# How does the pH of an acid change when an alkali is added to it?

Jun 12, 2016

$p H$ increases; sometimes it rises substantially.

#### Explanation:

$p H$, $\text{potenz hydrogen}$ is a measure of hydrogen ion concentration, i.e. $\left[{H}_{3} {O}^{+}\right]$.

By definition, $p H$ $=$ $- {\log}_{10} \left[{H}_{3} {O}^{+}\right]$, so that acidic solution have low (or even negative) values of $p H$, and alkaline solutions have high values of $p H$.

In water, the following equilibrium occurs:

"H_2O(l) rightleftharpoonsH_3O^+ + HO^-

At $298 \cdot K$ we can measure the value of this equilibrium very accurately:

[H_3O^+][""^(-)OH] $=$ ${10}^{-} 14$

And taking ${\log}_{10}$ of both sides:

log_10[H_3O^+] + log_10[HO""^-] $=$ ${\log}_{10} \left({10}^{-} 14\right)$ $=$ $- 14$

On rearrangement, 14=-log_10[H_3O^+] -log_10[HO""^-]

To give (finally):

$14 = p H + p O H$