Ionic strength-independent potentiometric cation concentration sensing on paper using a tetrabutylammonium-based reference electrode

Abstract

Ion-selective electrodes (ISEs) are known not to strictly measure the concentration of ions, but their activities. The relationship between the two varies with ionic strength, which may result in measurement errors if samples of variable ionic strength are measured and concentration is chosen as the sensor output. Traditionally, concentration readings from potentiometric signals have been possible if the ionic strength of the solution matches that of the calibrant or is made constant by an ionic strength adjusting buffer. In this work, a direct concentration output is realized for a potassium-selective electrode by using a TBA+ (tetrabutylammonium)-selective membrane as the reference electrode and preloading a TBA+ salt in the solution channel. It is shown that the activity coefficients for K+ and TBA+ change similarly to effectively cancel out the influence of ionic strength, allowing for direct potentiometric concentration measurements with deviations from ideality of < 1 mV. The approach was realized on paper-based analytical devices (PADs). The potentiometric concentration data on paper were found to be independent of ionic strength and, owing to their geometric feature, of sample volume. K+ concentration in grape juice, infused water and green tea analyzed by the newly developed PADs showed good correspondence with Atomic Emission Spectroscopy (AES).