22 Van Name — Influence of Hydrochloric Acid on the 



forty-eight hours or more before filtering, the final volume 

 being in every case 200 cm3 . 



The precipitates were collected upon asbestos in a perforated 

 platinum crucible, washed thoroughly with a decinormal solution 

 of ammonium sulphocyanide and then with alcohol to remove 

 any excess of the ammonium sulphocyanide, dried at 105° and 

 weighed. It should be mentioned in this connection that no 

 distinct instance of the apparent tendency of cuprous sulpho- 

 cyanide to increase slightly in weight on prolonged heating at 

 110°, described in the article before referred to, was noticed 

 during the present investigation. 



The tests for copper in the filtrates were made by evaporat- 

 ing the solution with nitric acid to a small bulk, heating in a 

 platinum crucible over a radiator to expel sulphuric acid and 

 decompose interfering substances, dissolving the residue in 

 nitric acid, filtering, electrolyzing and weighing the copper. 



As the gravimetric results thus obtained were not sufficiently 

 accurate for small amounts of copper, the copper was also 

 determined by the following colorimetric method. 



The electrolytic deposit was dissolved in nitric acid and the 

 solutiou, contained in a small beaker of about 15 cm3 capacity 

 placed against a white background, was neutralized with ammo- 

 nia and made faintly acid with acetic acid. A dilute solution 

 of potassium ferrocyanide was then added, a few drops at a 

 time, until no further deepening of the brown color was 

 observed. Another beaker of the same size containing the 

 same amount of potassium ferrocyanide was placed beside the 

 first, and after diluting its contents to a like volume a copper 

 sulphate solution containing '00025 grm. of copper per cubic 

 centimeter was added drop by drop from a pipette graduated 

 to hundredths of a cubic centimeter until the brown color was 

 identical with that in the first beaker. With amounts of copper 

 less than *0001 grm., this method is correct to within -00002 

 grm. Its accuracy, however, naturally grows less as the 

 amount of copper increases. The results obtained by the 

 gravimetric and colorimetric methods appear in the table in 

 adjacent columns. 



Small amounts of copper were in nearly every case found in 

 the alcohol washings, due to traces of the precipitate passing 

 through the filter with the alcohol. Although it was difficult 

 to entirely prevent loss in this way, in only a few instances 

 was the quantity thus carried through large enough to cause 

 distinct cloudiness in the filtrate. Water might have been 

 used instead of alcohol to wash out the ammonium sulpho- 

 cyanide, but the chances of loss seemed on the whole to be 

 considerably less when alcohol was employed. 



The values obtained by the weighing as cuprous sulpho- 



