pH adjustment - tris HCl pH 6.8 (Sep/11/2006 )
WE HAVE ADJUST PH BEFORE OR AFTER DILUTION
If you mean do you adjust pH before or after making up to final volume, then you, obviously, have to pH before making up to final volume as otherwise you'll make a solution that is more dilute than is required.
Say you require 1 litre of solution:
Make up to approximately 800ml
Make up to 1000ml.
You should make pH before you go up to th etotal volume. When you add water, the pH doesn't change
There are two effects to keep in mind when preparing Tris and other buffers:
1. There is a temperature effect (dpKa/dT for Tris is -0.028). If you prepare Tris at pH 6.8 at 25C, then at 37C, the pH will be 6.46. Note that the temperature changes when you add HCl to Tris base to adjust the pH.
2. There is a dilution effect. The pH decreases approximately 0.1 unit upon a tenfold dilution.
once titrated, pH of Tris buffers will not be stable and should be adjusted again after 24h
Wow, I didn't no that last comment. We never check after 24 hrs. But I will now. Thanks a lot!