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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: mrflow3r on December 11, 2005, 05:08:35 PM

Title: Diluting a buffer has no effect on its pH?!
Post by: mrflow3r on December 11, 2005, 05:08:35 PM

Hello there, my text book says 'diluting a buffer has no effect on its pH', but I don't understand why though.

This is the K_a expression of a buffer solution
K_a = [Conjugate base][H₃O⁺ ]/[Conjugate acid]

My textbook says pH doesn't change since both [conjugate base] and [conjugate acid] are diluted equally.
But if you dilute this solution, wouldn't that change the concentration of everything including H₃O⁺?

Thank you, I appreciate your help.

Title: Re:Diluting a buffer has no effect on its pH?!
Post by: Borek on December 11, 2005, 06:18:01 PM

That's only approximation, but it holds very well.

Check this pH of buffers (http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch) lecture.

If you dilute 1M solution of acetic buffer 50/50 to 0.001M change in pH is less then 0.03 pH unit (assuming that the ionic strength of the solution have not changed).

Title: Re:Diluting a buffer has no effect on its pH?!
Post by: mike on December 11, 2005, 06:18:56 PM

http://www.chemicalforums.com/index.php?board=2;action=display;threadid=5915;start=msg26271#msg26271