Instruction manual

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SECTION 12.0
pH MEASUREMENTS
12.1 General
12.2 Measuring Electrode
12.3 Reference Electrode
12.4 Liquid Junction Potential
12.5 Converting Voltage to pH
12.6 Glass Electrode Slope
12.7 Buffers and Calibration
12.8 Isopotential pH
12.9 Junction Potential Mismatch
12.10 Sensor Diagnostics
12.11 Shields, Insulation, and Preamplifiers
12.1 GENERAL
In nearly every industrial and scientific application, pH is determined by measuring the voltage of an electrochemical cell.
Figure 12-1 shows a simplified diagram of a pH cell. The cell consists of a measuring electrode, a reference electrode, a
temperature sensing element, and the liquid being measured. The voltage of the cell is directly proportional to the pH of
the liquid. The pH meter measures the voltage and uses a temperature-dependent factor to convert the voltage to pH.
Because the cell has high internal resistance, the pH meter must have a very high input impedance.
Figure 12-1 shows separate measuring and reference electrodes. In most process sensors, the electrodes and the tem-
perature 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 electrode, and
the liquid junction potential. The potential of the measuring electrode depends only on the pH of the solution. The potential of
the reference electrode is unaffected by pH, 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
MODEL XMT-P pH/ORP SECTION 12.0
pH MEASUREMENTS
FIGURE 12-1. pH Measurement Cell.
The cell consists of a measuring and reference electrode. The voltage between the elec-
trodes is directly proportional to the pH of the test solution. The proportionality constant
depends on temperature, so a temperature sensor is also necessary.