Ln. Trevani et al., Copper(II)-ammonia complexation equilibria in aqueous solutions at temperatures from 30 to 250 degrees C by visible spectroscopy, J SOL CHEM, 30(7), 2001, pp. 585-622
The spectra of copper(II)-ammonia solutions in 2 mol-kg(-1) NH4NO3 (aq) wer
e recorded as a function of pH with anew UV-visible flow cell, capable of o
perating at conditions up to 325 degreesC and 300 bars. Equilibrium constan
ts 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 d
etermined by evolving analysis and nonlinear least-squares regression. Meas
urements at higher temperatures were limited by thermal decomposition of NH
4NO3 (aq). The formation constants of Cu(NH3)(n)(2+) decrease with temperat
ure, consistent with extrapolations of literature data from measurements be
low 100 degreesC. Measurements above 150 degreesC were carried out in 0.5 m
ol-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.