Rate Constant.

Arithmetic plots representing the change in absorbance vs time are linear,indicating that color loss is following zero order reaction.The slopes of these lines are obtained and are indicative of the rate of change of color with time.Most suitable example is the pH dependency of the hydrolysis of methyl DL-alpha-phenyl-2-piperidyl acetate at 80 degree C which shows the pH at which the maximum hydrolysis can be affected.
In order for rate constants or velocity of degradation to be of use in the formulation of pharmaceutical products,it is necessary to evaluate the temperature dependency of the reaction. This permits the prediction of the stability of the product at ordinary shelf temperature from data obtained under exaggerated conditions of testing.According to the rule of thumb method the rate of reaction was said to be double for each 10 degree C rise in temperature.Although the rule may serve as a fairly accurate method to estimate for certain preparations,it is not generally applicable therefore,to assign an overall factor for influence of temperature on the acceleration of the reaction is unwise.Some deterioration reactions are not measurably influenced over 10 degrees C temperature range,while other undergo rapid degradative changes.The recommended procedure is to set up a planned schedule of accelerated tests for each formulation in order to ascertain the temperature dependency of the chemical changes in the product undergoing evaluation.