MOLYBDENUM(VI) AND TUNGSTEN(VI) COMPLEX-FORMATION .5. THE REACTION WITH LACTATE IN 1.0 MOL DM-3 SODIUM-CHLORIDE MEDIUM

Complex formation of molybdate and of tungstate with lactate have been investigated in the pH(c) range 2-7.5 by potentiometric and enthalpimetric titrations at 25-degrees-C in 1.0 mol dm-3 NaCl. The potentiometric data were treated with the computer program SUPERQUAD taking into account the side-reactions of molybdate and tungstate with hydrogen ions. For both systems the 'best' reaction model comprises only one major complex and a number of minor complexes. Thermodynamic quantities have been determined for each of the major complexes. For the molybdenum(VI) complex, [MoO2(C3H4O3)2]2-, the values for the formation constant and enthalpy change are log beta122 = 15.71 and DELTAH(theta) = -72 kJ mol-1 respectively and for the tungsten(VI) complex, [WO2(C3H4O3)2]2-, the values are log beta122 = 17.47 and DELTAH(theta) = -80 kJ mol-1. The equilibrium constant and enthalpy change for the protonation of lactate have also been determined, log K = 3.60 and DELTAH(theta) = -1.7 kJ mol-1. The energetics of complexation is discussed in terms of the thermodynamic quantities.