pH determination (electrometric method)

Here's one of the topic presentation of Subject - Physical Pharmaceutics-1, UNIT - pH, buffers and Isotonic solutions (as per the syllabus of third semester according to Gujarat Technological university) pH, buffers and Isotonic solutions unit of this subject (as per the syllabus of third semester according to Gujarat Technological university).

This topic consists two methods i.e. electrometric and calorimetric methods, so this presentation includes only electrometric method.

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1. pH determination By, Jahanvi P. Tankaria B.Pharm. Sem 3 Akshar Preet Institute of Pharmacy

2. Electrometric method

3. Introduction: The pH of a solution can be determined by means of an electrode, whose potential depends on the hydrogen activity. The hydrogen and glass electrodes are the typical pH electrodes, though the later one is commonly used.

4. Glass electrode is a condensed or combined electrode, i.e., this incorporates a reference electrode junction next to the glass membrane (indicator electrode) so that both electrodes are kept in one unit. The pH sensitive glass bulb is made of lithia glass, which is suitable over entire range.

5. Principle: The potential difference exists at the interface between glass and a solution containing hydrogen ions. The magnitude of this difference varies with the concentration of the hydrogen ions. Glass electrode is used for the pH measurement. The glass electrode consists of a thin glass bulb of a special glass blown at the end of glass tube. The bulb is fitted with 0.1 N HCl , which furnishes a constant hydrogen ion concentration.

6. A silver-silver chloride electrode (a silver wire coated with silver chloride) makes the necessary electrical connection with the acid The standard reference buffer solutions are used to calibrate the pH meter, i.e., to set the reference pH. Then the pH of the unknown solution can be read directly from the panel of the pH meter.

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8. Method: The glass membrane electrodes is connected to the pH meter. Initially, the temperature is set to the solution temperature, as it affects the pH of the solution. All pH meters have provision for temperature compensation. The electrodes are immersed in a standard buffer solution of pH 7.0. The asymmetrical potential is altered until the meter reads the known pH values of the buffer solution. The instrument is further calibrated using standard buffers of pH 4.0 and/or pH 9.14.

9. The electrode is rinsed with water. The bulb of the glass membrane electrode is immersed in the test solution. The pH value of the solution is read from the meter directly.

10. Advantages: Glass electrode is not affected by oxidation-reduction systems, since there is no exchange of electrons. Sensitivity of electrometric method is high, as accurate measurements can be obtained. Electrode establishes equilibrium rapidly. The solution is uncontaminated, because the addition of indicator is avoided. pH range of measurement is high.

11. Disadvantages: Electrometric method is not suitable for viscous solutions and gels, because of poor ion mobility. Initial cost of pH meter is high to the colorimetric method. In this method, glass electrode is specified for the pH determination. Several types of electrode are available which can be used.

12. THANKS!