Half-cell potential

A half-cell (electrode) potential is the equilibrium voltage that develops across a single electrode-electrolyte interface due to its electrochemistry, defined relative to a reference electrode. In a recording montage the measured signal is the difference between two such half-cells, so if the two electrodes are not identical their unequal half-cell potentials appear as a large DC offset — often tens to hundreds of millivolts, dwarfing the microvolt neural signal — that the amplifier must reject or block.

More troublesome than the static offset is its slow drift: temperature, ion concentration, and interfacial chemistry change the half-cell potential over time, producing low-frequency baseline wander that overlaps the band of slow cortical potentials and can saturate high-gain DC-coupled amplifiers. This is why front-ends use matched electrode materials (for example Ag/AgCl for scalp EEG), AC coupling or high-pass filtering, and large input dynamic range, and why measuring true slow potentials (such as the contingent negative variation or DC shifts) is genuinely hard.