Making buffer solutions?

Weak acid buffer systems buffer best at pH within one unit of pKa. So to buffer at pH 6 you need an acid with pKa between 5 and 7. There are several that come close, but the best is the hydrogen citrate ion (pKa 5.40) or the hydrogen phthalate ion (pKa 5.41). Acetic acid and propionic acid are just outside this range.