The acid dissociation constant is a value defined for every acid HA and denoted by Ka = [H+][A-]/[HA]

The acid dissociation constant of oxalic acid is 5.60×10^-2 and 5.42×10^-5. There are two values as oxalic acid is a poly protic acid and has a chemical formula HOOCCOOH.

5.60×10^-2 = [H+][HC2O4-]/[HOOCCOOH]

5.42×10^-5 = [H+][C2O4 2-]/[C2O4H-]

From the Ka value the value of pH can be derived as :

pH = pKa + log[A-]/[HA]

pKa is defined as the negative of the log to the base 10 of Ka, or pKa = -log Ka

At half neutralization the pH of an acid is given by pH = pKa + 0 as log ([A-]/[HA]) = log 1 = 0. For oxalic acid this ranges between –log(5.60×10^-2 ) = 1.25 and –log(5.42×10^-5 ) = 4.267.