A solvent extraction method for the determination of microgram amounts of chromium

SHORT COMMUNICA’I’XONS

296

A solvent

extraction microgram

method for the determination amounts of chromium

of

BLUNDY~ has recently described a method for the determination of micro-quantities of chromium in the presence of iron and nickel, after extraction of chromium(V1) into hexone2.3. Back-extraction of chromium(V1) into aqueous solution is followed. by a calorimetric finish with diphenylpicrazide. The present work arose out of a study of the extraction of chromium(V1) into basic organic solvents, particularly tri-n-butyl phosphated. The species extracted from acid solution has been identified as a solvatecl chromium(V1) acid: similarly WHITE AND Ross6 have reported that in extractions with tributyl phosphine oxide the species in the organic phase is (solvatcd) HKra07. It has been shown that tributyl phosphate (TBP) extracts perchromic acid (CrOs) resulting from the reaction of chromium(V1) with hydrogen peroxide, as well as clzromium(V1) itself, from dilute acid solution. Perchromic acid is a Lewis acid which is extracted only by basic solvents 0; it has been found that the extraction into TBP is quautitative. Furthermore, since TBP extracts hydrogen peroxide from aqueous solutionr, CrOG can be formed directly in the organic phase. This solvent extraction of chromium(VI) and CrOs has been made the basis of a simple calorimetric method for estimating microgram quantities of chromium. The various methods available for the oxidation of Cr(II1) to Cr(VI) have been fully discussed by BLUNDY and were not investigated further. The possible interferences of Cu, Ce. Ni, Th and U (see ref. I) were examined; the results compare favourably with those of other methods. EXPERIMENTAL Materials. Tri-n-butyl phosphate was purified as described previously”. A standard solution of chromium(V1) was prepared by dissolving 186.4 mg of oven-dried AnalaR K&rO,t in 250 ml. of I h7 sulphuric acid: tenfold dilution gave a solution containing 20 ,~cgCr per ml. A suitable solution of hydrogen peroxide was made by diluting 20 vol. reagent with I N sulphuric acid to give a ca. r o/o solution. Throughout this work, the aqueous phase consisted of AnalaR grade I N H&04. In testing for interference by other metals, suitable solutions were made up in dilute sulphuric acid from the following AnalaR materials ; Cu as CuSO4 - 5Ha0, Cc as (NH&Ce(NO&, Ni asNiS04*7HaO, Th asTh(NOa)4*6HzO andU as UOe(NO3)2’6HaO. Procedzwe, Pipette suitable aliquots of the standard chromium(V1) solution into calibrated stoppered tubes: dilute to 7 ml with dilute sulphuric acid. Add I ml of TBP and equilibrate the contents by gently inverting the tube about 50 times. Add I ml of acid peroxide solution and re-equilibrate the solutions. After centrifuging, transfer a suitable volume of the blue (upper) organic layer to the micro-cell of a Spekker Absorptiometer and measure the optical density using OY2 and 0G3 filters. RESULTS AND DISCUSSION A plot of optical density against the concentration of chromium(VI) is linear over the range o-100 pg Cr to within jc 3%, the line passing through the origin. It is worth noting that Cr06 is much more stable in TBP solutions than is apparently the case with other solventsoll?; the optical density of the separated organic phase was found to be virtually constant for at least IO-r5 h. Annl. Clritn. Acla, 27 (19Gz) 29e-297:

SHORT COMMUNICATIONS

297

To examine the possible effects of other elements, 50 mg of cerium(IV), copper(II), nickel(H), thorium(IV) and uranium(V1) were added separately to 60 pg of chromium(V1) and the extraction and calorimetric procedure were carried out as described above. In Table I, the final optical density of the TBP phase is given as TARLE INTERPERENCE

I

FROMOTHERBLEIlENTS CeUY

IN CrDI3TERBlINATION -

(50 mg)

Ni(I1) Th(TV) U(VU Cu(I1) CuUI)

(50 (50 (50 (50 ( 5

mg) mg) mg) mg) mg)

0.44 x.01

o.g9 o.gg 0.74 1 .oo

a fraction of that resulting from 60 pg of Cr(VI) alone. The method is clearly unaffected by large excesses of h’i, Th and U; in the case of the latter two elements, this is no doubt due to the fact that the extraction into TRP is apparently completely suppressed by working in aqueous sulphuric acid solution. The interference from cerium(IV) was not unexpected but this can presumably be avoided by previous treatment with sodium azide 1. The marked interference by copper(H), also reported by BI,UNDY~, is more difficult to explain. Examination of the visible absorption spectra (in the 210-700 rnp region) of solutions of copper sulphate and potassium dichromate, andof an equimolar mixture of the two, revealed no evidence of chemical reaction between them, nor was any precipitate found on allowing such a mixture to stand for some days. Reasonably small quantities of copper do not seem to affect the method however; even 5 mg of copper represents in this case an almost Ioo-fold excess. This simple technique should be applicable to a variety of systems, both for analysis and for the rapid separation of chromium from other elements which are not extracted by TBP from dilute aqueous sulphuric acid. DR.

J. C. DALTON

(U.K.A.E.A.,

Windscale)

is thanked

for a useful discussion

of

this work.

Defiavtntent of Chemistry, University of Manchester,

Manchester

,4 _ ) D. G. TUCK* I.

(Great Britain)

1 I?. D.

RLUNDY, Rmzlysl, 83 (rg58) 555. 3 H. A. BRYAN AND J. A. DEAN, Airal. Chem.. zg (1957) 1289. 3 A. I%. WEINHARDT AND A. N. HIXSON, Ind. Eng. Clrem., 43 (rggr)

4 R.M.

DxhnroND

6 J. C. WHITE

0 D. I?. EVANS,

AND

D.G.TucK:D.G.TucK

W. J. Ross, U.S. Atomic j. Clrem. Sot., (1957) 4013.

AND

AND

Energy

D. e. TUCK, J. Clrem. Sot., (x959) 2x8. 8 D. G. TUCK, J. Chem. SOL, (x958) 2783. 0 R. 1<. BROOKSHIRE AND H. FREUND, Anal.Cltem., 10 A. GLASNERAND M. STEINBERG, AnaLChem.,27

1676.

R.M.WALTERS;

Comm.,

unpublished work.

ORNL-2326

(x957).

7

23 (x951) 11x0. (1955) 2008.

Received April 4th, 1962 * Present address: Depaftmcnt Britain)

of Chemistry, I,

Univcrsih

’ of Nottingham, 5) :, ,.A&i..

Gh&.

Nottingham

AC&; 27 (IgCz)

(Great

.

296297