On iontophoretic delivery enhancement: Ionization and mobility of lidocaine hydrochloride in propylene glycol |
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Institution: | 1. Showa Pharmaceutical University, 3-3165, Higashi-Tamagawagakuen, Machida, Tokyo 194-8543, Japan;2. Graduate School of Pharmaceutical Sciences, Kumamoto University, 5-1, Oe-Honmachi, Kumamoto 862-0973, Japan |
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Abstract: | The structure of lidocaine hydrochloride (LidHCl) in propylene glycol (PG), a solvent known to enhance transdermal delivery of drugs, and the mobilities of the different kinds of ionic species appearing in this system was investigated at 25.0°C by precision conductometry. The molar conductivity was determined at several concentration between 0.4 and 10 mM and the data analysed using the conductance equation of Fuoss-Hsia and Fernandez-Prini (FHFP equation). For concentrations of up to ≈1.2 mM no higher aggregates that LidH+ were found. Using a two-parameter analysis of the equilibrium, LidH+⇔Lid+H+, we obtained the acid dissociation constant, Ka(LidH+)=2.5·10−7 (molar scale); pKa=6.60, and the limiting molar conductivity, λ0(LidH+)=0.2675 cm2/Ω per mol. For concentrations above 1.2 mM there is strong evidence of formation of ion-pairs, LidH+Cl−. The ion-pair association constant was estimated to Kp≈40 indicating that about 15% of the electrolyte is in the form of LidH+Cl− at the highest concentration (10 mM) investigated. |
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