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.