posted by micole on .
If you have a list of acids or bases and their conjugate salts, and are asked which pairs create the best buffer, how would you know which are best?
The salt contains the conjugate base or acid of the corresponding acid or base on the list. Since most buffers are combinations of a weak acid and its conjugate base, or a weak base and its conjugate acid, the combination of the salt and the related acid or base make a buffer mixture. That is because one of the ions in the salt is conjugate to one of the substances on the list. Examples:
1) NaC2H3O2(salt) with HC2H3O2(acid) = HC2H3O2 / C2H3O2-
2) NH3(base) with NH4Cl(salt) = NH3 / NH4+
Usually the "best" buffer means one which is buffered at a particular pH. Look at the pKa of the acid or base and see if that pKa is near where you want to buffer. If so that is a good buffer for you. For example, the pKa for acetic acid is about 4.74. Therefore, acetic acid/sodium acetate will buffer in that range.