Instruction manual

105
MODEL XMT-P pH/ORP SECTION 13.0
ORP MEASUREMENTS
SECTION 13.0
ORP MEASUREMENTS
13.1 General
13.2 Measuring Electrode
13.3 Reference Electrode
13.4 Liquid Junction Potential
13.5 Relating Cell Voltage to ORP
13.6 ORP, Concentration, and pH
13.7 Interpreting ORP Measurements
13.8 Calibration
13.1 GENERAL
Figure 13-1 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 13-1 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 13.2, 13.3, and 13.4.
FIGURE 13-1. 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