Solid dosage forms kinetics of chemical decomposition

3 Solid dosage forms kinetics of chemical decomposition 

In spite of the importance of solid dosage forms, there have been relatively few attempts to evaluate the detailed kinetics of decomposition. Most of the earlier work was carried out with the sole objective of predicting stability, and data were treated using the rate equations derived for reaction in solution. More recently, the mechanisms that were developed to describe the kinetics of decomposition of pure solids have been applied to pharmaceutical systems and some rationalisation of decomposition behaviour has been possible. A comprehensive account of this topic has been presented by Carstensen on which the following summary is based. 

It is convenient to divide single component systems into two categories those solids that decompose to a solid product and a gas, and those that decompose to give a liquid and a gas. 

An example of this category is p-aminosali-cylic acid, which decomposes to a solid (p-aminophenol) and a gas (carbon dioxide)  

The decomposition curves which result from such a reaction show either (a) an initial rapid decomposition followed by a more gradual decomposition rate, or (b) an initial lag period, giving a sigmoidal appearance. The shape produced by (a) can usually be accounted for by topochemical (or contracting geometry) reactions and that produced in (b) by nucleation theories. 

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