124 EXPERIMENT STATION. [Jan. 



standard uranyl nitrate, using potassium ferrocyanide to deter- 

 mine the end point. The Kessler and Bodeker methods are ob- 

 jectionable in their requirement of an " outside " indicator. 

 The Buiisen and Bennett methods are lengthy, and demand very 

 careful manipulation. The permanganate titration is not as 

 sensitive as the iodine, and the Krickhaus method oifers no ad- 

 vantages in its application to arsenic acid over a similar reduc- 

 tion and titration with iodine. In other words, the iodine titra- 

 tion method (Mohr) seemed to us rather superior to any other 

 in point of accuracy, manipulation and time, and was adopted 

 for the work in \'iew. 



There arc a number of processes that are noted more particu- 

 larly as a means of eliminating impurities likely to effect the 

 arsenic determination, among which may be mentioned the dis- 

 tillation processes of rischer,^ Piloty and Stock,- Stead. '"^ and 

 Jannasch and Seidel,^ using hydrochloric acid in connection 

 with reducing substances such as ferrous salts, hydrogen sulfide, 

 and potassium bromide and hydrazine hydrochloride. The above 

 list of methods is far from complete, but attention has been called 

 to practically every type applicable to commercial products. 



loDixE Method (Mohr). 

 x\s previously stated, the iodine method appeared to offer the 

 greatest advantages, and was selected. A clear understanding 

 of the character and limitations of the reaction underlying the 

 method is necessary at the outset. Iodine is an indirect oxidizer, 

 acting on the elements of water with the formation of hydriodic 

 acid and the liberation of oxygen. 



AS2O3 + 41-^2 H2O = AsoOr, + 4 HI. 



The oxidation cannot be conducted in an acid or neutral solu- 

 tion because of the reversible action of the hydriodic acid. If 

 the latter is neutralized with sodium bicarbonate as rapidly as 

 produced, the reaction will proceed to completion. Caustic 

 alkali or carbonate cannot l)o employed, as thoy absorb iodine, 

 the former being especially active. The reaction between starch 



> Ztachr. Analyt. Cliem., 21, 266 (1882). 

 2 Ber. Deut. Chom. Gcsell., 30, 1649 (1897). 

 ' Sutton, Vol. .\nal., Edit. 9, 159 (1904). 

 * Ber. Deut. Chem. Gesell., 43, 1218 (1910). 



