1911.] PUBLIC DOCUMENT — No. 31. 123 



classification of this character is open to criticism, but will serve 

 the purpose intended. 



The gravimetric methods include the hydrogen sulfide pre- 

 cipitation of arsenous acid ^ weighable as arsenous sulfide after 

 removal of the excess sulfur; the jSTeher modification- of the 

 Bunsen method," precipitating arsenic acid with hydrogen sul- 

 fide, Aveighable as arsenic sulfide ; the modified Levol method, 

 precipitating arsenic acid with " magnesia mixture," weighable 

 as magnesium pyro-arsehate ; and the Werther method,^ precipi- 

 tatiug arsenic acid with uranyl acetate, weighable as uranyl 

 pj'ro-arsenate. The inherent faults of the sulfide methods render 

 them impracticable. The modified Levol method, the most 

 ])rominent of the gravimetric, is complicated, tedious and tends 

 towards low results. All of these methods are time consumers, 

 and none of them appear to have met with favor, having of late 

 been almost entirely superseded by volumetric. 



The volumetric methods include the Kessler method,^ oxidiz- 

 ing arsenous acid with potassium bichromate and titrating the 

 excess chromic acid with standard ferrous sulfate, using potas- 

 sium ferri-cyanide to determine the end point ; the permanganate 

 method, titrating arsenous acid with standard potassium perman- 

 ganate to a rose color; the ]\lohr method, titrating arsenous acid 

 with standard iodine in the presence of sodium bicarbonate, 

 using starch paste as indicator; the Bunsen method,*^ based on 

 the difference in amount of chlorine evolved from hydrochloric 

 acid by a given weight of potassium bichromate in the presence 

 of arsenous acid, the gas being conducted into potassium iodide 

 and the free iodine titrated with standard sodium thiosulfate, 

 using starch paste as indicator ; the Krickhaus method,^ reducing 

 arsenic acid to arsenous with hydrochloric acid and potassium 

 iodide, and titrating the free iodine with standard thiosulfate ; 

 the Bennett modification ^ of the Pierce method,^ precipitating 

 arsenic acid with silver nitrate and titrating the silver in the 

 precipitate with potassium sulphocyanate, according to Vol- 

 hard; ^^ and the Bodeker method,^ ^ titrating arsenic acid with 



» Fresenius, Quan. Chem. Anal. » Jour. Amer. Chem. Soc, 21, 431 (1899). 



= Ztschr. Analyt. Chem., 32, 45 (1893). » Proc. Col. Sci. Soc, Vol. 1. 



' Ann. Chem. Pharm., 192, 305. '" Liebig's Ann. Chem., 190, 1 (1878). 



* Jour. Prakt. Chem., 43, 346 (1848). •' Ann. Chem. Pharm., 117, 195. 



» Poggend. Ann., 118, 17, Series 4 (1863). 



s Ann. Chem. Pharm., 86, 290. 



' Engin. and Min. Jour., 90, 357. See Sutton for earlier references. 



