Solid-Contact Reference Electrodes Based on Lipophilic Salts

A novel solid-contact reference electrode (SCRE) for potentiometric measurements is studied. A lipophilic salt was introduced into a PVC membrane by two different approaches, i) using tetra(alkyl)ammonium-tetra(butyl or phenyl) borate, i.e. a single salt (Phi(+)Psi(-)) and ii) tetra (alkyl)ammonium chloride + potassium tetra(phenyl)borate i.e. two salts (Phi Cl-+(-)) and (K+Psi(-)) that presumably form (T(alkyl)A(+)T(phenyl)B-) and potassium chloride (KCl) in the membrane. The effect of the salt size on the performance of the SCRE was studied by preparing membranes using compounds with varying alkyl chain lengths. To get reference electrodes, these membranes were drop cast on two different substrates: glassy carbon (GC) disks covered with the conductive polymer (poly(3,4-ethylenedioxythiophene) doped with chloride (PEDOT-Cl), or silver disks coated with silver chloride (Ag/AgCl). The potential response of these electrodes was investigated in several potentiometric experiments using a conventional double junction silver chloride electrode as a reference. It was found that electrodes with the tetrabutylammonium tetrabutylborate (TBA-TBB, single salt) in the membrane were insensitive to changes in electrolyte concentrations but reached stable potentials slowly after changes in the sample electrolyte. The double salt approach, where KCl was also present in the membrane, improved the response time, but these electrodes were more sensitive to concentration changes.