Potentiometric

Potentiometric refers to a class of electrochemical measurements in which the potential difference between two electrodes is measured with negligible current flow. Potentiometric methods are used to determine the activity or concentration of ions in solution by relating the measured voltage to the ion activity through the Nernst equation.

The principle involves an indicator electrode that responds to the target ion and a stable reference electrode that provides a constant potential. The potential difference, measured with a high-impedance voltmeter, reflects the chemical potential of the ion in the solution rather than a current or charge transfer. The Nernst equation describes the relationship: E = E0 + (RT/zF) ln a_ion, where E is the measured potential, E0 a standard potential, z the ion’s valence, F Faraday’s constant, R the gas constant, T temperature, and a_ion the activity of the ion. At 25°C, the slope is about 59 mV per decade of activity for monovalent ions, with different slopes for multivalent ions.

Commonly used potentiometric devices include pH meters (glass electrode paired with a reference electrode) and ion-selective electrodes (ISEs) that use ionophores or membranes selective for a specific ion, such as potassium, calcium, nitrate, or chloride. Electrode pairs are connected to a high-impedance input to minimize current and prevent electrode polarization. Calibration with standards is essential to relate measured potentials to known ion activities.

Applications span environmental monitoring (water quality), clinical and laboratory analysis, and industrial process control. Potentiometric methods are valued for simplicity, rapid response, and relatively low cost, though they require careful calibration, temperature control, and attention to electrode selectivity and drift. Potentiometry also encompasses related techniques such as potentiometric titration, where potential change indicates a chemical endpoint.