# How is hydronium ion concentration used to classify a solution as neutral, acidic or basic?

Oct 8, 2015

Hydronium ion, ${H}_{3} {O}^{+}$, is conceived to be the characteristic cation of water. If $p H = 7$, the solution is neutral; if $p H > 7$, the solution is basic; if $p H < 7$, the solution is acidic.

#### Explanation:

We write the formula ${H}_{3} {O}^{+}$ or ${H}^{+}$. We mean the characteristic cation of water. This might be a cluster of 3 or 4 or 5 water molecules with an extra proton, ${H}^{+}$, to make ${H}_{7} {O}_{3}^{+}$ for instance. It is sufficient for calculations to write the ion product for water dissociation as:

${K}_{{H}_{2} O}$ $=$ $\left[{H}_{3} {O}^{+}\right] \left[O {H}^{-}\right]$ $=$ ${10}^{- 14}$. Taking $- {\log}_{10}$ of both sides, we get:

$p H + p O H = 14$. At neutrality, $p H$ $=$ $p O H$ $=$ $7$. (That is $\left[{H}_{3} {O}^{+}\right]$ $=$ $\left[O {H}^{-}\right]$). If $p H$ $<$ $7$, then the solution is acidic (because $\left[{H}_{3} {O}^{+}\right]$ $>$ $\left[O {H}^{-}\right]$). And of course the solution is alkaline, $p H$ $>$ $7$, when $\left[{H}_{3} {O}^{+}\right]$ $<$ $\left[O {H}^{-}\right]$.

I hope this is at the appropriate level.