On iontophoretic delivery enhancement:: ionization and transport properties of lidocaine hydrochloride in aqueous propylene glycol

The ionization and transport properties of lidocaine hydrochloride (LidHCl) in aqueous propylene glycol (PG) containing 20% PG by weight was studied by means of electrical precision conductometry. For drug concentrations exceeding about 1.7 mM a slight formation of LidH(+)Cl(-) ion-pairs is indicated; ion-pair association constant, K-p = 1.73 (molar scale). A two variable analysis of the experimental data yielded K-a = 1.5 x 10(-8) for the acid dissociation constant of LidH(+), i.e. pK(a) = 7.82, and the limiting ionic conductivity, lambda(o)(LidH(+)) = 21.73 cm(2) S mol(-1). To enable evaluation of single ion conductivities the proton transport number of HCl in the present solvent mixture was determined using the moving boundary method. (C) 2000 Elsevier Science B.V. All rights reserved.