gcsescience.com                                          33                                          gcsescience.com

Acids and Alkalis

Using Conductivity to find the End Point of a Titration.

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

       Links        Acids and Alkalis         Revision Questions       

gcsescience.com    The Periodic Table    Index    Neutralisation Quiz    gcsescience.com