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To accurately measure the con-centration of hydrogen ions (pH value) in a solution, a `glass electrode` is often used in chemistry laboratories. The electrode is con-structed on the principle of a galvanic cell, and the output voltage of the electrode is proportional to the pH value of the solution to be measured. Tha temperature of the solution considerably affects the pH value, thus a pH meter is effec-tively a temperature compensated millivoltmeter.