Kinetics and Mechanism of a Photochemical Reaction

Department of Pharmacy, The University of Sydney, Sydney, Australia 

The kinetic relationship most frequently applying to stability studies in solution is that of first-order kinetics in which the rate of degradation of a drug substance depends on the concentration raised to the first power, as in Eq. 1, 

The alternative kinetic situation that is often encountered experimentally in photochemical reactions is zero-order kinetics, corresponding to the situation where the rate of degradation of the drug is a constant, independent of concentration, as shown in Eq. 4 

Other more complicated kinetic relationships, such as second- or fractional-order reactions, arise very infrequently from complex reaction mechanisms, and do not need to be considered for photodegradation reactions. 

At first glance, it would seem that distinguishing between zero- and first-order kinetics is not a difficult task. However, a complication does arise because a reaction has to be taken to at least 50% conversion before an unequivocal 

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