ABSTRACT
Ordinary laboratory voltmeters cannot be used for the measurement of electron motive force (emf) of glass electrode cells because of the high electrical resistance of glass electrodes (typically, 10-200 MΩ). Special high-impedance voltmeter circuits are required, which draw 10-12 A or less from the circuit. The pH meter is a voltmeter but with several critical additional functions. It measures potential across the pH-sensing and reference electrode system, converts the potential difference measurement at a given temperature into pH terms, and provides (pH-metry) mechanisms to correct for the nonideal behavior of the electrode system. An operational amplier not only serves as a high-impedance voltmeter but provides stability and automatic operation through the use of a feedback loop. The operational controls on a pH meter are best understood by referring to the operational manual provided by the manufacturer for an individual instrument. Modern electronic techniques permit the production of a simplied pH meter that measures pH with an accuracy of +0.1 pH unit. The microprocessor-equipped pH meters include a temperature probe to display temperature compensation, a memory to store pH values of standard buffers, a waiting period to allow draft before taking pH readings, and built-in diagnostics to alert for electronic malfunctions or defective electrodes. Along with a pH meter, we also require a reference electrode and a glass membrane electrode, which act as indicator electrodes in pH measurement.
