G. Edgar — Iodometric Estimation of Yanadic Acid. 175 



with sulphur dioxide, boiling out the excess of reagent, adding 

 barium chloride to precipitate the sulphuric acid formed, and 

 then subjecting the solution to distillation with hydriodic 

 acid. The chromic acid and the iron being already reduced 

 by the sulphur dioxide, it is evident that the iodine liberated 

 in the distillation should correspond to a reduction of vanadium- 

 tetroxide to trioxide. This process, however, yielded faulty 

 results, due, as was ascertained, to the formation of some dithi- 

 onic acid during the treatment with sulphur dioxide. 



Another attempt was made to treat the residue after distil- 

 lation with hydriodic acid with an excess of standard iodine 

 solution and make alkaline with sodium bicarbonate, the excess 

 of iodine to be afterwards determined with standard arsenious 

 acid. In this process the vanadium trioxide should be oxidized 

 to pentoxide and the ferrous salt to ferric salt, thus yielding 

 the necessary data for the third equation. This method 

 was found to be unavailable, owing to the fact that in the 

 presence of the bulky precipitate of chromic and ferric hydrox- 

 ides the vanadium is reoxidized very slowly by iodine. 



The method which finally proved successful was the esti- 

 mation of the chromic acid in a separate portion by a modifi- 

 cation of Browning's method of reduction with standard arseni- 

 ous acid.* 



The experiments to be described were made upon mixtures 

 in varying proportions of solutions of sodium vanadate, potas- 

 sium bichromate and ferric chloride ; the entire process in 

 detail being the following : 



(I). The solution, of about fifty cubic centimeters volume, 

 was divided into two equal parts, one of which was placed in a 

 distillation flask. To this, one to two grams of potassium 

 bromide were added, together with twenty-five cubic centi- 

 meters of concentrated hydrochloric acid, and the mixture was 

 distilled until a volume of about twenty -five cubic centimeters 

 was reached, the reduction having always been found to be 

 complete in that time. The bromine liberated was absorbed in 

 alkaline potassium iodide, and, after cooling and acidifying, the 

 iodine liberated was estimated by titration with approximately 

 tenth normal sodium thiosulphate. The results are given in 

 (I), Table (II). 



(II). The absorption apparatus was replaced, and after the 

 addition to the solution in the distillation flask of one gram of 

 potassium iodide, ten cubic centimeters of concentrated hydro- 

 chloric acid and three to five cubic centimeters of syrupy phos- 

 phoric acid, distillation was again carried on until a volume 

 of ten cubic centimeters had been reached ; the iodine thus 

 liberated being estimated as before. The results are given 

 under (II), Table (II). 



* This Journal, vol. i, p. 35, 1896. 



