26 Van Name — Influence of Hydrochloric A cid, etc. 



amount of the concentrated acid exceeds about 05 per cent of 

 the total volume. By employing a considerable excess of 

 ammonium sulphocyanide the precipitation can be made prac- 

 tically complete, at least after a few hours standing, even 

 when the amount of acid is several times the above figure. 

 The use of a decided excess of the sulphocyanide is advisable 

 not only in the presence of acid but also when the solution 

 contains much ammonium salts, on account of the reduction in 

 the solubility of the precipitate thus brought about.* 



The result of an increase in the ammonium bisulphite in the 

 presence of hydrochloric acid is, except for the liberation of 

 sulphur dioxide, exactly the same as the effect produced by a 

 partial neutralization of the acid by ammonia, namely, the 

 formation of ammonium chloride at the expense of the acid. 

 Unless the amount of acid thus neutralized is quite large, the 

 solubility of the cuprous sulphocyanide in the ammonium salts 

 formed is too small to interfere materially with the complete- 

 ness of the precipitation. There is, moreover, no objection to 

 the use of ammonia in addition to the bisulphite to neutralize 

 an excess of acid, and where sulphur dioxide is employed in 

 place of ammonium bisulphite, ammonia is in many cases 

 desirable. If, finally, the amount of acid is very large it is 

 unquestionably better to remove the greater part before pre- 

 cipitating the copper. 



The helpful advice of Prof. F. A. Gooch throughout the 

 -course of this investigation is gratefully acknowledged. 



*See the last four determinations of Table II. 



