636 AGRICULTURAL ANALYSIS 



timeters of iodin corresponding to the arsenious oxid in the pre- 

 vious method gives the number of cubic centimeters of iodin 

 corresponding to the arsenic oxid in 0.3333 gram of the sample. 



Total Arsenic Oxid. Method II. The method is designed 

 to eliminate part of coloring matter. The solutions required are 

 the same as in Method I for total artenious oxid. 



Determination. Acidify 50 cubic centimeters of the solution, 

 prepared as directed in the preceding paragraph, with concen- 

 trated hydrochloric acid, heat to 80, add 50 cubic centimeters 

 more of hydrochloric acid and three grams of potassium iodid, 

 and proceed as described in Method I for total arsenic oxid in 

 london purple beginning with the second sentence. 



Water-Soluble Arsenious Oxid. Use the same solutions as 

 in Method I for total arsenious oxid. 



Determination. Extract one gram oi london purple in a stop- 

 pered flask with 500 cubic centimeters of cold carbon dioxid 

 free water for seven days, shaking eight times each day. Filter 

 through a dry filter; to 100 cubic centimeters of filtrate, add 

 sodium bicarbonate, and titrate with standard iodin, using starch 

 as indicator. 



Water-Soluble Arsenic Oxid. Use the same solutions as in 

 Method I for total arsenious oxid. 



Determination. Transfer an aliquot (about 200 cubic cen- 

 timeters) of the water extract from the determination of soluble 

 arsenious oxid to a flask, make slightly alkaline with sodium 

 hydroxid, and concentrate to about 25 cubic centimeters. Re- 

 move the flask and allow it to cool to about 80, and add an 

 equal volume of concentrated hydrochloric acid and three grams 

 of potassium iodid. Allow it to stand 15 minutes, dilute, exactly 

 use up the iodin set free with twentieth-normal thiosulfate (us- 

 ing starch if necessary), and neutralize the solution with sodium 

 carbonate. Again make slightly acid with hydrochloric acid, 

 taking care that all lumps of sodium carbonate are acted on, then 

 make alkaline with an excess of sodium bicarbonate, and titrate 

 with iodin, using starch as indicator. From this figure subtract 



14 Haywood, Journal of the American Chemical Society, 1900, 22 ' 800. 

 Bureau of Chemistry, Bulletin 107, 1907 : 29. . 



