IONIZATION CONDITIONS FOR IONTOPHORETIC DRUG-DELIVERY - ELECTRICAL CONDUCTANCE AND AGGREGATION OF LIDOCAINE HYDROCHLORIDE IN 1-OCTANOL AT 25-DEGREES-C

The behaviour of lidocaine hydrochloride (LidH(+) Cl-) in 1-octanol at 25.00 degrees C has been studied with respect to transport properties and aggregation by precision conductometry over the concentration int erval 0.2-6 mM. The analysis of the conductance data, based on the con ductance equation of Fuoss, Hsia, and Fernandez-Prini (FHFP equation), indicates that for concentrations up to 0.6 mM free LidH(+) and Cl- i ons are present in equilibrium with LidH(+) C- ion pairs. At higher co ncentrations there is strong evidence of formation triple ions. Any di ssociation of LidH(+) into Lid and Hi is shown to be negligible within the concentration interval studied. The values K-a=(127.6+/-2.7).10(3 ) and K-T=65 of the ion pair and triple ion association constants, res pectively, were obtained. The limiting molar conductivity, Lambda(0)(L idHCl)=1.969+/-0.031 cm(2) Omega(-1) mol(-1), is less by a factor of a bout 50 as compared with the corresponding value for water as solvent medium. This effect can be only partly ascribed to the difference in v iscosity between l-octanol and water. The mobility of the triple ions is found to be 1/3 of that of the free ions. (C) 1997 Elsevier Science B.V.