Buffer question

[Rhino1000]

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TBR, example 5.1.

In example 5.1, they say that dilution of an acidic buffer will yield no change in the pH of the buffer. In fact, they say that the answer suggesting that diluting the buffer will raise the pH "slightly" is wrong (they insist no change at all). How can this be, unless the buffer completely counteracts any changes in H+ concentration by dissociating more acid into the solution? How would this work? Would it have something to do with the fact that HA dissociates into two molecules (H+ and A-), thus making it tougher to dissociate at higher concentrations (analogous to gas reactions that are regulated by volume changes)? Also, even if this is the case, wouldn't adding water change the pH at least a tiny bit?

 

[t5Nitro]

I think since you know the pH of the acid you can determine that it is a weak acid; weak acids have small Ka values so when K << 1, reactants are favored. You know the acid dissociation reaction HA --> H(+) + A(-). I think they just want you to know that [HA] won't really change, and therefore [A-] won't really change. Then the ratio in the HH equation also does not change. At least until you exceed the buffering capacity, the pH should remain the same.

 

[gettheleadout]

TBR, example 5.1.

In example 5.1, they say that dilution of an acidic buffer will yield no change in the pH of the buffer. In fact, they say that the answer suggesting that diluting the buffer will raise the pH "slightly" is wrong (they insist no change at all). How can this be, unless the buffer completely counteracts any changes in H+ concentration by dissociating more acid into the solution? How would this work? Would it have something to do with the fact that HA dissociates into two molecules (H+ and A-), thus making it tougher to dissociate at higher concentrations (analogous to gas reactions that are regulated by volume changes)? Also, even if this is the case, wouldn't adding water change the pH at least a tiny bit?

It's an approximation, as is the Henderson-Hasselbalch equation. For the purposes of the MCAT, dilution of a buffer does not affect the pH.

For more info, see this thread and the links provided within: http://www.chemicalforums.com/index.php?topic=6154.0

 

[patrickmbyrne]

Ph=pka + log (B/A) if you change the dilution of A you change the dilution of B . Exactly the same as above.