Ka is the equilibrium constant for the dissociation reaction of a
weak acid. The value of K
a is used to calculate pH of weak acids. The pK
a value is used to choose a
buffer when needed. Choosing an acid or base where pK
a is close to the pH needed gives the best results.
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 |
