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Understanding pH Measurements: Techniques and Theories in Chemistry

This article explores the methods for measuring pH in solutions, focusing on the limitations of ion activity measurements. We discuss the coupling of hydrogen electrodes with reference electrodes like calomel, detailing half-cell reactions and cell reactions to derive pH values. The Nernst Equation is introduced along with practical problems related to ion activity accuracy. Additionally, we highlight the importance of standard solutions and the glass electrode's role in modern pH measurement, which has largely replaced the traditional hydrogen electrode.

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Understanding pH Measurements: Techniques and Theories in Chemistry

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  1. Chemistry 232 pH measurements

  2. Measuring the pH of Solutions • The activity of a single ion cannot be measured • Only measure our ‘best approximation’ to the hydrogen ion activity. • Couple a hydrogen electrode with another reference electrode, e.g., a calomel reference electrode.

  3. A Cell for ‘Measuring’ the pH • Half-cell reactions. HgCl2 (s) + 2e- Hg (l) + 2 Cl- (aq) E(SCE) = 0.2415 V 2 H+ (aq) + 2e- H2 (g) E (H+/H2) = 0.000 V • Cell Reaction HgCl2 (s) + H2 (g)  Hg (l) + 2 H+ (aq) + 2Cl- (aq) Ecell = (0.2415 - 0.0000 V) = 0.2415 V Pt H2 (g), f=1H+ (aq) HgCl2 Hg Cl- (aq), 3.5 MPt

  4. The Nernst Equation • For the above cell • Note – concentration of KClon one side of the liquid junction is so large, the magnitude of the junction potential should be small!

  5. The Practical Problem • The activity of the Cl- ion in the cell is not accurately known. • We try to place the cell in a reference solution with an accurately known pH (solution I). • Next place the solution whose pH we are attempting to measure into the cell (solution II).

  6. Assuming that the ELJ and the a(Cl-) are the same in both cases, • Substituting the definition of the pH into the above expression,

  7. Standard Solutions • Generally, two solutions are used as references. • Saturated aqueous solution of sodium hydrogen tartarate, pH = 3.557 at 25C. • 0.0100 mol/kg disodium tetraborate, pH = 9.180 at 25C.

  8. The Glass Electrode • The glass electrode has replaced the hydrogen electrode in the operational definition of the pH.

  9. Glass Electrodes • Measuring the pH – the glass electrode is immersed in the solution of interest. • Inner solution – generally a phosphate buffer with a sufficient quantity of Cl- (aq). • Silver-silver chloride electrode is sealed within the cell; calomel electrode is used as the reference electrode.

  10. Glass Electrodes and pH • The potential difference arises across the special glass membrane • Equilibrate the hydronium ions inside the membrane with those outside the membrane.

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