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Figure 4.14 shows the degradation of phen-tolamine hydrochloride at two different pH

values plotted according to equation (4.54). The gradients of these plots should be proportional to the product of the charge carried by the reactive species. Whether the gradient is positive or negative depends on the reaction involved. Reactions between ions of similar charge, for example the acid-catalysed hydrolysis of a cationic drug ion, will produce plots of positive slope (i.e. the reaction rate will be increased by electrolyte addition), whereas the base-catalysed hydrolysis of positively charged drug species will produce negative gradients. Investigations of the influence of ionic strength on reaction rate may therefore be used to provide confirmation of the type of reaction which is occurring. For example, the gradients of the two plots of Fig. 4.14 are 0.260 and -1.759 at pH3.1 and 7.2 respectively. Since the value of 2A at 90°C is 1.174, we can calculate values of the product zAzB as 0.221 and 1.498 at pH3.1 and 7.2 respectively. These values are not integers as they should be if the reactions involved were simple acid-and base-catalysed reactions. This has been explained by suggesting complex reactions between the buffer species and the phentol-amine at each pH.

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