# What is an acid?

Sep 6, 2017

What is an acid?

#### Explanation:

Well, it is a species that increases the concentration of the characteristic cation of the solvent. And since the normal solvent is water, we would say that an acid is a species that increases concentration of the hydronium ion, ${H}_{3} {O}^{+}$, the $\text{acidium species....}$

In water we know that $\text{autoprotolysis}$ occurs.....

$2 {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + H {O}^{-}$

And under standard conditions we can quantify this equilibrium, i.e.

${K}_{w} = \left[{H}_{3} {O}^{+}\right] \left[H {O}^{-}\right] = {10}^{- 14}$; and we can take ${\log}_{10}$ of both sides to give......

${\log}_{10} {K}_{w} = {\log}_{10} \left[{H}_{3} {O}^{+}\right] + {\log}_{10} \left[H {O}^{-}\right] = {\log}_{10} \left({10}^{-} 14\right) = - 14$

And on rearrangement......

$14 = {\underbrace{- {\log}_{10} \left[{H}_{3} {O}^{+}\right]}}_{p H} {\underbrace{- {\log}_{10} \left[H {O}^{-}\right]}}_{p O H}$

And so (finally!), our working relationship.....

$14 = p H + p O H$

And so at high acidity $p H$ is low or negative; at neutrality $p H = p O H = 7$, and under alkaline conditions, $p O H$ is low or negative.

Anyway a lot to digest here....I would expect a 1st or 2nd year undergrad to be able to reproduce this......A level students should know that $14 = p H + p O H$.