Dissolution of Hydrous Metal Oxides in Acid Solutions

抄録

The dissolution rate of hydrous oxides of iron, chromium and nickel in acid solutions was expressed as follows; v=ka_H+^la_A^m, where a_H+ is the proton activity, a_A the anion activity, l the reaction order for proton and m the reaction order for anion. The value of l is close to 0.5 irrespective of the species of hydrous oxides but the value of m depends upon the anion species. The dissolution mechanism of hydrous oxide was discussed from electrochemical point of view. The dissolution reaction may be regarded as consisting of the following two coupling and coupled reactions.
OH^-_(H.O.)+H⁺_aq→H₂O_aq
[Me(OH)_n(A^r-)_p]^(z₊-n-rp)+_(H.O.)→[Me(OH)_n(A^r-)_p]^(z₊-n-rp)+_aq
where[Me(OH)_n(A^r-)_p]^(z₊-n-rp)+ represents the intermediate metal-anion complex transferring across the Helmholtz layer at the hydrous oxide-electrolyte interface. The rate of these two reactions is controlled by the potential difference, φ_HL, across the Helmholtz layer, and the net-dissolution rate of hydrous oxides can be determined by superimposing the polarization curves (potential difference, φ_HL, vs. reaction rate) of the two coupling and coupled reactions. The intermediate metal-anion complex has been estimated from the comparison of experimental results with theoretical derivation by assuming the Freundlich isotherm for the adsorption of anions on hydrous metal oxide.