Copper(II)–Ammonia Complexation Equilibria in Aqueous Solutions at Temperatures from 30 to 250°C by Visible Spectroscopy

Abstract

The spectra of copper(II)–ammonia solutions in 2 mol-kg−1 NH4NO3(aq) were recorded as a function of pH with a new UV–visible flow cell, capable of operating at conditions up to 325°C and 300 bars. Equilibrium constants for the formation of copper(II)–ammonia complexes Cu(NH3)n 2+, 1 ≤ n ≤ 4, from 30 to 150°C were determined by evolving factor 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°C. Measurements above 150°C 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(NH3)n 2+ species to higher wavelength and increases in molar absorbance with increasing temperature are discussed in terms of the structure of the complexes.