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HEPES Buffer - (Apr/19/2011 )


I don't really understand what is the nature of HEPES (acid or base ?).
What is the difference between HEPES (free acid) and HEPES sodium salt...?
What will be the difference in adding HEPES between 2 samples, one with pH=7.5 and the other with pH=8.1.
Should i for each one prepare a HEPES solution specifically for these two samples? i mean should i before adding HEPES in my solution prepare 2 HEPES solutions with the 2 differents pH.
Same question: Must HEPES solutions have the same pH that there is in my samples ?

The aim of my experiment is to stabilish pH in 2 flasks where the pH of my seawater has been decreased with HCl to 7.5 and to 8.1(No HCl in this one it's ambiant seawater)

I hope i have been clear, sorry for my english...
Hope you could help me




HEPES is a buffer - this means that it will keep your solutions at or close to the pH you require. It seems to me that you should prepare a solution for each pH you require.

The difference between the Na and free acid is that the free acid does not release sodium when dissolved. The free acid form is an organic acid in that is has a COOH group, which you may(should) have learned about in chemistry. The Na salt has the form COO-Na+.


OK, thanks for your reply...
That means we can not decrease pH with sodium salt because it doesn't release H+...?
Could someone bring me more informations ?



on the contrary, you would increase the pH with the sodium salt. you can make equimolar solutions of free acid and sodium salt and add one to the other until you reach the desired pH. the final volume is not important (unless you make too little) but the concentration of the buffering species will remain the same.