Emerson Process Management 3081 pH/ORP instruction manual Section ORP Measurements, General

MODEL 3081 pH/ORP	SECTION 14.0
	ORP MEASUREMENTS

SECTION 14.0

ORP MEASUREMENTS

14.1General

14.2Measuring Electrode

14.3Reference Electrode

14.4Liquid Junction Potential

14.5Relating Cell Voltage to ORP

14.6ORP, Concentration, and pH

14.7Interpreting ORP Measurements

14.8Calibration

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.

Voltmeter

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.

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.

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