Congruence charge

This pH-congruence can be explained (Lyklema, 1987) by inferring that the process of charge formation is independent of the nature of the oxide hence it must be determined by the solution side of the double layer. On the other hand, the specific part, determined by the surface side, is determined by the chemical nature and is reflected only by the pHpzc. 

Discuss some of the consequences of Fig. 3.4. Why does the congruence of surface charge vs ApH imply the applicability of Nernst s law for oxides ... 

The phenomena of surface precipitation and isomorphic substitutions described above and in Chapters 3.5, 6.5 and 6.6 are hampered because equilibrium is seldom established. The initial surface reaction, e.g., the surface complex formation on the surface of an oxide or carbonate fulfills many criteria of a reversible equilibrium. If we form on the outer layer of the solid phase a coprecipitate (isomorphic substitutions) we may still ideally have a metastable equilibrium. The extent of incipient adsorption, e.g., of HPOjj on FeOOH(s) or of Cd2+ on caicite is certainly dependent on the surface charge of the sorbing solid, and thus on pH of the solution etc. even the kinetics of the reaction will be influenced by the surface charge but the final solid solution, if it were in equilibrium, would not depend on the surface charge and the solution variables which influence the adsorption process i.e., the extent of isomorphic substitution for the ideal solid solution is given by the equilibrium that describes the formation of the solid solution (and not by the rates by which these compositions are formed). Many surface phenomena that are encountered in laboratory studies and in field observations are characterized by partial, or metastable equilibrium or by non-equilibrium relations. Reversibility of the apparent equilibrium or congruence in dissolution or precipitation can often not be assumed. 

By "congruence analysis" is meant a rather formal and general procedure to find out whether A G reacts on the surface potential or on the surface charge. In the field of electrosorption on mercury this procedure is fairly well established ) to obtain "the primary electric variable". 

Frumkin, A.N., Damaskin, B.B., and Survila, A.A. (1968) The congruence of the adsorption isotherm with respect to the electrode potential or charge and the choice of an independent electric variable. /. Electroanal. Chem., 16 (4), 493-503. 

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