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Hello, in this video we will see how to perform a colorimetric titration and how to use a calibrated burette
To carry out a colorimetric titration
we need the following material
A graduated burette
a beaker
a magnetic bar
a magnetic stirrer
the solution to titrate
the solution to analyze
and coloured indicators
Let's first try to understand the principle of a colorimetric titration
The solution to titrate is placed in a beaker
and the titrating solution is placed in the graduated burette
To be a relevant titration reaction
this one have to be quantitative, unique and proceed rapidly
At the equivalence point it is necessary for the titrating species (titrant)
to react in a stoichiometric way with the titrated species (analyte)
The equivalence point, i.e. the volume of the titrating species added, is then determined
in order to deduce the amount of substance and the concentration of the analyte
How should we proceed experimentally to carry out a colorimetric titration?
An important point is the choice of the graduated burette
Here for example, you can see three burettes made of glass
but one can also use plastic burette
The size of the burette is important
We can find burettes of 50 ml, 15 mL, 25 mL
Here you have three burettes of 25 ml
What may differ between each two burettes is also the type of tap
At the top of the burette there is a funnel which can be of different sizes
and will facilitate the filling of the burette
Another important point between these two burettes
in one case you do not have a visualization help to read the volume
while an aid is present here (you can see it on the back)
It will give us more precision to determine the equivalence point
Then how do we choose the burette?
Generally the equivalent point is located intentionally at the middle of the burette
Typically for a 25 ml burette
an equivalent point in the range of 12 to 15 ml is searched
How do we experimentally carry out a colorimetric titration?
The first step is to rinse the graduated burette with distilled water
To do so, we will close the tap
place a beaker underneath as a waste beaker
and fill the burette with distilled water up to the top
Distilled water is passed through the tap
and we can check that there is no leakage
After this rinsing step with distilled water
we must rinse the graduated burette with the titrating solution
in order to remove the remaining distilled water
that could lead to a wrong value of equivalence point
As before we first check that the tap is closed
we place a waste beaker below
and we fill the burette to the top with the titrating solution
The solution is then run through the burette
After this second rinsing step of the burette with the titrating species
we will now fill the burette with the titrating species
As before, we check that the tap is closed
then we place a waste beaker
and we add the titrating solution up to the top
We go a little bit higher than the zero
in order to use the tap and check that there is no air bubble above or below the tap
If so
the procedure is to open and close the tap several times as quickly as possible
So we're going to open the tap and move to zero
Here we check that there are no air bubbles above or below the tap
Then we make sure that the meniscus is at the zero level
and we will have properly filled this burette with the titrating solution
When filling the burette and coming to the zero
attention must be paid to the meniscus
The eyes must be well in front of it
The look must not be too low or too high to avoid parallax errors
The next step is to place the solution to analyze in the beaker
Usually, the beaker will be rinsed with the solution to analyze
In the same way as for the burette
traces of water are removed in the beaker to avoid disturbing the titration
The beaker is rinsed
and the solution to analyze is dispensed in it using a volumetric pipette
We take the sample
We use a volumetric pipette to be as accurate as possible
in order to precisely determine the amount of substance of the analyzed species
and its concentration thereafter
A magnetic bar is then placed in the beaker
We check that the agitation is homogeneous and is not too strong
then we add a coloured indicator in the beaker
We add a few drops of coloured indicators to make sure that there is a colouration
Here the solution has a blue colour
and the equivalence point will be determined when there is a change of colour in the beaker
We will now carry out the titration and determine the equivalence point
That is the purpose of the coloured indicator
The solution now shows a pale blue colour difficult to distinguish
Usually, to better distinguish the colour
and in particular the change of colour that will occur during this titration
we place a white cloth or white paper under the beaker
This allows us to better see the colour
Then we will perform the titration
Generally three titrations are carried out to determine the equivalence point as precisely as possible.
First titration: We will titrate quickly
and roughly evaluate the equivalence point
to know where it is located
Then we will carry out two more precise titrations
where we will each time add the titrating solution more carefully
we will add millilitre per millilitre (or every 2 ml) around the equivalence point
So a second titration, more precise, will be carried out
where equivalence point will be determined
And then a third titration
where we will determine again an equivalence point
in order to check that the two equivalence points obtained in the second and third titrations are identical
At first, we carry out a fast titration
to try to roughly evaluate the equivalence point
After this, we have observed a change of colour at an equivalence point of 13 ml
We will now carry out a second titration, more accurate
We know that our equivalence point is located around 13 ml
and we will therefore do a quick drip up to 10-11 ml
A quick drip or a fast trickle
(you see here that it is quite fast)
with the titrating solution
So now we're around 8 ml
We're going to start making a much slower drip
The goal is to be as accurate as possible in determining the equivalent volume
and the colour change
trying to be precise down to the drop
Now we are getting at 11 ml
and we're going to slow down the flow a little more
The important point is to have a significant colour change
and we can see that through a good persistence of colour
Here we can see when adding the drops
that a yellow colour appears but quickly vanishes
since the solution is homogenized
We are really close to 13 ml
at exactly 12.4 ml
You can see here that the colour is almost permanent
there is a blue remanence
so we'll add one more drop
to really be as accurate as possible
Now we have a good persistence of the yellow colour
To be really sure we can add another drop
Now we really have a deep colour change
We are at an equivalence point of 12.5 ml
To read the equivalence point
I make sure as before that my eye is well at the level of the meniscus
and I read at the bottom of the meniscus
Once the titration is done
we must empty the burette
and then rinse this burette with distilled water for future use
so that it is not soiled with the titrating solution
We add distilled water at the top of the burette
and rinse it thoroughly
There are several types of coloured indicators
depending on the nature of the titration reaction
There may be acid-base, redox, complexation or adsorption indicators
In our case we performed an acid-base colorimetric titration with an acid-base indicator
The soda was titrated with hydrochloric acid and the bromothymol blue (BBT) was used as a coloured indicator
The BBT is blue in basic environments and yellow in acidic environments
For this titration of the soda by hydrochloric acid
one could have used another coloured indicator such as the phenolphthalein
Some reactions do not necessarily require the addition of a coloured indicator
especially when the reagents or products are coloured
This is for example the case of manganometry with KMnO4 (potassium permanganate)
where KMnO4 is purple in solution
KMnO4 reacts with Fe2+ for example
A reaction occurs between KMnO4 and Fe2+ leading to Mn2+ and Fe3+
The equivalence point is determined by the colour change when KMnO4 is in excess
The solution turns to purple
Interactive numerical educational resourcesfor the analysis of complex media