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Figure 4.11 The effect of pH on the photodegradation of ciprofloxacin. Radiation source: mercury lamp at wavelength of 313 nm.

Reproduced from K. Torniainen, S. Tammilehto and V. Ulvi, Int. J. Pharm., 132, 53 (1996).

drugs in solution, a fact which is used to good effect in the experimental studies of drug stability described above. Such studies are usually carried out at high temperatures, say 60 or 80°C, because the hydrolysis rate is greater at these temperatures and can therefore be measured more easily. Of course, if a formulation has to be heat sterilized then its stability will, in any case, have to be measured at elevated temperatures. Figure 4.12 shows the pH-rate profiles for the degradation of codeine sulfate at several temperatures and also the calculated values at 25°C. We will now see how these calculated values can be obtained.

The equation which describes the effect of temperature on decomposition, and which shows us how to calculate the rate of breakdown at room temperature from measurements at much higher temperatures, is the Arrhenius equation.

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