Kinetics of α- and β-molybdosilicic acid formation

Abstract

The kinetics of formation of α- and β-molybdosilicic acid have been investigated and rate equations proposed, together with possible reaction sequences involving individual molybdate species. The α-acid was found to form according to a sum of exponentials model [A_t=θ₁+θ₂exp(θ₃t)+θ₄exp(θ₅t)] relating absorbance with time. In contrast, the β-acid formed according to a simpler exponential model [A_t=θ₁+θ₂exp(θ₃t)], which is consistent with pseudo-first-order behaviour with respect to silicate concentration. For the first time in a kinetic study of heteropolymolybdate formation, the effects of changes in molybdate speciation upon the kinetics have been considered in detail. The concentrations of individual molybdate species at pH between 0.5 and 5.0, and total molybdate concentrations up to 0.20 M as molybdenum, have been calculated using equilibrium constants together with an existing computer program. The most interesting result of the study is that the observed α-acid kinetics demand the presence of a third silicon species in addition to the reactant silicate and the α-acid product. At this stage it is not known whether the third species should be another silicate condensation product or a molybdosilicic acid. The general application of the results of this study to silicate analysis is discussed. Methods of non-linear parameter estimation have been used extensively in fitting theoretical models to observed data.