This technique can be used to make pure crystals of a soluble salt. The burette is filled with hydrochloric acid. A known quantity of alkali (say 50 cm3 sodium hydroxide) is released from a pipette into the conical flask. The tap on the burette is turned open to allow the acid to be added drop by drop into the alkali. The alkali contains an indicator (phenolphthalein) which is pink in an alkali and colorless in an <span>acid. </span> When enough acid has been added to neutralize the alkali, the indicator changes from pink to colorless. This is the end point of the titration.
The titration<span> can be repeated using the </span><span>same amounts </span><span>of </span>acid<span> and </span>alkali<span> but </span>without<span> the </span>indicator. <span>Pure salt</span> crystals<span> which are </span>free<span> from </span><span>indicator </span><span>can then be crystallized </span><span> from the </span>neutral<span> solution.</span>