INTRODUCTION TO CHEMISTRY LABORATORY

GVSR Pavan Kumar

ANALYSIS

• Chemists are the people concerned with ANALYSIS.

• Analysis is the process of learning nature and composition of any substance.

CLASSIFICATION

• The THREE important questions generally studied in ANALYSIS are….

• WHAT ??????

• HOW MUCH????

• WHERE ???

But for a chemist………………

• WHAT does it consist?• HOW MUCH each of the constituent is ?• WHERE from it was bought?

• Answer for the first question………..• It consists of flour, sugar, cashew, ghee,

kishmish, ilachi……..

• Answer for the second question……500g of flour; 300g of sugar; 100g of cashew;

100g of kishmish; 500g of ghee; 5g of ilachi.Answer for the third question is…… bought from XYZ Sweet store (ISO:9000 certified)

God blessed us with the two wonderful organs for both the QUALITATIVE ANALYSIS

andQUANTITATIVE ANALYSIS

EYES are for Qualitative Analysis and TONGUE is for Quantitative analysis with limitations

• QUALITATIVE ANALYSIS is the process of identification of each of the constituent present in a sample provided.

• QUANTITAIVE ANALYSIS is the process of determination of the QUANTITY of each of the constituent present in the sample.

It was mainly based on classical gravimetric and titrimetric schemes followed by the invention of a precise analytical balance - extremely low throughputs and highly precise and accurate analyses with restricted sensitivity (detection up to fraction of a per cent only).

Gravimetry is the determination of an element or species through the measurement of the mass of a well-characterized insoluble product of a definite chemical reaction involving that element or species

Gravimetry

• When NaCl solution is mixed with a solution of silver nitrate it forms a precipitate of silver chloride

Titrimetry Titrimetry is the determination of an element or species through

the measurement of mass of a chemical necessary to exactly complete a definite chemical reaction in

a solution containing that element or species

End points generally are determined by observing a color change in an indicator.

Endpoints are also commonly determined by observations of potential differences

at electrodes, or changes in conductivity

TERMS USED IN VOLUMETRIC ANALYSIS

• TITRATION The process, operation, or method of determining the concentration of a substance in solution by adding to it a standard reagent of known concentration in carefully measured amounts until a reaction of definite and known proportion is completed, as shown by a color change or by electrical measurement, and then calculating the unknown concentration.

• TITRATE The reagent of known concentration in the process of titration.

• TITRANT the solution in a titration that is added from a burette to a measured quantity of another solution.

• INDICATOR a substance used in titrations to indicate the completion of a chemical reaction, usually by a change of colour.

END POINT

• ENDPOINT The point in a titration at which no more titrant should be added. It is determined, for example, by a color change in an indicator or by the appearance of a precipitate.

• (In the above picture we can clearly observe the endpoint)

• ERROR is defined as the difference between the true value and observed value. The following are the different types of errors generally seen.

• Physical error• Experimental error

• Parallax is a deceptive change of the position of an object which is observed while the position of the observer changes. Position of eye at all volumetric vessels must be at the same level as the meniscus. If not, the parallax will cause an error while reading the position of the meniscus of a liquid in a burette. It will be a positive mistake if the eye is lower, and negative if the eye is higher than the meniscus plane

Experimental error

• These errors are due to………..1.Preparation of solutions and reagents2.Concentration of reagents and indicators3.Weighing the required chemical substances4.Addition of reagents from graduated

apparatus such as burette pipette etc

AACURACY AND PRECISION

• ACCURACY is defined as the NEAREST approach to the true value……

• For example if the % of iron in iron ore is theoretically 74 and that of the reported is 73.8 then the reported value is said to be accurate. And if the reported value is found to be 67.5 then the value is not accurate.

• PRECISION is defined as REPEATABILITY AND REPRODUCIBILITY of the REPORTED value which is found to be accurate….

Accurate value

Day1 Day2 Week1 Week2 Month

0.049 0.049 0.049 0.049 0.049 0.048

0.048 0.048 0.048 0.048 0.048 0.047

0.049 0.049 0.049 0.049 0.049 0.048

0.049 0.049 0.049 0.049 0.049 0.048

0.049 0.049 0.049 0.049 0.049 0.048

ROLE OF INDICATOR……

• An indicator in a titration plays key role by detecting the END POINT. The indicator shows a different and distinct colour change at the point of addition of excess of drop of reagent from burette. It works accurately on some conditions such as pH, medium (acidic or alkaline) and on potentials.

ACID BASE TITRATIONS• An acid-base titration is the determination of the

concentration of an acid or base by exactly neutralizing the acid/base with an acid or base of known concentration.

• Before starting the titration a suitable pH indicator must be chosen.

• The pH of the equivalence point can be estimated using the following rules:

1. A strong acid will react with a strong base to form a neutral (pH=7) solution.

2. A strong acid will react with a weak base to form an acidic (pH<7) solution.

3. A weak acid will react with a strong base to form a basic (pH>7) solution.

REDOX TITRATIONS• Redox titration (also called oxidation-reduction

titration) is a type of titration based on a redox reaction between the analyte and titrant.

• Redox titration may involve the use of a redox indicator and/or a potentiometer.

• Oxidation is defined as the addition of oxygen or removal of hydrogen or loss of electrons. For example, in the extraction of iron from its

• Fe(II)

• Reduction id removal of oxygen or addition of hydrogen or gain of electrons

• A redox reaction is a simultaneous oxidation and reduction process

COMPLEXOMETRIC TITRATIONS

• Complexometric titration is a form of volumetric analysis in which the formation of a colored complex is used to indicate the end point of a titration. Complexometric titrations are particularly useful for the determination of a mixture of different metal ions in solution.

• The indicators used here are called as METALLOCHROMIC INDICATORS.

• Metal +In→ [M-In]• M-In is metal indicator complex• [M-In]+EDTA → [M-EDTA]+In

PRECIPITATION TITRATIONS

• The titration, which involves the formation of precipitate during the course of a titration.

• The first drop of titrant in excess will react with an indicator resulting in a color change and indicating the end point of the titration.

SELECTION OF INDICATORS

• ACID BASE INDICATORS: Acid - Base indicators (also known as pH indicators) are substances which change colour with pH. They are usually weak acids or bases, which when dissolved in water dissociate slightly and form ions.Consider an indicator which is a weak acid, with the formula HIn. At equilibrium, the following equilibrium equation is established with its conjugate base:HIn(Aq) +H2O -------H3O+ +In-

• Indicator ColourpKlnpH range AcidBase Thymol Blue – red yellow 1.5 1.2 - 2.8

• Methyl Orange red yellow3.7 3.2 - 4.4 Bromocresol Greenyellowblue4.7 3.8 - 5.4 Methyl Redyellowred5.1 4.8 - 6.0

• Bromothymol Blueyellowblue7.0 6.0 - 7.6 • Phenol Redyellowred7.9 6.8 - 8.4 • Thymol Blue - 2nd changeyellowblue8.98.0 - 9.6

Phenolphthaleincolourlesspink9.4 8.2 - 10.0

• A redox indicator (also called an oxidation-reduction indicator) is an indicator that undergoes a definite color change at a specific electrode potential.

• The requirement for fast and reversible color change means that the oxidation-reduction equilibrium for an indicator redox system needs to be established very fast. Therefore only a few classes of organic redox systems can be used for indicator purposes.

• There are two common type of redox indicators:• metal-organic complexes (Ex. phenanthroline)• true organic redox systems (Ex. Methylene blue)• Sometimes colored inorganic oxidants or reductants

(Ex. Potassium manganate, Potassium dichromate) are also incorrectly called redox indicators. They can’t be classified as true redox indicators because of their irreversibility.

• Almost all redox indicators with true organic redox systems involve a proton as a participant in their electrochemical reaction. Therefore sometimes redox indicators are also divided into two general groups: independent or dependent on pH.

• Internal indicator means that the substance used as an indicator is actually added to the reaction flask and this flask changes colour once the end point has been reached.........examples include diphenylamine and ferroin.

• External indicators are not added to the reaction flask....these are used ONLY WHEN A SUITABLE INTERNAL INDICATOR IS UNAVAILABLE because these indicators take a lot of time to calculate examples are potassium ferricyanide

Metal ion indicator……………

• To carry out metal cation titrations using EDTA, it is almost always necessary to use a complexometric indicator to determine when the end point has been reached. Common indicators are organic dyes such as Fast Sulphon Black, Eriochrome Black T, Eriochrome Red B or Murexide.een displaced from the metal cations in solution, and that the endpoint has been reached. Thus, the free indicator (rather than the metal complex) serves as the endpoint indicator. These are also called as METALLOCHROMIC INDICATORS