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pH and ORP Measurement (Part 4 of 4)

By David Mills

E-Zine February 2007

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Potentiometry is the field of electrochemistry where measurements of the electrical potential between two electrodes are made under conditions of negligible current flow. Both pH and ORP are potentiometric measurements that are expressed in millivolts of potential. In both cases (but especially for pH) the underlying electrochemical processes cannot maintain these potentials under conditions where a significant current is also passed. Therefore, high input-impedance amplifiers are typically used to restrict the amount of current that flows in the electrode circuit. In some instruments, the high input-impedance amplifier is part of the electronic unit. Other instruments include the amplifier as part of the sensor unit or as a separate device installed between the sensor and electronic units.

Potentiometric sensor units consist of a measurement half-cell and a reference half-cell. Combination sensors house both half-cells in the same unit. Some suppliers can provide separate measurement units and reference units that should be installed close to each other.

The measurement half-cell is designed to create an electrical potential that is related to the concentration of the ion or ions of interest. The reference half-cell is designed to maintain a constant electrical potential regardless of the ion concentration in the process fluid. The reference half-cell is used to complete the circuit. Both the measurement half-cell and the reference half-cell must be immersed in the process fluid to assure electrical continuity through the entire circuit.

Low conductivity process fluids such as purified water often require special techniques to assure that a sufficient conductivity is maintained in the region between the measurement half-cell and the reference half-cell so as to complete the circuit.

This article was excerpted from The Consumer Guide to Industrial pH and ORP Instrumentation

ISSN 1538-5280

Spitzer and Boyes, LLC
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