184 ANALYTICA CWIMICA ACTA

Short Communications

Determination of silicon in thorium

In the determination of silicon in thorium by the silicomolybdate blue method, MENES AND MANNING” adjust the final PH of the solution to 3.0. This value is very critical since it is not possible to redissolve thorium molybdate below PH 3, and the absorbance decreases when the pH is greater than 3, The curve for absorbance zrs. pH for silicon in the absence of thorium shows that between pH I and 3, the absorbance is constant. For thorium samples too, adjustment of PH to a value below 3 would give the best results, but for the difficulty of redissolution of thorium molybdatc.

This difficulty can be overcome with the help of EDTA, which is added to the sam- ple solution before adjustment of the initial PH. The precipitate which forms after the addition of ammonium molybdate solution can then be redissolved by adding tartaric acid and raising; the pw to 2.5. When once dissolved, the thorium molybdate can be kept in solution even at PH x. The addition of EDT.4 has no effects on the ab- sorbance of the silicomolybdate. The dotted line in Fig. I shows values obtained by

1 0.6’.

8 % 0.4. %

9

0.2 -

OO i 1 2 3 4 9 6 7 PH

Fig. I. The cffcct of plr 011 tlw colottr intensity of blue silicomolybrlrrtc complex: 0, Si upprox. 17 CG@ I% prcscnt; 0, Si qqwox. 20 pg, I% d.vscnt,

the modified method at PH lower than 3, The rest of the figure is as described by MENIS AND MANNING.

The modification considerably improves the precision of the method. The relative standard deviation for a sample containing 50 p.p.m. of silicon was found to be 4o/o as against 13% reported by MENIS AND MANNING. The other advantage of the modi- fied method is that even in the presence of phosphate impurity, it is no longer neces-

A?tflk Ct8i7Ft. .dCta, 3X (1964) X84-185

SHORT COMMUNICATIONS 185

sary to make absorbance measurements within 5 min of the time that the reduction is carried out, since the silicon heteropoly blue colour is found to be stable for at least 24 h.

Details of the modified procedure are being published in the form of a reportd.

Analytical Division, S. R. DESAI

Atomic Energy Estabmmrent Z’rombay, H. SAVITRI

Bombay-z8 (rftdia) M. S. VARDE

1 0. MENIS AND Il. L. MANNING, Amf. Chim. Acfn, 16 (1957) G7. 3 S. I<. DESAI, N. SAVITH~ AND M. S. VARDE. Determirtafion o~Silicorr irr ,Thoriunr Nilralc, Atomic

Energy Eetablishmcnt Trombay, Inrlio,‘XXcport No. A.E.E.T. (Anal.) 30, 1963.

(Received January xst, xgo4)

Atrul. Chim. Ada, 31 (1964) x84-185

Spectrophotomktric determination of iron with DTPA

Almost simultaneously, scvcral workers 1-a described a reaction between the Fe(III)- EDTA chelate and hydrogen peroxide in the PH region IO-II, which resulted in a

complex with a maximum molar absorptivity of cu. 500 in the visible rqion at 520 mp. The reaction was applied for the quantitative determination of iron. Copper, cobalt, nickel and chromium interfcrcz at 520 rnp, but at 450 rnp the intcrfercncc of an zoo-fold amount of copper is negligible a. The intcrfcrcnce of not more than twice the amount of cobalt can be eliminated by the addition of cyanidedeb.

Very recently, attention was given to this method by CHENG AN) GOYDISH~,

CLULEY AND NEWMAN~ and from this laboratory 7. In these procedures iron is dcter-

mined by measuring ‘the differcncc in .absorbancc of two tkolutiona containing iron,

the interfering elements and EDTA, to one of which hydrogen peroxide is added. The principle was earlier suggested by 1~xNcrjor.r 3, but with few experimental details. Good results were obtained in the prescncc of Iooo-fold amounts,*,,of coppeP+ an4 nickel’, up to IO-fold amounts of cobalt7 (after prior oxiclation to-the trivalent state) and, under carefully controlled conditions, up to a xo-fold amount of chrommmf.

In order to improve the analysis of iron by means of this procedure, we have investigated the same determination with various EDTA analogues, viz. HEDTA, DTPA, CyDTA and NTA. The iron(II.1) ‘chelates of these compoun’ds ‘all show 4 similar behaviour on the addition of hydrogen peroxide, but in all except one cast, the propcrtics of the resulting complex or of the ,interfcring ‘complexes make the ligand less suitable for analytical application than EDTA; oxygen evolution in the solution and instability of the resulting complexes arc the,main difficulties.

The compourtcl that gives better results than EDTA ‘in’some cases is DTPA.

* Tilt intcrfcrcncc of large anlounts of copper, mcntioncd in a previous paper’ lutcr nppcarcd not to occur.

Anal. C/rim. Acfn, 31 (1964) x85-186