HEPES buffer preparation - (Dec/19/2011 )

Hello!
I have to prepare a 20 mM HEPES buffer, using hepes free acid form and the potassium salt. How can I calculate their quantities? I think I should use pH (pH=7.4) and pKa which is equal to 7.5.
I could also use only hepes free acid form and than adjust pH using KOH or NaOH. Which protocol is better?

thank you

If you have both compounds, the mixing is easier. Make 1 M stock solutions of the free acid and of the salt. You can then mix these freely to obtain a 1 M solution of the desired pH. Finally, dilute 50x to obtain your 20 mM solution. This way will make it easier next time you need to make a HEPES buffer of a different pH or molarity. A possible problem is the solubility of the free acid , but it should be OK.

Thank you!!!