A potentiometric, spectrophotometric, and calorimetric investigation of molybdenum(VI)–oxalate complex formation

Abstract

Complexation between molybdate and oxalate has been investigated in the pH_c range 2.0–7.0 by potentiometric, spectrophotometric, and enthalpimetric titrations at 25 °C in 1.0 mol dm^–3 sodium chloride. The potentiometric data were treated with the computer program MINIQUAD taking into account side reactions of molybdate and oxalate with hydrogen ions. The ‘best’ reaction model comprises the three complexes (1,1,2)^2–, (2,2,5)^3–, and (2,2,6)^2– with formation constants log β₁₁₂= 13.619, log β₂₂₅= 31.20, and log β₂₂₆= 34.08; the numbers in parentheses refer to the values of p, q, and r in the general formula (MoO₄^2–)_p(C₂O₄^2–)_q(H⁺)_r. The spectrophotometric data were treated with the program SQUAD and the results obtained confirmed those obtained by potentiometry. The u.v. spectra of the complexes are reported. The enthalpy and entropy changes for complex formation were calculated from the enthalpimetric data using the values of the now known formation constants. The enthalpy values are: ΔH₁₁₂⊖=–59.5, ΔH₂₂₅=⊖–123.0, and ΔH₂₂₆⊖=–117.0 kJ mol^–1. Equilibrium constants for the successive protonation of oxalate in 1 mol dm^–3 sodium chloride have also been determined, log β₀₁₁= 3.52 and log β₀₁₂= 4.41.