ON THE IMPURITIES OF COMMERCIAL ZINC. 95 
Note on the Precipitation of Zine from Acid Solutions by Sulphuretted Hydrogen. — In the earlier steps 
of this investigation, we were somewhat embarrassed by the precipitation of sulphide of zine, when sul- 
phuretted hydrogen was passed for several hours through solutions of sulphate of zinc containing a consid- 
erable excess of sulphuric acid. In studying the best method of avoiding the difficult ; We made a series 
of experiments which seem to us to reconcile statements of previous observers which were apparently at 
variance, and to put the subject in a clearer light than that in which former observations had left it. From 
nine experiments with an aqueous solution of sulphate of zinc acidulated with sulphuric acid, and nine similar 
experiments with chloride of zinc and chlorhydric acid, Calvert * inferred that “zinc, even in very acid liquors, 
was freely and sometimes completely precipitated from them by sulphuretted hydrogen,” and that “the old 
method for the separation of salts of zinc from those of copper must in future be rejected as completely in- 
exact.” On the other hand, Grundmann f has deduced from four experiments with a solution of sulphate of 
zinc, acidulated with chlorhydric acid, a rule for the prevention of this precipitation of zinc by sulphuretted 
hydrogen, and he also maintains that the separation of copper or cadmium from zine can be satisfactorily per- 
formed by repeating two or three times the precipitation of the copper or cadmium by sulphuretted hydrogen, 
and taking care to maintain in the solution a certain specified excess of acid. Grundmann used in ath of 
his four experiments, 10 c.c. of a solution of sulphate of zinc (containing 0.2118 gram. of oxide of zinc in 
10 c. c.), and 100 c. c. of water; to this mixture he added in the first experiment 1 c.c. of chlorhydric acid 
(Sp. Gr. = 1. 1.) ; in the second, 2 c. c. ; in the third, 3 c. c.; and in the fourth, 4 c. c. In the first experiment 
all the zinc was precipitated, but in the last none. The following tables contain a series of experiments which 
we devised in order to prove what might be inferred from some of Calvert’s experiments ; namely, that this 
precipitation of zine by sulphuretted hydrogen was dependent, not so much upon the relation of the amount of 
' free acid to the zinc-salt in the solution, as upon the degree of dilution of the solution, and that the mere ad- 
dition of water might determine this precipitation in a solution which remained perfectly clear before such 
dilution. For convenience of comparison, we have used in the experiments of Table I. chlorhydric acid, and 
a solution of pure,chloride of zinc of the same strength, and in the same quantities which were employed by 
Grundmann, but have substituted for the sulphate of zinc which he used chloride of zinc, in order to avoid 
bringing together in the same solution the sulphate of zinc and free chlorhydric acid, — a complication which 
seemed unnecessary and undesirable. 
TABLE I. 
1 Solution; .2118 gram. : ` Remarks. 
Sp. ico LL m Zn Om 10 c. "deren Water. : 
S No precipitate at the end of half an hour; after 15 hours 
1 f 100 c. c. d an abundant but not complete precipitation, 
9 ee 10 c. c. l 200 c. c. At the end of 15 hours all the zinc was precipitated. 
i : The precipitation began in two hours, and when néxt 
3:13 500 c. c. d gien (us the end of 24 hours) was complete. — 
r No precipitate in 4 hours; in 24 hours there was a large 
4 200 c. c. d but not complete precipitate. 
No precipitate in 1 hour; after 18 hours an abundant 
5 Ir 2ac i 10 c. c. : 500 c. c. Í bl not complete precipitation. 
6 S e 1000 c. c. In 18 hours all the zinc was precipitated. 
J ; 
S HS was passed three times, and the last time during 18 
T H 100 c. c. consecutive hours; no precipitate. 
H S was passed twice; the last time during 12 consecu- 
451 Bee 10 c. c. 1 200 c. c. d tive hours; no precipitate. 
"diee Ea Began to precipitate in 3 hour; in 18 hours there was 
9 1j E 500 c. c. d a large but not complete precipitation. - 
: HS was passed three times, and the last time during 
10.11 ; f 500 c. c. d 18 consecutive hours; no precipitate. 
; H S was passed twice, during 15 hours each time; no 
1 |» 4ec 10 c. c. 4| 1000 ee, | precipitate. ` 
B to precipitate in 1 hour, and gave a dense but 
12 l) [| 1500 cc. d ee Ley precipitate. 
* Report of British Association for Adv. Sci., 1855, pP. 51. 1 Jour. f. pr. Ch., 1858, LXXII. 242, 
