pH measurement

pH measurement

Introduction

The pH measurement of aqueous solutions plays an important role in many industrial process plants. Among other applications, it is used to monitor the quality of products and the safety of equipment. It is also used to monitor compliance with environmental regulations regarding the composition of industrial wastewater discharges.

The pH scale measures the acidity or alkalinity of an aqueous solution. The notation pH is sometimes referred to as the power of hydrogen or the potential of hydrogen.

Measurement

The pH of aqueous solutions is a measurement of the hydrogen ion (hydronium H3O+ ) concentration in the solutions.

The values of pH range from about 0 (for a strongly acidic solution) to about 14 (for a strongly alkaline or basic solution). A neutral solution (neither acidic or basic), such as pure water (HOH), has a value of 7 at a typical room temperature and at atmospheric pressure, whereas the pH of an acidic solution is less than 7 and the pH of a basic solution is greater than 7.

The pH scale is logarithmic which means that a difference of one pH unit is equivalent to a ten-fold difference in hydrogen ion concentration.

For very dilute aqueous solutions, the pH value can be defined by this logarithmic expression:

Note that the above equation uses the notation H+ rather than H3O+ simply to indicate that the two notations are often assumed to be interchangeable.

The above logarithmic relationship results in a nonlinear relationship between the hydronium ion concentration and the pH value of an aqueous solution. The typical graph of a titration, as shown just to the right, clearly illustrates that non-linearity when performing a titration to conduct an aqueous solution from an acid pH point of about 3.8 to an alkaline (or basic) pH point of about 10.8.

The non-linear relationship makes it difficult to implement a reliable pH control system in an industrial process. Therefore,
advanced techniques are required to maintain a pH set point, as close as possible to the real pH value of a process solution. The diagram just below illustrates one such technique for providing a pH control system in an industrial plant:


The diagram below is of an electric power plant using a fuel-fired boiler to produce superheated steam that drives a steam turbine which, in turn, drives a generator that produces the electric power. A demineralizer train is used to remove impurities from the power plant's inlet raw water and the demineralized water is maintained at a desired pH value by a pH control system similar the one in the diagram just above. The diagram also indicates the many other streams within such a power plant that require the routine measuring of pH as well other chemical parameters.

Electrodes for measuring pH

When a liquid is brought into contact with another liquid, a voltage difference (i.e., an electrochemical potential) occurs due to their differences in electron mobility. However, a membrane is needed to keep the liquids apart. As shown in the adjacent diagram on the right, a pH meter measures essentially the electrochemical potential between a known liquid inside a thin glass electrode (membrane) and an unknown liquid outside. Because the thin glass bulb allows the small hydrogen ions to interact with the glass, the glass electrode measures the electrochemical potential of hydrogen ions or the potential of hydrogen.

Two electrodes are required to obtain a pH measurement, one to measure and other to work as a reference. The adjacent diagram depicts two separate electrodes, the measuring electrode on the left and the reference electrode on the right.

However, by far, most pH measurements are made using what is commonly referred to as a combination electrode such as depicted in the diagram just below. As can be seen in that diagram, the measuring and the reference electrodes are combined within a single glass device.

References

• Pedro Estéfano Cohn (2006), Analisadores Industriais, Editora Interciência, ISBN 9788571931473.
• Brian LaBelle, Sensing pH, controlling pH, May 1, 2006. From the website of the International Society of Automation (ISA).
• F.G. Shinsky, Characterizers For Control Loops, May 1999. From the website of Exper Tune, Inc.
• Dr. J. Floor Anthoni, pH meter principles - What is pH and how is it measured? , 2005. From the website of the Seafriends Marine Conservation and Education Centre.
• Brochures of Emerson Project Management.
• Brochures of Yokogawa Electric Corporation.

Publishing note:

This article was written by Geano Elias Rocha, a member of this wiki, and uploaded by him. Some minor format editing was then provided by Milton Beychok, the organizer of this wiki.