I2 He (Cs GOOLE: 
The fact that the solubility of the sulphides is so much greater 
in the presence of ferric sulphate, but is not at all proportional to 
the concentration of the latter salt, is probably to be explained by 
the hypothesis that the active agent is really the sulphuric acid, 
while the ferric sulphate exerts little direct action. The reaction 
of sulphuric acid on a sulphide may be represented by the equation 
Ag,S+H,SO,=Ag,SO,+H,S. This reaction, like most others, 
comes to an equilibrium. It must stop as soon as the product of 
the concentrations of the silver ion and the sulphide ion in the solu- 
tion becomes equal to the solubility product of the sulphide acted 
upon (in this case Ag,S). As the solubility products of most of the 
insoluble sulphides are very small,’ the reaction in these cases can 
proceed but for an almost immeasurably short distance before 
stopping. But if some substance present remove the hydrogen 
sulphide as fast as formed, the reaction may proceed till all the 
ferric sulphate is used up. This is probably the main function of 
the ferric sulphate, which reacts thus with hydrogen sulphide: 
Fe,(SO,);+H.S = 2FeSO,+H.SO,+S. If this be the true explana- 
tion of the action of the ferric sulphate, then it is to be expected that 
a small amount of ferric sulphate would cause as powerful a solvent 
action as a larger amount. ‘The increased action in solutions more 
concentrated in ferric sulphate, shown especially in the case of the 
galena, may be ascribed either to a direct oxidizing effect of the 
sulphate on the sulphide or to the increased speed with which the 
more concentrated sulphate solution destroys the hydrogen sulphide 
formed. 
If this explanation of the function of the ferric solution be the 
true one, two questions naturally arise: (@) Why is a measurable 
loss obtained experimentally when pure sulphuric acid acts on the 
sulphides ? and (6) Was the reaction really at an end when the 
measurements taken in the experiments were made? In answer 
to question (a) the writer suggests that the oxygen of the air acts 
like the ferric sulphate, in removing hydrogen sulphide from solu- 
tion, thus: H,S+-O=H,0O-+S; its action is much slower than that 
of the ferric sulphate, however, so that the action of the acid on the 
* Knox, Trans. Faraday Soc., IV (1998), Part I, p. 29. Knox determined the 
solubility product of AgaS as 3.9 10-5; of CuS as 1.2X10—4; of PbS as 2.6 X10—%, 
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