What are the "Universal" buffer mixtures?

1 Answer
Ernest Z.
Oct 16, 2015

Universal buffer mixtures are mixtures of buffers.

Explanation:

No, that's not a facetious answer.

A buffer is a mixture of a weak acid and its conjugate base (or vice versa).

It has its best buffering capacity in the range of #"p"K_a±1#.

If you combine substances with #"p"K_a# values differing by only 2 #"pH"# units or less, you can prepare a wide range of buffers.

Citric acid is a useful component of a buffer mixture because it has three #"p"K_a# values (3.13, 4.76, and 6.40), and these are separated by less than two #"pH"# units.

If you add another acid such as #"H"_2"PO"_4^- //"HPO"_4^-# (#"p"K_a = 7.2#), you can extend the range by another two #"pH"# units.

By adjusting the relative amounts of #"Na"_2"HPO"_4# and citric acid, you can prepare a buffer solution for any #"pH"# between #"pH"=3# and #"pH"=8#.

The mixture is called a universal buffer.

If you add more acids like diethylbarbituric acid (#"p"K_a = 7.98#) and boric acid (#"p"K_a = 9.27#) and adjust the #"pH"# by adding #"NaOH"#, you can prepare a buffer for any #"pH"# in the range from #"pH"=2# to #"pH" = 12#.

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