[manbot]

Does diluting a buffer change its pH? It seems to me that as long as the buffer strength is relatively high and the pKa is not too low, diluting it wont affect it because the relative proportion of the conjugate base and acid will still be pretty much the same. Is this assumption right?

thanks for any help

kevin

[AWK]

Dilution of buffer affects pH but slightly. But dilution of buffer 1000 times can change pH even by 1

[Borek]

Dilution of buffer affects pH but slightly. But dilution of buffer 1000 times can change pH even by 1

Much more - limit is pH 7.00

See the table in my post here:

http://www.chemicalforums.com/index.php?topic=5915.msg26271#msg26271

[manbot]

well, ill be only diltuing by a factor of 2 from a starting concentration of 3M, so I guess it should be OK. Also, the buffer happens to be set at pH 7, so i guess adding water wont effect it anyway right?

Also, another question i had was if you add ONLY the conjugate base of something (specifically, i was makling a sodium cacodylate buffer at about pH , is it more vulnerable to pH changes from dilutions? It doesnt seem like it would becuase of the saem reason as before - the relative concentrations of the acid and conjugate base will remain the same, but at the same time, adding twice as much of a base will cuase twice as much to dissociate.

[Borek]

1. Pure water (even distilled) rarely have pH 7, more likely it'll be something like 5.6-5.8 (due to the contact with atmospheric CO₂). Luckily the amount of dissloved CO₂ is small.

2. See buffer capacity discussion.