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Ana&ca Chuntca Acta, 257 (1992) 129-133 Elsevier Science Pubhshers B V , Amsterdam

Spectrophotometric determination of chromium in geological samples D P S Rathore * and P K Tarafder Chemzcal ~~ra~o~,

Atonzzc Mznerab Dzvzswn, Deparimerzi of Atmzc kzpzu-3OWlJ (Indza)

Energy, 5, Jaz Jawan Colony, Tank Road,

(Received 4th June 1991, revised manuscript received 8th September 1991)


A method for the determmatlon of chrommm 1s presented, based on the oxldatlon of hydroxylamme hydrochlonde to mtrous acid by chrommm(V1) m acetlc acid medrum followed by dmzotuatlon of the nitrite produced with p-ammophenylmercaptoacettc acid and subsequent couplmg of the d!azonmm salt w~tb ~-(l-naphthyi~thylenedIamme ~hydr~hIorIde m acidic medmm to form a stable bluezsh azo dye The method IS suttable for the determmatzon of chrommm(VI) from 0 04 to 12 mg 1-l m a 1 O-cm cuvette The molar absorptrvtty and Sandell’s sensitwity are 3 65 X lo4 1 mol-’ cm-’ and 0 0014 fig cmb2, respectively I@words Spectrophotometry,

Chromium, GeologIcal materials,, Rocks

Several photometric methods for the determlnation of chrommm have been reported, mcludmg those mvolvmg dlphenylcarbazlde [ 1,2], azobased reagents 13-71 and other chelating orgamc reagents [S-11] Most of these methods are not sufflclently selecttve, and some of them require close control of temperature and long reaction tunes In addltlon, the stab&y of the coloured products 1s not satrsfactory Recently, p-ammophenylmercaptoacetlc acid has been used as a diazotxzable amme for the spectrophotometrlc dete~natlon of nitrite [12151 In this work, the mtrrte produced by the oxldatlon of hydroxylamme by chromlum(V1) (either chromate or dlchromate) m acetlc acid medmm dlazotlzed p-ammophenylmercaptoacetic acid and subsequently coupled unth N-(1naphthyl)ethylenedi~lne dihydr~hlo~de (NED) m acrdlc medium to give a bluelsh azo dye



Spectral measurements were made with a VarIan Model 634-S double-beam spectrophotometer wrth 1 O-cm quartz cuvettes Reagents

p-Ammophenyhnercaptoacetlc acid of 98 1% purity (Evans Chemetlcs, New York) was used as received All other chemtcals were of analytlcalreagent grade ~ta~ar~ ~~r~rnl~~~~ ~~~t~n ~~~ pg mE -I) Prepare a stock solution by dlssolvmg

0 9338 g of potassmm chromate (or 0 7072 g of potassmm dlchromate) m 250 ml of dlstdled water Ddute this solution for use m the constructlon of the cahbratlon graph p-Atrunophenylmercaptoacetu: d

act& 0 5% (w /

Dissolve 0 5 g of p-ammophenylmercapto-

OCJO3-2670/92/$0500 Q 1992 - Elsevler Science Publishers B V All rights reserved



acetlc acid m 100 ml of dlstllled water contammg 0 5 ml of concentrated hydrochloric aad N-(I-NaphthyOethylenedlamme drhydrochlonde, I% (W/U) Dissolve 1 g of N-(l-naphthyl)ethyl-

enedlamme dlhydrochlorlde m 100 ml of distilled water contammg 5 ml of concentrated hydrochloric aad Store this solution m a brown bottle This reagent solution 1s stable for 1 week Hydroxylamme hydrochloride, 10% (w/v) Drssolve 10 g of hydroxylamme hydrochloride m 100 ml of dlstllled water Sodrum acetate-acetrc

acui buffer

t’pH = 3)

Dissolve 2 g of sodium acetate trlhydrate m 60 ml of drstllled water and add 40 ml of glacial acetic acid Procedure

In a 25-ml cahbrated flask, place 2 5 ml of 10% hydroxyhumne hydrochloride solution, 5 ml of sodium acetate-acetic acid buffer solution, 2 5 ml of 0 5% p-ammophenylmercaptoacetlc acid, a known volume of sample solution [containing 1 O-30 p.g of chrommmWI)J and 2 ml of 1% N-(l-naphthyljethylenedmmme dlhydrochlorlde solution Shake the flask, set it aside for 20 mm, then dilute to the mark with dlstllled water A bluelsh azo dye develops which remains stable for > 24 h Measure the absorbance at 570 nm m a l-cm cuvette agamst a reagent blank prepared m the same manner but contammg no chrommm(VI) Prepare a calibration graph for l-30 pg of chrommm m a smular manner



was obtained with 2 5 ml of lo-15% solution of hydroxylamme hydrochlonde per 25-ml final volume In the determmatlon of mtrlte [121, a sultable acldlty of hydrochloric acid for dlazotlzatlon of p-ammophenylmercaptoacetx acid to give maximum absorbance was found to be 4-5% In the present Instance for chrommm determmatlon, if the hydrochloric acid concentration was mcreased to 4-5% for complete dlazotlzatlon of the amme, it resulted m a decrease m the absorbance of the &our formed A 2 5-ml volume of 0 5% hydrochloric acid (used for dlssolvmg the amine) and 5 ml of sodmm acetate-acetic acid buffer solution per 25-ml final volume were found to be optimum for maximum absorbance The effect of the NED concentration and acidity on the couplmg reaction was studied The best results were obtamed with 2-3 ml of 1% NED solution m 4-5% hydrochloric acid per 25-ml final volume This optimized NED concentration 1s sufficient for up to 12 0 mg 1-l of chromnnn(VI) The absorbance of the azo dye was found to be constant n-respective of the time mterval between oxldatlon-dlazotlzatlon and addition of NED Hence the oadatlon-dlazotuation reactions were very fast at room temperature A composite reagent system was also tried but this did not provide satisfactory results The colour development was found to be independent of temperature over the range 15-40°C Beer’s law, sensltuxty, precLslon and accuracy


A lO+g amount of chronuum(V1) was taken and the final volume was 25 ml m the followmg expernnents Effect of reactwn condztzons The coloured azo dye showed an absorption

maXlmum at 570 nm The absorption of the reagent blank at 570 nm 111a 1 O-cm cuvette measured agamst dIstIlled water was m the range 0 003-O 005 The concentration of hydroxylamme was varled from 0 5 to 30% The maxnnum absorbance

Beer’s law was obeyed from 0 04 to 12 0 mg 1-l of chronuum(VI) More concentra(ed solutions can be diluted with dlstllled water to brmg them wlthm the operatmg range of the mstrument provided that the hydrochloric acid concentration 1skept at the optimum level Other reagent concentrations are sufficient for up to 12 0 mg 1-l of chromunn(V1) The apparent molar absorptlvity (referred to chrommm) and Sandell’s sensltlvlty m the region of mmmmm photometric error were found to be 3 65 X lo4 1 mol-’ cm-’ and 0 0014 pg cm-‘, respectively Under the optnmzed condltlons, the reproduclblhty of the method was checked by nme replicate determmatlons of chromnun(VI) over a






period of mne consecutive days The relatrve standard devlatlons were found to be 2 08, 140 and 114% for concentrations of 0 2, 0 4 and 10 mg I-’ of chrommm(VI), respectively Effect of varwus cons The effect of various ions which generally ac-

company chrommm was studied accordmg to the recommended procedure at a 0 4 mg 1-l concentration of chrommmW1) and the results obtained are given m Table 1 Prior to the determmatlon of chrommm, various elements, viz, copper( n-on(III), manganese, cobalt, nickel, tltaruum, zircomum, magnesium and calcium, are separated from chrommm by filtration of then hydrous oxides, after digestion of the sodium perorude or the mixed sodium carbonate and sodium peroxrde melt m water The high tolerance towards catlons, m particular chrommm(II) [formed durmg the oxldatlon of hydroxylamme by chrommm(VI) m acetic acid medmm], can be attributed to the presence of the ammo, thlo and carboxyl groups m the dlazotlzable amme, which generally faahtate chelate formation of these ions This results m a decreased avallabdlty of these ions m solution to interfere m the determmatlon of chrommmW1) All other ions can be tolerated to an appreciable extent Peroxide interferes because it reduces chronuumW1) to the trivalent state m acidic solution, hence the residual peroxide must be destroyed by boding the sample solution for 30 mm before an ahquot of the subsequent filtrate 1s acidified If any colour due to permanganate ion 1s observed during the bollmg period, 1 ml of ethanol 1s added to reduce permanganate and the solution 1s boiled for an addltlonal 10 mm [16]

TABLE 2 Chrommm content of geologwi samples (n = 8) Sample

Chromate-1 Chromite-2 Magnetite-l Magnetite-2 ASK-l ASK-2 a By m&I) ues

Chrommm present (%I Volumetnc method a

Dlphenylcarbazlde method

Proposed method

2850 3126 -

28 46 3189 2 15 0 17 00040 0 0081

28 71 30 94 2 17 0 17 00040(0005)b OO082(OO09)b

via dlchromate tltratlon b Recommended val-

Sample solution preparatwn

Chromate and magnetite were decomposed by fusion with sodium peroxide and rocks with a mtiure of sodium carbonate and sodmm peroxide, to convert chrommm to soluble chromate ion [16,171 Lktermmatron ples

of chromrum m geologzcal sam-

In order to check the vahdlty of the method for the determmatlon of chrommm m geological samples, two sets of experiments were done In the first, the recovery of chrommm was checked by adding chrommm(V1) to the different sample solutions In the other, various amounts of the sample solution were taken and dduted to a constant volume with dlstdled water For all samples the determmatlon of chromium was quantitative The results obtained are given m Tables 2 and 3 and show that the proposed method 1s comparable to the dlphenylcarbande method

TABLE 1 Effect of diverse Ions on the determmatlon of 0 4 mg 1-l of chromnun(VI) Ion (tolerance hmlt m mg 1-l) a NH: @SO),CH,COO-, La3+, UO$+ (4001,Cl-, Na’, K+ QOOO),SO:- (4000), F- (900), NO;, oxalate, PO:-, SO:-, Cr3+, co’+, LI+, NI*+, Th4+ (200), HCO;, CO:-, A13+ Ba*+, Ca*+, Ce4+, Cd’+, Mg’+, Hg’+, Se4+, Ag+, Sr2+WO),M@ (351, sdlcate (300), I-, peroxodlsulphate (401, T14’, V5+ (41, As5* (81, Cu”, BI3+, IO; (20), Fe3+ (lo), Pb2+ (SO),Zr4+ (801, Zn2+ (60), W6+ (2) a Amount of ion causmg an error of less than 2% m the determmation








Chronute-1 Chromate-2 Magnetite-1 Magnetite-2

of chrommm

m four different

Chrommm(VI) added (~3


0 10 0 10 0 10 0 10

14 16 24 12 15 58 25 66 17 55 27 44 4 25 13 97



acid medium chromnun(I1)

Chronuum found (cLg)

oxtdlzes hydroxylamme m acetic to generate nitrous acid and

CrOi- + NH,OH + 4H++ Cr’++ HNO, + 3H,O Chromate ion m acidic medmm 1s converted mto dlchromate 2CrOi- + 2H+~t Cr,O;-

rock samples by the method

+ H,O

Dlchromate also oxldlzes hydroxylamme m acetic acid medium to generate nitrous acid and chrommm(I1) + 2NH,OH + 6H++

of standard



Proposed reactron mechanism



Recovery (%o) 99 60 100 80 98 90 97 20

Chrormum found &g) 14 23 2421 15 95 25 96 17 53 27 42 425 13 99


(n = 8)

method Recovery (%o) 99 80 loo 10 98 90 97 40

therefore suitable for the determmatlon of chrommm(V1) m micro amounts The senatlvlty, simplicity, temperature mdependence, stability of the azo dye formed, broad range of chrommm determination and high tolerance towards a large number of foreign ions are the advantages of the proposed method The authors are grateful to Evans Chemetlcs, New York, for the gift of a sample of pammophenylmercaptoacetlc acid Thanks are due to Shrl B N Tlkoo, Head, Chermstry Group, for provldmg the necessary faclhtles Thanks are also due to Shrl A C Saraswat, Director, Atomic Mmerals Dlvlslon, for permlsslon to publish this work

2Cr2++ 2HN0, + 5H,O The assumption of the reduction of chrommm(VI) to chrommm(I1) by hydroxylamme hydrochlonde m acetic acid medium 1s supported by the results of a qualitative test On adding chromate or dlchromate solution to hydroxylamme hydrochloride solution m the presence of acetic acid, a blue colour IS produced, mdlcatmg the presence of chrommm(I1) ion, chrommm(I1) ions are blue whereas chrommm(II1) Ions are green The nitrous acid thus produced dlazotrzes pammophenylmercaptoacetlc acxd to form a dlazomum cation Immediately, which 1s subsequently coupled with N-(l-naphthyljethylenedlamme &hydrochloride m hydrochloric acid medium to produce a bluelsh azo dye 1121 The method 1s

REFERENCES 2 Marczenko, Spectrophotometrlc Determmatlon of Elements, Horwood, Clnchester, 1976, p 215 E B Sandell and H Omshl, Photometnc Determmatlon of Trace Metals, Wiley, New York, 1978, pp 389-391 T Yotsuyanagl, Y Takeda, R Yamaslnta and K. Aomura, Anal Cinm Acta, 67 (1973) 297 S G Nagarkar and MC Eshwar, Inchan J Technol, 13 (1975) 377 B Subrahmanyam and M C Eshwar, Mlkrochlm Acta, 11 (1976) 578 F -S Sun, Talanta, 30 (1983) 446 G -Z Fang and C -Y Miao, Analyst, 110 (1985) 65 E Jacobsen and W Lund, Anal Chum Acta, 36 (1966) 135 F Buscarons and J Artigas, Anal Chum Acta, 16 (1957) 452





10 S K. MaJumdar and AK. De, Anal Chem ,32 (1960) 1337 11 W-B Qi and L -Z Zhu, Talanta, 33 (1986) 694 12 P K Tarafder and D P S Rathore, Analyst, 113 (1988) 1073 13 D P S Rathore and P K Tarafder, J In&an Chem Sot, 66 (1989) 185 14 D P S Rathore and P K. Tarafder, Acta Clenc Indxa, 16, P(l) (1990) 21 15 DP S Rathore and PK Tarafder, J In&an Chem Sot, 67 (1990) 231

133 16 M E Donaldson, Methods for the Analysis of Ores, Rocks and Related Matenals, Monograph 881, Mines Branch, Department of Atormc Energy, Mmes and Resources, Ottawa, 1978, p 47 17 A I Vogel, Text Etook of Quantltatrve Inorgamc Analysis Includmg Elementary Instrumental Analysis, hngman, New York, 1978, p 359

Rathore d p s spectrophotometric determination of chromium in geological samples 1992