Infrared spectroscopy of metal carboxylates: I. Determination of free acid in solution

High-purity metal oxides can be produced by hydrolysis at high temperatures of metal carboxylates in a process called hydrolytic stripping. The process has potential application both in the field of extractive metallurgy and in the production of oxide materials such as pigments, catalysts, ceramics and magnetic oxides. The nature of the solids precipitated has a strong dependence on the composition and structure of the metal carboxylates involved in the reaction. In this work, infrared spectroscopy has been used for the quantitative analysis of organic solutions containing Versatic 10™, as a first step in the speciation of the metal carboxylate solutions used in the hydrolytic stripping process for the precipitation of magnetic oxides. Based on Beer's law, the characteristic νC=6;O bands for the monomeric and dimeric forms of Versatic acid were used to measure the concentration of the acid in Escaid 110™ solutions at temperatures in the range 20-200 °C, and thus obtain equations allowing the estimation of the free acid present in metal carboxylate solutions. It was found that the molar absorptivity (ε_D) corresponding to the fundamental C=O stretching vibration of the dimeric form of Versatic acid depends strongly on temperature over the range 20-200 °C, while that corresponding to the monomeric form (ε_M), remains essentially constant with respect to temperature over the same range. Both species obey Beer's law over the entire range of concentration investigated. According to Beer's law, equations of the form [V-10]=αA_νM + βA_νD were obtained for a given temperature. The accuracy of these equations was tested against Versatic acid solutions of known concentrations; the error in the prediction was always found to be less than 8%. Thus, the equations obtained allow the estimation of the free Versatic acid present in metal carboxylate solutions at given specific temperatures.