Hydrophilic polyurethane coated silver silver chloride electrode for the determination of chloride in blood

Potentiometric properties of hydrophilic polyurethane (HPU) coated Bg/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 resp onse to bromide; fast initial response to bromide, followed by gradual pote ntial changes (from -2 to -25 mV/min), and an abrupt potential drop (betwee n about -60 and -80 mV) after some period of time. Fitting the potentiometr ic response curves to the diffusion model proposed initially by Hulanicki a nd Lewenstam, and modified later by Morf, it was possible to estimate the r elative permeability of HPU membranes: the concentration of bromide that re ached the surface of AgCl layer From the bulk solution was reduced by a fac tor of 1/20 with HPU-A, 1/12 with HPU-B and 1/3 with HPU-C membrane. The ra tio of peak currents measured at the HPU coated Pt electrode and that at th e bare Pt electrode for various neutral and anionic species indicated that the HPU membranes effectively discriminate the passage of those molecules b y their size (e.g., M-w < 400 for neutral and M-w < 200 for anionic molecul es). As the HPU membranes substantially reduce the diffusion of larger anio ns than chloride, e.g., bromide and thiocyanate, the HPU modified Ag/AgCl e lectrodes accurately determine the chloride level in clinical samples even in the presence of those interfering anions. Since the HPU membrane also ef fectively prevent the surface fouling of the electrode from protein adsorpt ion, the HPU coated Ag/AgCl electrode was successfully employed to measure the chloride in serum or whole blood.