pKa

pKa is a measure of acid strength, with smaller numbers indicating stronger acids. Very strong acids have negative values for pKa, while weaker organic acids have values of about 5. Extremely weak acids, like alkanes, have pKa values as high as 50.

Definition

When an acid, AH is added to water, the acid becomes depronated to become the acid's conjugate base, A^-, and a hydronium ion, H₃O⁺, is formed. The equilibrium constant, K_eq, for this reaction can be expressed as

K_eq = ${\frac {[A^{-}][H3O^{+}]}{[HA][H2O]}}$

In the special case of dilute acids, the concentration of water is very large and essentially constant, allowing for the definition of the acidity constant, K_a, as follows

K_a = ${\frac {[A^{-}][H3O^{+}]}{[HA]}}$

Because the range of values of K_a range from 10¹⁴ to 10^-50, it is more convenient to express such extreme values using a logrhythmic scale. Thus, just like pH is defined as the negative log of the hydronium concentration (pH = -log [H3O]+), one defined pKa in a similar fashion.

pK_a = -log K_a