Solid state reactions studied by isothermal microcalorimetry; The solid state oxidation of ascorbic acid

Abstract

Abstract This paper reports a new method for the analysis of isothermal calorimetric data, for reactions in the solid state. As an illustration for this method of analysis, the solid state oxidation of L-ascorbic acid (vitamin C) has been studied. The investigation considers the role that water plays in the solid state oxidation of ascorbic acid and hence poses questions of the generality of water in solid state reactions. Isothermal heat conduction microcalorimetry (e.g., TAM, Thermometric, Sweden) has, for some time, been proposed as a general and rapid analytical technique that allows the calculation of the kinetic and thermodynamic parameters of chemical reactions. A new method for the determination of the kinetic and thermodynamic parameters from calorimetric data allows the quantitative study of reactions without preconceptions of the reaction mechanism and quantities of material reacting. This allows the quantitative study of reactions, especially reactions in the solid state, which, because of the inherently complex nature of reaction, are very difficult to analyse by any other means. From the analysis of the calorimetric data for the solid state oxidation of ascorbic acid at 298.15 K, we have determined that the reaction has a change in enthalpy of −195 ±10 kJ mol−1 with an associated rate constant of 4.1 × 10−6 s−1. The study was carried out as a function of the quantity of water, between dry (i.e. under ambient conditions) to 200μl, added to 0.5 g ascorbic acid. The dry sample of ascorbic acid was used directly from the container, no special attention was paid to prevent atmospheric moisture entering the sample before loading the sample. Between these water quantities the kinetic and thermodynamic parameters remained the same, indicating that the reaction mechanism was unchanging throughout the range studied.