[Fe^{3+}]^2 × [I^{−}]^2 / ([Fe^{2+}]^2 × [I_{2}]) = K_{eq}, and for this constant the relation[551] K_{eq} = K_{I^{−}, Iodine} / (K_{Ferro, Ferri})^2 = 5.6E29 / (10^{17})^2 = 5.6 / 10^5 [p275] can be established. ‹It is evident, from the value of the constant, that the chief tendency of the reversible reaction will be toward the reduction of the ferric ions and the liberation of iodine›, which is in accord with experience (‹exp.›, p. 251). It is interesting to note, again, that the reduction of the ferric salt depends on the reduction of the ‹ferric-ion›: the ferric-ion may be ‹suppressed›, with the aid of potassium fluoride (see p. 255), and the addition of potassium iodide to a mixture of ferric chloride and potassium fluoride leads to the formation of ‹traces›, only, of free iodine (‹exp.›). «Action of Chlorides on Ferric Salts.»—Now, when a chloride is used in place of an iodide, we have to do with an ion, Cl^{−}, which has an enormous affinity for its charge, as compared with that of iodide-ion. The equilibrium relation for the reversible reaction 2 Cl^{−} ⇄ Cl_{2} has the form [Cl^{−}]^2 : [Cl_{2}] = K_{Cl^{−}, Chlorine}, and the value[552] of the constant is 2E60. For the reaction of chloride-ion on ferric-ion we would have, as in the case of the action of iodide-ion, 2 Fe^{3+} + 2 Cl^{−} ⇄ 2 Fe^{2+} + Cl_{2} and [Fe^{3+}]^2 × [Cl^{−}]^2 / ([Fe^{2+}]^2 × [Cl_{2}]) = K_{eq}. For this equilibrium constant we have the relation, as determined above (p. 274), K_{eq} = K_{Cl^{−}, Chlorine} / (K_{Ferro, Ferri})^2 = (2E60) / (10^{17})^2 = 2E26. [p276] It is evident, from the value of the equilibrium constant, that the action of chloride-ion on ferric-ion must result quantitatively so differently from the action of the analogous iodide-ion (p. 275), that the net qualitative results are entirely dissimilar. Whereas in the case of the iodide, liberation of iodine and reduction of the ferric-ion are bound to be the chief and obvious actions, in the case of the chloride-ion, on the other hand, the equilibrium constant demands that there should be no ‹appreciable› reduction of the ferric-ion or liberation of chlorine—which is in accordance with our experience (‹exp.›, p. 256). It is noteworthy, however, that the equilibrium relations demand that at least ‹traces› of chlorine be liberated, and ‹traces› of ferrous salt be formed, since neither [Fe^{++}] nor [Cl_{2}] may have the value 0. If we add some sodium chloride to a solution of sodium sulphate, connected electrically, in the usual way, with a solution of ferric sulphate, a very slight momentary current is produced (‹exp.›). The liberation of the first traces of chlorine and of ferro-ion on the electrodes is necessary, and also sufficient, to satisfy the conditions for equilibrium as expressed by the constant, until diffusion from the electrodes removes these traces.