Molybdenum(VI) complex formation. Part 4. Equilibria and thermodynamic quantities for the reactions with tartrate in 1.0 mol dm-3 sodium chloride
Complex formation between molybdate and tartrate has been investigated in the pHc 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 βpqr, where the subscripts p,q, and r refer to the stoicheiometric coefficients in the general formula [(MoO4)p(C4H4O6) qHr](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.