Describe the principle of operation of a pH meter.
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Glass Electrode: The heart of a pH meter is a glass electrode. This electrode is typically filled with a solution containing a known concentration of hydrogen ions. The glass membrane of the electrode is selectively permeable to hydrogen ions.
Reference Electrode: In addition to the glass electrode, a pH meter also includes a reference electrode. This electrode is typically filled with a solution of known constant pH, often potassium chloride (KCl) solution.
Electrochemical Potential: When the glass electrode is immersed in a solution, a potential difference is established between the glass electrode and the reference electrode. This potential difference is proportional to the logarithm of the hydrogen ion concentration in the solution, according to the Nernst equation.
pH Measurement: By measuring the voltage between the glass electrode and the reference electrode, the pH meter determines the pH of the solution. The meter then displays the pH value on its screen.
Calibration: To ensure accuracy, pH meters need periodic calibration using buffer solutions with known pH values. Typically, two or more buffer solutions with different pH values are used to calibrate the meter.