Molybdenum(VI) complex formation. Part 4. Equilibria and thermodynamic quantities for the reactions with tartrate in 1.0 mol dm–3 sodium chloride

Abstract

Complex formation between molybdate and tartrate has been investigated in the pH_c range 1.2–7.5 by potentiometric and enthalpimetric titrations at 25 °C in 1.0 mol dm^–3 sodium chloride. The potentiometric data were treated with the computer program SUPERQUAD taking into account all side reactions of molybdate and tartrate with hydrogen ions. The ‘best’ reaction model comprises ten complexes, representing four different molybdate–tartrate stoicheiometries. The formation constants of the complexes, denoted by β_pqr, where the subscripts p,q, and r refer to the stoicheiometric coefficients in the general formula [(MoO₄)_p(C₄H₄O₆)_qH_r]^{(2p+2q–r)–}, have the values: log β₁₂₂= 16.33, log β₁₂₃= 19.99, log β₁₂₄= 22.92, log β₂₁₄= 24.81, log β₂₁₅= 26.16, log β₄₄₇= 56.22, log β₄₄₈= 61.53, log β₄₄₉= 63.98, log β₄₂₆= 43.4, and log β₄₂₇= 48.2. Enthalpy and entropy changes for all the complexes, except (4,2,6)^6– and (4,2,7)^5– which are minor species, were evaluated from the enthalpimetric data using the values of these formation constants. Equilibrium constants as well as enthalpy and entropy changes for the protonation of tartrate in 1.0 mol dm^–3 NaCl have also been determined, log β₀₁₁= 3.69 and log β₀₁₂= 6.38.