A Study on Diflunisal: Solubility, Acid Constants and Complex Formation with Calcium(II) and Magnesium(II)

The properties of diflunisal, a widely used analgesic, were studied in physiologic solutions, 0.15 mol.dm(-3) NaCl. Solubility and protonation constants were determined and its behavior as ligand towards Ca(II) and Mg(II) was investigated. Solubility and protonation constants of diflunisal at 25 degrees C and 0.15 mol.dm(-3) were obtained from electromotive force measurements of galvanic cells using coulometric titrations. The experimental data yielded the solubility, s, of -log(10) s = 3.86 +/- 0.02 and the protonation constants log(10) K-1 = 11.98 +/- 0.10 and log(10) K-2 = 3.86 +/- 0.03. Equilibria between diflunisal and Ca(II) and Mg(II) were investigated by means of electromotive force measurements and by comparing solubilities of diflunisal in the presence and absence of Ca(II) or Mg(II), respectively. Experimental data were explained by assuming the formation of 1:1 complexes for Ca(II) and Mg(II) along with evaluating the relative stability constants.