AN-1852| Application Note

Designing With pH Electrodes

Designing With pH Electrodes
A pH electrode measures hydrogen ion (H+) activity and produces an electrical potential or voltage. The operation of the pH electrode is based on the principle that an electric potential develops when two liquids of different pH come into contact at opposite sides of a thin glass membrane. This was originally discovered in 1906 by Max Cremer1. His discovery laid the foundation for Fritz Haber and Zygmunt Klemensiewicz, who published their findings in 1909, to create the first glass electrode which measured hydrogen activity2. Today, modern pH electrodes use the same principles to measure pH in a variety of applications including water treatment, chemical processing, medical instrumentation, and environmental test systems. The modern pH electrode is a combination electrode composed of two main parts, a glass electrode and a reference electrode as shown in Figure 1. pH is determined essentially by measuring the voltage difference between these two electrodes. At the tip of the electrode is the thin membrane wh
ich is a specific type of glass that is capable of ion exchange. It is this element that senses the hydrogen ion concentration of the test solution. The reference electrode potential is constant and is produced by the reference electrode internal element in contact with the reference-fill solution which is kept at a pH of seven.

National Semiconductor Application Note 1852 Jason Seitz September 25, 2008

pH Electrode Characteristics
When designing with a pH electrode, as with any sensor, it is important to understand the sensor characteristics and how they affect a specific application. These characteristics include whether the sensor is active or passive, unipolar or bipolar, and whether it has a voltage or current output. Sensor sensitivity, linearity, full scale range, and source impedance should also be considered. The pH electrode is a passive sensor which means no excitation source (voltage or current) is required. Because the electrode's output can swing above and below the reference point, it is classified as a bipolar sensor. It produces a voltage output which is linearly dependent upon the pH of the solution being measured. The source impedance of a pH electrode is very high because the thin glass bulb has a large resistance which is typically in the range of 10 M to 1000 M. This means that the electrode can only be monitored by a high-impedance measuring device. The transfer function of the pH electrode is:

where pH(X) = pH of unknown solution(X) pH(S)= pH of standard solution = 7 ES = Electric potential at reference or standard electrode EX = Electric potential at pH-measuring electrode F is the Faraday constant = 9.6485309*104 C mol-1, R is the universal gas constant = 8.314510 J K-1 mol-1 T is the temperature in Kelvin The transfer function in Figure 2 and Figure 3 shows that as the pH of the solution increases, the voltage produced by the pH-measuring electrode decreases.

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FIGURE 1. Typical pH Glass Electrode

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FIGURE 2. pH-Electrode Transfer Function
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