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)=1.5x10(-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.