634 AGRICULTURAL ANALYSIS 



"ic" and "ous" arsenic in the form of the uncombined oxids. 

 Dirt in the form of sand may also be present in the mixture; to 

 a limited extent. It will thus be seen that the analysis of Ion- 

 don purple should include determinations of moisture, total ar- 

 senic oxid, total arsenious oxid, soluble arsenic oxid, soluble ar- 

 senious oxid, calcium oxid, sand and the dye by difference. 



Following are the methods of analysis commonly used in deter- 

 mining the above constituents. 



Moisture. Dry from one to two grams for from 10 to 12 

 hours at a temperature of 105 to no . 11 



Total Arsenious Oxid. Method I. Solutions Required. Pre- 

 pare starch and iodin solutions by either of the methods given 

 under paris green. 



Determination. Dissolve two grams of london purple in a 

 mixture of about 80 cubic centimeters of water and 20 cubic cen- 

 timeters of concentrated hydrochloric acid at a temperature of 

 from 60 to 70 ; filter and wash to a volume of 300 cubic cen- 

 timeters. Treat 100 cubic centimeters of this solution with so- 

 dium bicarbonate in excess and make up to the mark in a 500 

 cubic centimeter flask, using a few drops of ether to destroy the 

 bubbles. Pass a portion through a dry filter, and to 250 cubic 

 centimeters, add starch water, and titrate the solution with stand- 

 ard iodin to the appearance of a blue color. The result is the 

 arsenious oxid, as such, in 50 cubic centimeters of the original 

 solution, or in 0.3333 gram of the original london purple. 



Total Arsenious Oxid. Method 7/. 12 The method is designed 

 to eliminate part of the coloring matter. The solutions re 

 quired are as in Method I for total arsenious oxid. 



Determination. Place two grams of london purple in a beak- 

 er and dissolve in about 80 cubic centimeters of water and 20 

 cubic centimeters of concentrated hydrochloric acid at a tem- 

 perature of 60 to 70, cool and add sodium carbonate in slight 

 excess, transfer to a 250 cubic centimeter flask, bring to the 

 mark, shake, and filter through a dry filter. Acidifv 50 cubic 

 centimeters of the filtrate with hydrochloric acid and make al- 



11 Haywood, Journal of the American Chemical Society, 1900, 22 : 800. 

 Bureau of Chemistry, Bulletin 107, 1907 : 28. 



12 Bureau of Chemistry, Bulletin 81, 1904 : 199; Bulletin 107, 1907 : 29. 



