96 ON THE IMPURITIES OF COMMERCIAL ZINC. 
TABLE II. 
Sp. Eti d eA TOC Water. Bees, 
H S was passed for several hours, and the solution satu- 
1 3 c. c. Dee 0 rated with the gas stood over night; a slight cloudi- 
r : ness, not amounting to a precipitate, appeared. 
2 5 c. c. 9 c.c. 0 Do. do. . do. 
3 ZUG * 5 c. c. 0 Do. do. do. 
HS was passed steadily for 24 hours.  Precipitate 
4 3 e. c. : 5 e.c. 25 e. c. | larger than before dilution, but still very slight. 
5 5 c. c. 5 c.c. 25 c. c. Do — do. do. 
6 8 e. c. bet 25 c. c. Do. do. do. 
H S was passed steadily for 28 hours, producing a dis- 
7 10 c. c. 5 e.c. 0 | tinct cloudiness. A 
HS produced in 6 hours a marked cloudiness, which at 
8 15 c. c. 9 c.c. 0 d the end of 40 hours had somewhat increased. 
$ HS was passed steadily for 28 hours, producing a dis- 
9 20 c. e. 5e. oi - 0 d tinet cloudiness. : 
HS was passed 6 hours and over night; a perceptible 
10 25 c. c. 5 e. c. 0 d but very small precipitate. : 
Five experiments, of which the results were not distin- 
11-15 | 30-50 c. c. 5 c. c. 0 guishable; H S was passed thoroughly, and in each 
, case a very slight precipitate was produced. 
16 10 e. c. 40 c. c. 0 H S was passed steadily for 40 hours, producing a very 
i : slight precipitate. : 
The experiments of Table I. sufficiently prove that this precipitation is caused by too great dilution, and that 
an amount of free acid which prevents the precipitation of zinc from a concentrated solution becomes insuf- 
` ficient when the solution is very much diluted. The precipitates produced in dilute solutions are readily soluble: 
in strong acid. This precipitation of zinc from acid solutions seemed a more serious difficulty to Calvert than 
it did to Grundmann, because Calvert's solutions were, on the average, very much more dilute than Grund- 
mann's, both with regard to the acid and to the zinc-salt which they contained. Calvert’s strongest solution 
contained only as much free acid as Grundmann's weakest, and hence in Calvert's experiments there was al- 
ways a free precipitation of zine which was often complete, while Grundmann soon found a limit beyond which 
the precipitation was too trivial to be noticeable. | | 
The precipitates obtained in the experiments of Table II. were so minute, that they could not be certainly 
proved to be sulphide of zinc, and were utterly insignificant in comparison with the amount of zinc-salt con- 
tained in the very concentrated solutions. The gradual increase in the amount of free acid from the first 
experiment to the fifteenth, did not prevent the formation of this slight precipitate, and in the sixteenth ex- 
periment, the cloudiness produced was no greater than in the fifteenth. In a concentrated solution, therefore, 
a small amount of free acid will practically prevent the precipitation of the zinc. We have observed that in 
almost all cases, even when there is no actual precipitate, sulphide of zinc is deposited on any scratches on the 
sides of the beaker, and forms an adhering ring at the end of the tube which delivers the sulphuretted hydrogen. 
With regard to the value of the process of separating copper or cadmium from zinc by means of sulphuretted . 
hydrogen, it seems perfectly possible to avoid such a dilution of the solution under treatment as would cause 
the precipitation of the zinc, without falling into the opposite and equally dangerous error of having too much 
free acid in the solution. Martin * has shown that copper, and several other metals of the same group, are not ` 
completely precipitated by sulphuretted hydrogen in presence of a certain excess of concentrated acid. Be- 
tween these two extremes, it is undoubtedly possible to find a safe mean, which may be rendered doubly sure 
by resorting to the tedious reprecipitation recommended by Grundmann and by Fresenius; but in most cases 
other methods of analysis can be used with less trouble and greater accuracy. : 
* Jour. f. pr. Ch., 1856, LXVII. 371. f Anleitung zur Quantitativen Chem. Analyse, 4te Aufl., 1858, p. 423. 
. Caxenrpor, May, 1860. 
