The output of heat flow and heat compensation
calorimeters is proportional to the rate of all
physical and chemical processes taking place.
The large sample size associated with microcalorimeters
makes it feasible to study reactions in dilute solutions
(1-10 mg/ml MW ~ 300) as well as in the solid state.
This is a powerful and rapid way to screen reactivity of compounds at
different conditions including pH, oxidizing agents,
cosolvent effects, etc. In the example shown note that under basic conditions the reaction
was endothermic while it was exothermic
in acid buffer. This gives the
chemist insight into what drives the degradation reaction (entropy
versus enthalpy driven).
The same data can be plotted as the log of
heat flow versus 1/T to obtain the apparent activation
energy. Note that at pH 10
there were multiple activation energies associated with the decomposition
(i.e., non Arrhenius).