Question

What are the "Universal" buffer mixtures?

Answer 1

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`.