LONDON PURPLE; 635 



kaline with sodium bicarbonate. Titrate the amount of arsen- 

 ious oxid present with the standard iodin solution. 



Total Arsenic Oxid. Method 7. 13 Solutions Required. Use 

 the same solutions as described above for total arsenious oxid. 



Determination. Heat 50 cubic centimeters of the hydrochloric 

 acid solution of london purple prepared by the preceding meth- 

 od, to 80 on the water bath, remove and add 50 cubic centi- 

 meters of concentrated hydrochloric acid and three grams of po- 

 tassium iodid. Allow the mixture to stand for at least 15 min- 

 utes, the arsenic acid thus being reduced to the arsenious con- 

 dition and the iodin, set free. 



Then rinse the solution into a large beaker, dilute well, and add 

 twentieth-normal sodium thiosulfate, drop by drop, to eliminate 

 the free iodin. The end point here is rather difficult to read en 

 account of the very dark color of the solution, but with a little 

 practice the chemist can determine it by proceeding as follows : 



Run in the sodium thiosulfate a little at a time, occasionally 

 withdrawing a drop of the solution and adding it to a drop of 

 starch paste. This will give a blue color of varying intensity, 

 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 thio- 

 sulfate. 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 of 

 the sodium carbonate on the bottom are neutralized by the acid, 

 and finally make alkaline with sodium bicarbonate. Add starch 

 paste and titrate with the standard iodin solution. 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 centimeters of iodin corresponding 

 to the total amount of arsenic in the solution expressed as ar- 

 senious oxid. Subtracting from this the number of cubic cen- 

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



