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Acids and Alkalis

Using Conductivity to find the End Point of a Titration.

At the start of this titration the conical flask contains
strong alkali that is fully ionised in water.
electrodes are placed inside the conical flask
ions in the water
conduct electricity and a current will flow.
Conductivity is a measure of how well the solution in
the conical flask
conducts electricity.
more ions there are the better the conductivity and
higher the current will be.
current can be measured using an ammeter.

As acid is added to the alkali hydrogen ions and hydroxide ions
react together to form water molecules.
The number of ions in the conical flask starts to decrease
and the current flowing through the solution will decrease.

At neutralisation all of the hydrogen ions and hydroxide ions
have reacted together to form water molecules.
neutral solution contains
salt ions dissolved in water molecules.
The solution will still
conduct electricity because of the salt ions
but the current will be at a minimum.

As more acid is added the current will start to increase
because there will now be unreacted hydrogen ions
in the solution as well as the salt ions.
The solution is now
no longer neutral but has become acidic.
If you draw a
graph of current against the amount of acid added
you can see where the
minimum is.
This is the
end point of the titration at neutralisation.

The End Point for Titration using Conductivity

The titration can be repeated
with the same amounts of acid and alkali used at the end point.
Pure salt crystals can then be crystallised from the neutral solution.

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