I have an interesting problem....I know I learnt about this in Chem classes,
but that was a long time ago and can't remember!
Why does the pH of a dibasic sodium phosphate buffer solution increase upon
dilution? For example, a 10-fold dilution in the buffer increased the pH
from 7.25 to 7.35. If you just look at the Henderson-Hasselbach equation, as
long as the ratio of the base to the conjugate acid stays the same (as with
dilution), the pH should not change.
I presume that the aw (water activity coefficient) increases with dilution
for dibasic phosphate (relatively more than monobasic phosphate) resulting
in the above change. However, I cannot find any hard data on the actual aw
values at various dilutions.
Or am I on the wrong track?
Please help. Google did not return any useful matches.