[539] ‹Vide› Ostwald's ‹Lehrbuch der allgemeinen Chemie›, 2d Ed., Vol. II, p. 874, for the historical data on this action. ‹Vide› Küster's experiments, ‹Z. Elektrochem.›, «4», 503 (1897). [540] See the footnote, p. 267, in regard to the form the equilibrium ratio assumes when metals producing ions of different ‹valence› are used. [541] The value of the constant is calculated from the data given by Peters, ‹Z. phys. Chem.›, «26», 193 (1898). [542] The fact that this equilibrium relation has been proved to hold for the action Fe^{2+} ⇄ Fe^{3+} and that it must be taken into account in all oxidation-reduction reactions involving these ‹ions›, in no wise excludes the possibility that other equilibrium relations can also exist between ferrous and ferric compounds. For instance, ferrous hydroxide Fe(OH)_{2} may well have a characteristic tendency of its own to assume a further positive charge (lose an electron) according to Fe(OH)_{2} ⇄ Fe(OH)_{2}^{+}, the potential of which action may, under given conditions, be a ‹main determining factor› in the course of an action, ‹e.g.› in alkaline mixtures. It is not impossible, even, that we also must consider negative ions FeO_{2}^{2−} and their tendency to be oxidized. Evidence would ‹suggest› that ferrous hydroxide, ‹or its negative ion› FeO_{2}^{2−}, may have, indeed, a very ‹great tendency› to be oxidized, possibly much greater than the tendency of Fe^{2+} to form Fe^{3+}. (‹Cf.› Manchot, ‹Z. anorg. Chem.›, «27», 419 (1901), and McCoy and Bunzel, ‹J. Am. Chem. Soc.›, «31», 370 (1909)). Closer investigations of these relations, from a quantitative viewpoint, would probably determine this question and bring exceedingly important relations to light. [543] ‹E.g.› by the potential of the action Cl_{2} ⇄ 2 Cl^{−}.