Several papers published in this Journal (1) present the buffering effect in a pedagogical way. However, an experimental aspect deserves to be emphasized. Our first-year students measure the pH of acetate buffer solutions and compare them with the calculated values. They also compare the measured and calculated pH of 0.1 M acetic acid and sodium acetate. The agreement is good in all cases except one: the measured pH of the sodium acetate solution is always lower than the calculated value. We found that the explanation of this discrepancy is an excellent illustration of buffering power. The measured pH of CH₃COONa is too low because of the dissolution of CO₂. This can be shown theoretically and experimentally.

As the CH₃COONa solution is basic, we consider the following equilibria to calculate its pH. The second dissociation of carbonic acid can be neglected.

CH₃COO^- + H₂O CH₃COOH + OH^-

CO₂·H₂O + OH^- HCO₃^- + H₂O

This gives

and

Neglecting [H⁺], the mass balance equations give

[CH₃COO^-] + [CH₃COOH] = 0.1

[CH₃COOH] = [OH^-] + [HCO₃^-]

Without carbonic acid we get [CH₃COOH] = [OH^-] and [CH₃COO^-] = 0.1; then [OH^-]² = 0.1 x 10^-9.24 and pH = 8.9. In aqueous solutions in equilibrium with air containing 350 ppm of CO₂, the dissolved CO₂ concentration is approximately [CO₂·H₂O] = 10^-5 mol/L. Solving the above equations, we get [HCO₃^-] = 10^2.6[OH^-] and [CH₃COOH] = 4 x 10²[OH^-]; then 4 x 10²[OH^-]² = 0.1 x10^-9.24 and pH = 7.6. This is very close to the value obtained by the students. The CO₂ dissolution decreases the pH of the sodium acetate solution by more than one unit. An experimental proof of this effect can be given. When pure N₂ is bubbled through the solution, the pH increases. However, this increase is very slow. Even with high N₂ flow and vigorous stirring, we only got pH = 8.4 after two hours. An additional proof can be given. If we stop the N₂ flow and leave the stirred solution in contact with air, we see that the pH decreases slowly.

The students are surprised to see that a small quantity of CO₂ can so greatly affect the pH of the sodium acetate solution but has no effect on the pH of its mixtures with acetic acid. This unexpected observation gives them a more striking illustration of the buffering effect than the classical addition of a small quantity of dilute HCl.

Literature Cited

1. Urbansky, E. T., Schock, M. R. J. Chem. Educ. 2000, 77, 1640; Gould, E. S. 1999, 76, 1511; Barnum, D. 1999, 76, 938; Ramette, R. W. 1998, 75, 1504; Chiriac, V.; Balea, G. 1997, 74, 937.