pKa Definition in Chemistry

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pKa can be used to describe strength of acids. Adrianna Williams / Getty Images

If you're working with acids and bases, two familiar values are pH and pKa. Here is the definition of pKa and a look at how it relates to acid strength.

pKa Definition

pKa is the negative base-10 logarithm of the acid dissociation constant (Ka) of a solution.
pKa = -log10Ka
The lower the pKa value, the stronger the acid. For example, the pKa of acetic acid is 4.8, while the pKa of lactic acid is 3.8. Using the pKa values, one can see lactic acid is a stronger acid than acetic acid.

The reason pKa is used is because it describes acid dissociation using small decimal numbers. The same type of information may be obtained from Ka values, but they are typically extremely small numbers given in scientific notation that are hard for most people to understand.

Key Takeaways: pKa Definition

  • The pKa value is one method used to indicate the strength of an acid.
  • pKa is the negative log of the acid dissociation constant or Ka value.
  • A lower pKa value indicates a stronger acid. That is, the lower value indicates the acid more fully dissociates in water.

pKa and Buffer Capacity

In addition to using pKa to gauge the strength of an acid, it may be used to select buffers. This is possible because of the relationship between pKa and pH:

pH = pKa + log10([A-]/[AH])

Where the square brackets are used to indicate the concentrations of the acid and its conjugate base.

The equation may be rewritten as:

Ka/[H+] = [A-]/[AH]

This shows that pKa and pH are equal when half of the acid has dissociated. The buffering capacity of a species or its ability to maintain pH of a solution is highest when the pKa and pH values are close. So, when selecting a buffer, the best choice is the one that has a pKa value close to the target pH of the chemical solution.