Molybdenum(VI) complex formation—8. Equilibria and thermodynamic quantities for the reactions with citrate

Abstract Complex formation between molybdate and citrate has been investigated in the pH range 1.0–9.0 by potentiometry, spectrophotometry, differential pulse polarography and calorimetry in NaCl (1.0 mol dm−3). The “best” reaction model comprises mononuclear, dinuclear and tetranuclear complexes. 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 [(MoO4)p(cit)qHr](2p+3q−r)−, have the values (at 25°C): log β111 = 8.35, log β112 = 15.00, log β113 = 19.62, log β114 = 21.12, log β224 = 31.02, log β225 = 35.86, log β226 = 40.08, log β124 = 25.34, log β125 = 29.54, log β126 = 33.34, log β213 = 21.73, log β214 = 26.90, log β215 = 31.53, log β429 = 60.76, log β4210 = 64.69 and log β4411 = 77.45. A set of stability constants pertaining to 2°C has also been determined. These constants were used to calculate species distribution curves needed to evaluate the differential pulse polarographic data obtained at 2°C. The polarographic results were consistent with the reaction model derived from the potentiometric data. Enthalpy and entropy changes for complexes occurring in sufficiently high concentrations were calculated from the calorimetric data. The enthalpy changes are as follows: ΔH1110 = −48.5, ΔH1120 = −55.6, ΔH1130 = −66.5, ΔH1140 = −69, ΔH2240 = −140, ΔH2250 = −137, ΔH2260 = −142, ΔH1260 = −92, ΔH2140 = −117, ΔH2150 = −117, ΔH42100 = −251 and ΔH44110 = −305 kJ mol−1. Approximate values for the following enthalpy changes were calculated from the two sets of constants determined at 25 and 2°C: ΔH1240 = −77, ΔH1250 = −83, ΔH2130 = −81 and ΔH4290 = −252 kJ mol−1.