Today's ion meters are based on the ion-selective electrode technique. These are measuring instruments that have a sensitive membrane capable of measuring ionic activities. That is, they measure the concentration of a given ion in a solution using both a sensing electrode and a reference electrode.
The methodology used by ion meters with selective ion electrodes is ion exchange, which measures the potential difference produced by the contact of the electrode with the ion being analyzed.
To carry out the ion measurement, the ion selective electrodes must come into contact with a certain ion present in the solution. Since the main concern of this measurement is the concentration, and not the activity of the ions in the solution, many times salts are used that increase the ionic strength to facilitate the relationship between the potential of the electrode and the concentration. The activity of the specific ion is converted to electric potential when it comes into contact with the active membrane of the ion-selective electrode. Once this conversion is carried out, the electrical potential can be measured with a voltmeter or with a pH meter.