Ion-Exchange and Lipophilicity Limitations of Ionic Liquid Reference Electrodes

Reference electrodes (REs) based on moderately lipophilic ionic liquids (ILs) are an attractive approach to realize integrated potentiometric sensing systems that do not require a traditional liquid junction. However, it is not sufficiently understood how lipophilic sample ions can result in interference. Less lipophilic ILs should partition more easily into the aqueous phase where they can compete with sample ions and suppress ion-exchange, but since a decreased IL lipophilicity favors ion-exchange interference the overall effect is not immediately clear. We use here the established phase boundary potential model to describe failure of IL-based REs upon exposure to lipophilic solution ions. The traditional definition of the detection limit is employed to find the intersection of the separate Nernstian responses to IL and solution interference, which is mathematically simpler to process. The approach focuses on cation interference and considers ion pairing in the polymeric phase together with appropriate ion-exchange and coextraction equilibria. The range of RE potential stability is shown to depend on 1) the concentration of the IL in the membrane; 2) the lipophilicity of the IL expressed through its partition constant; 3) the relative lipophilicity of the lipophilic interference and the IL ion of the same charge expressed through the corresponding ion-exchange constant; 4) the stability constant of ion pairs between the interfering ion and the counterion from the IL. For the experimental evaluation of the model, two commercially available ionic liquids were selected: [C8mim+][C1C1N-] (IL1) and [TBMOEP+][C1C1N-] (IL2). Partition constants between water and a polymeric phase based on poly(vinyl chloride) (PVC) and dodecyl 2-nitrophenyl ether (NPDE) were determined for the first time for both IL1 (3.55 ± 0.05) and IL2 (4.68 ± 0.22) using so-called sandwich membrane experiments. Based on these values, the expected reference electrode lifetime is discussed. Tetramethylammonium, tetraethylammonium and tetrabutylammonium cations were selected as model interfering ions and the corresponding exchange constants were obtained experimentally. The model provides excellent RE failure prediction in which calculated critical activities for the tetrabutylammonium cation closely match the experimental values for both ILs.