Browning — On the Interaction of Chromic^ etc. 35 



Art. VIII. — On the Interaction of Chromic and Arsenious 

 Acids ; by Philip E. Browning. 



[Contributions from the Kent Chemical Laboratory of Tale College — XLVIIL] 



Kessler* has shown that arsenious acid may be determined 

 by treating it, in the presence of hydrochloric acid, with an 

 excess of a chromate solution of known strength, by which 

 treatment the arsenious acid is oxidized and the chromic acid 

 reduced. The excess of the chromic acid is then determined 

 by the addition of a ferrous salt of known strength until a 

 drop taken from the solution gives a blue color with a ferri- 

 cyanide. Chromic acid is then added again until the blue 

 color disappears. The amount of the chromate originally 

 used less the excess determined by the ferrous salt gives the 

 amount of the chromate used for the oxidation, from which 

 may be calculated the amount of arsenious acid originally 

 present. Despite the use of a ferrous salt and the numerous 

 steps involved in the manipulation, Kessler claims very satis- 

 factory results for his method. The object of this paper is to 

 give the results of some experiments in which Kessler's reac- 

 tion was used for the reverse process, the arsenious acid being 

 used in excess, according to the reaction : 



4Cr0 3 + 3As 2 3 + (X)As 2 3 = 2Cr 2 3 + <3As 2 5 + (X)As 2 3 



For the work, a solution of potassium dichromate was made, 

 the standard of which was determined by evaporating definite 

 portions measured from a burette into previously weighed and 

 counterpoised crucibles, heating just to the melting point, and 

 weighing after cooling. A decinormal solution of arsenious 

 acid was made in the usual manner, as was also a decinormal 

 iodine solution which was standardized frequently in course of 

 the work against the arsenious acid. The method of manipu- 

 lation was as follows : Definite portions of the chromate solu- 

 tion were measured from a burette into counterpoised flasks 

 and weighed, as a check on the burette reading, about 10 cm 3 

 of dilute hydrochloric or sulphuric acid (1-4) added, and a 

 carefully measured amount of the arsenious acid solution — 

 care being taken to have an excess of the amount necessary for 

 the reaction. After a few moments the reduction seemed com- 

 plete, the solution having taken on the bluish green color 

 characteristic of the chromium salts. The application of heat 

 was found to be unnecessary, the reaction going on quickly 



*Pcgg. Anal., xcv, 204, 1855. 



