Abstract The interaction of H + - and Cu 2+ -ions with Ca-montmorillonite was investigated in 0.1 mol/dm 3 solutions of Ca(CIO 4 ) 2 at 298.2 K by Potentiometrie titrations using both glass electrodes (for H + ) and ion specific electrodes (for Cu 2+ ). The experimental data were interpreted on the basis of the surface complexation model. The calculations were performed with the least-squares program FITEQL (Westall, 1982) using the constant capacitance approximation. The best fit was obtained with a set of equilibria of the general form pH++qCu2++≡SOH⇔(H+)p(Cu2+)q(≡SOH)(p+2q)+βp,q(int)s=[HpCuq(≡SOH)(p+2q)][H+]p[Cu2+]q[≡SOH] and the constants log β 1,0(int) S = 8.16 (± 0.04), log β-1,0(int) S = −8.71 (± 0.08), log β 0,1(int) S = 5.87 (± 0.06), log β−1,1(int) S = −0.57 (± 0.12), log β −2,1(int) S = −6.76 (± 0.02). An appropriate modeling of the H + adsorption data requires the introduction of a second surface group ≡ TOH with the acidity constant ≡TOH−H+⇔≡TO−logβ−1,0(int)S=−5.77(±0.07). In addition, the ion exchange equilibria Ca 2+ − Cu 2+ and Ca 2+ − H + had to be taken into account. Arguments are presented to identify the groups ≡ SOH and ≡ TOH as surface aluminol groups =Al(OH)(H 2 O) and surface silanol groups ≡ Si-OH, respectively.

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