Nernstian

Nernstian describes the expected or observed behavior of an electrochemical system that follows the Nernst equation, relating electrode potential to the activities or concentrations of the species involved. In a half-cell or sensor, the potential E is governed by E = E0' + (RT/nF) ln(a_ox/a_red). At standard room temperature (about 25°C), this becomes E = E0' + (0.05916/n) log10(a_ox/a_red). Consequently, a one-electron transfer yields a theoretical slope of about 59 mV per decade change in concentration, while a two-electron process yields about 29.6 mV per decade, and so on.

In practice, “Nernstian” refers to electrodes or sensors whose potential changes with ion activity or concentration in close agreement with the Nernst equation over a defined range. This behavior is particularly sought in ion-selective electrodes, where a near-ideal linear response versus the log of activity indicates proper membrane selectivity and kinetics. The pH electrode is a common example, ideally showing about a −59 mV change per unit change in pH at 25°C, reflecting a one-electron process for H+.

Deviations from Nernstian behavior are described as non-Nernstian and can arise from various non-idealities, including temperature deviations, activity coefficient changes, slow electrode kinetics, interference by other ions, membrane saturation, or uncompensated junction potentials. Such factors can alter the slope, linearity, or response time of the sensor.

The concept is named after Walther Nernst, whose equation linking electrochemical potentials to species activities underpins much of electrochemistry and sensor design.