1910.] PUBLIC DOCUMENT — No. 31. 141 



tation of a very insoluble form of copper and the loss of the 

 determination. Twenty-five cubic centimeters of a saturated 

 solution of zinc acetate and 20 cubic centimeters of potassium 

 iodide (165 grams to 1,000 cubic centimeters) are added, and 

 the free iodine titrated with X/IO sodium thiosulfate solution 

 (24.83 grams per liter). The thiosulfate is run in gradually, 

 with constant shaking, until the brownish yellow color (iodine) 

 has been largely destroyed ; then 2 cubic centimeters of starch 

 paste (1 gram to 200 cubic centimeters) are added and the 

 titration continued until the blue particles have entirely disap- 

 peared. Towards the end of the reaction the flask should be 

 stoppered and shaken thoroughly. 



The copper equivalent of the thiosulfate is determined by 

 diluting 25 cubic centimeters of a standard copper solution with 

 water, evaporating and titrating exactly as in the test. The 

 standard solution is prepared by dissolving 10 grams of pure 

 dry metallic copper in 200 cubic centimeters of concentrated 

 nitric acid, and making up to a liter with water at 20° C The 

 solution should be analyzed gravimetrically, and will keep almost 

 indefinitel}'. From this data the reducing action of the sugar 

 solution can be readily calculated in terms of copper, and by 

 conversion tables the corresponding amount of sugar. The 

 method has been more recently applied to the determination of 

 copper in Paris green and arsenite of copper, and found equally 

 satisfactory. The copper is precipitated from a hydrochloric 

 acid solution in the presence of sodium acetate with a slight 

 excess of sodium hydrate. The resulting cuprous oxide is trans- 

 ferred to a sugar tube and determined as above. While the 

 first reading of this method might give the impression that it 

 was rather difficult, in reality it is extremely simple, can be 

 carried out rapidly and the titration is very sensitive. 



