30 METHODS OF ANALYSIS. 



(g) WATER-SOLUBLE ARSENIC OXID.Q PROVISIONAL. 



(1) SOLUTIONS REQUIRED. 



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



(2) DETERMINATION. 



Transfer an aliquot (about 200 cc) of the water extract from the determina- 

 tion of soluble arsenious oxid to a flask, make slightly alkaline with sodium 

 hydroxid, and concentrate to about 25 cc. Remove the flask and allow it to 

 cool to about 80 C., and add an equal volume of concentrated hydrochloric 

 acid and 3 grams of potassium iodid. Allow it to stand 15 minutes, dilute, ex- 

 actly use up the iodin set free with twentieth-normal thiosulphate (using starch 

 if necessary), and neutralize the solution with sodium carbonate. Again make 

 slightly acid with hydrochloric acid, taking care that all lumps of sodium 

 carbonate are acted on, then make alkaline with an excess of sodium bicar- 

 bonate, and titrate with iodin, using starch as indicator. From this figure 

 subtract the figure representing the amount of soluble arsenious oxid, and cal- 

 culate the remainder as arsenic oxid. 



4. Copper Carbonate, 

 (a) COPPER OXID. OFFICIAL. 



(1) SOLUTIONS REQUIRED. 



Use the same solutions as described under the determination of total copper 

 in Paris green (page 27). 



(2) DETERMINATION. 



Dissolve a weighed quantity of the substance in dilute nitric acid and 

 employ one of the methods of analysis given under the determination of total 

 copper in Paris green (page 27). 



5. Potassium Cyanid. 

 (a) CYANOGEN. & OFFICIAL. 



(1) SOLUTIONS REQUIRED. 



Prepare a twentieth-normal solution of silver nitrate. 



(2) DETERMINATION. 



Weigh a large quantity of the sample in a weighing bottle, dissolve in water, 

 and make up to a definite volume. To an aliquot add twentieth-normal silver 

 nitrate, a drop at a time, with constant stirring, until one drop produ 

 permanent turbidity. In calculating the results, one equivalent of silver ia 

 equal to two equivalents of cyanogen, according to the following equation : 



2KCN+AgNO 3 =KCN.AgCN-fKNO, 



6. Soap, 

 (a) GENERAL STATEMENT. PROVISIONAL. 



In most soaps, not considering resin soaps, it is necessary to know three con- 

 stituents In order to judge of their value for spraying purposes, namely, rnois- 



J. Amer. Cnem. Soc., 1000, 22 (12) : 800. 

 6 Button's Volumetric Analysis, Oth ed., p. 200. 



