In pursuit of calibration-free all-solid-state ion-selective electrodes (ISEs), we propose a coulometry strategy based on thin-layer samples confined adjacent to the ion-selective membrane (ISM) surface, with the system being controlled under a cathodic potential sweep. The ion-to-electron transducer in the ISE is the conducting polymer poly(3-octylthiophene) (POT), the oxidation state of which changes upon the application of a cathodic sweep and triggers the accumulation of the preferred cation in the ISM. This accumulation is provided of absolute nature (i.e., the cation concentration is totally depleted in the sample) when the capacity of the membrane encompasses the charge of the cation of interest in the sample (K