The pH Reference Electrode
The reference electrode represents a defined electrical connection between the measured liquid medium and the pH meter. The overall accuracy of the pH measurement is often determined by the reference electrode and therefore the choice of the reference electrode design is of significant importance. An ideal reference electrode should produce a predictable voltage potential, which should respond only in accordance with the Nernst equation. A good and stable reference electrode should also have a low temperature coefficient and possess no temperature hysteresis.
A typical pH reference electrode consists of an internal electrode (similar to the design of the measurement electrode) which is immersed into a defined electrolyte. This electrolyte must be in contact with the measured medium. Over the years various reference systems have been employed, but only two systems, the mercury-mercurous chloride (calomel) and the silver-silver chloride reference (Ag-AgCl) electrodes were found reliable with respect to an accurate and stable potential. Hamilton has standardized on the Ag-AgCl reference system. A brief overview of different pH reference designs and their benefits can be found in a separate article.
While at low and stable temperatures (max. 80°C) the calomel electrode has a high potential stability and a high accuracy down to a hundredth of a millivolt. But today the Ag-AgCl electrode has gained practical acceptance and is by far the most frequently employed reference system. It is easy to manufacture, its voltage potential rapidly attains equilibrium between -30°C and 135°C, and is very reproducible. The Ag-AgCl reference electrode remains stable and accurate especially with wide temperature fluctuations and at high temperatures up to 135°C.
The internal electrode of an Ag-AgCl reference electrode consists of a silver chloride coated silver wire which is immersed into potassium chloride of 3 molar concentration situated in a large chamber formed by the glass body of the reference electrode.
A liquid junction or “diaphragm” (normally a small porous ceramic disk) is fitted at the bottom of this chamber to permit the potassium chloride electrolyte to diffuse or leak into the measured medium. This diffusion of the electrolyte with the liquid medium is required to create an electrical path between the inside of the reference electrode and the measurement electrode.
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