# What are the "Universal" buffer mixtures?

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 $\text{p} {K}_{a}$ values differing by only 2 $\text{pH}$ 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 $\text{p} {K}_{a}$ values (3.13, 4.76, and 6.40), and these are separated by less than two $\text{pH}$ units.

If you add another acid such as ${\text{H"_2"PO"_4^- //"HPO}}_{4}^{-}$ ($\text{p} {K}_{a} = 7.2$), you can extend the range by another two $\text{pH}$ units.

By adjusting the relative amounts of ${\text{Na"_2"HPO}}_{4}$ and citric acid, you can prepare a buffer solution for any $\text{pH}$ between $\text{pH} = 3$ and $\text{pH} = 8$.

The mixture is called a universal buffer.

If you add more acids like diethylbarbituric acid ($\text{p} {K}_{a} = 7.98$) and boric acid ($\text{p} {K}_{a} = 9.27$) and adjust the $\text{pH}$ by adding $\text{NaOH}$, you can prepare a buffer for any $\text{pH}$ in the range from $\text{pH} = 2$ to $\text{pH} = 12$.