# What is water on the pH scale?

Oct 16, 2016

Distilled water has $p H = 7$.

#### Explanation:

$p H$, $\text{pouvoir hydrogene}$, is a measure of hydrogen ion concentration in water (I think there is an accent in there somewhere but nevermind).

Aqueous acidity is conceived to be a measure of hydrogen ion concentration according to the following equilibrium:

$2 {H}_{2} O r i g h t \le f t h a r p \infty n s {H}_{3} {O}^{+} + {H}_{3} {O}^{+}$

And at $298 K$, ${K}_{w} = \left[{H}_{3} {O}^{+}\right] \left[H {O}^{-}\right]$ $=$ ${10}^{-} 14$.

Taking $- {\log}_{10}$ of both sides, we get the defining relationship:

$p H + p O H = - {\log}_{10} \left({10}^{-} 14\right) = 14$.

At neutrality under standard conditions, thus $p H = p O H = 7$.