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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.

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for the reasons that alan outlines below you usually prepare buffers and then adjust them with a pH meter rather than preparing them from a recipe. – shigeta Feb 4 '13 at 22:42
Yes, I did exactly that; I was just curious why there are two different values :). Also, I didn't want to introduce a lot of ions by using excessive acid/base when adjusting pH. It is best to start with the correct recipe to end up with the correct molarities of ions in the solution. – Engin Yapici Feb 5 '13 at 22:32
I used to mix 1M stocks of each H2KPO4 and HK2PO4 to avoid unknown NaCl from NaOH and HCl, but this was also wasteful... – shigeta Feb 6 '13 at 5:27
up vote 3 down vote accepted

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

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


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.

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Thank you for the great answer! – Engin Yapici Feb 5 '13 at 22:30

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