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Potentiometry is a method used in electroanalytical chemistry to measure electrochemical potential of charged particles. An electrode system is used to measure this potential and detect ions while other substances are also present. Two electrodes are placed in an analyte solution and connected to a potentiometer. Measurements are always made when no or very little current is present, so the composition of the substance measured is not altered, making quantitative analysis possible.
In an electrochemical cell linked by the potentiometer, a reference electrode is used that has a potential independent from any temperature or analyte variables. Electrodes used for reference in potentiometry usually include hydrogen, saturated calomel, or silver chloride. An indicator electrode is exposed to the analyte solution, and its potential varies depending on the ions present in the solution. Each electrode is placed in a separate solution and connected to a single potentiometer instrument, while a salt bridge is exposed to each sample, completing an electrical circuit.
The purpose of measurements with electrochemical cells is to calculate ion concentrations using electrode potential, the universal gas constant, the Faraday constant, the charge of the ions, and the temperature in Kelvin. Variables and constants are used to determine the potential of a substance using the Nernst equation, first created in the late 1800s. By the early 1900s, it was discovered that the potential could be different on each side of a glass membrane if concentrations of hydronium ions varied from one side to another.
Instruments used in potentiometry also include pH meters. The pH defines whether a substance is an acid, base, or neutral, which can greatly affect potentiometry measurements. In an electrochemical cell, a reference electrode, called an anode, has a potential independent from any temperature or analyte variables. An indicator electrode, often referred to as a cathode, is exposed to the analyte solution, and its potential varies depending on the ions present in the solution.
Indicator electrodes can be metallic types or membrane versions, which are also called ion-selective electrodes. The potentiometer is used to conduct measurements of circuits with low resistance. For use with high-resistance glass electrodes, pH meters are suitable for making electrochemical measurements of low- and high-resistance circuits. A pH electrode has to be soaked in water to function and has an important purpose in potentiometry. Under the right conditions, the electrochemical properties of any substance can be determined by an electrochemical cell.