Goochand Peirce — Iodometric Determination of Acids. 31 



Aet. VII. — The Iodometric Determination of Selenious 

 and Selenic Acids ; by F. A. Gooch and A. W. Peirce. 



[Contributions from the Kent Chemical Laboratory of Yale College— XLVIL] 



It has been shown in a recent paper from this laboratory* 

 that the simple contact of solutions of selenious acid, potas- 

 sium iodide, and hydrochloric acid according to the recom- 

 mendation of Muthman and Schaeferf is not enough to effect 

 the liberation of the theoretical amount of iodine when the 

 assumption is made that the selenium of the selenious acid is 

 all reduced to the elementary condition. On the other hand, 

 it was found that the yield of iodine is complete when such 

 mixtures are submitted to distillation under well-defined con- 

 ditions. It is necessary, however, to estimate not only the 

 iodine which passes to the distillate, but that which is retained 

 in small proportion in the residue, and, though this method of 

 proceeding yields closely accurate analytical results and is by 

 no means difficult, it is obvious that a process so contrived 

 that the reduction of the selenious acid should be registered 

 entirely in the residue would possess the advantage in point of 

 convenience. We have made the attempt, therefore, to apply 

 in this case a principle of action laid down in a method elabo- 

 rated in this laboratory for the estimation of chlorates.^ When 

 a solution of arsenic acid containing potassium iodide and sul- 

 phuric acid is boiled under defined conditions§ the arsenic 

 acid is reduced to arsenious acid with liberation of iodine. 

 When the arsenic acid is in excess the whole of the iodine is 

 evolved and the arsenious acid produced is its exact measure. 

 Upon making the solution alkaline with acid potassium carbo- 

 nate, the arsenious acid may be re- oxidized by standard iodine, 

 and the amount of iodine thus used will be the exact equiva- 

 lent of that set free in the reduction-process. If, however, any 

 other substance more easily reducible than arsenic acid is 

 present, such substance should, naturally, take its part in 

 liberating iodine from the iodide and the reduction of the 

 arsenic acid should be correspondingly less. This was found 

 to be the case when a mixture containing a chlorate, arsenic 

 acid, potassium iodide, and sulphuric acid was boiled under 

 regulated conditions, so that, with a knowledge of the amount 

 of iodide employed and the determination of the quantity of 



* Gooch and Reynolds, this Journal, 1, 254. 



fBer. d. Chem. Gesell., xxvi, 1008. 



± Gooch and Smith, this Journal, xlii, 220. 



§ Gooch and Browning, this Journal, xxxix, 188. 



