An alkali such as sodium hydroxide or potassium hydroxide dissolves in water, so it is a soluble reactant. We cannot add it in excess and filter, because the extra alkali would stay dissolved in the solution and could not be filtered out. A titration must be used instead.
In the titration, an indicator shows when exactly the alkali has been neutralised, and it tells us the exact volume of acid needed to react with a measured volume of alkali. For example, if 25 cm3 of alkali needs 20 cm3 of acid, then those are the volumes that react completely.
The acid and the soluble reactant are then mixed in the correct proportions, using the same volumes as before but without the indicator. If the indicator were added it would contaminate the salt. After reaction, the solution left contains only salt and water. Some of the water is evaporated to make the salt crystallise.
