MOLYBDENUM(VI) COMPLEX-FORMATION .8. EQUILIBRIA AND THERMODYNAMIC QUANTITIES FOR THE REACTIONS WITH CITRATE

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 beta(pqr), where the subscripts p, q and r refer to the stoichiometric coefficients in the general formula [(MoO4)(p)(cit)(q)H-r]((2p+3q-r)-), have the values (at 25 degrees C): log beta(111) = 8.35, log beta(112) = 15.00, log beta(113) = 19.62, log beta(114) = 21.12, log beta(224) = 31.02, log beta(225) = 35.86, log beta(226) = 40.08: log beta(124) = 25.34, log beta(125) = 29.54, log beta(126) = 33.34, log beta(213) = 21.73, log beta(214) = 26.90, log beta(215) = 31.53, log beta(429) = 60.76, log beta(4210) = 64.69 and log beta(4411) = 77.45. A set of stability constants pertaining to 2 degrees 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 degrees 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: Delta H-111(0) = -48.5, Delta H-112(0) = -55.6, Delta H-113(0) = -66.5, Delta H-114(0) = -69, Delta(224)(0) = -140, Delta H-225(0) = -137, Delta H-226(0) = -142, Delta H-126(0) = -92, Delta H-214(0) = -117, Delta H-215(0) = -117, Delta H-4210(0) to = -251 and Delta H-4411(0) = -305 kJ mol(-1). Approximate values for the following enthalpy changes were calculated from the two sets of constants determined at 25 and 2 degrees C: Delta H-124(0) = -77, Delta H-125(0) = -83, Delta H-213(0) = -81 and Delta H-429(0) = - 252 kJ mol(-1).