# Question #34d98

Mar 7, 2016

A solution that will mantain the pH of the system fixed.

#### Explanation:

The buffer solution is a mix of an acid ($H A$) and its conjugated base (${A}^{-}$), or a base ($B$) and its conjugated acid ($B {H}^{+}$), which has a simple, intuitive mechanic.

If you put an acid in there, the base will be protonated, the equilibrium restored and the pH will be returned; if you put a base in there, the acid will protonate it, the equilibrium restored and the pH will be returned. And if you pour something indifferent to acid-base mechanisms, the additional volume will dilute both acid and base equally so the pH won't change.

They'll still give way if a big amount of volume is added so they're diluted or if enough acid or base is reacted to react with it all.

Buffer solutions follow the following equation:

$p H = p {K}_{a} + \log \left(\frac{\left[{A}^{-}\right]}{\left[H A\right]}\right) = p {K}_{a} + \log \left(\frac{\left[B\right]}{\left[B {H}^{+}\right]}\right)$

Where $p {K}_{a}$ is minus log the acidity constant of $H A$ or $B {H}^{+}$ and the square brackets represent concentration in moles per liter.

So we can see that usually we make a buffer solution near the $p K a$ of the species in question because we need to have a decent concentration of the species (around 2 mol/L of each) or the buffer will give way too quickly, and to raise or lower the pH by 2 points, you need to have 100 times more of one than the other.

Common buffers are ${H}_{3} P {O}_{4} \frac{}{} {H}_{2} P {O}_{4} N a$ (phosphoric) for strong acid pH (around 1), $C {H}_{3} C O O H \frac{}{} C {H}_{3} C O O N a$ (acetic) for slightly acidic pH (around 4 or 5), $N a H C {O}_{3} \frac{}{} N {a}_{2} C {O}_{3}$ (carbonate) for neutral to slightly basic pH (from 7 to 8), $N {H}_{4} C l \frac{}{} N {H}_{3}$ (ammoniac) for basic pH (from 8 to 10).

Buffers are useful for making sure a reaction will stay in a given pH range, which can be extremely useful when dealing with compounds that can easily precipitate or react with $O {H}^{-}$ or won't be able to happen in acidic medium (like the reaction of magnesium with EDTA, which needs to happen in pHs from 9 and up), or in cases which involve microorganisms or enzymes which will die or stop working outside a certain pH range. Or, like the one in your stomach that makes sure drinking some acid lemonade won't cause intestinal trouble.