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In nearly every industrial and scientific application, pH is determined by measuring the voltage of an electrochem-ical cell. Figure 2-1 shows a simplified diagram of a pH cell. The cell consists of a measuring electrode, a refer-ence electrode, a temperature sensing element, and the liquid being measured. The voltage of the cell is directly
The aim of this book is to give a representative description of pH measurement in the process industries. The actual sensor, the pH electrode, is therefore the main focus of the text. Correct sensor use is fundamental for a meaningful pH measurement.
principle is understood and an accurate measurement made possible. The first section (practical considerations) of the book describes the sensor, and the other elements that constitute a pH measurement sys-tem. Together with a troubleshooting diagram, this section gives the in-formation needed in order to ensure the correct working of the pH elec-
pH electrode measurements are made by comparing the readings in a sample with the readings in standards whose pH has been defined (buffers). When a pH sensing electrode comes in contact with a sample, a potential develops across the sensing membrane surface and that membrane potential varies with pH.
pH measurement is based on the use of a pH sensi-tive electrode (usually glass), a reference electrode, and a temperature element to provide a temperature signal to the pH analyzer.
To be able to measure pH one needs to have a measurement tool which is sensitive to the hydrogen ions that define the pH value. The principle of the measurement is that one takes a sensor with a glass membrane which is sensitive to hydrogen ions and observes the reaction between it and a sample solution.
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.
