Before filling a burette we should make sure we've placed a small glass funnel in
the top. We should also make sure we've got the rest of the equipment we need
including solutions, distilled water, a suitable indicator, in this case phenolphthalein,
a 25 millilitre (ml) volumetric pipette, three clean conical flasks and a
pipette filler. Two examples of these are shown on the screen.
Here we're going to use a more traditional rubber bulb pipette filler,
which should be attached carefully to the end of the pipette.
It's important to hold the pipette close to the end attached to the pipette filler to
reduce the risk of breakage.
Now we can press the appropriate button on the pipette filler and gently squeeze
the bulb to expel the air and we are now able to draw up some of our solution
into the pipette. We place the pipette into the solution and we press the appropriate
button on the filler and this draws the solution into the pipette.
Initially we are going to take more solution then we need. Here you can see
the line on the pipette and you can see the solution is clearly above that.
Now we can gently remove the rubber bulb and we can place our thumb over the end of the
pipette. Now by gently moving your thumb we can release the solution in a carefully
controlled manner. Until the bottom of the meniscus is touching the line on the pipette.
Once the solution is on the line,
we know that we have exactly 25 cm3 of our solution.
This can now be transferred to a clean conical flask.
We allow the solution to run under gravity into the flask by removing our thumb.
It's important to recognise that the pipette is calibrated in such a way that you should
always touch the pipette onto the surface of the liquid in the flask to ensure that
the correct volume has been transferred. There will always be a small amount of
liquid left in the pipette but that's meant to be there so don't worry about it.
One safe way of filling a burette is to clamp it and place the clamp stand on
top of a stool.
This allows you to look at the burette at eye level. You will be given guidance on
what's expected of you at this point.
After checking the tap on the burette is closed you can proceed to fill the burette.
It's sensible to lift the funnel out slightly is this prevents spluttering of
the solution.
It's not essential to fill the burette to the zero line, although you must make
a note of the burette reading for later on.
Before carrying out the titration we need to add a few drops of indicator, which in this case is phenolphthalein.
It's important not to add more than two or three drops
of indicator as you don't need it.
Now we can carry out a rough titration sometimes called an overshoot titration.
To find out roughly how much of our other solution is needed to neutralise
the solution in the conical flask.
This is done by adding the solution from the burette until we see a permanent
colour change.
We can now work out the volume of solution which we added from the burette
by noting the final burette reading and performing a simple subtraction.
In this case we've added just over 16 cm3 all together so we can
reasonably expect our accurate titrations to require somewhere in the region of 15 cm3 of solution.
So we are going to add about 14 cm3 by going very quickly
from 19 cm3 to 33 cm3 on the burette
Here you can see the solution running in quickly
and once we've added those 14 cm3 we can adjust the tap on the
burette to add the remaining solution very slowly in a dropwise fashion.
This should be continued with regular swirling until the colour has been
permanently discharged.
Now we can work out accurately how much acid has been added by
performing a subtraction and the experiment should then
be repeated until concordant results are obtained.