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, Fe3+ ⇄ Fe2+, and that the oxidation of chloride ions to chlorine is also a reversible process, 2 Cl− ⇄ Cl2. We will deal first, in some detail, with the action Fe3+ ⇄ Fe2+. For this reversible action we have an equilibrium constant[541] [Fe2+] : [Fe3+] = KFerro, Ferri = 1017, 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 1017 times as great as the concentration of ferric-ion.
If we connect a 0.1-molar solution of ferric chloride with a 0.1-molar solution of ferrous chloride, by means of a "salt bridge" and a pair of platinum electrodes dipping into the solutions and connected with the voltmeter (see p. [253]), a current is produced, the positive current entering the voltmeter from the electrode placed in the ferric chloride solution (exp.). It is evident that, in the effort to establish equilibrium, ferric ions in the ferric chloride solution are reduced at the expense of the oxidation of ferrous ions in the ferrous chloride solution. If we consider only the ratio of the concentration of the ferro-ion to that of the ferric-ion in each of the salt solutions and leave out of consideration, for the moment, other, secondary, electrical forces,[544] it is clear that the ratio [p271] [Fe2+]1 : [Fe3+]1 in the ferrous salt solution, considered by itself, is far closer to the point of equilibrium[545] than the ratio [Fe2+]2 : [Fe3+]2 in the ferric chloride solution, in which the concentration of ferric-ion is enormously greater than that of ferro-ion, while the equilibrium constant demands that the ferro-ion should be in great excess. The strongest tendency to change must be toward a reduction of the concentration of the ferric-ion in the solution of ferric chloride, which is in agreement with the observed direction of the current. Equilibrium, it may be added, will be reached when the ratio of the concentration of ferro-ion to that of ferric-ion is the same in both solutions.[546]
The addition of potassium fluoride to the ferric chloride solution converts the ferric-ion into the rather stable complex ferrifluoride-ion FeF63−, whose potassium salt K3FeF6 is formed. The [p272] concentration of ferric-ion being decidedly reduced, the system must be nearer to the condition of equilibrium, the potential must fall (exp.). It is again evident (p. [255]) that the oxidizing agent is clearly the ferric-ion, and not the total quantity of the ferric salt in the solution.
Intensity of Reactions.
Reduction of Ferric Salts by Iodides.
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− ⇄ I2. The constant[550] KI−, Iodine for the equilibrium ratio [I−]2 / [I2] is 5.6E29 at 25°. [p274]
The reduction of ferric salts by iodides is a reversible reaction: 2 Fe3+ + 2 I− ⇄ 2 Fe2+ + I2, and the ultimate condition of equilibrium will depend on the values of the constants, KFerro, Ferri and KI−, Iodine, and on the concentrations of the components used. For the condition of equilibrium we have
[Fe3+]2 × [I−]2 / ([Fe2+]2 × [I2]) = Keq,
and for this constant the relation[551]
| Keq = | KI−, Iodine | = | 5.6E29 | = | 5.6 |
| (KFerro, Ferri)2 | (1017)2 | 105 |