Three-Endpoints Method for Isothermal Weibullian Chemical Degradation

Consider the degradation of a heat-labile compound during isothermal storage or exposure to a high constant temperature. Assume that this process follows Weibull kinetics. If the temperature dependence of the rate parameter follows the exponential model, a simpler alternative to the Arrhenius equation, then the compound's diminishing concentration is specified by three kinetic parameters. These can be extracted from three concentration ratios determined at three different constant temperatures at the same or at different times.

The solution of the corresponding three simultaneous nonlinear equations starts with an attempt to match three entered time-temperature-concentration ratio points with reconstructed degradation curves generated with the combined Weibull-exponential model. Once a match is found visually by using sliders, their positions provide estimates of the values of the corresponding parameters and/or proper ranges for the initial guesses that can be used in numerical solution of the equations.

The kinetic parameters so obtained can then be used to reconstruct the entire three degradation curves at the three temperatures, and to predict the compound's degradation patterns at temperatures outside the observed range.