3.3. Measurement of the pH

Traditional methods using pH-sensitive dyes (e.g. indicator papers) are suitable only for the estimation of the pH. Precise measurement is possible only on electrochemical basis. However, the hydrogen (H/H+) electrode is not among the simple possibilities because the use of this electrode is complicated and dangerous. In the laboratory practice, the measurement of pH is based on the ability of glass for fast and reversible H+ adsorption, which generates a [H+]-dependent potential. pH electrodes are therefore made of glass. At the tip of the electrode, a globular glass membrane encloses an inner liquid. The pH of this liquid and, thus, the number of H+ ions absorbed on the inside surface of the glass membrane is constant. When the globular tip of the electrode is immersed into a solution to be measured (outside solution), the outer surface adsorbs or desorbs protons, the amount of which depends on the pH. The potential of the glass electrode is proportional to the difference between the amounts of protons adsorbed at the two sides. This, in turn, will depend only on the pH of the outside solution. The pH can be precisely determined via comparing this potential to the potential of a reference electrode (which is a non-polarisable electrode usually made of silver or calomel, built inside the glass electrode) following a pH calibration of potential differences.

Control questions and exercises

  1. Explain the position of the equilibrium in the reaction of Equation 3.7. Why is it opposite to those in reactions of Equations 3.1 and 3.4?

  2. Prove the connection between Ka and Kh and use it to interpret the relationship between the strength of a conjugate acid and base.

  3. Derive Equation 3.13 for a buffer made of a weak base (BOH).

  4. Explain why the Henderson-Hasselbalch equation cannot be applied on single-component systems.

  5. How does the pH of a buffer change upon dilution?

  6. Explain why pH is not 7.0 at the neutralisation point (100 % titration) of acetic acid.

  7. Draw the titration curve of a 0.1 M solution of a weak base (BOH) with a pK of 9.8, and specify its ionisation states.

  8. Does the concentration influence the pH at the centre of the titration plateau?

  9. Draw the titration curves of lysine, arginine, glutamic acid, aspartic acid and histidine using the pKa values specified in Figure 3.2.

  10. Determine the isoelectric point of amino acids listed in Exercise 9.

  11. Calculate the charge of a histidine side chain at pH 7.4 and pH 7.6.

  12. Estimate the isoelectric point of the following peptide: H2N-Gly-Ala-Asp-Arg-Val-His-His-Glu-Met-COOH.

  13. Calculate the ratio of the components in the H2CO3/HCO3- buffer system at the pH of tissues (7.4) and that of the lungs (7.6).