Copper(II)-ammonia complexation equilibria in aqueous solutions at temperatures from 30 to 250°C by visible spectroscopy

The spectra of copper(II)-ammonia solutions in 2 mol-kg(-1) NH4NO3 (aq) were recorded as a function of pH with anew UV-visible flow cell, capable of operating at conditions up to 325 degreesC and 300 bars. Equilibrium constants for the formation of copper(II)-ammonia complexes Cu(NH3)(n)(2+), 1 less than or equal to n less than or equal to 4, from 30 to 150 degreesC were determined by evolving analysis and nonlinear least-squares regression. Measurements at higher temperatures were limited by thermal decomposition of NH4NO3 (aq). The formation constants of Cu(NH3)(n)(2+) decrease with temperature, consistent with extrapolations of literature data from measurements below 100 degreesC. Measurements above 150 degreesC were carried out in 0.5 mol-kg(-1) CF3SO3H (aq), at the very high ammonia concentrations required to avoid the precipitation of CuO(s). The spectra are consistent with Cu(NH3)(4)(2+) as the predominant species, based on extrapolations of peak maxima and molar absorptivities from lower temperatures. Shifts in the spectra of Cu2+ and the Cu(N-H-3)(n)(2+) species to higher wavelength and increases in molar absorbance with increasing temperature are discussed in terms of the structure of the complexes.