# What does the pH scale measure?

Jul 2, 2018

It is a measure of the concentration of hydronium ion....

#### Explanation:

See here and here, and here....

In aqueous solution under standard conditions, ${K}_{w} = \left[{H}_{3} {O}^{+}\right] \left[H {O}^{-}\right] = {10}^{- 14}$...and when we take ${\log}_{10}$ of both sides...we get...

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

And so in aqueous solution....$p H + p O H = 14$...under standard conditions...

And why do we use such a scale? Well, logarithms are a throwback to the days BEFORE ready access to electronic calculators....for scientists, and engineers, and for accountants and actuaries...when they were faced with large and small division and multiplication, and calculation of compound interest … it was EASIER to do addition i.e. $x \times y \equiv \log x + \log y$...than long hand multiplication, and then take the antilog to get the answer.