Potentiometry is an electroanalytical technique that measures the potential difference (voltage) between two electrodes in an electrochemical cell with no or negligible current flow.
The potential difference is related to the concentration of ions or molecules in a solution and can be determined using the Nernst equation or other relevant relationships.
Potentiometry is widely used in fields such as pharmaceutical analysis, environmental monitoring, and quality control of food and beverages.
Components of a Potentiometric Measurement
In a typical potentiometric measurement, an electrochemical cell consists of two electrodes:
1.Working Electrode (Indicator/Sensing Electrode)
This electrode is sensitive to the analyte of interest, responding to its concentration in the solution.
2.Reference Electrode
The reference electrode maintains a stable and well-defined potential, serving as a reference point for measuring the potential difference between the two electrodes.
Types of Potentiometric Electrodes
1. Ion-selective electrodes (ISEs):
ISEs are sensitive to specific ions in a solution, responding to their concentration by developing a potential according to the Nernst equation.
The most common ISE is the glass electrode, which is selective for hydrogen ions (H+) and used for pH measurements.
Other ISEs include solid-state electrodes, liquid membrane electrodes, and gas-sensing electrodes, which are selective for various cations (e.g., Na+, K+, Ca2+) and anions (e.g., Cl-, NO3-).
2. Redox electrodes:
These electrodes are used to measure the potential of redox-active species in a solution, which is related to their concentration and redox state.
Redox electrodes typically consist of an inert metal, such as platinum or gold, and are often combined with a reference electrode to form a single, combined electrode.
3. Potentiometric biosensors:
These sensors incorporate a biological recognition element, such as an enzyme, antibody, or receptor, which selectively binds to the analyte of interest and triggers a change in potential that can be measured.
Potentiometric biosensors have been developed for various pharmaceutical and biomedical applications, including drug monitoring, diagnostic tests, and environmental analysis.
Application of Potentiometry
1.pH Measurement
Used to measure pH using a glass electrode as the indicator and a reference electrode (e.g., silver/silver chloride or calomel).
The potential difference between the electrodes indicates the pH of the solution.
2.Ion-Selective Electrode (ISE) Measurements
ISEs selectively respond to specific ions such as fluoride, chloride, potassium, sodium, and calcium, and are used in water quality testing, blood analysis, and environmental monitoring.
3.Titration
Potentiometry monitors titrations by measuring the potential difference between the indicator and reference electrodes, allowing for accurate determination of the titration end point.
4.Quality Control
Common in pharmaceuticals, food, and beverages.
For example, chloride ion concentration in saline can be measured using ISEs, and acidity in fruit juices can be determined using pH electrodes.
5.Electroplating
Potentiometry controls metal deposition in electroplating by measuring the potential difference between the metal electrode and the ion-containing solution, ensuring consistent plating quality.