Edgar — Estimation of Chromic and Yanadic Acids. 333 



xIet. XXXYII. — The Iodometric Estimation of Chromic and 

 Yanadic Acids in the Presence of One Another; by 

 Graham Edgar. 



[Contributions from the Kent Chemical Laboratory of Yale Univ. — cxci.] 



The difficulties which are met in the separation and gravi- 

 metric estimation of chromium and vanadium have led to various 

 attempts to accomplish the estimation of these elements in the 

 presence of one another. In 1898 Hillebrand* proposed a 

 method based on the fact that chromium may be accurately 

 estimated when present in small quantities by comparison of 

 the color of the solution containing an alkali chromate with 

 that of standard solutions of potassium monochromate. If 

 then the solution be reduced with sulphur dioxide and the 

 excess removed by boiling, vanadium, if present, may be esti- 

 mated by titration with standard potassium permanganate. 

 This latter process, however, is open to the objection that if 

 titration be made in the hot solution an appreciable amount of 

 chromic salt is oxidized, while if the solution be cold the end 

 point is more or less uncertain. If, however, a correction be 

 made for the permanganate used up in partially oxidizing the 

 chromium, the method is accurate for the small quantities of 

 vanadium found in rocks. 



A method has been also proposed by Campagnef for the 

 estimation of these elements in the same solution, in which the 

 vanadium is estimated by titration with potassium permanga- 

 nate after reduction with hydrochloric acid and subsequent 

 treatment with concentrated sulphuric acid to convert the 

 oxychloride into oxysulphate. The solution is then boiled 

 with an excess of strong potassium permanganate, the excess 

 of reagent destroyed by the addition of a piece of filter paper, 

 and the oxides of manganese filtered off, the filtrate containing 

 the vanadium as vanadate and the chromium as chromate. 

 Ferrous ammonium sulphate in excess is now added and this 

 excess determined by titration with potassium permanganate, 

 the difference being of course the amount of ferrous salt used 

 up in reducing the chromic acid, thus allowing the chromium 

 to be calculated. 



The dehniteness with which vanadic acid is reduced to 

 different stages of oxidation by means of varied reducing 

 agents makes it especially suited for processes of differential 

 reduction, in which the substance to be analyzed is treated 

 with two or more reducing agents whose action upon the 

 constituents to be estimated is different. The latter are then 

 calculated from the determination of the total reducing effect 

 in each case. 



* Journal Amer. Chem. Soc, 20, 461-465. 

 {Bull. Soc. Chim. (3), xxxi, 962-965. 



Am. Jour. Sci.— Fourth Series, Vol. XXVI, No. 154.— October, 1908. 

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