Potentiometry and Ion-Selective Electrodes

Potentiometry measures the equilibrium potential of an indicator electrode at zero current to determine the activity of a target ion, with ion-selective electrodes providing species-specific response.

Definition

An electroanalytical method in which the potential of an indicator electrode, measured against a reference at negligible current, is related through the Nernst equation to the activity of a specific ion in solution.

Scope

This topic covers potentiometric analysis: the Nernstian relationship between measured potential and ion activity, the construction and response of ion-selective electrodes including the glass pH electrode and membrane-based sensors, selectivity coefficients and interferences, the role of the reference electrode and liquid junction, and calibration. It is the basis of pH measurement and many clinical and environmental ion sensors.

Core questions

• How does the equilibrium potential of an indicator electrode encode the activity of a target ion?
• What gives an ion-selective membrane its preference for one ion over others?
• How are interferences from competing ions quantified through selectivity coefficients?
• Why does potentiometry respond to activity rather than concentration?

Key theories

Nernstian potentiometric response

The cell potential varies linearly with the logarithm of the target ion's activity at a slope near 59/z mV per decade, allowing activity to be read directly from a calibrated potential measurement.

Selectivity and the Nikolsky–Eisenman relation

Real ion-selective electrodes also respond to interfering ions; selectivity coefficients in the Nikolsky–Eisenman equation quantify this cross-sensitivity and define the usable range of a sensor.

Clinical relevance

Potentiometric sensors are ubiquitous in clinical chemistry analyzers for blood pH, sodium, potassium, calcium, and chloride, in environmental water monitoring, and in industrial process control, prized for simplicity, wide dynamic range, and direct activity measurement.

History

The glass electrode for pH was developed by Cremer (1906) and Haber and Klemensiewicz (1909); the mid-20th century brought solid-state and liquid-membrane ion-selective electrodes, and ionophore-based sensors expanded the range of detectable ions through the late 20th century.

Key figures

• Fritz Haber
• Max Cremer
• Ernő Pretsch
• Eric Bakker

Seminal works

• wang2006
• bard2001
• bakker1997

Frequently asked questions

Why does a pH electrode measure activity rather than concentration?

The membrane potential responds to the chemical potential of hydrogen ions at the membrane surface, which depends on their activity; this is why pH is formally defined in terms of hydrogen-ion activity, not molar concentration.

What is a selectivity coefficient?

It expresses how strongly an interfering ion contributes to the electrode signal relative to the target ion; a small coefficient means the electrode is highly selective, while a large one means interferences can corrupt the measurement.