Emerson Process Management 3081 pH/ORP instruction manual PH Measurements

MODEL 3081 pH/ORP	SECTION 13.0
	pH MEASUREMENTS

SECTION 13.0

pH MEASUREMENTS

13.1General

13.2Measuring Electrode

13.3Reference Electrode

13.4Liquid Junction Potential

13.5Converting Voltage to pH

13.6Glass Electrode Slope

13.7Buffers and Calibration

13.8Isopotential pH

13.9Junction Potential Mismatch

13.10Sensor Diagnostics

13.11Shields, Insulation, and Preamplifiers

13.1 GENERAL

In nearly every industrial and scientific application, pH is determined by measuring the voltage of an electrochemical cell. Figure 13-1 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 13-1. 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.

Figure 13-1 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

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