«Applications; Reduction of Ferric Salts and Oxidation of Ferrous Salts.»—It will not be difficult to arrive now at definite conceptions as to why certain reactions of oxidation and reduction do not seem to take place, although they are, qualitatively, entirely analogous to reactions which take place readily. The study of one of the questions previously raised (see p. 256), namely as to why ferric ions apparently are not reducible by chloride ions, while they are easily reduced by iodide ions, will be sufficient to illustrate the application of the principles. In considering the question of the possible reduction of ferric to ferrous ions, at the expense of the oxidation of chloride ions to chlorine, we must bear in mind the fact that the reduction of the ferric ions is a ‹reversible process›, Fe^{3+} ⇄ Fe^{2+}, and that the oxidation of chloride ions to chlorine is also a ‹reversible process›, 2 Cl^{−} ⇄ Cl_{2}. We will deal first, in some detail, with the action Fe^{3+} ⇄ Fe^{2+}. For this reversible action we have an ‹equilibrium constant›[541] [Fe^{2+}] : [Fe^{3+}] = K_{Ferro, Ferri} = 10^{17}, which must be [p270] taken into account in all oxidation and reduction reactions involving these ions.[542] In a system containing the two ions, the tendency towards reduction of ferric-ion and the tendency toward oxidation of ferro-ion ‹would be directly in equilibrium› (‹i.e.› without the intervention of other opposed forces, such as an electric potential, produced by an opposing cell or produced by an opposing action[543] of other components in the solution) ‹only when the concentration of ferro-ion is 10^{17} times as great as the concentration of ferric-ion›.