How does salt change the pH of water?

Jun 16, 2016

Common salt, $N a C l$, the stuff with which you season your fish and chips, does not change the $p H$ of water.

Explanation:

$N a C l$ is the salt of a strong acid and a strong base. The salt does not affect equilibrium values of $\left[{H}_{3} {O}^{+}\right]$ and $\left[H {O}^{-}\right]$:

$H C l \left(a q\right) + N a O H \left(a q\right) \rightarrow N a C l \left(s\right) + {H}_{2} O$

The reaction as written lies strongly to the right.

On the other hand we could perform the equivalent reaction with the weak acid, hydrogen fluoride:

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

The sodium fluoride product would participate in the back reaction to some extent, and a solution of $N a F$ in water would give a slightly basic solution: