PH-Meters are instruments that measure the electrical potential between two electrodes, reference and measured, indicating if the solution is acid or alkaline. Both electrodes are placed inside neutral electrolytes with known number of hydrogen ions, usually 3M KCl.
The measured electrode is placed next to a glass membrane that reacts with the ions from the measuring solution and makes difference in the electrical potential. This difference expressed through Nernst’s equation gives the pH value of the measured solution.
