Influence of Water on the Stability of Cobalt Bromide Complexes in Aqueous Calcium Nitrate–Ammonium Nitrate Melts at T = 328.15 K: A Spectrophotometric Approach

This study focuses on the analysis of the absorption spectra of cobalt(II) nitrate and cobalt(II) bromide complexes in the xCa(NO₃)₂–(1 − x)NH₄NO₃–H₂O system. Measurements were conducted within the wavelength range of 400–800 nm, at a constant temperature of T = 328.15 K and atmospheric pressure of p = 101.3 kPa. The spectra were recorded for systems with a fixed salt composition of 0.3Ca(NO₃)₂–0.7NH₄NO₃–H₂O, but with varying water content, described by the mole ratio of water to salt (R = H₂O/salt), where R was set to 1.0, 1.2, and 1.6. Additionally, the study investigated systems with varying calcium nitrate-to-ammonium nitrate ratios, represented as xCa(NO₃)₂–(1 − x)NH₄NO₃–H₂O, where x takes the values 0.3, 0.4, and 0.6. In these systems, the mole ratio of water to total salt was kept constant at R = 1.6. For systems with x = 0.7 and 0.9, a higher water-to-salt mole ratio of R = 3.6 was used. The analysis of absorption spectra revealed a shift of the maximum absorption peak toward shorter wavelengths (blue shift) as the water content (R) increased. This behavior suggests that both water molecules and nitrate ions participate in the coordination around the cobalt(II) ion. Based on the spectral data, the presence of the following complexes was confirmed: [Co(NO₃)₄(H₂O)₂]²⁻, [Co(NO₃)₂Br₂]²⁻ and [CoBr₄]²⁻. The overall stability constants of these complexes were calculated, and the corresponding resolved spectra of the species were determined at T = 328.15 K.