of solid potassium iodid added, about 2 or 3 grams. When it is all dissolved by 

 shaking, the free iodin is titrated with the twentieth-normal thiosulphate, using 

 starch as indicator. 



TOTAL COPPER OXID, METHOD III (PROVISIONAL). 



SOLUTIONS REQUIRED. 



A fifth-normal solution of potassium cyanid is prepared, which is standardized 

 against a known weight of copper dissolved in nitric acid, the method being the 

 same as that described in the following paragraph. 



METHOD. 



An aliquot portion of the nitric acid solution used in Method I for total copper 

 oxid is neutralized with sodium carbonate and a trifling excess of the carbonate 

 added. One cubic centimeter of 0.960 sp. gr. ammonia is added and the dark 

 blue solution titrated to the disappearance of the blue color with standard potas- 

 sium cyanid. 



LONDON PURPLE. 



MOISTURE (PROVISIONAL). 



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

 to 110 C. 



TOTAL ARSENIOUS OXID (PROVISIONAL). 



SOLUTIONS REQUIRED. 



A starch solution and a twentieth-normal iodin solution, prepared by either of 

 the methods given under " paris green." 



METHOD. 



Two grams of london purple are dissolved in about 80 cc of water and 20 cc 

 of hydrochloric acid at a temperature of 60 to 70 C, filtered, and washed to a 

 volume of 300 cc. One hundred cubic centimeters of this solution are treated 

 with sodium bicarbonate in excess and made up to the mark in a 500-cc flask, 

 using a few drops of ether to destroy bubbles. Two hundred and fifty cubic 

 centimeters are then filtered off through a dry filter, starch water added, and the 

 solution titrated 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 0.3333 

 gram of the original london purple. 



TOTAL ARSENIC OXID, METHOD I (PROVISIONAL). 



SOLUTIONS REQUIRED. 



The same solutions as those described above for total arsenious oxid are used. 



METHOD. 



Fifty cubic centimeters of the hydrochloric acid solution of london purple, 

 prepared by the preceding method, is heated to 80 C on the water bath and 

 then taken off and 50 cc of concentrated hydrochloric acid and 3 grams of potas- 

 sium iodid added. The mixture is allowed to stand for at least 15 minutes, the 

 " ic " arsenic thus being reduced to " ous" arsenic and the iodin set free. The 

 solution is then rinsed out into a large beaker, well diluted, and tenth-normal 

 sodium thiosulphate added 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 practice the chemist can determine it by proceeding as 

 follows : 



