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MODEL 3081 pH/ORP SECTION 14.0
ORP MEASUREMENTS
SECTION 14.0
ORP MEASUREMENTS
14.1 General
14.2 Measuring Electrode
14.3 Reference Electrode
14.4 Liquid Junction Potential
14.5 Relating Cell Voltage to ORP
14.6 ORP, Concentration, and pH
14.7 Interpreting ORP Measurements
14.8 Calibration
14.1 GENERAL
Figure 14-1 shows a simplified diagram of an electrochemical cell that can be used to determine the oxidation-
reduction potential or ORP of a sample. The cell consists of a measuring electrode, a reference electrode, the liq-
uid being measured, and a temperature-sensing element. The cell voltage is the ORP of the sample. In most indus-
trial and scientific applications, a pH meter is used to measure the voltage. Because a pH meter is really a high
impedance voltmeter, it makes an ideal ORP meter.
Figure 14-1 shows separate measuring and reference electrodes. In most process sensors the electrodes and the
temperature element are combined into a single body. Such sensors are often called combination electrodes.
The cell voltage is the algebraic sum of the potential of the measuring electrode, the potential of the reference elec-
trode, and the liquid junction potential. The potential of the measuring electrode depends on the ORP of the solu-
tion. The potential of the reference electrode is unaffected by ORP, so it provides a stable reference voltage. The
liquid junction potential depends in a complex way on the identity and concentration of the ions in the sample. It is
always present, but if the sensor is properly designed, the liquid junction potential is usually small and relatively
constant. All three potentials depend on temperature.
The construction of each electrode and the electrical potential associated with the electrode are discussed in
Sections 14.2, 14.3, and 14.4.
FIGURE 14-1. ORP Measurement Cell.
The cell consists of a measuring and reference electrode. The voltage between the elec-
trodes is the ORP of the test solution. Because ORP depends on temperature, the tempera-
ture at which the measurement is made must be reported.
Voltmeter