Titration and spectrophotometric investigations were carried out to reveal the effect of supporting sulphate electrolyte on the equilibrium characteristics of Cu(II) and maleic acid (LH₂) solutions. A consideration of ligand protonization and an account for dilution effects makes it possible to perform a rigorous quantitative description of experimental titration curves containing two well-defined jumps of pH. The cumulative stability constants of LH^– and LH₂ species (β₁^H and β₂^H, respectively) established in the presence of 0.3 M K₂SO₄ (log β₁^H = 5.75 ± 0.09, log β₂^H = 7.30 ± 0.08) are lower as compared with solutions containing no supporting electrolyte (log β₁^H = 6.05 ± 0.07, log β₂^H = 7.48 ± 0.07). This effect seems to arise from the higher ionic strength of the former solutions. Very close spectrophotometric data were obtained for Cu(II) sulphate and perchlorate solutions. In both cases, absorption maxima are observed at the same wavelength λ_max = 806–808 nm, and they linearly depended on the concentration of Cu²⁺ aqua-ions. The latter results provide no data on the interaction between Cu²⁺ and SO₄^2– ions.

Keywords: copper, maleic acid, stability constants, titration, spectrophotometry