Acids and Bases - Weak Acid Ka Values

Table of Common Ka values

pKa values are used to help select buffers.
pKa values are used to help select buffers. Cl4ss1cr0ck3R/Creative Commons/CC 2.0

Ka is the equilibrium constant for the dissociation reaction of a weak acid. A weak acid is one which only partially dissociates in water or an aqueous solution. The value of Ka is used to calculate 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

NameFormulaKapKa
aceticHC2H3O21.8 x 10-54.7
ascorbic (I)H2C6H6O67.9 x 10-54.1
ascorbic (II)HC6H6O6-1.6 x 10-1211.8
benzoicHC7H5O26.4 x 10-54.2
boric (I)H3BO35.4 x 10-109.3
boric (II)H2BO3-1.8 x 10-1312.7
boric (III)HBO32-1.6 x 10-1413.8
carbonic (I)H2CO34.5 x 10-76.3
carbonic (II)HCO3-4.7 x 10-1110.3
citric (I)H3C6H5O73.2 x 10-76.5
citric (II)H2C6H5O7-1.7 x 1054.8
citric (III)HC6H5O72-4.1 x 10-76.4
formicHCHO21.8 x 10-43.7
hydrazidicHN31.9 x 10-54.7
hydrocyanicHCN6.2 x 10-109.2
hydrofluoricHF6.3 x 10-43.2
hydrogen peroxideH2O22.4 x 10-1211.6
hydrogen sulfate ionHSO4-1.2 x 10-21.9
hypochlorousHOCl3.5 x 10-87.5
lacticHC3H5O38.3 x 10-43.1
nitrousHNO24.0 x 10-43.4
oxalic (I)H2C2O45.8 x 10-21.2
oxalic (II)HC2O4-6.5 x 10-54.2
phenolHOC6H51.6 x 10-109.8
propanicHC3H5O21.3 x 10-54.9
sulfurous (I)H2SO31.4 x 10-21.85
sulfurous (II)HSO3-6.3 x 10-87.2
uricHC5H3N4O31.3 x 10-43.9