Ionizing transductive elements transform the change of measurable value into change of ionization current that runs, for example, through the liquid, located between two electrodes (Fig. 7). The typical example for use of ionizing principle is a pH meter. The degree of acidity solution is defined by concentration of hydrogen positive ions in it, which is called hydrogen potential (more known in shortened form as pH). Hereby,
рН= - lоg[Н+],
where |H+] – concentration of hydrogen ions in grams per liter.
pH value is 0 for purely acid solution, 7 – for neutral acid (for example, pure water) and 14 – for purely alkaline solution.
Typical pH-probe has the electrodes, being in gelatin, with known value of hydrogen potential. They are formed by special glass membrane, being in contact with solution, which pH value is changed. The difference of potentials between two electrodes reflects pH value of solution (about 59 mV per pH unit).