[ptryon]

Hi,

I am writing some questions on calculating the pH of buffer solutions for an A2 class. Acidic buffers should have a pH lower than 7 (they are comprised of an acid after all!) However, when I answer my own question I am getting a pH greater than 7. I can't see how this could be the case with a solution that is comprised of an acid an a salt of its conjugate base.

Calculate the pH of a buffer made up of 2.0moldm^-3 phenol and 1.0 moldm^-3 sodium phenoxide. The pKa of phenol is 9.6.


Using the Henderson-Hasslebach equation:

pH = pKa + log([A^-]/[HA]) = 9.6 + log (1.0/2.0) =  9.90

Am I wrong claiming that all acidic buffers should have a pH below 7? Or is this something to do with the assumptions we make when we use the Henderson-Hasslebach equation? (i.e. phenol is a very weak acid and therefore the assumptions don't hold to be true?) or is there a mistake in my calculations or question? Any help would be greatly appreciated...

[AWK]

You mixed wery weak acid (pK_a=9.6) with medium weak base (pK_b=4.4). Such a mixture within buffer ratios (10:1 to 1:10) will always show pH over 7.

[Borek]

Am I wrong claiming that all acidic buffers should have a pH below 7?

If by "acidic buffer" you mean buffer made from a weak acid and its conjugate base - yes, you are wrong.

Please remember that it is pretty easy to reverse the thinking - why not treat your buffer made from phenoxide as the one made from a relatively strong base PhO^-? Using your terminology this is a "basic buffer", so its pH is over 7.

In general, trying to classify buffers as "acidic" and "basic", taking what they are made of as a classification parameter, is a tricky and dangerous idea (despite looking reasonably on the surface). Each buffer contains both an acid and its conjugate base, neither is "more important". How would you call buffer made from a substance with pKa = pKb = 7?

[ptryon]

Thank you- both explanations are consistent and most helpful 
I really appreciate your time and effort

[snorkack]

Calculate the pH of a buffer made up of 2.0moldm^-3 phenol and 1.0 moldm^-3 sodium phenoxide. The pKa of phenol is 9.6.


Using the Henderson-Hasslebach equation:

pH = pKa + log([A^-]/[HA]) = 9.6 + log (1.0/2.0) =  9.90

I get 9,3 by that equation.