RA (1962) a Modified Single Solution Method for the Determination of Phospate in Natural Waters

8/12/2019 RA (1962) a Modified Single Solution Method for the Determination of Phospate in Natural Waters

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ANALYTICA CHIMICA ACTA 31

A MODIFIED SINGLE SOLUTION METHOD FOR THEDETERMrNATION OF PHOSPHATE IN NATURAL WATERS

J . MURII-IY AND J , I. f


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32 J . MURPHY, J . I. RILEYEspcriments made by adding various amounts of antimony to the single solutionreagent showed that the optimum antimony concentration was 0.4 mg per 50 mlfinal volume. At this concentration the colour attained its maximum intensity inboth distilled water and sea water in about IO min and remained constant for atleast 24 h. Greater concentrations of antimony did not lcacl to more rapid colour

development. At concentrations of above ca 4 mg of antimony per 50 ml of solution,a turbidity of basic antimony salt was formed.The absorption mnximum of the phosphomolybdcnum blue formecl in the presenceof antimony is at 882 mfx, The phosphomolybdenum blue yiclclcd by either thestannous chloride or the ascorbic acid method has its absorption masima at lowerwavelengths (Fig. I). This suggested that antimony might be a component of the

I600 I I700 600Wvelength mp- III 50-1111 flilsks; ~.Gz-~111 CCIISwith ascorbic acicl ; rcclucccl with ascorbic acid -t_ irntimony.

molybdenum blue compound absorbing at 882 m,u. In order to determine the atomicratio of antimony to phosphorus in the complex, the comples formecl from IO ,ug ofphosphate-phosphorus was quantitatively estrfxtcd with iso-butyl methyl ketone.lk solvent was evaporated and the r&due was treatecl with 9 M hydrochloric acidand zinc. The stibinc and hydrogen formed were passed into 0.2~/~ w/v mercuricchloricle solution. After the gas evolution had continued for 30 min, the antimonyabsorbed in the mercuric chloricle solution was determined photometrically usingmethyl fluoronc. Duplicate csperiments gave antimony: phosphorus atomic ratiosof I :0.5)7 and I :o.g4 for the complcs.

ESPERIMENTAJ .All measurements of optical clensity were made with a Unicam S.P. 500 spectrophoto-


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DETERMINATION OF PHOSPHATE IN NATURAL WATERS 33meter, moclifiecl to USC 7.62 and 15.24 cm cells (3 and 6 inch respectively). Distilledwater was used in the compensating cell.

(I ) Szcl+.wic acid (5 N). Dilute 70 ml of concentrated sulphuric acid to 500 ml.(2) Ammonizim vnolybdate. Dissolve zo g of A.R. ammonium molybdate in waterand dilute to 500 ml. Store the solution in a Pyres glass bottle.(3) Ascorbic acid (0.1 M) Dissolve 1.32 g of ascorbic acid in 75 ml of water. Thissolution should, if possible, be prepared on the day it is required as the ascorbic acideasily becomes oxidised. If it is necessary to keep the solution it can probably bestabilised by addition of 25 mg of ethylenediaminetetraacetic acid (clisodium salt)ancl 0.5 ml of formic acid per 75 ml of solution.(4) ~otassizcnt. antivvtonyl tuvtvwte (I vng Sb/vvtl). Dissolve 0.2743 g of potassiumantimony1 tartrate in distilled water and dilute to IOO ml.(5) Mixed reagent. Mis thoroughly 125 ml OF 5 N sulphuric acid and 37.5 ml ofammonium molybdate. Add 75 ml of ascorbic acid solution and 12.5 ml of potassiuman timonyl tartrate solution. This reagent shoulcl be prepared as required as it cloesnot keep for more than 24 11.

Standavd fi lr osfi hate sol& ionsStock fihosfihate sohition. Prepnsc a solution containing 0.1757 g of potassiumclihyclrogen phosphate per 1. This solution contains 40 mg P (as phosphate)/l. Prepare,as recluired, a solution containing 0.2 ,ug P (as phosphate)/ml by dilution of the stockphosphate solution.

Treatment of aj5bavatusFill the calibrated flasks, which arc to be used for the determination, with concen-trated sulphuric acid, allow them to stand overnight, ancl then rinse thoroughly. It is

preferable to keep a set of flasks to be used only for the determination of phosphate,and after use to wash them and to keep them filled with water until required again.If this is clone the treatment with sulphuric acid is only required occasionally.Pr o cedu r e

Pipette 40 ml of the sea water sample into a 5o-ml calibratecl flask, add 8 ml of themisecl reagent from a tilt measure, dilute to volume with water, and mis well. Afternot less than I0 min measure the optical density of the solution at 882 rnp using cellsof an appropriate length. Determine the reagent blank in the same manner usingfreshly distilled water. Calibrate the methorl using I, 2 and 4 pugphosphorus as phos-phate (these amounts are appropriate for a r5-cm cell), and make up to approsi-mately 40 ml with distilled water before adding the reagent. The calibration curveonly needs occasional checking as it remains constant ancl appears to be inclependentof changes in the batches of reagents. For normal purposes the salt error is negligible(less than 1%).Beer s law und re wodztcibiL ity

Replicate calibration runs were ma& using I-IO ,ug of phosphorus (as phosphate)in 40 ml of distilled water. The optical densities of the resulting molybdenum blue.L1 ra l . Chim. Ada 27 (1962) 31-36


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34 J. hlUIWHY, J. 1. RILEYcomplexes were measured at 882 mp in 7.62-cm cells. The results (Table I) show thatBeers law is obeyed over this concentration range, nd that the method gives anexcellent reproducibility.As a further test of the reproducibility of the method with sea water, seven replicateanalysts were made on a natural sea water, poor in phosphate. The average opticaldensity (measured in a rg.q.-cm cell), less reagent blank, was 0.164 rf= 0.0007, corre-sponding to a concentration of 18.6 f 0.1 ,xg P(as phosphate)/1 in the sea water.

C:ALII1RATlON 01 hl151HOD

nf CUW.._. .--. - -_-. - ._._. -_ -_-_-.__-- _.._.._,_.___ _. ._ ._.. ._ ._. ._.. . ..__-.__-__- _.....I 0.110 0.110 0.110 0.1 IO2 0.220 0.220 0.22 I 0.2203 0.33 0.33 0.33 1 O-3314 0.442 0.442 0.44 I o*+t2j: 0.554 ~~.554 0*555 o.s.540.884 o.H84 0.885 0.884IO I#lOO I.IOOb l.IOlb I.lOdJ--------- ---.-..._.-. --_------.- ._..__._._- _.-. .-- . .-. _---_-.. ._... _.._ .._.__--.- _I _ .___.

Opf i d drrlsityiJlcrcJnrJtf/Jl~ PO,-1-. - ..- . . _ . _-_-_- .._._._0.11000.11000.1103o.r1og0.x 10x0.11og0.1100

-_ ..-. - _..... ..-_.,_-..-__* Mcnsurccl at 882 fq.4 in 7.0~~cm cells (Icsurcagcnt blank 0.02 I).11 Mwsurcd in .+-cm cdl calculntcci for 7.G2-~111 cel l

DlSTl3RMINAT1ON OF SALT ERROR 013 hlF,THOI~..-- ..-_._--___ .-- ___._- -.-_.--_.-_-. --- . -_._--.-I_

__--_-__-___ . .I - ____-____._ ____---.____-_ _.....__ . -_.-_-0.00 0.0 0.042 -- --0.00 1.0 0.102 0.210 -

IS).Gl 0.0 o,o.+fj 0.003 -.t.()o 1.0 O.ZGi?. O.ZlCJ -0.59.80 1.0 0,203 o.z20 4-0.54.70 1.o 0.2GG 0:222 -I_o.g

lg.6 I 1.0 0.287 0:222 -t_o.g0.00 .I.0 o.gz7 0.885 -4.00 4.0 o.c)2(i 0.88{ -0.19.w 4t.o 0.922 0.879 -0.7

I+.70 .I,0 0.922 0.873 -0.7lr)*dl 4.0 0.g2CJ 0.8S.t -0.1.-__ _-_.. ___.- -~_.I-.__-.--_ _I.. --___ -._.__.__._--__a Mcasurcd in 15.24-cm cells nt 882 mp.IJ IJlmk rcckonctl proportional to chloriuity.

Salt rr orIn most molybdenum blue tnethods for the determination of silicon and phosphorus,less intcnsc colours arc clcvclopccl with the same amount of silicon or phosphorus,in the presence of sea water salts, than in clistillcd water. The cffcct is particularlypronouncecl when the stnnnous chloride method is usecl for the determination ofphosphate in oceanic waters; in this case the salt error amounts to ca. Is/ .The salt error of the rapid single solution method was studied. Filtered sea water,


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D~T~I~~iINATION OF PHOSPHATE IN NATURAL WATERS 35which had been stored in polyethylene bottles to remove phosphate7, was dilutedwith distilled water to give waters having chlorinities ranging between 4.90 andxg,60/,0. Duplicate phosphate determinations were made on 40-ml portions of distilledwater and of the original sea water, and on 4o-ml aliquots of the diluted sea watersenriched with I ,ug and 4 pg of phosphorus as phosphate. The results (Table II) showthat at both levels of phosphate concentration the salt error was below I*/~~. Therate of colour development in sea water was similar to that in distilled water, and thecolour once formed, was stable for at least 24 1 .

The interference of several ions, which are known to intcrferc in molybdenum bluemethods for the determination of phosphate, was investigated. Determinations weremade using 4o-ml nliquots of distilled water containing these ions, both alone and inthe presence of IO pg of phosphate - 1. The results are shown in Table III : they in-clicatc that no interfcrencc is causccl by copper, iron or silicate at concentrationsmany times greater than their greatest reported concentration in sea water. Arsenateproduces a similar colour to phosphate, but since according to recent work+10 itsconcentration in sea water is only CCE:.5 ,ug As (as arsenato)/l, it will not interfereseriously in the determination (a SCSI, ntcr containing this amount of arsenic will givean optical density in a rg-cm cell only 0.003 greater than the reagent blank).

-_._--_-___-.- I_..__. ---^ ..--.-_..._-_. - . . --..--.-- . _r.--. -.-- I-_-.-.-_..-_._... .. . ..-..- ._ _ _^____ - -.--Critmvktrul irmrrttr of errmrni Opfiid trsiIy ui tit2 m/a (~.rm rdllsl_-._ -._. * . ._.--- - . _.-- -- .-_.._- ---. . _.I_.__-.-_. ___-l___._flucl.5~~ *lit) Nrr ncidd icw - - I c to pfiP0P -I f..if~tvr?rcL~--_..1-1. .-___...-t_- .._. _..-._-*- _.. . _..___I ..-_.-._-- .I__.._... ~.. .-_.-_.~.--.- . . _.I..--.---..._-_---- ..__.- - 0.012 0*503 0.g ICopper( 1 I) 500 0.01x 0.95 0*5G7Ir~n(~ll) 2500 0.017 0,5g8 0.58 t

Silicate 500 0.01.t 0.594 0. 30Amcnatc 50 I . I *+.tu 1,752 0.008Arlicnatc I 0.020 0.614 a..594- -__--_ -I _I..___ -._._._._-______.. ..__ ---_ _. .._ __..____._-.__-_- _.__. I. .- ___._-_-_-____ _.0 Jfcasurcd in r-cm ccl1 culculatcci for .*-cm cells.

~1single solution reagent is dcscrihcd for the clctcrmination of phosphorus in sea water. It consistsof an acidified solution of :tmmonium molyhctatc containing ascorbic ncicl and ix small amount ofantimony. This rcagcnt rcnctu rapidly with phosphate ion yielding in blue-purple compound whichcontnins nnCimony and phosphorus in a I : I atomic ratio. Ihc coniplcx is very staLlc ant1 obeysBeers law up to R phosphntc concentration of at least z ml.Thc sensitivity of the proccdurc iscumparablc with that of the stannous chloride mcthocl. The snll error is less then I %.

Unc mdthode spcctrophotom~triquc cst tldcritc pour tc dosage clu phosphate chins lcnu de mer,WI moycn de molybdatc dirmmonium, cn prdscncc claciclc ascorbiquc et dantimoinc. 11 SC formcrapicicmcnt nn composb violet blcu, runfurmant ;Lntimoinc ct phosphorc tlirns un rnpport ato-miqut? dc I : I .13cschrcibuny cincr Mcthodc zur 13estimmung von Phosphat in Mecrwasscr mii, i-lilfc von Ammo-niummolybdat in Gcgcnwart von AscorbhWiurc uncl Antimon, Der gcbiidclc l u-violcttc Kom-plox wird ~pcktropho~ometrisch gemcsscn,

Anal.~Ckiet. Rcla, 27 (1962) 31-30


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6 J. i%lURPHY, Je I. RILEY

1 J. MUXPHY AND J. I. RIIXY, J. Mw ine Bi ol . Assoc. Un i t ed Ki ngdom, 37 (19.58) 9.2 I- 1. W. I-IARVJiY, j. Muri?te 13iol . Assoc. Un i t ed JCingdom. 27 (1948) 337.3 J. 1). II. STRICKLAND AND I

RA (1962) a Modified Single Solution Method for the Determination of Phospate in Natural Waters

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