INSECTICIDES AND FUNGICIDES. 



29 



The end point is easily read if the beaker is placed on a white surface 

 between the eye and the light and the iodin solution run in until a distinct 

 purple color appears. The figure thus obtained gives the number of cubic centi- 

 meters of iodin corresponding to the total amount of arsenic In the solution 

 expressed as arsenicus ox id. Subtracting from this the number of cubic centi- 

 meters of iodin corresponding to the arsenious oxid in the previous method 

 gives the number of cubic centimeters of iodin corresponding to the arsenic oxid 

 in 0.3333 gram of the sample. 



(d) TOTAL ARSENIOUS OXID,<* METHOD II (HAY WOOD-DAVIDSON). PROVISIONAL. 

 (Designed to eliminate part of coloring matter.) 



(1) SOLUTIONS REQUIRED. 



Use the same solutions as in Method I for total arsenious oxid. 



(2) DETERMINATION. 



Place 2 grams of London purple in a beaker and dissolve in about 80 cc of 

 water and 20 cc of concentrated hydrochloric acid at a temperature of GO to 

 70 C., cool and add sodium carbonate in slight excess, transfer to a 250-cc 

 flask, bring to the mark, shake, and filter through a dry filter. Acidify 50 cc 

 of the filtrate with hydrochloric acid and make alkaline with sodium bicar- 

 bonate. Titrate the amount of arsenious oxid present with the standard 

 iodin solution. 



(e) TOTAL ARSENIC OXID,& METHOD II (HAY WOOD-DAVIDSON). PROVISIONAL. 

 (Designed to eliminate part of coloring matter.) 



(1) SOLUTIONS REQUIRED. 



Use the same solutions as in Method I for total arsenious oxid. 



(2) DETERMINATION. 



Acidify 50 cc of the solution, prepared as directed in the preceding para- 

 graph, with concentrated hydrochloric acid, heat to 80 C., add 50 cc more of 

 hydrochloric acid and 3 grams of potassium iodid, and proceed as described 

 in Method I for total arsenic oxid (page 28), beginning with the second 

 sentence. 



(f) WATER-SOLUBLE ARSENIOUS OXID.& PROVISIONAL. 



(1) SOLUTIONS REQUIRED. 



Use the same solutions as in Method I for total arsenious oxid. 



(2) DETERMINATION. 



Extract 1 gram of London purple in a stoppered flask with 500 cc of cold 

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

 through a dry filter, to 100 cc of filtrate add sodium bicarbonate, and titrate 

 with standard iodin, using starch as indicator. 



a U. S. Dept. Agr., Bureau of Chemistry, Bui. 81, p. 199. 

 *J. Amer. Chem. Soc., 1900, 22 (12) : 800. 



