20 Van Name — Influence of Hydrochloric Acid on the 



Art. II. — The Influence of Hydrochloric Acid on the 

 Precipitation of Cuprous Sulphocyanide ; by R. G. Van 



Name. 



[Contributions from the Kent Chemical Laboratory of Yale University. — CV.] 



In a previous contribution to this Journal* the writer gave 

 the results of a series of experiments upon the estimation of 

 copper by precipitating and weighing as cuprous sulphocyanide. 

 It was concluded from this work that the process did not lose 

 greatly in accuracy even when quite large amounts of sulphuric 

 or hydrochloric acid were present. In discussing the results 

 obtained, however, no distinction was made, as should have 

 been done, between free acid originally present in the solution 

 and that remaining in the free state after the addition of the 

 precipitants, ammonium bisulphite and ammonium sulpho- 

 cyanide. • 



With hydrochloric acid the effect of the bisulphite, as will 

 be shown, is to convert a definite quantity of acid into ammo- 

 nium chloride, so that by increasing the bisulphite the free acid 

 present when the copper is precipitated may be diminished to 

 any desired extent. 



The results obtained by precipitating copper in solutions con- 

 taining various amounts of hydrochloric acid, weighing as 

 cuprous sulphocyanide, and determining the copper in the fil- 

 trate, are given in Table I. 



In neutral or faintly acid solutions cuprous sulphocyanide is 

 precipitated in a very finely divided state and often shows a 

 tendency to pass through the filter with the wash water as soon 

 as the latter becomes nearly free from dissolved salts. This 

 may be prevented by washing with a salt solution. For this 

 reason the precipitates were washed with decinormal ammo- 

 nium sulphocyanide, a medium in which cuprous sulpho- 

 cyanide is even less soluble than in pure water. 



An electrolytically standardized copper sulphate solution of 

 about -J- normal strength was measured out in portions of 50 cm3 

 for the first seven determinations. For the last four, like 

 quantities of approximately ^ normal copper sulphate were 

 taken. The required amount of strong hydrochloric acid of 

 specific gravity l-lT-1'18 was then added, followed, after dilut- 

 ing, by ammonium bisulphite and ammonium sulphocyanide 

 in the order named, the last in measured portions of a stand- 

 ardized decinormal solution. The ammonium bisulphite solu- 

 tion was prepared by saturating aqueous ammonia with sul- 

 phur dioxide. 



After precipitation the mixture was allowed to stand for 



* Vol. x, 451. 



