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Acids and Alkalis

Titration - Using an Indicator or a pH Meter.

The exact amount of acid needed to neutralise an alkali
can be found by titration.
This technique can be used to make pure crystals
of a soluble salt (one which dissolves in water).
A titration uses a burette, pipette and a conical flask.

In the example below, an acid and an alkali react to make sodium chloride.

hydrochloric acid   +  sodium hydroxide sodium chloride  +  water.
HCl_(aq)         +            NaOH_(aq)               NaCl_(aq)      +   H₂O_(l)

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 colourless in an acid.

When enough acid has been added to neutralise the alkali
the indicator changes from pink to colourless.

The titration can be repeated using the same amounts of acid and alkali
but without the indicator.
Pure salt crystals which are free from indicator
can then be crystallised from the neutral solution.

Alternatively a pH meter can be used to find the end point.
At neutralisation the pH is 7.

Either Indicator can be used for a Strong Alkali + Strong Acid.

Universal indicator is not usually used for a titration
because it changes gradually giving different colours for a different pH.
Methyl orange or phenolphthalein are used
because they give a sudden change in colour at neutralisation
which makes it easier to see the end point of the titration.

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