Analytical Potentiometry and Ion-Selective Electrodes
Analytical potentiometry measures the equilibrium potential of an indicator electrode to determine the activity of a target ion, most familiarly through the glass pH electrode.
Definition
Analytical potentiometry is an electroanalytical method that determines an ion's activity from the equilibrium potential, measured at near-zero current, of an ion-selective indicator electrode relative to a reference electrode.
Scope
This topic covers potentiometric measurement as practiced in analysis: reference electrodes, indicator and ion-selective electrodes including glass, solid-state, liquid-membrane, and gas-sensing types, the Nernstian response, selectivity coefficients, and potentiometric titrations. It addresses calibration, junction potentials, and the practical limits of selectivity and detection. Within NaturalAtlas it is the analytical-chemistry treatment of potentiometry, complementing the electrochemistry subfield's coverage of cell thermodynamics.
Core questions
- How does the Nernst equation relate electrode potential to ion activity?
- What gives an ion-selective electrode its selectivity for one ion over others?
- Why must potentiometric measurements be made at essentially zero current?
- How are reference electrodes and junction potentials managed for accuracy?
Key theories
- Nernstian electrode response
- An ideal ion-selective electrode's potential changes by a fixed amount per decade change in the activity of its target ion, as the Nernst equation predicts; real electrodes approach this response but are limited by interfering ions described through selectivity coefficients.
- Selectivity of ion-selective membranes
- Carrier-based and other ion-selective membranes respond preferentially to one ion through specific binding chemistry; their cross-sensitivity to interfering ions is quantified by selectivity coefficients that set the usable concentration range.
Mechanisms
An ion-selective membrane develops a boundary potential that depends on the activity of the target ion on each side; measured against a stable reference electrode at negligible current, this potential follows the Nernst equation. Selectivity arises from the membrane chemistry, which responds preferentially to one ion. Calibration with standards of known activity, and control of liquid-junction potentials, converts the measured cell voltage into a concentration or pH.
Clinical relevance
Potentiometry with ion-selective electrodes is fundamental to clinical electrolyte and blood-gas analysis, pH measurement throughout science and industry, and environmental and process monitoring of ions such as fluoride, nitrate, and chloride.
History
The glass electrode's pH response was discovered by Cremer and characterized by Haber and others in the early 20th century, while Nernst's equation provided the quantitative basis. The mid-20th century saw the development of selective membranes for ions beyond hydrogen, and ionophore-based sensors later broadened potentiometry into a versatile analytical family.
Key figures
- Walther Nernst
- Fritz Haber
- Max Cremer
- Ernő Pretsch
Related topics
Seminal works
- harris2020
- skoog2017
- bakker1997
Frequently asked questions
- Why is potentiometry measured at zero current?
- Drawing current would drive a net electrode reaction and disturb the equilibrium being sensed; measuring at essentially zero current with a high-impedance meter lets the electrode report the true equilibrium potential and thus the ion activity.
- What does an ion-selective electrode's selectivity coefficient mean?
- It quantifies how strongly an interfering ion contributes to the measured potential relative to the target ion; a small coefficient means the electrode responds mainly to its intended ion, giving more reliable results in complex samples.