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Determination of dissociation constants of some pharmaceutical compounds using derivative spectrophotometry

Derivative spectrophotometry is applied to the determination of the dissociation constants (pKa) of some weakly dissociating compounds. The pKa value(s) can be calculated by measuring the amplitude of derivative maxima (D) at the selected wavelengths for the dissociated, undissociated and partially dissociated forms of the compound. A graphical technique, based on plotting the derivative spectrophotometric titration curves and interpolating pKa at D1/2, is also presented. Both methods are applied to pharmaceutical compounds with single and double dissociation constants. The results are in good agreement with the reported pKa values of the investigated compounds.