On the influence of temperature and background concentration on thermodynamic stability constants of piperidinedicarboxylate complexes of cadmium

The stability of mixed piperidine-cadmium dicarboxylates was determined as the function of the chain length in the dicarboxylate ligand in a wide concentrational range of ionic medium and temperatures. The formation of the complex was studied potentiometrically at 15-50 deg C in aqueous solutions, with the concentration of the supporting electrolyte being 0.5, 0.75, 1.0, 1.5 and 2.0 mole/l. The stoichiometric (βsub(b)sup(c)) and thermodynamic (βsub(n)sup(0)) stability constants were calculated and the composition of the complexes was determined. It was shown that the composition is independent on the temperature and on the concentrations of cadmium dicarboxylate and the supporting electrolyte, and depends only on the concentration of piperidine. Mixed complexes of cadmium are quite stable at all concentrations of the ionic medium. The main thermodynamic functions, ΔZ, ΔH, ΔS were calculated from the temperature dependence of the stability constants. The change in entropy had a positive sign, and ΔS increased (10.3-32.6 cal/mole) with an increase in the stability constant of the complex formed in solution. For mixed complexes the change in the isobaric thermodynamic potential (from -4.40 to -12.28kcal/mole) and in 'H (from -1.15 to -2.34 kcal/mole) caused by an increase in the carbon chain length of the dicarboxylate ligand becomes more negative. However, the heat of formation was very small in comparison with the heat of hydration of Cd2+ (440 kcal/mole)