Thin layer coulometry ion sensing protocol with potassium-selective membrane electrodes

We recently presented a calcium-selective thin-layer coulometric system in view of realizing recalibration free ion sensors. A multi-pulse protocol allowing for a significant background current correction was further modified here by a numerical evaluation of the electrolysis end-point. A potassium-selective system was studied here within the 10 μM to 1 mM KCl concentration range with a 10 mM LiCl as a background solution. A residual 20 nA s−1 signal change was used to indicate the end of exhaustive ion transfer from the sample across the ion-selective membrane. Optimized electrolysis times were found from 13.4 s for a 10 μM sample to 270 s for a 1 mM sample. The intercept of the calibration curve of observed ion transfer charge vs. concentration was found to be just 2.3 ± 2.0 μC. The observed slope of 1.18 ± 0.02 C M−1 was very similar to the one calculated based on the known volume of the sample, 1.19 ± 0.12 C M−1, suggesting that the method is promising as an absolute measurement tool.