Titrations

                             

In a lab or in the classroom should be demonstrated and discussed a simple acid base titration + calculation.
This could be done as follows:
  1. First the titration is executed with a pH meter and a plotter, when the titration liquid (titulant) is added regularly dropwise from a buret, until passing the end point.
  2. Then the titration has to be repeated with an indicator to stop exactly in the end point.
  3. Finally, the calculation must be done and discussed.
On internet you can find nice examples of titrations, for example: Titrations on wikipedia

A titration is a rather fast method to determine the concentration of dissolved substances.
We use certain apparatus where the volumes of liquids can be read accurately, like: pipettes, burettes, graduated flasks, and more.
That's why we also talk about volumetric analysis.

The reaction equatiopn shows the mol proportion of the reactants.
When the reactant are joined exactly in that mol proportion, we say that the equivalence point has been reached.

In most cases of titration, two dissolved substances react carefully; the addition is stopped, if possible, exactly at the moment of equivalent amounts of reactants.

The concentration of the titulant (mostly in the buret) must be well known.
The concentration of the other substances can be calculated, starting with the data.

At the moment of having joined equivalent amounts of substances, the reaction must stop; so stop adding from the buret at that very moment.

Normally we stop this process just a bit too late; with the last added drop of titrant solution a bit too much was added.
The practical end point of the titration in many cases does not coincide 100% with the theoretical equivalence point. We add automatically a bit too much.
For a reliable determination, this error may not be more than 0.5%. In practice this means that you may add at most one drop too much on a total of 200 drops.

To be able to observe the endpoint, the changing moment, we often need an indicator
At acid base titration: an acid base indicator
At redox titration: a redox indicator


An indicator mostly is....:
a weak organic acid a weak organic reductor
HIn H+ + In- RedIn n e- + OxIn
HIn has another color than In- RedIn has another color than OxIn
The color of HIn or In- The color of RedIn or OxIn
can only be observed if the equilibrium is sufficiently to the left or the right.
The color of an indicator depend on the environment:
In acid environment (or in reducing environment)
the above equilibria are at the left,
so the color of HIn dominates (or the color of RedIn).
Per indicator this can vary substantially.




For data about more indicators, you can find table VIII.

At acid base titration, the pH of the end solution not necessarily must be 7; the end pH depend on the properties of the products.
These products can contain, in their turn, weak acids or bases.

The following fundamental rules can be applied:

titration of strong acid with strong base: pH = 7
titration of strong acid with weak base: pH < 7
titration of weak acid with strong base: pH > 7


The right choice of indicator depend on the properties of the products ánd of the changing zone of the indicator (see table VIII).
The indicator must match with the titration.

indirect titrations
Sometimes it turns out to be impossible to determine the concentration of a certain substance by titration; for example when such a substance is unstable of not soluble.
In such cases we can execute a so called indirect titration.
The substance to be determined must react beforehand with another reactant (en intermediate or a substitute). The original substance finally does not participate in the actual titration.
The substitute is titrated and its number of mols calculated.
after that, by calculating, you can determine how much original substance was present.

example:
Marble has a high percentage of Calcium carbonate, and you want to determine that percentage.
An option is to add a substance that reacts with the Calcium carbonate, so to remove all carbonate.
Imagine that you would use Nitric acid to do so; make sure that uyou know exactly how much nictric acid you added at the start. Part of that acid will react with the carbonate, a rest of it remains.
So, if you know how much acid you added in the beginning, and you know how much acid is left, then the difference is exactly the amount of acid that reacted with the carbonate.
The acid that remained can be determined by titration with a base, could be: NaOH(aq).

Often we use, in the case of redoxtitrations, the couple: I3/S2O32-(Iodine with tiosulfate).
This titration mostly is applied at indirect titration: the substance to investigate is replaces by Iodine or Tio, dependent on the question: was the unknown substance an oxydator or a reductor?
At these so called Iodometric titrations we use the cheap and well working indicator: starch(aq).
Fresh starch has spiral shaped molecules (see also module 12, biochemistry), wherein the tri-Iodide ions exactly fit and cause a dark blue color.
the tri-iodide itself also has a color (yellow brownish), but the (dis)appearance of that color is not as clear to observe as is the dark blue color

If you want to execute an acid base titration without any indicator, then you could use a pH meter. On the meter you must continuously check the (changing) pH values: In the equivalence point, the pH change is at its maximum; a sudden big pH change shows up.
You can plot the course of the pH change in a graph (by hand or automatically).

Every time you add, for example 0.5 ml titrant, you read the pH value, or the potential (in the case of redox reaction) and you put that value on paper. That's how you create a titration curve. In such a curve you can easily derive the equivalence point.

There are several titration methods, partly already discussed, partly to be discussed later.