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E-Zine January 2007Click here to go to Part 1 Click here to go to Part 2 Another difficulty with the theoretical foundation for pH measurement is that activity coefficients for particular ionic species do not represent physical reality because the ions do not exist in the solution by themselves. In order to maintain electrical neutrality, they are accompanied by other ions of opposite sign and these other ions affect their activity. Therefore, an activity coefficient measured for cation C in solution with anion A would not be valid when that cation C was found at the same concentration in solution with a different anion AA. Practical industrial applications are generally more difficult because they often involve more concentrated solutions. However, there are a few electrochemical cells that when used with simple dilute solutions, the activity of the hydrogen ion can be determined in a manner that is relatively independent of the accompanying anion. While the application of thermodynamic theory to solutions is beyond the scope of this text, it is important to note that a small set of standard solutions have been identified and measured to determine reference pH values over a useful temperature range. In this report, the operational definition of pH is that recommended by the International Union of Pure and Applied Chemistry and endorsed by most national standards groups. This definition states that the pH of an unknown solution is found by measuring the electrochemical potential of a pH measurement system immersed in the solution and calculating the pH by interpolation using similar potential measurements made on two of the standard solutions as follows:
This operational definition is based on measurements of the electrochemical potential of the hydrogen ion in solution. A number of different pH measurement systems (also known as electrochemical cells) have been developed. Some, such as the hydrogen cell, are complex to construct but yield results that are dependent on relatively few assumptions. Glass membrane and ion-sensitive field effect transistor (ISFET) cells are simpler to use but are dependent on more assumptions. The hydrogen cell is limited to theoretical studies and will not be discussed further. This article was excerpted from The Consumer Guide to Industrial pH and ORP Instrumentation ISSN 1538-5280 |
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