nearNernstian

Near-Nernstian describes an electrochemical sensor or electrode response that closely approximates, but does not exactly follow, the Nernst equation. In ion-selective and redox sensors, the potential is expected to vary with the activity of a target species according to the Nernst equation, producing a linear response when plotted against the logarithm of activity. A near-Nernstian response is one whose slope is close to the ideal Nernstian value but shows small deviations due to non-idealities.

At 25°C, the Nernst slope for a monovalent ion is about 59.2 mV per decade of activity. Near-Nernstian electrodes typically exhibit slopes within roughly 10% of this value, often in the range around 50–60 mV per decade, though exact values depend on ion valence, temperature, and sensor design.

Factors that can cause deviations include temperature changes, deviations from ideal activity–concentration relationships (activity coefficients), interfering ions, membrane properties of ion-selective electrodes, junction potentials, long-term aging, and incomplete equilibration of the sensing phase. These factors may shift the slope or the intercept, or affect linearity across the measurement range.

In practice, near-Nernstian behavior indicates usable, near-ideal quantitative performance, with calibration capturing the actual slope and intercept. The term is contrasted with sub-Nernstian behavior (significantly weaker slopes) and super-Nernstian behavior (slopes that exceed the ideal value).