A kinetic description of monomolecular photochemical reaction with the use of polychromatic irradiation

Any kinetic description of a (photo)chemical reaction primarily involves a mathematical analysis showing how the concentration of the (photo)reactant depends on time. The desired dependence, namely the time dependence of concentration of (photo)reactant, is obtained by integration of differential rate equation. The simplest case of (photo)reaction is a (pseudo)monomolecular one. Unlike the kinetic model of (pseudo)monomolecular chemical reaction, the general analytical solution of mathematical model of photochemical reaction is, depending on experimental conditions, much more complicated or even impossible. In the present study, an approximation of the integral kinetic description of (pseudo)monomolecular photolysis has been developed, using high conversion and polychromatic irradiation. It was found that the reaction rate of photolysis of the model compound used can be described by a simple exponential asymptotical equation with two parameters, a linear and an exponential one. The former parameter means the maximum reaction rate and also characterises an overlap of absorption spectrum of starting photo-reactant and the spectrum of incident light. The physical meaning of the latter one is, unfortunately, more complex. Unlike the linear parameter, the value of exponential parameter has to be optimised on basis of the measured dependence of concentration of photo-reactant on time. Integration of the empirical rate equation proposed gave a simple relationship between concentration and reaction time. Potassium ferrioxalate was used as a model photo-reactant. Its photolysis was carried out both in a differential through-flow and an annular integral reactor. Medium pressure Hg arc lamps were used as polychromatic light sources in both cases. The maximum conversion of ferrioxalate was about 90%. It is supposed that the model can be extended to account for any (pseudo)monomolecular irreversible photochemical reaction.