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 interval 0.2-6 mM. The analysis of the conductance data, based on the conductance equation of Fuoss, Hsia, and Fernandez-Prini (FHFP equation), indicates that for concentrations up to 0.6 mM free LidH(+) and Cl- ions are present in equilibrium with LidH(+) C- ion pairs. At higher concentrations there is strong evidence of formation triple ions. Any dissociation 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, respectively, were obtained. The limiting molar conductivity, Lambda(0)(LidHCl)=1.969+/-0.031 cm(2) Omega(-1) mol(-1), is less by a factor of about 50 as compared with the corresponding value for water as solvent medium. This effect can be only partly ascribed to the difference in viscosity 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.