Hydrophilic Polyurethane Coated Silver/Silver Chloride Electrode for the Determination of Chloride in Blood

Abstract

Potentiometric properties of hydrophilic polyurethane (HPU) coated Ag/AgCl electrodes of the second kind have been examined. Three types of HPUs which have 42% (HPU-A), 100% (HPU-B), and 206% (HPU-C) of water uptake have been used as coating membranes for Ag/AgCl electrodes. Both HPU coated and bare Ag/AgCl electrodes exhibited virtually the same potentiometric response to chloride. On the other hand, all the electrodes exhibited a three step response to bromide; fast initial response to bromide, followed by gradual potential changes (from –2 to –25 mV/min), and an abrupt potential drop (between about –60 and –80 mV) after some period of time. Fitting the potentiometric response curves to the diffusion model proposed initially by Hulanicki and Lewenstam, and modified later by Morf, it was possible to estimate the relative permeability of HPU membranes: the concentration of bromide that reached the surface of AgCl layer from the bulk solution was reduced by a factor of 1/20 with HPU-A, 1/12 with HPU-B and 1/3 with HPU-C membrane. The ratio of peak currents measured at the HPU coated Pt electrode and that at the bare Pt electrode for various neutral and anionic species indicated that the HPU membranes effectively discriminate the passage of those molecules by their size (e.g., Mw < 400 for neutral and Mw < 200 for anionic molecules). As the HPU membranes substantially reduce the diffusion of larger anions than chloride, e.g., bromide and thiocyanate, the HPU modified Ag/AgCl electrodes accurately determine the chloride level in clinical samples even in the presence of those interfering anions. Since the HPU membrane also effectively prevent the surface fouling of the electrode from protein adsorption, the HPU coated Ag/AgCl electrode was successfully employed to measure the chloride in serum or whole blood.