling . . . . . . . . . . . . . . . . 10-211-1 Sample Handling, Preservation, Methodology, and Major In strume ntationRequired . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11-2

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Chapter 1IMPORTANCE OF QUALITY CONTROL

1.1 GeneralThe analytical laboratory provides qualitative and quantitative data for use in decision-making. To be valuable, the d ata m ust accura tely describe th e characteristics andconcentrations of constituents in the samples submitted to the laboratory. In many cases,because they lead to faulty interpretations, approxim ate or incorrect results are worse thanno result a t all.Am bient w ater quality stand ards fo r pH, dissolved oxygen, heavy metals, and pesticides areset to establish satisfactory conditions for drinking water, fishing, imgation, powergeneration, or oth er water uses. The laboratory data define whether conditions are beingmet and whether the water can be used for its intended purposes. In wastewater analyses,the laboratory d ata identify th e characteristics of th e treatmen t plant influent and th e finalload imposed upon receiving water resources, as well as the effectiveness of steps in t h etreatment process. Decisions on process changes, plant m odifications, or th e constru ction ofnew facilities may be based upon the results of water laboratory analyses. The financialimplications of su ch decisions suggest tha t e xtr em e care be taken in analysis.

.Effective research in w ater pol lution co ntrol also depends upon a valid labor atory da ta base,which in turn may contr ibute to sound evaluations of bo th the progress of the research itselfand the viability of available water pollution-control alternatives.The analytical data from water and wastewater laboratories may also be used to determinethe ex tent of com pliance of a polluting ind ustry with discharge or surface water standards.If th e lab orato ry results indicate a violation o f a standard , remedial action is required by theresponsible parties. Both .legal and social pressures can be brought to bear to protect theenvironment. The analyst should realize no t only tha t he has considerable responsibility forproviding reliable laboratory descriptions of the samples at issue, bu t also that his profes-sional competence, the validity of the procedures used, and the resulting values reportedmay be challenged (perhaps in court). For the analyst to meet such challenges, he shouldsupport the laboratory data with an adequate documentation program that provides validrecords of the control measures applied to all factors bearing on the final results of investi-gations.1.2 Quality Assurance ProgramsBecause of the imp ortan ce of labo ratory analyses in determining practical courses of actionthat may be followed, quality assurance programs to insure the reliability of the water andwastewater d ata are essential. Althou gh all analysts practice quality con trol (QC) in amountsdepending upon their training, professional pride, and the importance of their particularprojects, under actual working conditions sufficiently detailed QC may be neglected. Anestablished, rou tine, quality assurance program applied to each analy tical test can relieveanalysts of the necessity of originating individual QC efforts.~

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Quality assurance programs have two primary functions in the laboratory. First, theprograms should continually monitor the reliability (accuracy and precision) of the resultsreported; i.e., they should continually provide answers to the question How good (accurateand precise) are the results obtained? This function is the determination of quality. Thesecond function is the control of qua lity (t o meet the program req uireme nts for reliability).As an example of the distinction between the two functions, the processing of spikedsamples may be a determination of measurement quality, but the use of analytical gradereagents is a con trol m easure.Each analytical method has a rigid protocol. Similarly, QC associated with a test mustinclude definite required steps for monitoring the test and insuring that its results arecorrect. The steps in QC vary with the type of analysis. For example, in a titration,standardization of the titrant on a frequent basis is an element of QC. In any instrumentalmethod, calibration and checking out of instrumen tal response are also QC functions. All ofthe experim ental variables that affec t the final results sho uld be cons idered, evaluated, andcontrolled.In sum mary , labora tory data , in qu antitativ e terms, e.g., in milligrams per liter, are reportedby the analyst. These values are interpreted by industrial plant engineers to showcompliance or noncom pliance with permits fo r discharge, by s tate pollution c ontro l agenciesto-define the need for additional sampling and analysis to confirm violations, or by EPA t odemonstrate th at prescribed waste treatm ent was sufficient to protect the surface watersaffected by th e discharge.This handbook discusses the basic factors of water and wastewater measurements thatdetermine th e value of analytical results and provides recomm endations for th e control ofthese factors to insure that analytical results are the best possible. Quality assuranceprograms initiated fro m, and based upon , these recom men dations should increaseconfidence in th e reliability of th e rep orted analytical results.Because ultimately a lab orato ry direc tor must assume full responsibility for th e reliability ofthe analytical results sub mitte d, th e la bora tory dire ctor must also assume full responsibilityin bot h design and imp lemen tation fo r the correspo nding qua lity assurance program.1.3 Analytical MethodsMany analytical metho ds for com mo n water pollutants have been in use for many years andare used in most environmental laboratories. Examples are tests for chloride, nitrate, pH,specific conductance, and dissolved oxygen. Widespread use of an analytical method inwater and wastewater testing usually indicates that the method is reliable, and thereforetends to sup port th e validity of the rep orted test results. Conversely, the use of little-know nanalytical techniques forces th e w ater and wastewater data user t o rely on the judg me nt ofthe laboratory analyst, who must then defend his choice of analytical technique as well ashis conclusions. Present Federal regulations, notably section 304(h) of Public Law 92-500(Federal Water Pollution Co ntrol Am endm ents of 19 77 ) and th e Interim Drinking WaterRegulations specifically require th e use of EPA-approved me thod s of analysis.Uniformity of methodology within a single laboratory as well as among a group ofcooperating laboratories is required to remove methodology as a variable when there aremany data users. Uniformity of methodology is particularly important when several

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laboratories provide da ta t o a comm on data bank (such as STORET*) or cooperate in jointfield surveys. A lack of uniformity of methodology may raise doubts as to the validity ofthe reported results. If the same constituents are measured by different analyticalprocedures within a single laboratory, or by a different procedure in differen t laboratories,it m ay be asked which procedure is superior, why the superior method is not usedthroughout, and what effects the various methods and procedures have on the data valuesand their interpretations.Physical and chemical measurement methods used in water or wastewater laboratoriesshould be selected by the following criteria:

a.

b.

C.

d.

The selected meth ods should measure desired constituents of water samples in thepresence of normal interferences with sufficient precision and accuracy to meet thewater da ta needs.The selected procedures should use equipment and skills ordinarily available in theaverage water pollution co ntrol laboratory or w ater supply laboratory .The selected m etho ds sho uld be sufficiently tested t o have established their validity.The selected metho ds should b e sufficiently rapid t o permit repetitive routine use inthe examination of large numbers of water samples.

The restriction to the use of EPA meth ods in all laboratories providing data to EPA permitsthe combination of data from different EPA programs and su ppor ts the validity of decisionsmade by EPA.Regardless of w hich analytical metho ds are used in a laboratory, th e m ethodology should becarefully documented. In some reports it is stated that a standard method from anauthoritative reference (such as ref. 1) was used thro ugh out an investigation , when closeexamination has indicated, however, that th is was no t strictly true. Stan dard meth ods maybe modified or entirely replaced because of recent advances in the state of the art orpersonal preferences of the laboratory s taff . Documentation of measurement proceduresused in arriving at labo ratory d ata should be clear, honest, and adequately referenced; andthe p rocedures should be applied exactly as documented .Reviewers can app ly th e associated precision and accuracy of each specific m etho d wheninterpreting the laboratory results. If the accuracy and precision of the analyticalmethodology are unknow n o r uncertain, th e data user may have t o establish the reliabilityof th e result he or she is interpreting before proceeding with th e interpretation.The necessarily strict adherence to accepted methods in water and wastewater analysesshould no t stifle investigations leading t o impro vem ents in analytical procedures. Even withaccepted and documented procedures, occasions arise when the procedures must bemod ified; e.g., to elimin ate unusual in terferences, o r t o yield increased sensitivities. When a*STORET is the acronym used to identify the computer-oriented U.S. EnvironmentalProtection Agency water quality control inform ation system; STORET stands for STOrageand RETrieval of data and inform ation.

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modification of a procedure is necessary, i t should b e carefully formu lated. Data shouldthen be assembled using both the original and the modified procedures to show thesuperiority of the latter. Such results can be brought to the attention of the organizationsresponsible for standardization of procedures. To increase the benefit, the modifiedprocedures should be written in a standard format for routine use as applicable. Thestanda rd form at usually includes sco pe and app lication, principle, eq uip me nt, reagents,proce dure, calculation of results, an d ex pec ted precision and accuracy.Responsibility for the results obtained from use of a nonstandard procedure (i .e. , one thathas not becom e accepted through wide use) rests with the analyst and his supervisor.In field operations, because it may be difficult to trans port samples to the lab oratory, or toexamine large numbers of sam ples (e.g., fo r gross characteristics), th e use of rapid fieldmethods yielding a pprox imate answers is sometimes required. Such m ethods should be usedonly with a clear understanding that t he results ob tai ne d- ar e no t as reliable as thoseobtained from standard lab oratory procedures. Th e fact t ha t such m ethods have been usedshould be documented, and the resul tsShould not be reported in the same context withmore reliable analytical information. When only approximate values are available, perhapsobtained for screening purposes in th e field only, t he da ta user would then be so informed.1.4 Reference1. Standard Methods for the Examination of Water and Wastewater, 14th Edition,Am erican Public Hea lth Association, New York (19 75 ).


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Chapter 2LABORATORY SERVICES

2.1 GeneralQuality control of water and wastewater laboratory analyses involves consideration andcontrol of the many variables that affect the production of reliable data. The quality of th elaboratory services available to the analyst must be included among these variables. Anabu ndan t sup ply of distilled water, free from interfer ences and oth er undesirablecontaminants, is an absolute necessity. An adeq uate source of clean, d ry, compressed air isneeded. Electrical pow er for r outin e labo rator y use and voltage-regulated sources fordelicate electronic instrumentation must be provided. This chapter, therefore, will bedevoted t o describing me thod s of maintaining th e quality of these services, as used in waterand w astewater laboratory operations.2.2 Distilled WaterDistilled or demineralized water is used in the labor atory f or diluti on, preparation of reagentsolutions, an d final rinsing of glassware. Ordinary distilled w ater is usually n ot pu re. It maybe c onta mi nated by dissolved gases and by m aterials leached from th e conta iner in which ithas been stored. Volatile organics distilled over from the feed water may be present, andnonvolatile impurities may occasionally be ca m ed over by t he steam, in th e form of a spray.The conc entra tion of th ese cont am inan ts is usually qui te small, and distilled wa ter is usedfor many analyses without further purification. However, it is highly important that thestill, storage ta nk, and any associated piping be carefully selected, installed, and m aintainedin such a way as to insure minimum contamination.

*

Water puri ty has been defined in many different w ays, bu t one generally accepte d definitionstates tha t high-p urity wa ter is water th at has been distilled or deionized, or both , so that i twill have a specific resistance of 500,000 S l or greater (or a conductivity less than 2.0pmholcm). This definition is satisfactory as a base to work from, but for more criticalrequirements, the breakdown shown in table 2-1 has been suggested t o exp ress degrees ofpurity (1).

Table 2-1WATER PURITYApproximateMaximum ConcentrationConductivity of Electrolyteegree of Punty (mgll)pmho/cm)

Pure 10 2-5Very Pure 1 0.2-0.5Ultrapure 0.1 0.0 1-0.02Theor etically Pure 0.055 0.00

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The American Society for Testing and Materials (ASTM) specifies four different grades ofwater for use in m ethods of chemical analysis and physical testing. The method ofpreparation of the various grades of reagent water determines the limits of impurities. Thevarious types o f reagent water an d ASTM re quirem ents are listed in table 2-2.Type I grade water is prepared by t he distillation of feed wa ter having a maximumconductivity of 20 pmho/cm at 25C followed by polishing with a mixed bed ofion-exchange materials and a 0 .2-pm m emb rane filter.Type I1 grade water is prepared by using a still designed to produce a distillate having aconductivity of less than 1.0 pmho/cm at 25OC. This may be accomplished by doubledistillation or the use of a still incorporating special baffling and degassing features .Ty pe 111 grade w ater is prepared by distillation, io n exchan ge, o r reverse osmosis, followedby polishing with the 0.45-pm m emb rane filter.Type IV grade water ,is prepared by distillation, ion exchange, reverse osmosis, o relectrodialysis.Properly designed metal stills from reputable manufacturers offer convenient and reliablesources of distilled water. These stills are usually con struc ted of cop per, brass, and bronze.All surfaces that contact the distillate should be heavily coated with pure tin to preventmetallic contamination. The metal storage tank should be of sturdy construction with atight-fitting cover, and have a filter in th e air vent t o remove airbo rne dust, gases, and fumes.For special purposes, an all-glass distillation unit may be p referable to th e me tal still. Thesestills are usually smaller, and of more limited capacity than the metal stil ls. An actualcomparison in which the distillates from an all-glass still and a metal still were analyzedspectrographically for certain trace m etal con tam inan ts is given in table 2-3. It can be seenthat the all-glass stil l produced a produ ct th at h ad substantially lower contam ination fromzinc, copper, and lead.All stills require periodic cleaning to remove solids that have been deposited from the feedwater. Hard water and high-dissolved-solids co nte nt pro m ote scale form ation in th e

Tab le 2-2REQUIREMENTS FOR REAGENT WATERMinimumaximumMaximum Electrical Electrical

at 25OC at 25 CMinimum ColorRetent ion Time

(min)Total Co ndu ctivity Resistivity* pH at 25OC of KMn04(mg/l) (pmho/cm) ( M a cm)

Gradeof Water Matter

Type 1 0.1 0.06 16.67 - 60Type I1 0.1 1 o 1 o - 6 0Type 111 1 o 1.o 1 o 6.2-7.5 10Type IV 2.0 5 O 0.2 5.0-8.0 10*See reference 2.

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Table 2-3COMPARISON OF DISTILLATES FROM GLAS S ANDMETAL STILLSElement and Concen tration (pg/l)Source

Zn B Fe Mn A1 Cu Ni PbAll-Glass Still


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A piping system for delivering distilled water to the area of use within the laboratory is aconvenient feature. in this case, special care should be taken tha t the qua lity of the water isnot degraded between the still and the point of use. Piping may be of tin, tin-lined brass,stainless steel, plastic, or chemically resistant glass, depending on the quality of the waterdesired, its intended use, and on available funds. Tin is best, but is also very expensive. As acompromise, plastic pipe or glass pipe w ith Te flon* O-rings at all conn ecting joints issatisfactory fo r m ost purposes. The glass pipe has an obvious advantage when freed om fro mtrace amou nts of organic materials is impo rtant.When there is no piped-in supply, distilled water will probably be transported to thelaboratory and stored in polyethylene or glass bottles of about 5-gal capacity. If stored inglass containers, distilled water will gradually leach the more soluble materials from the glassand cause an increase in dissolved solids. On the other hand, polyethylene bottles containorganic plasticizers, and traces of these materials may be leached from t he conta iner walls.These are of little consequence, except in some organic analyses. Rubber stoppers oftenused in storage containers contain leachable materials, including significant quantities ofzinc. This is usually no problem , because the water is not in direct contact with the stop per.However, the analyst should be aware of the potential for contamination, especially whenthe supply is not replenished by freq uen t use.The delivery tu be m ay consist of a piece of glass tubing th at exten ds almo st to th e bo tto mof'the bottle, and that is bent downward above the bottle neck, with a 3- to 4-ft piece offlexible tub ing attac hed for mobility. Vinyl tubing is preferable to latex rubb er, because it isless leachable; however, a short piece of latex tubing may be required at the outle t fo r bettercontrol of the pinchcock. The vent tube in the stopper should be protected against theentrance of dust.Ordinary distilled w ater is quite adeq uate for many analyses, including the dete rmin ation ofmajor cations and anions. Certain needs may require the use of double- or eventriple-distilled water. Redistillation from an alkaline permanganate solution can be used toobtain a water with low organic background. When determining trace organics by solventextraction and gas chrom atograph y, distilled water with sufficiently low backg round may b eextremely difficult to obtain. In this case, preextraction of the water with the solvent usedin the respective analysis may be helpful in eliminating undesirable peaks in the blank.Certain analyses require special treatment or conditioning of the distilled water, and thesewill now b e discussed.2.2.1 Ammonia -F ree WaterRemoval of amm onia can be accomplished by shaking ordin ary distilled w ater with a strongcation exchanger, or by passing distilled w ater through a colum n of suc h material. Forlimited volumes of ammonia-free water, use of the Quikpure (Box 254, Chicago, Ill.)500-ml bottle is highly recommended. The ion-free water described in section 2.2.3 is alsosuitable for use in the determina tion of ammonia.2.2.2 Carbon-Dioxide-Free WaterCarbon-dioxide-free water may be prepared by boiling distilled w ater for 15 min and coolingto room temperature. As an alternative, distilled water may be vigorously aerated with a*Trademark of E. 1. duP ont de N emours & Co.

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stream of inert gas for a period sufficient t o achieve satur ation and CO, removal. Nitrogen ismost frequently used. The final p H of the water should lie between 6.2 an d 7.2. It is notadvisable to sto re CO z-free water fo r extended periods.2.2.3 ion-Free WaterA multipurpose high purity water, free from trace amounts of the common ions, may beconveniently prepared by slow ly passing distilled water t hrou gh an ion-exchange colum ncontaining one part of a strongly acidic cation-exchange resin in the hydroxyl form. Resinsof a quality suitable fo r analytical work mu st be used. Ion-exchange cartridges of theresearch grade, available from scientific supply houses, have been found satisfactory. B yusing a fresh colum n and high-quality distilled water, a water corresponding to th e ASTMdesignation for typ e I reagent water (2 ) (maximum 0.1 mg/l of total matter and maximumconductivity of 0.06 mho/cm) can be obtained. This water is suitable for use in thedetermination of ammonia, trace metals, and low concentrations of most cations andanions. It is not suited to some organic analyses, however, because this treatment addsorganic contam inants to the wa ter by con tact w ith the ion-exchange m aterials.2.3 Compressed Air

a The quality of compressed air required in the laboratory is usually very high, and specialattention should be given to producing and maintaining clean air until it reaches the outlet.Oil, water, and dirt are undesirable contaminan ts in compressed air, and it is importa nt toinstall equipment that generates dry, oil-free air. When pressures of iess than 50 psi arerequired, a ro tary- type com pressor, using a water seal and no oil, eliminates any add ition ofoil that would subsequently have to be removed from the system. Large, horizontal,water-cooled compressors will usually be used when higher pressures are required.Compression heats air, thus increasing its tendency to retain moisture. An aftercooler istherefore necessary to remove water. Absorption filters should be used a t the compressor t oprevent moisture from entering the piping system. Galvanized steel pipe with threaded,malleable-iron fittings, or solder-joint co pper tubin g should be used fo r piping the air to th elaboratory.When the compressed air entering the lab or ato ry is of low quality, an efficient filter shouldbe installed between the outlet and the point of use to trap oil, moisture, and othercontaminants. As an alternative, high-quality compressed air of the dry grade iscomm ercially available in cylinders when n o o the r source exists.2.4 VacuumA source of vacuum in th e chemical laboratory, while no t an absolute necessity, can be amost useful item . W hile used primarily as an aid in filtra tion, it is also some times used inpipetting and in speeding up t he drying of pipets.2.5 Hood SystemAn efficient hood system is a requirement for all laboratories. In add ition t o removing thevarious toxi c and hazardou s fum es that may be generated when using organic solvents, orthat may be form ed durin g an acid digestion step , a hood system may also be used t oremove toxic gases th at may be formed during atom ic absorption analyses o r oth er

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reactions. A regular fume hood should have a face velocity of 100 ft/m in (linear) with thesash fully open.2.6 Electrical ServicesAn adequate electrical system is indispensable to the modern laboratory. This involveshaving both 1 15- and 230-V sources in sufficient capacity for the type of w ork that must bedone. Requirements for satisfactory lighting, proper functioning of sensitive instruments,and ope ration of high-current devices must be considered. Any specialized equip me nt ma ypresent unusual demands on the electrical supply.Because of the special typ e of w ork, requirements for a laboratory lighting system are quitedifferent from those in oth er areas. Accurate readings of glassware gradu ations, balan ceverniers, and other measuring lines must be made. Titration endpoints, sometimes involvingsubtle changes in color or shading, must be observed. Levels of illumination, brightness,glare, and location of l ight sources should be controlled to facilitate ease in m aking thesemeasurements and t o provide m aximum com fort for the employees.Such instrumen ts as spectrop hotom eters, flame photo meters, atomic absorption equipm ent,emission spectrographs, and gas chromatographs have co mplicated electronic circuits tha treqhire relatively constant voltage to maintain stable, drift-free instrument operation. If thevoltage of these circu its varies, ther e is a resulting chan ge in resistance, te mp eratu re, cur ren t,efficiency, light ou tp ut , and co mp on ent life. These characteristics are interr elate d, and onecannot be changed without dffecting the others. Voltage regulation is therefore necessary toeliminate these conditions.Many instruments have built-in voltage regulators that perform this function satisfactorily.In the absence of these, a small, portable, constant-voltage transformer should be placed inthe circuit between the electrical o utlet an d th e instrumen t. Such units are available fromSola Basic Ind ustries, Elk Grove Village, Ill., and are capable of supplying a constan t outp utof 118 V from an inp ut that varies between 9 5 and 130 V. When requirements are morestringen t, special transformer-regulated circuits can be used t o supp ly cons tant voltage. Onlythe instrument receiving the regulated voltage should be operated from such a circuit at anygiven time. These lines are in addition to, and separate from, the ordinary circuits used foroperation of equipment with less critical requirements.Electrical heating devices provide desirable h eat s ource s, and shou ld offe r continuou slyvariable temperature control. Hot plates and muffle furnaces wired for 230-V current willprobably give bette r service than those th at o perate on 115 V, especially if th e lowe r voltagecircuit is only marginally adequate. Water baths and laboratory ovens with maximumoperating temperatures of abo ut 200C perform well at 115 V. Care must be taken toground all equipm ent tha t co uld c ons titute a shock hazard. The three-pronged plugs th atincorporate grounds are best for this purpo se.2. 7 References1. Applebaum, S . B. , and Crits, G. J., Producing High Pu rity Water, Indus trial WaterEngineering (Sept./Oct. 1964).2.- Water, Part 31 of 1977 Book of ASTM Standards, p. 20, American Society forTesting and Materials, Philadelphia (19 77 ).

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Chapter 3INSTR UMENT SE LECTl0N

3.1 IntroductionThe m odern analytical labo ratory depends very heavily up on instrumen tation. Thisstatement may be completely obvious, but it should be remembered that the exceptionalemphasis on electronic equipm ent has really begun since th e d evelopment of th e transistorand the computer. To a certain extent, analytical instrumentation is always in thedevelopment stage, with manufacturers continually redesigning and upgrading theirproducts, striving for miniaturization, better durability and sensitivity, and improvedautomation. For laboratory supervisors and staff members the net result is a bewilderingstream of advertising brochures, ann oun cem ents, and catalogs of newly available equ ipm ent.Con sequen tly, the selec tion of analytical equ ipm ent is always difficult.The instrum ents commonly used in water and wastewater analysis include the following:

Analytical balancepH/selective-ion me terConductivity meterTurbidimeterSpec trometers (visible, ultraviolet (IN),nfrared (IR), and atomic absorption (AA))Total carbon analyzerGas chromatograph (GC)Gas chromatograph/mass spectrom eter (GCIMS)Tem perature devices (such as ovens and water baths)RecordersThese devices represent basic e quipm ent used in routin e w ork an d should be the subject ofcareful consideration before purchase. F urther, their o peration and maintenance ough t to beprimary considerations in sustained produc tion of satisfactory data. Obviously, fund ame ntalunderstanding of instrume nt design will assist the analyst in th e correct use of instrumentsand in so me cases will aid in dete cting instru me ntal failures. Calibration of all laboratoryinstruments with primary standards is encouraged whenever practical. This normallyinvolves a National Bureau of Standards standard reference material or calibration andcertification procedures. Calibration checks w ith secon dary standard s, mad e in eachlaboratory or available from private sources, are encouraged on a frequent basis if notrequired by th e analytical me thod each tim e an analysis is made.In the pages th at follow an atte m pt is made to discuss basic instrumen t design and to offersome remarks abo ut desirable instrum ental features.3.2 Analytical BalancesThe most important piece of equipment in any analytical laboratory is the analyticalbalance. T he degree of accuracy of th e balance is reflected in the accuracy of all data relatedto weight-prepared standards. Although the balance should therefore be the most protectedand cared-for instrument in the laboratory, proper care of the balance is frequentlyoverlooked.

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There are many fine balances on the market designed to meet a variety of needs. Types ofbalances include top-loading, two-pan, microanalytical, electroanalytical, semianalytical,analytical, and o the r special-purpose instrum ents. Each t yp e of balance has its own place inthe scheme of laboratory operation, but analytical single-pan balances are by far the mostimportant in the production of reliable data.Single-pan analytical balances range in capac ity from the 20-g to the popular 200-g modelswith sensitivities from 0.01 to 1 mg. Fe atures of single pan balances may include mechanicaland electronic switching of weights, digital readout, automatic zeroing of the emptybalance, and automatic preweighing and taring capabilities. Even with all the designimprovements, however, modern analytical balances are still fragile instruments, theoperation of which is subject to shock, temperature, and hum idity changes, to mishandling,and to various other insults. Some of the precautions to be observed in maintaining andprolonging the depen dable life of a balance are as follows:

a. Analytical balances sho uld be mou nted o n a heavy, shockproof table, preferably onewith an adequately large working surface and with a suitable drawer for storage ofbalance accessories. The balance level should be checked frequently and adjustedwhen necessary.

b. Balances should be located away from labora tory traffic and protected from su ddendrafts and humidity changes.c. Balance tempe ratures should be equilibrated w ith room tem pera ture; this isespecially im portant if building heat is shut o ff or reduced during nonworking hours.d. When the balance is not in use, th e beam should be raised from the knife edges, theweights returned to the beam, objects such as the weighing dish removed from thepan, and the weighing com partm ent closed.e. Special precautions should be taken to avoid spillage of corrosive chemicals on the

pan o r inside the balance case; th e interior of th e balance housing should be keptscrupulously clean.f. Balances should be checked a nd adjusted periodically by a comp any service man o rbalance c ons ultan t; if service is not available locally, th e manufacturers instructionsshould be followed as closely as possible. Service con tracts , including an automa ticpreventive maintena nce schedule, are encouraged.g. Th e balance sho uld be operated at all times according to the manufacturersins ructions.

Standardized weights to be used in checking balance accuracy, traceable to the NationalBureau of Standards, may be purchased from various supply houses. A complete set ofdirections for checking the performance of a balance is contained in part 41 of ASTMStandards (1 ).Because all analytical balances of the 200-g capacity suitable for water and wastewaterlaboratories have a bo ut t he sam e design specifications with reference to sensitivity,precision, convenience, an d price, i t is safe t o assume th at the re is no clear preference for acertain mode l, and selection can be mad e on the basis of availability of service.

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3.3 pH/Selective-Ion MetersThe concept of pH as a means of expressing the degree of effecfive acidity or alkalinityinstead of total acidity o r alkalinity was developed in 190 9 by Sorenson (2). It was not untilabout 1940 that commercial instruments were developed for routine laboratory measure-ment of pH .A basic meter consists of a voltage source, amplifier, and scale or digital readout device.Certain additional refinem ents prod uce varying perform ance characteristics betwe en m odels.Some models incorporate expanded scales for increased readability, solid state circuitry foroperating stability and extre me accuracy, and temp erature and slope adjustment to correctfor asymmetric potential of glass electrodes. Other features are scales that facilitate use ofselective-ion electrodes, recorder output, and interfacing with complex data-handlingsystems.In routine pH measurements the glass electrode is used as the indicator and th e calomelelectrode as th e reference. Glass electrodes have a very fast response tim e in highly bufferedsolutions. However, accurate readings are obtained slowly in poorly buffered samples, andparticularly so when changing from buffered t o unbuffered samples. Electrodes, both glassand calomel, should be well rinsed with distilled water after each reading, and should berinsed with, o r dip ped several t imes in to, th e n ext test sample before the final reading istaken. Weakly buffered samples should be stirred during measurement. When not in use,glass electrodes should not be allowed to become dry, but should be immersed in anappropriate solution consistent with the manufacturers instructions. The first steps incalibrating an instrument are to immerse the glass and calomel electrodes into a buffer ofknown pH, set the m eter to the pH of the buffer, and adjust the proper controls to bring thecircuit int o balance. T he temperature-compensating dial should be set at th e temperature ofthe buffer solution. For best accuracy, the instrument should be calibrated against twobuffers tha t bracket th e expected pH of the samples.

*

The presence of a faulty electrode is indicated by failure to obtain a reasonably correctvalue for the pH of the second reference buffer solution after the meter has beenstandardized with the first reference buffer solution. A cracked glass electrode will oftenyield pH readings that are essentially the same for both standards. The response ofelectrodes may also be impaired by failure to maintain the KCl level in the calomelelectrode, by imp rope r electrode m aintenance, or by certain materials such as oilysubstances and precipitates th at may c oat the electrode surface. Faulty electrodes can oftenbe restored to normal b y an approp riate cleaning procedure. Co mplete and d etailed cleaningmetho ds are given in part 31 of ASTM Standards (3), and are also usually sup plied by theelectrode manu facturer.Because of the asym metric potential of the glass electrode, most pH meters are built with aslope adjustment that enables the analyst to correct for slight electrode errors observedduring calibration with two different pH buffers. Exact details of slope adjustment andslope check may vary with different models of instruments. Th e slope adjustment must bemade whenever electrodes are changed, subjected to vigorous cleaning, o r refilled w ith freshelectrolyte. The slope adjustment feature is highly desirable and recommended forconsideration when purchasing a new meter.Most pH meters now available are built with transistorized circuits rather than vacuumtubes, which greatly reduces the warmup time and increases the stability of the meters.

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Also, many instrum ents are designed with a switching circuit so that the en t i re conventional0 to 14 scale of pH may be used to read a single pH u nit w ith a corre spon ding increase inaccuracy.This expanded-scale feature is of definite value when the m eter is used for poten tiometrictitrations and selective-ion work. It is of dubious value, however, in routine analyses,because pH readings more precise than kO.1 are seldom required. Primary standard buffersalts are available from the National Bureau of Standards* and should be used in situationswhere extreme accuracy is necessary. Preparation of reference solutions from these saltsrequires some special precautions and handling (3 ,4) such as the use of lowconduct ivi tydilution water, drying ovens, and carbon-dioxide-free purge gas. These solutions should bereplaced at least once each m onth.Secondary stan dard buffers m ay be prepared from NBS salts or purchased as solution s fromcomm ercial vendors. R outine use of these com mercially available solution s, which have beenvalidated by comp arison t o NBS standards , is recommended.The electrometric measurement of pH varies with tem perature because of two effects. Thefirst effect is the change in electrode output with temperature. This interference can becontrolled by use of instruments having temperature compensation or by calibrating theinstrument system including the electrode at the tem perature of the samples. The secondeffect is the change of pH of the sam ple with temperature. This error is sample depen dentand cannot be control led; i t should therefore be noted by report ing both the pH andtemp erature at th e time of analysis.Typical characteristics of a conventional expanded-scale meter are show n in table 3-1.3.3.1 pH ElectrodesA wide variety of special- and general-purpose pH electrodes are now available to meet allapplications in th e general analytical laboratory. A survey through any labo ratory supplycatalog may confuse more than clarify the selection process. A rugged, full-range, glass- orplastic-bodied combination electrode is a good choice for routine use. An addedconvenience is an electrode that contains solid geltype filling materials not requiring thenormal maintenance of an e lectrode co ntaining liquid filling solutions .3.3.2 Selectivelon ElectrodesElectrodes have been developed to measure almost every com mo n inorganic ion norm allymeasured in the water and wastewater laboratory. Application of these electrodes hasprogressed at a m uch slower pace and c urrently only three are approved for EPA monitoringapplications.Reference 5 includes methods for use of fluoride, ammonia, and dissolved oxygenelectrodes. Various techniques for use of these and other electrodes are reviewed inreferences 6 through 9 . A major problem in m easuring the to tal parameter with electrodes isthat of relating the ion activity to ion co ncen tration. Because th e electrodes only measure*NBS, Office of S tandard Reference Materials, Institute for Materials Research, Washington,D.C. 0234.

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Table 3-1PERFORMANCE CHARACTERISTICSOF TYPICAL pH/SELECTIVE-ION METERSReproduci- Powermallest Requirementsccuracy bility Temperature Inputange ScaleScale Division Comt>e nsationl Impedance

Normal 0-14 k 1 , 4 0 0 0.1 10 k0.05 +5 k0.02 +2Expanded 0-1 k100 -005 .5 k0.002 ( 3 ) k0.002 k0.2 0-1000-100 >10 1 15/220 50/60>10 1 15/220 50/601 Manual or automatic.3+2 percent of reading.May also be pow ered by self-contained batteries.


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activity, the challenge is to put all of the parameters of interest into the same measurableionic form and then to modify th e activity to be proportional to the conc entration. Thetechnique of known addition (spiking of samples) is recomme nded when unproven electrodemethods are being used or when sample matrix problems are suspected or not controlled byprior distillation or separa tion technique s.3.4 Conductivity MetersSolutions of electrolytes conduct an electric current by the migration of ions under theinfluence of an electric field. For a constant applied EMF, the current flowing betweenopposing e lectrodes immersed in the elec trolyte will vary inversely with th e resistance of thesolution. The reciprocal of the resistance is called cond ucta nce and is expressed in reciprocalohms (mhos). For natural water samples where the resistance is high, the usual reportingunit is micromhos.Most condu ctivity m et e p on th e market today use a cathode-ray tube , commonly known asthe magic eye, f o r indicating solution conductivity. A stepping switch for varyingresistances in steps of 1OX facilitates reading conductivities from a bo ut 0.1 t o ab ou t250,000 mho. The sensing element for a conductivity measurement is the conductivitycell, which normally consists of two thin plates of platinized me tal, rigidly supp ort ed with avery precise parallel spacing. For protection, th e plates are m ounted inside a glass tub e w ithopenings in the side walls and submersible end for access of sample. Variations in designshave included use of hard nbbber a nd plastics for protec tion of th e cell plates. Glass may bepreferable, in tha t the plates m ay be visually observed for cleanliness and possible damage,but the more durable encasements have the advantage of greater protection and reduced cellbreakage. Selection of various cell designs is normally based on personal preference withconsideration of sample typ e a nd durability requirements.in routin e use, cells should be frequently exam ined to insure tha t (a) the platinized coatingof plates is intact; (b) plates are n ot coated with suspended m atter; (c) plates are not b ent,distorted, o r misalined; and ( d) lead wires are properly spaced.Temperature has a pronounced effect on the conduc tance of solutions, and must becorrected for when results are reported. T he specified tem perature for reporting data usedby most analytical groups (and all EPA laboratories) is 25C.Data correction may beaccomplished by adjusting sample temperatures to 25C, r by use of mathematical orelectronic adjustment.Instrumental troubles are seldom encountered with conductivity meters because of thedesign simplicity. When troubles occur, they are usually in the cell, and for most accuratework th e following proced ures should be used:

a. Standa rdize the cell and establish a cell fac tor by measuring the cond uctiv ity of astandard potassium chloride solution (standard conductivity tables may be found invarious handboo ks).b. Rinse the cell by rep eated immersion in distilled water.

~ c. Again, immerse the cell in the sam ple several times before obtaining a reading.

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d. If the meter is equipped with a magic eye, determine the maximum width of theshadow at least twice, once by approaching the endpoint from a low readingupward, and onc e from a high reading dow nward.Because the cell constants are subject to slow change even under ideal conditions, andsometimes t o m ore rapid change under adverse conditions, i t is recommended tha t the cellcon stant be periodically established. T able 3-2 can be used for this operatio n.Fo r instrum ents reading in mhos, th e cell constan t is calculated as follows:

whereL = cell constant

K , =conductivity, in micromhos per centimeter, of the KC1 solution at the

K z = conductivity, in micromhos per centimeter, of the KCl solut ion at the sametemperatu re of measurement

temperature as the distilled w ater used to prepare the reference solutionK , = measured cond uctance, in mhos

Many different m anufacturers produce condu ctivity meters tha t perform well on water andwastewater samples. Selection should be made consistent with sampling requirements,availability of service and sales, and individual persona l preference.3.5 Turbidimeters (Nephelometers)Many different instrum ent designs have b een used for the optical measurement of turbid ityby measurement of either transmission or reflection of light. An equal or even greater

Table 3-2ELECTRICAL CONDUCTIVITY O F POTASSIUM CHLORIDE REFERENCE SOLUTIONSSolution Normality Method of Preparation Temperature Conductivity("C) (pmholcm)

A 0.1 7.43 65 g/1 KC1 at 20C 0 7,13818 11,16725 12,856B 0.01 0.7440 g/l KCl at 20C 0 77318 1 ,22025 1,408C 0.001 Dilute 100 m l of B t o 1.O 1 at 20C 25 147

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Each of the essential features listed, especially the mon ochrom ator and the photod etectorsystem, varies in design principles from one instrument to another. Some of thecharacteristics of the comm only used Perkin-Elmer model 124 double-beam gratingspectrom eter are the following:Light source

Visible region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TungstenLamp UV region. .................................... Deuterium lampWavelength accu racy . ..................................... k0.5 nmSpectral band width. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.5, 1 .O , and 2 .0 nmPhotom etric presentationLinear transmittance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Linear absorb ance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 to 100 percent0 t o 1A or 0 t o 2APhotodetector R-136. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 to 800 nmSam ple cells. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.0 to 10 cm3.6.1 Visible RangeDesirable features on a visible-range spectrometer are determined by the anticipated use ofthe instrument. Simple, l imited programs requiring use of only a few parameters canprobably be supported by inexpensive but reliable filter photometers. On the other hand, ifthe laboratory programs require a wide variety of measurements o n diverse samples at lowconcentrations, m ore versatile instrumen ts may be needed. One of the prime co nsiderationsis adaptability to various samp le cell sizes from 1.0 to 10.0 cm.3.6.2 Ultraviolet RangeA W spec trom eter is similar in design to a visible-range instru me nt exce pt for d ifferences inthe light source and in th e optics. The UV l ight source is a hyd rogen- or deuterium dischargelamp, which emits radiation in the UV portion of the spectrum, generally from about 200nm to the low-visible-wavelength region. T he o ptical system and samp le cells mu st beconstructed of UV-transparent material, which is usually quartz. A grating used in a Wsystem may be specially cut (blazed) in th e UV region fo r greater sensitivity.3.6.3 Infrared RangeA number of instrumen tal modifications are required in th e construction of spectrometersfor measurements in the IR region because materials such as glass and quartz absorb IRradiant energy, and ordinary photocells do not respond to it. Most IR spectrometers usefront-surfaced mirrors to eliminate the necessity for the transmission of radiant energythrough quartz, glass, or o the r lens materials. These mirrors are usually parabolic to focusthe diffuse IR energy. Such instruments must b e protected from high humidities and watervapor to avoid deterioration of the optical system and the presence of extraneousabsorption bands in th e IR.

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The energy or light source for a n IR instrum ent may be a Nernst glower or a Globar. Eachof these sources has certain chara cteristics tha t reco mm end i t f or use, but the more ruggedGlob ar is m ore c omm only used because it also has a more stab le emission. The receiving ordetection unit may be a the rmoc ouple, bolometer, therm istor, or photocon ductor cell. Thetype of analysis being performed dictates the degree of sophistication required in the IRinstrumentation selected t o acquire usable data .3.6.4 Proper Use of SpectrometersThe spectrometer manufacturer's instructions for proper use should be followed in all cases.Several safeguards against m isuse of the instrume nts, however, are mandatory.Instruments should be checked for wavelength alinement. If a particular colored solution isto be used at a closely specified wavelength, considerable loss of sensitivity can beenco untered if the w avelength co ntr ol is misalined. In visible-range instr um ents , an exce llentreference point is the maximum absorption for a diluted solution of potassiumpermanganate, which has dual peaks at 526 and 546 nm. On inexpensive instruments withless resolution the permanganate peak appears at 525 to 55 0 nm as a single, flat-toppedpeak.For bo th UV and IR instruments, standa rd absorp tion curves for many organic materialshave been published so that reference material for standard peaks is easily available.Standard films of styrene and other transparent plastics are available for IR wavelengthchecks. A very good discussion of factors that affect the proper performance ofspectrometers and standard reference materials available to calibrate them can be found inpublications of the National Bureau of Standards.* Use of certified s tandards is encouragedwhenev er practical.Too much e mphasis cann ot be placed on c are of abso rption cells. All absorp tion cells sho uldbe kept scrupulously clean, free of scratches, fingerprints, smudges, and evaporated filmresidues. Matched cells should be checked to see tha t they are equivalent , and anydifferences should be .accounted for during use or in the final data. Directions for cleaningcells are given in ch apte r 4.Generally speaking, trained technicians ma y ope rate a ny of the spectrom eters successfully;however, because interpretation of data from both the UV and IR instrume nts is becomingincreasingly complex, mere compliance with the operations manual may not be sufficientfor completely accomplishing the special techniques of sample preparation, instrumentoperation , and interpreta tion of absorption curves.3.6.5 Atomic AbsorptionThere are a number of differences in the basic design and accessories for atomic absorption(AA) equipment tha t require consideration before purchase and during subsequent use.These choices co ncern t he light sources, nebulizer burne rs, optica l system s, reado ut devices,and mod e conversions. Because some of these choices are no t readily obvious, the purchaseror user must be familiar with the types a nd numbe rs of samples to be analyzed and the

*NBS,Office of Stand ard Referenc e Materials, Institute for M aterials Research, Washington,D.C. 20234.

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specific elements t o be measured before making th e choice. For a program analyzing a widevariety of samples for a num ber of elements at varying concen trations, an instrument ofmaxim um versatility would be best. Most o f the discussion tha t follows applies to use ofinstrumentation in absorption, emission, or fluorescence modes.3.6.5.1 LampsHollow-cathode (H C) lam ps or electrodeless-discharge lamps (E DL) are available for over 70elements with single-element or multielement capability. Multielement lamps are consid-erably cheaper per eleme nt than single-element lamps, but the savings may n ot be realized ifthe lamps are not used strategically, because all the elements in the cathode deterioratewhen the lamp is used, regardless of which element is measured. The deteriorationphenomena result from the different volatilities of metals used in the cathode. One metalvolatilizes (sputters) more rapidly th an the others and redeposits upon an area of the othercathode metals. Thus, with continual use, a drift in signal will be noted with at least onemetal increasing and the other (or others) decreasing. If one can ignore the dubious costsavings of mu ltielement lamp s, use of single-element lamps could result in more precise andaccurate data.The line intensities of on e element in a mu ltielement HC lamp will usually b e less tha n th oseof a lamp containing a pure cath ode of th e same element because this element m ust sharethe discharge energy with the o the r elements present. However, this reduction shou ld notaffect the output by a factor of more than one-sixth to one-half, depending on thepart icular combinat ion and the number of elements combined. The output can be evengreater in so me mu ltielement lamps because alloying may permit a higher ope rating curr entthan for the case of th e pu re cathod e. All HC lamps have life expectancies that are related t othe volatili ty of th e cath ode me tal, and for this reason th e manufacturers recomm endationsfor ope rating shou ld be closely followed.Recent improvements in design and manufacture of HC lamps and EDLs have resulted inlamps with more c onstant o utp ut and longer life. Under normal cond itions an HC lamp maybe e xpec ted to ope rate satisfactorily for several years. HC lamps used to be guaran teed fo r acertain minimum ampere-hour period, but this has been changed to a 90-day warranty. It isgood practice to date newly purchased lamps and to inspect them immediately upon receipt.The operating current and voltage indicated on the lamp should no t be exceed ed during use.An increase in backg round noise or a loss of sensitivity are signs of lam p deterio ration .A basic design feature of AA spectrom eters is the convenience of the HC lamp changeoversystem. Some instruments provide for as many as six lamps in a rotating turret, allelectronically stabilized and ready for use by simply rotating the lamp turret. Otherinstruments provide for use of only one lamp at a time in the lamp housing, and requiremanual removal and replacement whenever more than one element is to be measured. Aquick-changeover system en ables frequent lamp changes during the period of operation. Ifnecessary lamp changes are infreque nt, however, multilamp moun ts do n ot represent a greatconvenience.After the proper lamp has been selected, the HC curren t should be adjusted according to themanufacturers recomm endations and allowed to electronically stabilize (warm u p) befo reuse. During this 15-min period, the monochromator may be positioned at the correctwavelength, and the proper slit width may be selected. For those instrum ents employing amultilamp turret, a warmup current is provided to those lamps not in use, thereby

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minimizing the warmup period after the turret is rotated. In a single-lamp instrument, theinstability exhibited during warmup is minimized by the use of a double-beam opticalsystem.3.6.5.2 Burner TypesThe most difficult and inefficient step in the AA process is converting the metal in thesample from an ion or a molecule to the neutral atomic state. It is the function of theatomizer/bumer to produce the desired neutral atomic condition of the elements. Withminor mod ifications burners are the same as those used for flame photometry.Basically there are two different types of burners. They are the total-consumption, orsurface-mix burner, and the laminar-flow or premix burner. There are many variations ofthese tw o basic types, such as the Boling, the high-solids, the turbulent-flow, the triflame,the nitrous oxide burners, and many others. As one might expect, there are manysimilarities among the various burners, the different names resulting from the differentmanufacturers. The element being determined and the type of sample solution dictate thetype of burner to be used. Generally, all types and makes of burner can be adjustedlaterally, rotationally, and vertically for selection of th e most sensitive area of th e flame forthe specific elem ent sought.Nonflame techniques have gained wide acceptance in AA analysis because of the ex t remesensitivity and the capability to directly introduce very diverse sample matrices. Thesesystems, which replace the conventional flame burner, come in various designs usingelectrical resistance to prod uce te mpe ratures as high as 3,500"C.3.6.5.3 Single- and D o u b l e (Spiit-) Beam InstrumentsThere is a great deal of existing uncertainty among instrument users about the relativemerits of single- and do uble-beam instrum ents. Neither sys tem is app ropr iate for all cases.With a single-beam instrument the light beam from the source passes directly through theflame t o the detector. In a double-beam system the light from the source is divided by abeam splitter into two paths. One path, th e reference bea m, goes directly t o th e detector.The second path, the sample beam, goes through the f lame to t he detector. The chopperalternate ly reflects a nd passes each beam , crea ting two eq ual beams falling alter nate ly upo nthe detector. If the be ams are equal, they cancel the alternate impulses reaching t hedetector, and no signal is generated. If the beams are different, the resulting imbalancecauses the detector to generate a signal that is amplified and measured. The differencebetween the reference a nd sam ple beams is then de termined as a direct function of absorbedlight. The advantages of the double-beam design are tha t any variations in th e source are ofreduced importance, and smaller dependence is placed upon the stability of the powersupply. Conversely, stabilization of the power supply can eliminate the apparent need forthe split-beam system. F urtherm ore, the beam splitter requires additional mirrors or opticalaccessories that cause some loss of radiant energy.A single-beam system does not monitor source variations, but offers certain otheradvantages. It allows use of low-intensity lamps, smaller slit settings, and smaller gain. As aconsequence, the singlebea m instrume nt, properly designed, is capable of operating withlower noise and better signal-to-noise ratio, and therefore with better precision andimproved sensitivity. Because the simplified optical system conserves radiant energy,

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especially in the shorter wavelengths, it facilitates operation in the low-wavelength range.With this advantage, it should be possible t o obtain bet ter sensitivity for those elements w itha strong resonance line below 350 nm and particularly those below 300 nm. Backgroundcorrection techniques are also available in single-beam systems.Double-beam instruments, however, offer the opportunity to perform more sophisticatedtechniques like background correction, two-channel multielement analysis, use of internalstandards, and element rationing. If one of these techniques is necessary, a double-beaminstrument must be considered.3.6.5.4 Readout DevicesReadout devices in even the lower cost AA instruments include digital concentrationdisplay. Using high-speed digital electronics, data-handling techniques encompass multiplecalibration standards, regression analysis to characterize the calibration curves, meanvariance and standard deviation statistical programs for sample calculations, and variousforms of printout reports in addition to recorder output. Choice of a readout system ispredicated largely upon laboratory needs and availability of budget. In general, any steptoward autom ation is desirable, bu t t he degree of automa tion should be compatible with thelaboratory program.3.6.5.5 Miscellaneous AccessoriesA number of instruments contain a mode selector, making an instrument usable for eitherabsorption or emission. The conversion to emission may be a desirable feature becausecertain elements are more amenable to analysis by this method. Some models offer anoption of atom ic fluorescence and can also be used as a W /visible spectrometer.Automatic sample changers are offered for almost all instruments on the m arke t, and as hasbeen previously stated, any automation feature is desirable. However, unless a laboratoryprogram performs a large number of repetitious measurements daily, an au tomatic samplechanger is not required. As a practical measure, other commonly used sample-changingdevices, although not expressly designed for AA use, can be interfaced w ith most AAinstruments.3.7 Organic Carbon AnalyzersA number of instruments designed to measure total organic carbon (TOC) in waters andwastes have appeared on the market in the past several years. The first of these unitsinvolved pyrolysis followed by IR measurement of the carbon dioxide formed. Sampleinjection of 20 to 200 p1 in a carrier gas of air or oxygen was performed with a syringe.Combustion a t 800C to 900C followed by IR analysis was performed automatically withfinal out pu t o n an a nalog recorder. System s using these principles are still produce d andrepresent a large part of th e TOC mark et.Other techniques of TOC analysis that modify every phase of the original TOC instrum entshave been introduce d. Sample presentation in small metallic boa ts and purging of CO, fromsolution are two new techniques. Wet chemical oxidation, either external to the instrum entor within th e instr um en t, using various oxida nts including ultraviolet ir radia tion is now inwide use. Measurement of t he CO, by reduction to methane (CH,) and quantitatio n with aflame ionization de tec tor are also available. Various meth ods of da ta handling are now used,

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ranging from recorder out pu t to direct readings and printouts of conc entration. Techniquesare also available for measuring materials like soil and sludges, and also the volatilecomponent of the TOC. Sensitivity on som e systems has been extended down to themicrogram per liter level. The major problems associated with TOC measurements areinterference from forms of inorganic carbon and the difficulty of obtaining a representativesample in the presence of particulate matter. Each system has its own procedure for samplepretreatment or for accounting for these problems. When choosing a TOC instrument,consideration should be given to the types of samples to be analyzed, the expectedconcentration range, and the forms of carbon t o be measured.3.8 Gas ChromatographsBecause GCs are available from a large number of manufacturers, selection of a particularmanufacturer may be based on convenience. No single multipurpose GC instrument permitsanalysis of a wide range of com pou nds . In this case, a GC/MS could be considered (1 1). If,however, relatively few types of enviro nme ntally significant com pound s are being surveyed,an inexpensive system equipped with a glass-lined injection port, electrolytic conductivitydetector, and analog recorder is a good choice. A review of the organic methods (1 2) to beused will give the analyst all the necessary information on the specific instrument,apparatus, and materials necessary for each type or class of compounds. A discussion ofvarious quality c ontr ol con siderations in GC analysis is given in chap ter 8 of this manual.Data handling requirements vary widely, and the need to automate GC data collection isdetermined by the extent of the sample load. In a routine monitoring laboratory, GCsystems incorporating their o wn microprocessers and report generating capabilities would beuseful in solving this problem. Because such systems greatly increase the cost, the overalleconomy of this choice mus t be considered.3.9 References

1. Single Arm B alances Testing, Pa rt 41 of 19 76 Book of ASTM Stan dards, Ame ricanSociety fo r Testing and Materials, Philadelphia ( 1976).2. Sorenson, S. P. L., Biochem. Z., 21, 201 (1909).3. pH of Water and W astewater, from Part 31 of 1976 Book of ASTM Standards,American Society for T esting and Materials, Philadelphia (1 976 ).4. Catalog of NBS Standard Reference Materials, NBS Special Publication 260, NationalBureau of Standa rds (1 975-76).5 . Methods for Chemical Analysis of Water and Wastes, U.S. EPA, Office of Research andDevelopm ent, EMSL, Cincinnati ( 1978).6. Rechnitz, G . A., Ion-Selective E lectrode s, Chem. Eng. News (June 12, 1967 ), p. 146.7. Riseman, J ean M., Me asurement of Inorganic Water Pollutants by Specific IonElectrode, American L aborato ry (July 19 69), p. 32.8. Koryta, Jiri, Theory and Application of Ion-Selective Electrodes, Anal. Chim. Acta,61 , 32 94 1 1 (1972).9. Covington, A. L., Ion-Selective Electrodes, CRC Critical Review in Anal. Chem . (Jan.10. Black, A. P., and Hannah, S . A., Measurement of Low Turbidities, J. Am. WaterWorks Assoc., 5 7, 90 1 (1 965).-11. EPA GC/MS Procedural Manual, Budde, W. L., and Eichelberger, J. W., Editors, 1stEdition, Vol. 1, U.S. EPA, O ffice of Research and D evelopment, EMSL, Cincin nati (inpress).

1974), pp. 355-406.

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12. Methods for Organic Analysis of Water and Wastes, U.S. EPA, EMSL, EnvironmentalResearch Center, Cincinnati (in prep aration).

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Chapter 4G LASSWAR E

4.1 GeneralThe measurement of trace constituents in water demands methods capable of maximumsensitivity. This is especially true for metals and trace organics such as pesticides, as well asfor th e d etermina tion of amm onia and phosphorus. In addition t o sensitive methods ,however, there are o ther areas that require special consideration. One suc h area is that of th ecleanliness of labo ratory glassware. Obviously, the very sensitive ana lytical systems are mo resensitive to errors resulting from the improper use or choice of apparatus, as well as tocontaminat ion effects due to an improper method of cleaning the a pparatus . The purpose ofthis chapter is to discuss the kinds of glassware available, the use of volumetric ware, andvarious cleaning requiremen ts.4.2 Types of GlasswareLaboratory vessels serve three functions : storage of reagents, measurement of solutionvolumes, and confinement of reactions. For special purposes, vessels made from materialssuch as porcelain, nickel, iron, aluminum, platinum, stainless steel, and plastic may beemployed to advantage. Glass, however, is the most widely used material of construct ion.There are many grades and types of glassware from which to choose, ranging from stu den tgrade to othe rs .possessing specific properties su ch as super s t rength, low boron content , andresistance to thermal shock or alkali. So ft glass containers are no t recomm ended for generaluse, especially f or sto rage o f reag ents because of th e possibility of dissolving of th e glass (o rof some of th e const i tuents of th e glass). Th e mainstay of the mo dem analytical laboratoryis a highly resistant borosilicate glass, such as tha t m anufactu red by Corning Glass Worksunder the name Pyrex or by Kimble Glass Co. as Kimax. This glassware is satisfactoryfor all analyses include d in reference 1.

-

Depending on the particular manu facturer, various trade names are used for specific brandspossessing special properties such as resistance to heat, shock, and alkalies. Examples ofsom e of these special brands follow:a. Kimax- o r Pyrex-bran d glass is a relatively ine rt all-purpose boro silicate glass.b. Vyc or-bran d glass is a silica glass (96 percent) made to withstand cont inuoustemperatures up t o 9OO0C and can b e down-shocked in ice water withou t breakage.c. Corning-brand glass is claimed to be 50 times more resistant to alkalies thanconventional ware and practically boron-free (maximum 0.2 percent) .d. Ray-Sorb- or L ow-Actinic-brand glass is used when the reagents o r materials are lightsensitive.e. Corex-brand labwa re is harde r tha n convention al borosilicates and therefo re be tterable to resist clou ding and scratching.

The use of plastic vessels, containers, and other apparatus made of Teflon, polyethylene,polystyrene, and polypropylene has increased markedly over recent years. Some of these

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materials, such as Teflon, are quite expensive; however, Teflon stopcock plugs haveavoid sticking or freezing is not required. Polyp ropylen e, a me thylp ente ne polym er, isavailable as laboratory bottles, graduates, beakers, and even volumetric flasks. It is crystalclear, sha tterp roof , autoclavable, and chemically resistant.

practically replaced glass plugs in burets, separatory funnels, etc., because lubrication to

The following are som e points t o consider in choosing glassware or plasticware:a.

b.

C.

d.

e.

f.

The special types of glass listed above, othe r than Pyrex or K imax, generally are no trequired to perform the analyses given in Me thods f or Chemical Analysis of Waterand Wastes (1).Unless instructed otherwise, borosilicate o r polyethylen e bottle s may be used for thestorage of reagents and stan dard solu tions.Dilute metal solutions are prone t o plate o ut on containe r walls over long periods ofstorage. Thus, dilute m etzl standard solutions m ust be prepared fresh at the tim e ofanalysis.For some operations, disposable glassware is entirely satisfactory. One example isthe use of disposable test tubes as sample containers for use with the Techniconautom atic sampler.Plastic bottles of polyethylene and Teflon have been found satisfactory for theshipment of water samples. Str ong mineral acids (such as sulfuric a cid) and organicsolvents will readily a tta ck polye thylene and a re to be avoided.Borosilicate glassware is not completely inert, particularly to alkalies; therefore,standard solutions of silica, boron, and the alkali metals are usually stored inpolyethylene bottles.

For additional information the reader is referred to the catalogs of th e various glass andplastic manufacturers. These catalogs contain a wealth of information such as specificprope rties, uses, an d sizes.4.3 Volumetric AnalysesBy common usage, accurately calibrated glassware for precise measurements of volume hasbecome known as volumetric glassware. This group includes volumetric flasks, volumetricpipets, and accurately calibrated burets. Less accurate types of glassware includinggraduated cylinders and serological and measuring pipets also have specific uses in theanalytical laborato ry when exac t volumes are unnecessary.The precision of volumetric w ork depends in part upon th e accuracy with which volumes ofsolutions can be measured. There are certain sources of error that must be carefullyconsidered. The volumetric apparatus must be read correctly; that is, the bottom of themeniscus should be tangent to the calibration mark. T here are oth er sources of error,however, such as changes in tem perature, w hich result in changes in th e actual capacity ofglass ap paratus and in th e volume of th e solutions. The capa city of an ord inary glass flask of4000-ml volum e increases 0.025 ml/deg with rise in tem perature, bu t if the flask is made ofborosilicate glass, the increase is much less. One thousand milliliters of water or of mostO.1N solutions increases in volume by approximately 0.20 ml/deg increase at room

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temperature. Thus solutions must be measured at the temperature at which the apparatuswas calibrated. This temperature (usually 20C) will be indicated on all volumetric ware.There may also be errors of calibration of the apparatus; that is , the volume marked on theapparatus may not be the true volume. Such errors can be eliminated only by recalibratingthe ap paratus or by replacing it.Volumetric apparatus is calibrated to contain or to deliver a definite volume of l iquid. T h swill be indicated on th e appa ratus with th e letters TC (t o conta in) or TD (t o deliver).Volum etric flasks are calibrated to con tain a given volume and are available in various shapesand sizes.Volumetric pipets are calibrated to deliver a fixed volume. The usual capacities are 1through 100 ml although micropipets are also available. Micropipets are most useful infurnace wo rk and are available in sizes ranging from 1 to 100pl .In emptying volumetric pipets, they should be held in a vertical position and the outflowshould be unrestricted. Th e tip of t he p ipet is kept in contac t with the wall of th e receivingvessel for a second or tw o a fter th e free flow has stopped. The liquid remaining in the tip isnot removed; his is most impo rtant.Measuring and serological pipets should also be held in a vertical position for dispensingliquids; however, the tip of the pipet is only touched to the wet surface of the receivingvessel after the outflow has ceased. For those pipets where the small amount of liquidremaining in the ti p is to b e blown o ut and add ed, indication is made by a frosted band nearthe top .Burets are used t o deliver definite volumes. The m ore comm on typ es are usually of 25- or50-ml capacity, graduated to tenths of a milliliter, and are provided with stopcocks. Forprecise analytical methods in microchemistry, microburets are also used. Microburetsgenerally are of 5- or 10-ml capacity, graduated in divisions of hundredths of a milliliter.Automatic burets with reservoirs are also available ranging in capacity from 10 to 100 ml.Reservoir capacity ranges from 10 0 to 4,000 ml.General rules in regard t o th e manipulation of a bure t are as follows: Do no t a t t empt t o d rya b uret tha t has been cleaned for use, but rinse it two or three times with a small volume ofthe solution with which it is to be filled. Do no t allow alkaline solutions to stand in a buretbecause the glass will be attacked, and the stopcock, unless made of Teflon, will tend tofreeze. A 50-mI buret shou ld not be emptied faster than 0.7 ml/s, otherwise too muc h liquidwill adhere to the walls and as the solution drains down, the meniscus will gradually rise,giving a high false reading. It should be emphasized that improper use or reading of buretscan result in serious calculation errors.In the case of all app aratu s fo r delivering liquids, the glass must be ab solutely clean so tha tthe film of liquid never breaks at any point. Careful attention must be paid to this fact orthe required amount of solu tion will no t be delivered. Th e various cleaning agents and th eiruse are described later.4.4 Federal Specifications fo r Volumetric GlasswareReference 2 con tains a description of Federal specifications for vo lumetric glassware. T heNational Bureau of Standards no longer accepts stock quantities of volumetric apparatus

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from manufacturers or dealers for certification and return for futur e sale to consum ers. Thiscertification service is still available, but apparatus will be tested only when submitted bythe ultimate user, and then only after an agreement has been reached with the Bureauconcerning the work to be done.Conseque ntly, the

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