2 replies to this topic

[claire84]

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

[phage434]

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.

[claire84]

Thank you!!!