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

1 Answer
Jun 12, 2016

pH increases; sometimes it rises substantially.

Explanation:

pH, "potenz hydrogen" is a measure of hydrogen ion concentration, i.e. [H_3O^+].

By definition, pH = -log_10[H_3O^+], so that acidic solution have low (or even negative) values of pH, and alkaline solutions have high values of pH.

In water, the following equilibrium occurs:

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

At 298*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(10^-14) = -14

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

To give (finally):

14=pH+pOH