Rao, V. M.

Sastri, M. N.

English

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Extraction of Cr(VI) fromOrthophosphortc Acid Solutions byTrilauryl AmineV. M. RAO & M. N. SASTRIDepartment of Chemistry, Andhra University, WaltairReceived 25 August 1975; accepted 20 N ovember 1975The extraction of Cr(VI) from orthophosphortc acidsolutions by trilauryl amine has been studied. Thenature of the species extracted is established to beCr20;-.QUANTITATIVE extraction of Cr(VI) as c.o.o-from hydrochloric. acid media by tertiaryamines and related systems has been reported='.Deptula+ reported the Cr(VI) extraction fromorthophosphoric acid media by tri-s-octyl amine.No phosphate was found in the organic phase andthe extracted species was characterized as Cr20~.The results of our studies on the extraction of Cr(VI)by trilauryl amine (TLA) from orthophosphoric acidmedia are reported in this note.Trilauryl amine (Mis General Mills, Kankakee,Illinois) was used as such. All other chemicalsused were of analytical grade or samples purifiedaccording to the standard methods, Chromium-51in the form of sodium chromate in isotonic salinesolution (15 mCi/mg specific activity) and phos-phorus-32 carrier-free in the form of orthophosphoricacid in hydrochloric acid were obtained from theIsotope Division, BARC, Bombay.Extraction of orthophosphoric acid by T LA -Phosphoric acid extractions were carried out using25 ml of the aqueous phase containing appropriateconcentrations of orthophosphoric acid and added32p tracer by equilibrating with an equal volume oftrilauryl amine in chloroform and the ~-activity inboth the phases was measured after separationwith a liquid GM counting assembly.Moores observed that the extraction of ortho-phosphoric acid varied with aqueous volume. Ourstudies of the extraction of orthophosphoric acidinto 0'025M TLA in chloroform are shown in Fig. 1.0·05'"3 0.03~o•••o~ <>02.:..°0~--~0~'4~--~0~~~--~e~.2~--~I~.6r---~2'O[H3PO.].'1 (N)Fig. 1 - Extraction of orthophosphoric acid into TLA(0'025M) in chloroformNOTESIt is seen that 1: 1 salt formation between TLA andHaP04 occurs in the organic phase only from solu-tions containing phosphoric acid up to sixty timesthe concentration of amine. At higher concentra-tions of phosphoric acid there is an increase in theextraction indicating that the organic phase takesup more acid than that required for the formationof (RaNH+H2P04).Chromium (VI) extraction - Chromium(VI) distri-bution studies were made using appropriate concen-tration of chromic acid, orthophosphoric acid andadded tracer by equilibrating with an equal volumeof trilauryl amine in chloroform, pre-equilibratedwith O'IM H3P04. The Y-activities in the twoseparated phases were measured with a singlechannel analyser coupled with a 3W well-typeNaI(Tl) scintillation detector.For finding out the effect of acidity, extractionswere carried out using O'05M trilauryl amine inchloroform pre-equilibrated with O'IM HaPO" and0'025M chromic acid with added tracer at variousconcentrations of HaP04. Up to 3M HaPO"concentrations the extractions are quantitative andindependent of acid concentration beyond whichCr(VI) undergoes reduction to Cr(III).Composition of the extracted species - In order todetermine the composition of the extracted speciesthe extraction isotherm method'' was employed.Extractions were carried out using different concen-trations of Cr(VI) in O'lM H3P04 keeping [TLA]constant. A plot of [Crorg.] V5 [Cr"q.] gave anextraction isotherm which rises steeply to a maxi-mum value and remains practically constant there-after. The ratio of concentrations of trilaurylamine to the maximum concentration of Cr(VI)extracted corresponds to a value of 1·05.In view of the very high distribution coefficientvalues the distribution ratio method of Hesford andMckay? could not be adopted for the evaluation ofthe composition of the extracted species.Extractions of varying concentrations of Cr(VI)from H3P04 solutions containing constant uptracer were carried out using 0'05M TLA pre-equilibrated with O·lM HaPO". Fig. 2 shows the[TLA] '" 0.05 M[H3P04]c 0·1M1000E... 600U'eOL07 400••L!!....20000 1-0 2·0 ·0 400[el'CV1)].1 C~.llt.•••-I)Fig. 2 - Distribution of lip activity as a function of [Cr(VI)]635INDIAN J. CHEM.;VOL. 14A, AUGUST 1976distribution of activity of 31p in the organic phaseas a function of Cr(VI) concentration in the aqueousphase. It is seen that as lcng as the Cr(VI) concen-tration is significantly les s than that required for1: 1 salt formation, the 3ip activity remains unalteredin the organic phase. However, as the Cr(VI)concentration in the aqueous phase approaches theamine concentration, the 32p activity in the organicphase shows a decrease and finally as the Cr(VI)concentration becomes equal or greater than theamine concentration, all the S2p activity is found inthe aqueous phase. This experiment clearly con-firms the absence of phosphate in the Cr(VI) speciesextracted into amine.In aqueous systems Cr(VI) exhibit S8-10 variousequilibria giving rise to different species. Under theexperimental conditions the predominant speciesHCrO, and Cr20~- arise from the equilibria:HCrO, ~ CrO:-+H+; Kl = 3·2x 10-72HCrO, ~ CrZO~-+H20; Kz = 98These two species can be extracted 2.S (RsNH+HCrO:;)or (R3NH+)2Cr20~- respectively by the extractant.In both the cases the extraction isotherm is expectedto give a Cr(VI)-amine ratio of l: 1.The UV spectrum of the extracted species showsmaxima at 280 and 355 nm with a peak ratio of 1·8(approximately two). These spectral characteristicsare attributed toCr20~- (ref. 2, 3).References1. (a) SASTRI, M. N., SUNDAR, D. S. & PRASADA RAo,T. S. R., CU1'1'.su., 35 (1966), 307. (b) PRASADARAo,T. S. R.; Studies on the blue pe1'oxychromic acid and itsderivatives, Ph.D. thesis, Andhra University, 1967.2. TUCK, D.G.&WALTERS, R. M.,]. chem. Soc., (1963), 1111.3. DEPTuLA, C., J. inorg, nucl. Chem., 30 (1968), 1309.4. DEPTULA, C., Chem: A bstr. , 69 (1968), 62035.5. MOORE, F. L., Analyt. Chem., 29 (1957), 1660.6. COLEMAN,C. F., BROWN, K. B., MOORE, J. G. & ALLEN,K. A., Proceedings of second into con]. on peaceful uses ofatomic energy, Geneva (United Nations Publications),1958, C-I0, paper 510.7. HESFORD, E. & MCKAY, H. A. C., Trans. Fa1'aday Soc.,54 (1958), 573.8. DAVIES, W. G. & PRUE, J. E., Trans. Faraday Soc., 51(1955), 1, 45.9. NEUSS, J. D. & RIEMAN, W., J. Am. chem, Soc., 56(1934), 2238.10. TONG, J. Y. & KING, E. L., J. Am. chem. soc., 51 (1955),1045.Extractive Titrimetric Determination ofHg(II), Cu(II) & Ag(I) Using LeadDiethyldithiocarbamateA. L. J. RAO & CHANDERSHEKHARDepartment of Chemistry, Punjabi UniversityPatiala 147002Received 23 May 1975; accepted 22 December 1975A procedure for the selective detenninations of H~,A~, Ha-Cu and Aa-Cu is described. Lead diethyldi-thiocarbamate [Pb(DDC),l undergoes quantitative,stoichiometric and fast exchange reactions with HI1(II),Ag(I). and Cu(II), even near the equivalence point,makinl1 it a suitable real1ent for their detenninations.Cobalt does not interfere in the detennination of theseCations.636KREIMER et at.1 used chloroform solution ofCu(DDC), for extractive titration of Ag(I)~Lead diethyldithiocarbamate has also been usedby many workers-+ for the spectrophotometricdetermination of Cu(U}. In this note Pb(DDC)ll.has been used as a potential extractive titrimetricreagent for the selective determination of Hg(U) andAg(I) alone and in binary combinations with Cu(II).A stock solution of Pb(DDC)2 (2 X 10-4M) was pre-pared by dissolving 101 mg /litre of Pb(DDC)2 in chloro-form and diluting with carbon tetrachloride (1 :20,v/v) to make up to the mark. Similarly, its 1 X lo-3Msolution was prepared. Dissolving Pb(DDC)2 incarbon tetrachloride takes a long time and chloro-form makes the preparation of titrant solution easy ..Metal ion solutions were prepared by dissolvinganalytical grade metal salts, standardizing by volu-metric methods and then preparing more dilutesolutions from these standardized solutions.Acetate buffer was prepared by mixing 2Msolutions of sodium acetate and acetic acid in theratio 4:1 (v/v).Erma photoelectric colorimeter AE-ll was.used for absorbance measurements. Titrations wereperformed using a specially constructed titrationapparatus similar to that proposed by Galik'.Determination of Hg and Ag can be done by locat-ing the end points visually, and also colorimetrically,The diethyldithiocarbamate of Hg and Ag formedduring the titrations does not give any absorbancechanges with the filter 420 nm used for measuringabsorbance due to Cu(DDC)2 formed during titrations.Procedure for Hg(I I) and Ag(I) - An appropriateamount of the metal ion to be analysed was taken,diluted to 20 m1 with distilled water and the pHadjusted to 4·0-5·5 by adding 2 ml of acetate buffer.About 2 !-tg of Cu was added as indicator. Titrant(2 X lQ-4M) was added in 1 ml portions, shaking forhalf a minute each time and separating the extractafter each addition. The titration was continueduntil the extract showed brownish colouration dueto the formation of Cu(DCC)z. The titration wasrepeated, adding in one lot the reagant solution(1 mlless than that is required in the first titration),retaining about 0·5 ml of the organic phase, thencontinuing with the addition of reagent in 0:1 ml por-tions until the extract showed brownish colouration.For locating the end points colorimetrically, about50 !-tg of copper was added to the aqueous phase·taken in the titration apparatus containing 20 mlof CCl,. Aqueous phase made nearly 20 ml andthe PH adjusted to 4·0 to 5·5 as before. Thetitrant solution (Ix 1o-3M) was added in 0·1 ml,portions with shaking of about 1 min each time.After setting the phases, absorbance of the organicphase was measured against CCI. blank. Absor-bance of the org-anic phase does not change duringthe exchange of Hg or Ag with the titrant. Afterthe end point, absorbance of the organic phase,started increasing owing to the formation ofCu(DCCh giving a break in the titration curve.Procedure for Hg-Cu and Ag-Cu- Simultaneousdeterminations of Hg-Cu arid Ag-Cu were done .bylocating the end points colorimetrically andadoptmgthe procedure as given for single cations. After

Extraction of Cr(VI) from Orthophosphoric Acid Solutions by Trilauryl Amine

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Extraction of Cr(VI) from Orthophosphoric Acid Solutions by Trilauryl Amine

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