Potentiometry: Types of Electrodes

Reference electrodes serve as a stable reference point for potentiometric measurements, while indicator and working electrodes react to variations in the composition of a solution.

The Standard Hydrogen Electrode (SHE) is a widely used reference electrode that maintains zero potential across all temperatures. However, its need for a continuous hydrogen gas supply renders it impractical for everyday use.

An alternative to SHE is the Saturated Calomel Electrode (SCE). This electrode features an H-shaped glass body that contains a platinum electrode immersed in mercury saturated with mercury(I) chloride and potassium chloride salts. Despite its widespread use, SCE is sensitive to temperature fluctuations and raises environmental concerns due to its mercury content.

Silver-silver chloride electrodes, which rely on the reduction of AgCl to Ag, present another option. These electrodes comprise a silver electrode submerged in a KCl solution saturated with silver chloride. Although more temperature-resistant than SCE, they may produce inaccurate measurements if silver ions react with sample components, leading to junction plugging.

To ensure accurate potentiometric measurements with reference electrodes, it is crucial to maintain the internal liquid level above the sample solution level, preventing contamination and junction plugging. Regular maintenance and cleaning are also essential for reliable results.

Metallic indicator electrodes can be classified into three categories: Class I electrodes directly respond to solutions containing their ions; Class II electrodes form slightly soluble salts with the ions being measured; and Class III electrodes react to various cations. Inert metallic electrodes function as sources or sinks of electrons in redox reactions.

Meanwhile, membrane electrodes, including ion-selective and pH-sensitive glass electrodes, are designed to measure specific ion concentrations within solutions.