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 200mul, 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.
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