The pKa for phosphoric acid is 2.15 for the first dissociation, 7.20 for the second dissociation and 12.35 for the third dissociation. These values are based on an ambient temperature of 25 degrees Celsius, and are known as Ka1, Ka2 and Ka3, respectively.
The pKa value measures the acidity of a solution based on how the hydrogen ions of the acid dissociate when added to an aqueous solution. Since phosphoric acid, represented by the molecular formula H3PO4, has three hydrogen atoms, it dissociates three times, into H2PO4-, HPO42- and PO43-. The pKa value is based on the Ka value, which is called the acid dissociation constant.