«Intensity of Reactions.»—‹Vice versa›, any oxidizing agent, which has the power to oxidize ferro-ion to ferric-ion, ‹does so the more readily and vigorously›, the more completely any ferric-ion, present or formed, is suppressed. If ferrous sulphate is added to a solution of silver nitrate, a ‹slow›[547] reduction of the silver-ion, and oxidation of the ferro-ion, takes place according to Fe^{2+} + Ag^{+} → Fe^{3+} + Ag ↓. Now, if a little potassium fluoride is added to the mixture, so as to suppress the ferric-ion, which is always present, by contamination, in the original ferrous sulphate solution, and which is formed in the action by the silver nitrate, the oxidation of the ferrous salt and the precipitation of metallic silver is very much ‹accelerated›,[548] and a heavy black precipitate of silver is formed instantly (‹exp.›). The experiment is an illustration of the rôle of potential in oxidation-reduction reactions, the potential and the reducing power of ferro-ion being decidedly diminished by the presence of its oxidation product, the ferric-ion.[549] It is also a further illustration of the rôle the ‹ions› play in these actions, the total amount of ferric salts not being changed by the introduction of the fluoride, which simply suppresses ‹ferric ions›. «Reduction of Ferric Salts by Iodides.»—In the study of the oxidation of the ferro-ion and the reduction of the ferric-ion, we [p273] have thus far considered only the reversible tendencies of the two ions to change into each other, tendencies which would be ‹directly› balanced, in a given solution, without the intervention of other forces, when the ratio of the concentrations of the ions is that of the equilibrium constant, 10^{17}. In reactions involving the oxidation of a ferrous salt, we have to deal, however, in exactly the same way, with the ‹reversible tendency› of the ‹oxidizing substance› to act as oxidizing agent, and, similarly, in every reduction of a ferric salt, we have to deal also with the ‹reversible tendency› of the ‹reducing agent› to act as such. In order to reach some definite conceptions as to the influences of these conflicting tendencies, we shall consider, next, the reduction of ferric salts by iodides, and then contrast this reduction with the action of chlorides on ferric salts, and we shall thus complete the study of this action (see p. 269). For the reduction of ferric salts by iodides (p. 251), we have to consider the reversible tendency of iodide-ion to form iodine and to be formed from iodine: 2 I^{−} ⇄ I_{2}. The constant[550] K_{I^{−}, Iodine} for the equilibrium ratio [I^{−}]^2 / [I_{2}] is 5.6E29 at 25°. [p274] The reduction of ferric salts by iodides is a ‹reversible› reaction: 2 Fe^{3+} + 2 I^{−} ⇄ 2 Fe^{2+} + I_{2}, and the ultimate condition of equilibrium will depend on the values of the constants, K_{Ferro, Ferri} and K_{I^{−}, Iodine}, and on the concentrations of the components used. For the condition of equilibrium we have