28 METHODS OF ANALYSIS. 



shaking, titrate the free iodin with thiosulphate, using starch as indicator 

 toward the end of the reaction. 



3. London Purple, 

 (a) MOISTURE. PROVISIONAL. 



Dry from 1 to 2 grams for from 10 to 12 hours at a temperature of 105 to 

 110 C. 



(b) TOTAL ABSENIOUS Oxn>,<* METHOD I (HAYWOOD). PROVISIONAL. 



(1) SOLUTIONS REQUIRED. 



Prepare starch and iodin solutions by either of the methods given under 

 " 2. Paris green," pages 25 and 26. 



(2) DETERMINATION. 



Dissolve 2 grams of London purple in a mixture of about 80 cc of water and 

 20 cc of concentrated hydrochloric acid at a temperature of from 60 to 70 C. ; 

 filter and wash to a volume of 300 cc. Treat 100 cc of this solution with sodium 

 bicarbonate in excess and make up to the mark in a 500 cc flask, using a few 

 drops of ether to destroy the bubbles. Pass a portion through a dry filter, and 

 to 250 cc add starch water, and titrate the solution with standard iodin to the 

 appearance of a blue color. The result is the arsenious oxid, as such, in 50 cc 

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



(c) TOTAL ARSENIC OXID,<* METHOD I (HAYWOOD). PROVISIONAL. 



(1) SOLUTIONS REQUIRED. 



Use the same solutions as described above for total arsenious oxid (b) (1). 



(2) DETERMINATION. 



Heat 50 cc of the hydrochloric-acid solution of London purple, prepared by 

 the preceding method, to 80 C. on the water bath, remove and add 50 cc of 

 concentrated hydrochloric acid and 3 grams of potassium iodid. Allow the 

 mixture to stand for at least 15 minutes, the arsenic acid thus being reduced 

 to the arsenious condition and the iodin set free. Then rinse the solution 

 into a large beaker, dilute well, and add twentieth-normal sodium thiosulphate, 

 drop by drop, to eliminate the free iodin. The end point here is rather difficult 

 to read on account of the very dark color of the solution, but with a little prac- 

 tice the chemist can determine it by proceeding as follows : 



Run in the sodium thiosulphate a little at a time, occasionally withdrawing 

 a drop of the solution and adding it to a drop of starch paste. This will, of 

 course, give a blue color with the starch, which becomes fainter as the iodin is 

 used up. Finally, when a drop of the solution gives only the slightest blue 

 color with the starch, add a little starch paste directly to the whole solution 

 and dissipate the blue color with a few drops of thiosulphate. With a little 

 practice the chemist can in this way get the exact end point Immediately 

 make the solution alkaline with solid sodium carbonate. Again make it 

 slightly acid with hydrochloric acid, taking care that all of the solid particles I 

 of the sodium carbonate on the bottom are neutralized by the acid, and finally 1 

 make alkaline with sodium bicarbonate. Add starch paste and titrate with the I 

 standard iodin solution. 



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



