Complexation of tungsten(VI) with citrate
Complex formation between tungstate and citrate (citric acid, H4cit = 3-carboxy-3-hydroxypentane-1,5-dioic acid) has been investigated in the pHc range 1.5-9.5 by potentiometric and enthalpimetric titrations at 25°C in 1.0 mol dm-3 NaCl. The potentiometric data were treated with the computer program SUPERQUAD taking into account side reactions of tungstate and citrate with hydrogen ions. The 'best' reaction model comprises eight complexes, representing four different tungstate-citrate stoichiometries. The formation constants of the complexes, denoted by βpqr, where the subscripts p, q and r refer to the stoichiometric coefficients in the general formula [(WO4)p(Hcit)qHr] (2p+3q-r)-, have the values log β111 = 10.21, log β112 = 17.03, log β113 = 21.67, log β114 = 22.82, log β224 = 34.89, log β225 = 39.3, log β126 = 34.51 and log β214 = 31.7. Enthalpy and entropy changes for the complexation reactions were calculated from the calorimetric data using the values of these formation constants. The enthalpy values for the major complexes are ΔH°111 = -66, ΔH°112 = -67 and ΔH°113 = -78 kJ mol-1.