Cobalt halide complex formation in aqueous calcium nitrate-ammonium nitrate melts. II. Cobalt(II) bromide

In this work absorption spectra of cobalt(II) bromide containing variable amounts of ammonium bromide in the same systems were recorded at different temperatures from 45 to 65 °C in the wavelength range 400-800 nm. In the absence of bromide the solutions show spectra typical for octahedral co-ordination of cobalt(II). Addition of bromide caused a shift of the absorption maximum toward lower energies and an increase of the molar absorption coefficient with the increasing bromide concentration. The position of the absorption maximum and the intensity of absorption indicate tetrahedral or severely distorted octahedral co-ordination. Stability constants for [Co(NO3)4]2−, [Co(NO3)2Br2]2− and [CoBr4]2− complex formation in xCa(NO3)2∙4H2O-(1-x)NH4NO3 are reported. Based on these stability constants, the thermodynamic data, i.e. enthalpy, entropy and free energy changes, were calculated for all complexation reactions. Also, the dependence of the thermodynamic parameters on the solvent composition has been analysed.