Ion selective electrode

Ion selective electrodes are sensors used in the measurement of specific chemical ions in solution. They have been called ion specific electrodes, but there can be interferences which do not make them completely specific, hence the selective description. The most common type is a hydrogen-sensing electrode used in pH measurements. They are increasingly common as real-time physiological sensors in critical care, providing real-time levels of sodium, potassium, bicarbonate, and other biologically significant ions. In general analytical chemistry, they usually are a cheaper, faster alternative to other methods such as atomic absorption spectrometry.

Design of the measurement system

A typical measurement system has a millivoltmeter that measures the difference between the measuring and reference electrode. There are several fundamental designs for the electrodes, with a given design being optimal for a specific ion.

Glass membrane

This is the usual type used for pH measurement.

Liquid membrane

"Liquid membrane electrodes have non-glass, solid-state crystals or pellets as the membrane component of the electrode.The most successful example is the excellent electrode for fluoride ion, which is based on a crystal of lanthanum fluoride doped with erbium to create crystal defects to improve conductivity."

Gas-sensing and combination

The electrodes consist of an ion-selective electrode, usually pH, in contact with a thin layer of solution that is held in place with a membrane permeable to the desired gas such as ammonia or carbon dioxide. When the gas passes through the membrane, the change of pH in the thin layer of solution is sensed by the glass membrane pH electrode.

Applications

The electrodes are not affected by the color of the solution, a highly desirable property for blood analysis. Measurement systems can be quite portable in comparison with other ion-sensing electrodes, allowing point of care or internal measurements in medicine, but also for such applications as environmental contaminant measurement in the field.