Why is second pKa value of phosphoric acid different across different sources? Am I missing something very fundamental here for phosphoric acid's second pKa value? * I check the Wikipedia page and these are the values: (1) 2.148, **(2) 7.198** , (3) 12.377 * Then I check Sigma-Aldrich website and it says: (1) 2.15, **(2) 6.82** , (3) 12.38 Why is the second pKa value different? Any ideas? I need to prepare potassium phosphate buffer for my proteins and got stuck with this simple problem.

Someone will probably come along and move this to "chemistry", but here goes anyway.

**The answer is: it depends upon how you measure it**.

See:

> Jungas, RL (2006) Best literature values for the pK of carbonic and phosphoric acid under physiological conditions. Analytical Biochemistry 349 1-15

The author states that _"..the best value for the true thermodynamic second dissociation constant of phosphoric acid in pure water at 38 °C is 7.180, rounded to the nearest 0.005."_

He then goes on to re-analyse the data reported in:

> Cohn, EJ (1927) The activity coefficients of the ions in certain phosphate solutions: a contribution to the theory of buffer action. J Am Chem. Soc. 49 173-193

and concludes that under physiological conditions of ionic strength and pH the best estimate is 6.795.