Ways of Measuring pH
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The methods for measuring pH fall roughly into the following three categories:
(A) Indicator methods
(B) Metal-electrode methods(including the hydrogen-electrode method, quinhydron-electrode method and antimony-electrode method)
(C) Glass-electrode methods
A brief description of each method follows: |
This category basically includes two methods: One involves comparing the standard color corresponding to a known pH with the color of an indicator immersed in the test liquid using buffer solution. The other method involves preparing pH test paper which is soaked in the indicator, then immersing the paper in the test liquid and comparing its color with the standard color. This method is simple, but prone to error. A high degree of accuracy cannot be expected.
* Various errors include;
- Error due to high salt concentration in the test liquid
- Error due to the temperature of the test liquid
- Error due to organic substances in the test liquid
A hydrogen electrode is made by adding platinum black to platinum wire or a platinum plate. It is immersed in the test solution and an electric charge is applied to the solution and platinum black with hydrogen gas.
 The hydrogen-electrode method is a standard among the various methods for measuring pH. The values derived using other methods become trustworthy only when they match those measured using hydrogen electrode method.
However, this method is not appropriate for daily use because of the effort and expense involved, with the inconvenience of handling hydrogen gas and great influence of highly oxidizing or reducing substances in the test solution.
This method involves immersing the tip of a polished antimony rod into a test solution, also immersing a reference electrode, and measuring pH from the difference in potential between them. This method was once widely used because the apparatus is sturdy and easy to handle. However, its application is now quite limited because results vary depending on the degree of polish of the electrode, and reproducibility is low.
Note: Quinhydron solution of a certain pH is sometimes used to check whether an ORP meter is operating normally. The principle of the quinhydron electrode is applied in such a case.
This method involves immersing the tip of a polished antimony rod into a test solution, also immersing a reference electrode, and measuring pH from the difference in potential between them. This method was once widely used because the apparatus is sturdy and easy to handle. However, its application is now quite limited because results vary depending on the degree of polish of the electrode, and reproducibility is low.
Note: This method is now used only in cases where a high degree of accuracy is not required (only for industrial use) and the test solution contains F-.
In this field, pH measurement is applied to plating, etching of metal surfaces and the manufacture of batteries. This method is most widely used for pH measurement because the balancing time of electrical potential is short, it has high reproducibility, it is rarely affected by oxidizing and reducing agents, and it can measure pH of various solutions.  This method is used not only in industry but in a wide variety of fields.
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