in Gravimetric Analysis. 



453 



Experiments 5 to 10 were carried out in the presence of 

 various amounts of free sulphuric acid up to 12 per cent of 

 the total volume of liquid. The acid, at least within this 

 limit, does not exert a sufficient solvent effect upon the cuprous 

 sulphocyanide to interfere materially with the accuracy of the 

 process, but it retards the precipitation, making it necessary to 

 increase the time of standing before filtering in proportion to 

 the amount of acid present. In several of these determina- 

 tions the precipitation was visibly incomplete even after sev- 

 eral hours standing. This effect of the acid, however, hardly 

 shows in the results of the table because the standing was 

 always prolonged until the copper appeared to be all down 

 before filtering. 



The low results of No. 7 was probably due chiefly to incom- 

 plete precipitation, although No. 9 shows that even with a 

 much larger amount of acid precipitation may be complete 

 within three hours. In general, however, it is safer to allow 

 ample time (twelve hours or more) for the precipitation when 

 there is much free acid present. 



Comparispn of Nos. 5 and 6, for which only a bare excess 

 of ammonium sulphocyanide was used, with Nos. 7 to 12 

 shows an apparent advantage in the larger excess in the pres- 

 ence of acid. Hydrochloric acid, judging from the results of 

 No. 11 and 12, has no greater disturbing influence than sul- 

 phuric acid, although in No. 12, where the concentrated acid 

 constituted one-fourth of the entire volume, there was appar- 

 ently a slight solvent action. The filtrate from this determi- 

 nation when concentrated to about 25 cm3 and treated with potas- 

 sium ferrocyanide gave a strong test for copper, as did also the 

 filtrate from No. 6. Several of the other filtrates were tested 

 in the same way, but none showed more than an insignificant 

 trace of copper. No. 7, however, was not tested. 



Table II contains the results of a series of experiments con- 

 ducted as before, except that larger amounts of copper were 



Table II. 



Cu 

 taken, 

 grm. 



•3175 

 •3175 

 •3175 

 •3175 



•3175 



H 2 S0 4 

 cone. 

 cm 3 . 



none 



10 

 HC1 

 cone. 

 cm 3 . 



20 



NH 4 SCN 

 approx. 



N/10. 



cm 3 . 



60 



60 



60 



100 



100 



Final 



volume. 



cm 3 . 



500 

 500 

 500 

 500 



500 



Cu 2 S 2 (CN), 



found 

 calc. as Cu. 



grm. 



•3176 

 •3177 

 •3176 

 •3175 



•3165 



Cu in 

 filtrate. 



Error, 

 grm. 



+ 0-0001 



+ 0-0002 



+ 0-0001 



0-0000 



— 0-0010 distinct 



