Ka is the equilibrium constant for the dissociation reaction of a weak acid. A weak acid is one that only partially dissociates in water or an aqueous solution. The value of Ka is used to calculate the pH of weak acids. The pKa value is used to choose a buffer when needed. Choosing an acid or base where pKa is close to the pH needed gives the best results.
Relating pH, Ka, and pKa
pH, Ka, and pKa are all related to each other. For an acid HA:
Ka = [H+][A-] / [HA]
pKa = - log Ka
pH = - log([H+])
At the halfway point on an equivalence curve, pH = pKa.
Ka of Weak Acids
Ka of Weak Acids | |||
---|---|---|---|
Name | Formula | Ka | pKa |
acetic | HC2H3O2 | 1.8 x 10-5 | 4.7 |
ascorbic (I) | H2C6H6O6 | 7.9 x 10-5 | 4.1 |
ascorbic (II) | HC6H6O6- | 1.6 x 10-12 | 11.8 |
benzoic | HC7H5O2 | 6.4 x 10-5 | 4.2 |
boric (I) | H3BO3 | 5.4 x 10-10 | 9.3 |
boric (II) | H2BO3- | 1.8 x 10-13 | 12.7 |
boric (III) | HBO32- | 1.6 x 10-14 | 13.8 |
carbonic (I) | H2CO3 | 4.5 x 10-7 | 6.3 |
carbonic (II) | HCO3- | 4.7 x 10-11 | 10.3 |
citric (I) | H3C6H5O7 | 3.2 x 10-7 | 6.5 |
citric (II) | H2C6H5O7- | 1.7 x 105 | 4.8 |
citric (III) | HC6H5O72- | 4.1 x 10-7 | 6.4 |
formic | HCHO2 | 1.8 x 10-4 | 3.7 |
hydrazidic | HN3 | 1.9 x 10-5 | 4.7 |
hydrocyanic | HCN | 6.2 x 10-10 | 9.2 |
hydrofluoric | HF | 6.3 x 10-4 | 3.2 |
hydrogen peroxide | H2O2 | 2.4 x 10-12 | 11.6 |
hydrogen sulfate ion | HSO4- | 1.2 x 10-2 | 1.9 |
hypochlorous | HOCl | 3.5 x 10-8 | 7.5 |
lactic | HC3H5O3 | 8.3 x 10-4 | 3.1 |
nitrous | HNO2 | 4.0 x 10-4 | 3.4 |
oxalic (I) | H2C2O4 | 5.8 x 10-2 | 1.2 |
oxalic (II) | HC2O4- | 6.5 x 10-5 | 4.2 |
phenol | HOC6H5 | 1.6 x 10-10 | 9.8 |
propanic | HC3H5O2 | 1.3 x 10-5 | 4.9 |
sulfurous (I) | H2SO3 | 1.4 x 10-2 | 1.85 |
sulfurous (II) | HSO3- | 6.3 x 10-8 | 7.2 |
uric | HC5H3N4O3 | 1.3 x 10-4 | 3.9 |