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Chemistry 232

Chemistry 232. pH measurements. 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.

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Chemistry 232

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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 KCl on 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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