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Acids and Alkalis

A Titration using an Acid and an Alkali.

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 that dissolves in water).

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

hydrochloric acid sodium hydroxide arrow sodium chloride +  water.
  HCl(aq)         +            NaOH(aq)          arrow     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. This is the end point of the titration.

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.

Using a pH meter.

You can use a pH meter to find the end point.
At neutralisation the pH is 7.
A pH meter allows you to make a pure salt sample faster
than an indicator because you do not have to repeat the titration.

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