Cadmium Malonate Complexation in Aqueous Sodium Trifluoromethanesulfonate Media to 75°C; Including Dissociation Quotients of Malonic Acid

Authors

Moira K. Ridley, University of Nebraska-LincolnFollow
Donald A. Palmer, Oak Ridge National Laboratory
David J. Wesolowski, Oak Ridge National Laboratory
Richard Kettler, University of Nebraska-LincolnFollow

Document Type

Article

Date of this Version

1998

Comments

Published in Journal of Solution Chemistry. Vol. 27, No. 3, 1998, pp. 195-216.

Abstract

The molal formation quotients for cadmium-malonate complexes were measured potentiometrically from 5 to 75°C, at ionic strengths of 0.1, 0.3, 0.6 and 1.0 molal in aqueous sodium trifluoromethanesulfonate (NaTf) media. In addition, the stepwise dissociation quotients for malomc acid were measured in the same medium from 5 to 100°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 function of temperature and ionic strength with empirical equations formulated such that the equilibrium constants at infinite dilution were consistent, within the error estimates, with the malonic acid dissociation constants obtained in NaCl media. The equilibrium constants calculated for the dissociation of malonic acid at 25°C and infinite dilution are log K_1a = -2.86 ± 0.01 and log K_2a = -5.71 ± 0.01. A single Cd-malonate species, CdCH₂C₂O₄, was identified from the complexation study and the formation quotients for this species were also modeled as a function of temperature and ionic strength. Thermodynamic parameters obtained by differentiating the equation with respect to temperature for the formation of CdCH₂C₂O₄ at 25°C and infinite dilution are: log K = 3.45 ± 0.09, ΔH° = 7 ± 6 kJ-mol^-1, ΔS° = 91 ± 22 J-K-1-mol^-1, and ΔC°_p = 400 ± 300J-K^-1-mol^-1.