# What is a volatile buffer?

Jun 22, 2016

I can guess that is a pH buffer in which one of the species is volatile.

#### Explanation:

A "normal" pH-buffer is a solution formed by comparable concentrations of both the weak acid and its conjugated weak base.

These concentrations warrant that upon addition of small amounts of strong acids or bases these would be neutralized entailing only a little change of ratio between the concentrations ${C}_{a} / {C}_{b} \cong 1$, which is equal to the ratio [${H}^{+}$] /${K}_{a}$. Then, as long as ${C}_{a}$ remains almost equal to ${C}_{b}$, we get [${H}^{+}$] $\cong$ ${K}_{a}$, that is $p H$ is almost constant, or "buffered".

If the buffer is " volatile ", that means that its weak base (e.g. ammonia, $N {H}_{3} \left(a q\right)$ in buffer $N {H}_{4}^{+} + N {H}_{3}$) or the weak acid (e.g. carbonic acid, $C {O}_{2} \left(a q\right)$ in buffer $C {O}_{2} + H C {O}_{3}^{-}$) can evaporate as gaseous substances ($N {H}_{3} \left(g\right)$ and $C {O}_{2} \left(g\right)$ in the two examples).

Evaporation of $N {H}_{3} \left(g\right)$ makes the $p H$ lowering in the long time, whereas the escape of $C {O}_{2} \left(g\right)$ from the buffer makes its $p H$ slowly increasing. Eventually, the depletion of base or acid reserve in these buffer solutions will cause the impairment of their buffer capacity.