Determination of diclofenac salts in pharmaceutical formulations

Two methods for the determination of the diclofenac salts [sodium (DS) or diethylammonium (DA)] in three pharmaceutical formulations (tablets, suppositories and gel) are presented. In the first, diclofenac salt is determined both, by measuring the absorbance of the solutions at a fixed wavelength (lambda = 276 nm) and using a multi-wavelength computational program to process the spectrophotometric data in a selected range (lambda = 230-340 nm). The above method can not be applied to the analysis of gel formulations because of the opalescence of these solutions. In this case, the analysis is performed measuring the peak-to-peak amplitude in the first-derivative UV spectrum (D-1 261.296). In the second method, diclofenac (DH) is precipitated in acid medium and determined by the analysis of the endothermic peak (t(p) = 182 degrees C) in the DSC curve obtained in nitrogen atmosphere. Finally, some aspects of chemical (solubility, acid-base equilibria, redox reaction), spectroscopic (UV, LR) and thermoanalytical (TG, DSC) behaviour of DS and DH and the values of the parameters which enable to calculate the UV spectrum of DS in aqueous solution are reported.