Its easy to calculate the pH of an acetic acid solution by using the following equation.

[H^{+}] [Ac^{-}] = K_{a}= 1.8 x 10^{-5}

For a dilute acid and not a buffer [H^{+}] = [Ac^{-}]

The pH is the log_{10}[H^{+}].

An indicator cannot be used to determine a pH if the solution is outside of its range. So if bromcresol purple yields a yellow color we only know that the pH is equal or less than 5.2.

If the pH of a stop bath is 2.8 and we double its volume with water the pH increases by the log_{10}2 = 0.3. So the resultant pH is 3.1. The capacity of such a bath is half what the original bath had.