212 W. Gibbs—Contributions to Chemistry. 
plete when a salt of copper is also present and the liquid con 
tains free chlorhydrie acid. I have endeavored to 
this reduction a method for determining iron volumetrically, but 
all the experiments failed in consequence of the difficulty of de- 
termining the exact point at which the reduction of the iron is 
complete. Sulphocyanid of potassium, proposed for this purpose 
by Winkler® in his process with subchlorid of copper as a redu- 
cing agent, was not found to give sharp indications. When eo 
per and iron are present together as chlorids, the addition of 
ypophosphite of magnesium simply reduces the copper to sub- 
chlorid as above stated. If in this case we add an alkaline chlo- 
rid to keep the subchlorid of co per dissolved, the copper may 
easily precipitated as subeafohid by sulphydric acid gas. 
When arsenic or antimony are present with copper these must 
first be separated before recipitating the copper as hydruret, as 
careful experiments by Mr. ©, Lilly have shown that both arsenic 
and antimony are precipitated with the copper. Mr. Lilly ob- 
tained the following analytical results when arsenous acid was 
t. 
12690 gr. sulphate of copper gave 03279 gr. metallic copper =25°83 pr. ct. pad <r 
15127 gr. “ 0-8905 gr. . =2577 
09638 gr. “ ‘ 0°2509 gr. “ #=26°03:. 
The formula ee 25°42 of metallic copper. In presence of 
antimonious acid— 
0-7100 gr. sulphate of copper gave 0°2454 gr. copper = 34°56 per cent 
After addition of Sb, ©, and Rochelle salt 
0°9875 gr. sulphate of copper gave 0°2426 gr. copper = 24°56 per cent. 
sulphuric acid to convert all the sulphate of potassium 
¥ Zeitschrift fiir Analytische Chemie, Bd. iv, p. 423. 
