Influence of Water on the Equilibria of Protonation of Mesoporphyrin IX Dimethyl Ether in Acetonitrile

The equilibria of protonation of mesoporphyrin IX dimethyl ether (H₂MP), namely, H₂MP + H⁺ H₃MP⁺ and H₃MP⁺ + H⁺ H₄MP²⁺, in the H₂MP(1.4 × 10^–5 mol/l)–HClO₄(0–0.01 mol/l)–H₂O(0.01–1.65 mol/l)–CH₃CN system at 298 K were studied using the spectropotentiometry method. It was established that the first-step equilibrium (logK ₁ = 11.95) is not affected by water. The dication forms two hydrates (H₂O)H₄MP²⁺ and (H₂O)₂H₄MP²⁺ with the step formation constants K _{h 1} = 8.50 and K _{h 2} = 1.39. The conventional constant of the second protonation step is related to the water concentration through the equation logK ₂ ^" = logK ₂ + log(1 + K _{h 1}H₂O] + K _{h 1} K _{h 2}H₂O]²). In anhydrous acetonitrile, logK ₂ = 7.51. The electronic absorption spectra of H₄MP²⁺ and of its hydrates are identical; therefore, K ₁ and K ₂ ^" cannot be distinguished in photometric determination of H₄MP²⁺] in the presence of water.