Evolution of electrochemical potentials mediated by lipophilic salts at the buried membrane interface of solid contact ion selective electrodes

Abstract

In this work, the lipophilic salt tetradodecylammonium tetrakis(4-chlorophenyl)borate (ETH 500) was incorporated into a polyvinylidene fluoride (PVDF) matrix. This was subsequently used as a binder in a carbon composite solid contact for ion selective electrodes. The ability of ETH 500 to establish reproducible potentials when added to a batch of solid contact ion selective electrodes (SC-ISEs) is disputed in the literature. We address the discrepancy by showing that blending of the salt in binder prevents uncontrolled partitioning of solid contact components to the solvent during the membrane deposition step, resulting in reproducible SC-ISEs. Model potassium selective SC-ISEs in a batch reproduced their standard potentials EI0 with a standard deviation of 1.3 mV at a low salt loading of 3 wt% ETH 500 in the binder. We also show that establishing a SC composition that is far from equilibrium with the ISM can also introduce a slow partition of charged species over more than a month in storage. By further optimizing the composition of the binder, increased hydrophobicity, reduced drift during storage and a shortened time to ready can be obtained, verifying the versatility of such an approach.