62 ON THE IMPURITIES OF COMMERCIAL ZINC. 
95 gram. of pure sulphate of zinc in 100 c. c. of water acidulated with 5 gram. of 
pure sulphuric acid. The precipitated sulphate was first weighed on a tared filter, and 
afterwards ignited, and again weighed. The results of two experiments are given in 
the following table: — 
TABLE II. 
(1.) (2.) (3.) (4.) (5.) 
Weight of Weight of Weight of Excess of Pb O in Col.2 | Loss of PbO in Col. 3, 
PbOA PbO, 80; Pb O, 80s over Calculated Pb O from Calculated Pb O 
taken. on Tared Filter. after Ignition. in Col. 1. in Col. 1 
gramme. gramme. gramme. gramme. gramme. 
1.1013 0.8838 0.8692 
No. 1. containing containing ` containing 0.0021 0.0087 
0.6484 of PbO | 0.6505 of PbO 0.6397 of PbO 
1.0164 0.8159 0.8053 
No. 2. containing containing containing 0.002 0.0058 
0.5985 of PbO | 0.6005 of PbO | 0.5927 of PbO 
The filtrates from the sulphate of lead precipitates in these two experiments were 
saturated with sulphuretted hydrogen gas during forty hours, but not a trace of lead 
could be detected in either case. These experiments sufficiently prove that the ac- 
curacy of the determination of lead as sulphate is not impaired by the presence of 
sulphate of zinc, or, at least, that, if there be any error, it is an error on the safe side, 
causing a loss of lead, and not a gain.* | 
* The results given in Table II., and in the immediately preceding statement of the experiment with Pro- 
birblei, illustrate a fact which we have had frequent occasion to observe; viz. that the sulphate of lead can 
be weighed more accurately on a tared filter than by any process which subjects the filter and precipitate to 
ignition. There is an unavoidable loss in igniting this precipitate, in consequence of the volatilization of the 
sulphide of lead, which is reduced from the sulphate by the paper of the filter. The common objection to 
weighing this precipitate on a tared filter, is the difficulty of removing the last traces of the sulphuric acid 
used in washing; but careful washing with alcohol will overcome this difficulty, whereas no care can prevent 
the loss consequent upon ignition. We ignited 1.4004 gram. of absolutely pure sulphate of lead with a fine 
Swedish filter 10 centimetres in diameter, cut into small pieces, moistened, and mixed with the sulphate of lead. 
The sulphate, after ignition, weighed 1.3826 gram. 
Amount of lead in 1.4004 gram. PbO SO, = 0.95654 
Loss  “ by ignition (022001216 
yi e = in per cent — 1.271 
In this experiment the amount of sulphate of lead used was considerable; if this amount be small, the loss 
by ignition will be proportionally increased. It may be objected, that it is not usual to ignite the filter with 
the mass of the precipitate; but, on the other hand, it is often the case that the filter ashes cannot be burnt 
white on the crucible cover, and moreover the process of filtration produces a very intimate contact between 
the paper and the fine particles of sulphate of lead which is not imitated in the above experiment. - 
