The self association equilibrium of copper(II) hexanoate

Abstract

During work on the interaction between copper compounds and peroxides it became necessary to investigate the copper(^^) hexanoate-carbon tetrachloride system. Copper(11) alkanoates are known to exist as dimers in the solid state1 and electronic absorption spectra2 show that they may be predominantly monomeric or dimeric in certain solvents depending on the polarity and coordinating power of the solvent.3 The only previous study4 of the monomer-dimer equilibria of copper alkanoates gave an association constant of 103 1. mol-1, measured by a potentiometric method, for copper(^^) acetate in an acetic acid-water mixture. As no completely satisfactory method is available in the literature for the calculation of association constants and extinction coefficients from absorption spectra, in systems where the spectra of the species cannot be obtained independently, we give the method used to obtain these quantities in some detail. Dilute solutions (10-2-10-5~) of copper(^^) hexanoate in carbon tetrachloride containing 4 x 10-2 moll-1 of water do not obey Beer's law (Fig. 1). The isosbestic point at 26800 om-1 can be taken as a priori evidence for the existence of two species in solution. This was confirmed by the method of Wallace and ICatz5 who pointed out that the rank of a wavelength-concentration matrix of absorbances is equal to the number of species in solution whose absorbance is varying with concentration. Their method, which involves a matrix of experimental absorbance errors in the determination of rank, confirmed that the absorbance measurements are consistent with the presence of two species in solution for transmittance errors between 0.1 and 1.0%. The quoted transmittance error of the spectrophotometer used for the measurements is 0.3%. The absorbance measurements at five frequencies between 30500 and 14000 cm-1 for solutions from 8 x 10-511 to 2 x 10-2~~ were therefore fitted to a monomer-dimer equilibrium system using a modified direct search computer program.6