Critical influence of organic salt purity on the performance of reference electrodes based on highly lipophilic electrolytes

Liquid junction-free reference electrodes based on organic electrolytes are attractive because of their versatility and ease of miniaturization. Unfortunately, the common designs based on ionic liquids have been shown to exhibit poor electrode lifetimes and susceptibility to interference from solution species. These issues could be potentially mitigated via the incorporation of highly lipophilic electrolytes into reference membranes. However, while the increased lipophilicity significantly enhances lifetime and alleviates ion-exchange interference, the effect of the organic salt lipophilicity and purity on the electrode potential stabilization is currently not known, which limits their practical application in reference electrodes. In this study, we provide an in-depth analysis of how highly lipophilic salt composition influences electrode performance. Using commercially available ETH 500 as a model electrolyte, we demonstrate that even minimal excess of lipophilic cationic sites in the salt (0.11 ± 0.04 %) results in a significant potential response to anionic sample species, effectively converting the reference membrane used into an anion-exchanger-based ion-selective membrane. Furthermore, we show that trace amounts of moderately lipophilic charged impurities in the organic salt can completely dominate the interfacial potential because of the extremely high lipophilicity of the primary electrolyte. Both findings reveal the limitations of highly lipophilic organic salts in electrode design and highlight the need to identify an optimal lipophilicity range, that balances better lifetimes and interference resistance without sacrificing partitioning behavior. • Effect of organic salt lipophilicity and purity on reference electrodes based on lipophilic electrolytes remained unclear. • Reference electrodes based on a commonly used lipophilic salt ETH 500 are shown to exhibit residual selectivity to anions. • Potentiometric titrations are used to identify a slight cationic excess in the organic electrolyte. • Potential effect of minor cationic impurities on the response of reference membranes based on ETH 500 is considered. • The suitability of highly lipophilic electrolytes for reference electrode design is discussed.