The ion electrode (or, to use the formal term, ion selective electrode) is one that responds to specific ions in an aqueous solution. In this sense, the pH-sensitive glass electrode can be regarded as a hydrogen ion electrode. However, the term “ion electrode,” although referring to an electrode that responds to a particular type of ion, usually excludes the special case of the pH-sensitive glass electrode.
Research on ion electrodes has been actively pursued since the 1960s. From an early stage, HORIBA too has developed and put on sale various types of ion electrodes.
In order to measure the ion concentration of a specific ion, an ion meter—or a pH meter capable of measuring ions—is connected to the relevant specific ion electrode and a reference electrode, and these are immersed in the test solution. The specific ion concentration in the test solution is obtained by measuring the electromotive force between the two electrodes. In order to compensate for sensitivity (i.e. to determine the calibration curve), this method requires two types of reference solutions that have different concentrations of the specific ion. Just as with the pH-sensitive glass electrode, the sensitivity of ion electrodes is also affected by the difference in temperature, so measurement has to be carried out at a single constant temperature (25°C for example).
An ion electrode responds not only to the specific ion of interest, but also to other ions which have characteristics similar to those of that ion. The selectivity coefficient is a measure of this effect, i.e. that by which an electrode responds to types of ions other than the type of interest.
Response membranes for ion electrodes include glass membranes (for Na+ electrodes), silver-salt based solid state membranes (for the corresponding CN-, Cl-, S2-, I-, Br-, SCN-, Cu2+, Cd2+, Pb2+, or Ag+ electrode), single crystal membranes (for F- electrodes), liquid membranes and plastic solidification membranes (for No3-, K+, Na+, and Ca2+ electrodes), and diaphragm (for ammonia electrodes).
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