Water Pollution Continuous Monitoring Technology in Japan
Principles of Analyzers for Water Pollution Continuous Monitoring
In practice, a pH value is defined by the equation below:
This equation means that the pH value is a common logarithm expressing the reciprocal of the hydrogen ion concentration. The pH value of a neutral solution is obtained as 7 from the following calculation:
When the hydrogen ion concentration decreases (e.g., [H+]=10-10), the pH value is 10 showing that the solution is alkaline. When it increases (e.g., [H+]=10-3), the pH value is 3 showing that the solution is acid.
A large number of animals and plants inhabit lakes, rivers, and oceans. The pH values of those water areas differ depending on the kinds of animals and plants that can live there. In general, the pH values which allows animals and plants to live are in the neutral range. Animals and plants can no longer live if the pH values of their habitat become acidic or alkaline. The cause of such pH changes are mainly wastewater from factories and households.
The measuring methods for pH values include using an indicator reagent, the metal electrode methods (hydrogen electrode method, quinhydrone electrode method, and antimony electrode method), and the glass electrode method. The glass electrode method is considered to be the standard measuring method. In practice, however, pH values are obtained from hydrogen ion activity (thermodynamic activity) rather than hydrogen ion concentrations.
2. Measuring Method Using pH Electrodes
When two solutions with different pH values exist inside and outside a glass membrane, electromotive force occurs in that membrane (electrode membrane) in proportion to the difference between the two pH values. Since the solution typically used inside the glass membrane has a pH value of 7, the pH value of the solution outside the membrane can be obtained by measuring the electromotive force generated in the membrane.
The pH meter consists of a glass electrode and a reference electrode. It allows the pH value of the sample to be obtained by measuring the potential difference between the two electrodes with a potential difference meter.
To calibrate the pH meter, a standard solution with a known pH value is used. As standard solutions, phthalic acid (pH4.01), neutral phosphate (pH6.86), and borate (pH9.18) are mainly used.