Oxidation and reduction reactions are frequently met with in analysis, and we shall turn now to the consideration of such reactions, from the point of view of the modern theory of solution and the laws of equilibrium.
Leaving until later the discussion of the most important and most common oxidizing agents, such as oxygen, nitric acid, permanganate, etc., we shall, in order to develop the subject most simply, confine ourselves, for the moment, to the qualitative study of some oxidations and reductions met with early in the study of analytical reactions. One such reaction is the reduction of ferric salts by hydrogen sulphide, and the simultaneous oxidation of the latter to sulphur (exp.). The reaction may be expressed by the equation
2 FeCl3 + H2S → 2 FeCl2 + 2 HCl + S ↓.
If the action is considered to be the result of the interaction of the ionized ferric chloride and hydrogen sulphide, it would be represented by the equation
2 Fe3+ + 6 Cl− + 2 H+ + S2− → 2 Fe2+ + 6 Cl− + 2 H+ + S ↓.
It is then clear that the reacting components, according to such a conception, are the ferric and the sulphide ions, whose electrical charges mutually discharge each other. Considering only those components whose charges are changed, we have
2 Fe3+ + S2− → 2 Fe2+ + S ↓.
The reduction of a ferric to a ferrous salt would then be accomplished by the discharge of one of the three positive charges on the ferric ions; the oxidation of hydrogen sulphide to sulphur would be accomplished by the complete discharge of the sulphide ions.
Ferric salts are reduced, much in the same way, by iodides (exp.), iodine being liberated: 2 Fe3+ + 2 I− → 2 Fe2+ + I2.
Reduction then appears to involve a loss of positive charges by ions, oxidation a loss of negative charges. [p252]