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

Absorption spectra of cobalt(II) chloride in calcium nitrate-ammonium nitrate-water system of the composition xCa(NO3)2·4H2O-(1 − x)NH4NO3 (x = 0.30, 0.50 and 0.70) have been investigated in the wavelength range 400-800 nm at temperatures 45, 55 and 65 °C. Addition of chloride to cobalt nitrate solution caused a pronounced shift of the absorption maximum toward lower energies and a significant increase of absorption intensity, indicating a change in coordination geometry. The increase of the overall molar absorption coefficient with temperature in the chloride containing melts was attributed to mixed complex formation with solvent molecules (or ions) and chloride ions, and structural changes in the cobalt(II) complexes. From an analysis of the spectra it was concluded that the following complexes were formed: [Co(NO3)4]2−, [Co(NO3)2Cl2]2− and [CoCl4]2−. The overall stability constants of these complexes and the corresponding resolved species spectra were calculated at 45, 55 and 65 °C.