CADMIUM MALONATE COMPLEXATION IN AQUEOUS SODIUM TRIFLUOROMETHANESULFONATE MEDIA TO 75-DEGREES-C - INCLUDING DISSOCIATION QUOTIENTS OF MALONIC-ACID

The molal formation quotients for cadmium-malonate complexes were meas ured potentiometrically from 5 to 75 degrees C, at ionic strengths of 0.1, 0.3, 0.6 and 1.0 molal in aqueous sodium trifluoromethanesulfonat e (NaTr) media. In addition, the stepwise dissociation quotients for m alonic acid were measured in the same medium from 5 to 100 degrees C, at ionic strengths of 0.1, 0.3, 0.6, and 1.0 molal by the same method. The dissociation quotients for malonic acid were modeled as a functio n of temperature and ionic strength with empirical equations formulate d such that the equilibrium constants at infinite dilution were consis tent, within the error estimates, with the malonic acid dissociation c onstants obtained in NaCl media. The equilibrium constants calculated for the dissociation of malonic acid at 25 degrees C and infinite dilu tion are log K-1a = -2.86 +/- 0.01 and log K-2a = -5.71 +/- 0.01. A si ngle Cd-malonate species, CdCH2C2O4, was identified from the complexat ion study and the formation quotients for this species were also model ed as a function of temperature and ionic strength. Thermodynamic para meters obtained by differentiating the equation with respect to temper ature for the formation of CdCH2C2O4 at 25 degrees C and infinite dilu tion are: log K = 3.45 +/- 0.09, Delta H degrees = 7 +/- 6 kJ-mol(-1), Delta S degrees = 91 +/- 22 J-K-1-mol(-1), and Delta C-p(o) = 400 +/- 300 J-K-1-mol(-1).