Filter
Top Products
114
SECTION 13.0
pH MEASUREMENTS
13.1 General
13.2 Measuring Electrode
13.3 Reference Electrode
13.4 Liquid Junction Potential
13.5 Converting Voltage to pH
13.6 Glass Electrode Slope
13.7 Buffers and Calibration
13.8 Isopotential pH
13.9 Junction Potential Mismatch
13.10 Sensor Diagnostics
13.11 Shields, 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 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 3081 pH/ORP SECTION 13.0
pH MEASUREMENTS
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.