When a strong base such as NaOH is added to a weak acid (HA), the products are water and the salt of the acid:
HA + NaOH → Na+A- + H2O
As more and more of the free acid is converted to the salt, the ratio [salt]/[acid] rises as predicted by the Henderson-Hasselbalch equation. Between 0% and 10% titration, the pH changes dramatically with each added increment of base; the same is true between 90 and 100% titration. Convince yourself of this by plotting the titration curve for a weak acid of pK 7.0.
Notice that between 10 and 90% titration, the mixture of acid and its salt changes pH relatively little with each increment of base added. This is the defining property of a buffer: a solution of a weak acid and its salt that resist changes in pH upon addition of either strong acid or strong base. The central part of the titration curve is the buffering region; generally, buffering action is strongest between 1 pH unit below the pKa and 1 pH unit above it.
You saw in the living Henderson-Hasselbalch equation that when the concentration of acid and salt are equal, pH = pKa. This equivalence occurs at exactly the midpoint of the titration curve, when the acid has been titrated 50%. This fact can be used to determine the pKa of an unknown acid by its titration; its pKa is the pH at the exact midpoint of the titration, which is the inflection point.
Substitute several values of pKa, and notice the relationships between the resulting curves. How do their shapes compare? Their positions on the pH axis?
Which of these weak acids would be useful in the preparation of a buffer of pH 6.2?
| Weak acid | pKa |
|---|---|
| Lactic acid | 3.86 |
| Acetic acid | 4.76 |
| Histidine | 6.0 |
| Tris | 8.16 |
| Diethanolamine | 9.0 |
| Glycine | 9.8 |
To plot the titration curve for a weak acid enter the pKa and hit New Plot. Up to 5 plots can be displayed at one time. The Clear button will remove all plots. To see the pKa for each plot hit the Legend button. The Redraw button will refresh the graph. To see the value of each plot at a given point, move your cursor to the desired location then click and hold.
Original material from Lehninger Principles of Biochemistry, 5a edición, de D. Nelson and M. Cox; 2009. ISBN: 0-7167-7108-X.
Dr. José Antonio Encinar. (IBMC-UMH)