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An Ion selective electrode is a sensor which converts the activity of a specific ion (dissolved in a solution) into a voltage (potential), which can be measured by a mV or Ion meter. The voltage is theoretically dependent on the logarithm of the ion activity, as described by the Nernst Equation.
The sensing part of the electrode is usually made from an ion specific membrane, coupled together with a reference electrode (either separate or as a combination).
There are four main types of ion selective membranes which are detailed hereunder. Of these Ionode currently manufacture Ion selective electrodes using solid state (crystalline) membranes.
Glass membrane electrodes are formed from special ionically conducting glass. By altering the composition and constituents of the glass, changes can be made to selectivity, chemical resistance, response time, and impedance. The most common glass membrane ion selective electrode is the pH electrode. Another common one is the sodium selective electrode.
Crystalline or solid state membrane electrodes are made from relatively insoluble ionically conducting inorganic salts. These are available in homogeneous and heterogeneous forms. They have good selectivity since only ions which can introduce themselves into the crystal lattice can interfere. Examples include the Fluoride electrode which uses a doped LaF3 crystal, and the Chloride electrode which uses silver chloride powder.
Polymer Membrane Electrodes are based on special organic polymer membranes which contain various ion-exchange ionophores incorporated into an inert matrix. These are used in electrodes to measure ions such as Potassium, Calcium, and Nitrate.
Gas sensing electrodes have gas permeable membranes and an internal solution. Gas molecules diffuse across the membrane and react with the solution. This pH change is then detected by the pH electrode. Common gas sensing electrodes include ammonia and carbon dioxide.
Enzyme electrodes are based on the reaction of an enzyme reacting with a specific substrate, and the resultant product of this reaction (usually H+ or OH-) is detected by an electrode such as a pH electrode. These reactions occur within a special membrane. An example is the Glucose electrode.
Ion selective electrodes or ISE’s are commonly used in environmental, food and agriculture, power plants and in clinical applications.
Examples from these areas include Fluoride and Calcium in drinking water, Calcium in beer, Nitrates in plants, and Chloride in blood.
The main reasons Ion selective electrodes are popular:-
- the initial set up is inexpensive (typically one only needs a pH/mV meter or Ion meter, the electrodes, a stirring stand, and some basic chemicals)
- the measurements are unaffected by colour or turbidity in the sample
- the sample pre-treatment is usually simple
- the measurements can be done in “real time”, and can be easily automated.
- wide dynamic range
- selectivity/interferences
- accuracy, typically 2-5% relative
- calibrating solutions and samples need to have the ionic strength adjusted
- lifetime and maintenance requirements of polymer membrane electrodes
- sample pre treatment is sometimes necessary for non-ideal samples
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