Protolytic equilibria in homogeneous and heterogeneous aqueous systems of methyl- and propylparaben and influence of β-cyclodextrin on pKa values

Abstract

Acidity constants (Ka) and intrinsic hydrosolubility (Ks0) of two parabens, methylparaben and propylparaben, were spectrophotometrically determined at 25oC and a constant ionic strength of I=0.1 M (NaCl). Based on the determined equilibrium constants Ks0 and Ka, equilibrium constant Ks1 in heterogeneous system of methyl-and propylparaben was calculated. Solubility of the two parabens as a function of solution acidity was calculated applying the equation: s1 s0 3 [ ] S [H O ] K K + = + In addition, the effects of varying β-cyclodextrin concentrations on pKa values of the two parabens were examined. The results showed that β-CD influenced absorption spectra of the parabens (a decreased absorptivity and a bathochromic shift of the absorption maxima), but their pKa values remained almost unchanged.

Spektrofotometrijski su određene kiselinske konstante (Ka) i osnovna rastvorljivost (Ks0) u vodi metilparabena i propilparabena. Određivanja su vršena na temperaturi od 25 oC i pri konstantnoj jonskoj sili I=0.1 M (NaCl). Na osnovu određenih ravnotežnih konstanti Ks0 i Ka izračunata je ravnotežna konstanta Ks1 u heterogenom sistemu parabena. Rastvorljivost parabena u funkciji kiselosti rastvora izračunata je primenom jednačine 1 0 3 [ ] [ ] s s K S K H O+ = + Pored toga, ispitan je uticaj različitih koncentracija β-ciklodekstrina na pKa vrednosti datih parabena.