pKa in chemistry relates to the acid dissociation constant Ka as well as to pH and the strength of acids. Here is the pKa definition, its relationship with Ka and pH, and how pKa indicates whether an acid is strong or weak.
What Is pKa? pKa Definition
pKa is the negative base 10 logarithm of the acid dissociation constant, Ka. In a way, both pKa and Ka give you the same information, but the Ka value is a small decimal number that is easy to work with, while Ka has exponents and uses scientific notation.
Relationship Between pKa and Ka
Here is the equation that relates pKa and Ka:
pKa = -log10Ka
Alternatively, you can solve for Ka:
Ka = 10-pKa
Table of pKa Values for Common Acids
This table lists approximate pKa values for common acids:
| Acid | Formula | pKa |
|---|---|---|
| Acetic | CH3COOH | 4.76 |
| Formic | HCOOH | 3.75 |
| Hydrofluoric | HF | 3.20 |
| Phosphoric | H3PO4 | 2.16 |
| Trifluoroacetic | CF3COOH | 0.52 |
| Nitric | HNO3 | -1.4 |
| Sulfuric | H2SO4 | -2 |
| Hydrochloric | HCl | -7 |
| Perchloric | HClO4 | -10 |
In comparison, the pKa value for water (H2O) is 14.00 at 25 °C. (Note, some texts use 15.74 as the pKa for water, based on the reaction between water and methoxide acid.)
pKa and Acid Strength
A smaller pKa value indicates a stronger acid; a larger pKa value indicates a weaker acid. Strong acids actually have negative pKa values. Weak acids have positive pKa values.
For example, acetic acid has a pKa value of 4.8, while lactic acid has a pKa of 3.8. Both numbers are positive, so you know both acetic acid and lactic acid are weak acids. But, the pKa of lactic acid is lower, so it is a stronger acid than acetic acid.
The acid dissociation constant (Ka) measures how completely an acid dissociates in an aqueous solution. Strong acids completely dissociate into their ions in water. So, a high acid dissociation constant or Ka value indicates a strong acid, while weak acids have low Ka values.
Relationship Between pKa and pH
pH is a measure of hydrogen ion concentration in an aqueous solution. The lower the pH value, the higher the hydrogen ion concentration and the stronger the acid.
pKa and pH are related based on the concentration of an acid ([A-] and its conjugate base ([HA]). This relationship is the Henderson-Hasselbalch equation:
pH = pKa + log10[A-]/[HA]
pKa predicts the pH value where a chemical species donates or accepts a proton or hydrogen ion.
Buffer Selection
In addition to using pKa for predicting acid strength, calculating pKa, and finding pH, pK helps with buffer selection. Use the equation relating pH and pKa to the concentration of an acid ([A–]) and its conjugate base ([AH]):
pH = pKa + log10([A–]/[AH])
A buffer helps maintain the pH of a solution. The best buffering capacity occurs when the pH and pKa are about the same as each other. This is because it takes a lot of added acid or base to change the pH when it is near the pKa. Setting pH and pKa equal gives another equation:
Ka/[H+] = [A–]/[AH]
When half of the acid dissociates, pH and pKa are equal. Select the best buffer for a situation by choosing one with a pKa value close to the target pH of the solution.
References
- Atkins, Peter; de Paula, Julio (2006). Physical Chemistry. Oxford. ISBN 978-0198700722.
- Denbigh, K. (1981). “Chapter 4.” The Principles of Chemical Equilibrium (4th ed.). Cambridge: Cambridge University Press. ISBN 978-0-521-28150-8.
- Himmel, D.; Goll, S. K.; Leito, I.; Krossing, I. (2010). “A Unified pH Scale for all Phases”. Angew. Chem. Int. Ed. 49 (38): 6885–6888. doi:10.1002/anie.201000252
- Silverstein, Todd P.; Heller, Stephen T. (2017). “pKa values in the Undergraduate Curriculum: What Is the Real pKa of Water?”. J. Chem. Ed. 94(6): 690-695. doi:10.1021/acs.jchemed.6b00623
- Shriver, D.F; Atkins, P.W. (1999). Inorganic Chemistry (3rd ed.). Oxford University Press. ISBN 0-19-850331-8.