### indirect titration

We also know special titrations:
For example, if direct determination of a concenctration is not possible. The substance to investigate could be a gas, or an insoluble solid, or it might be unstable. What to do then?

It is possible to execute an indirect titration.
The substance to determine must first completely react with a "substitute".
• You know exactly how much substitute you first had.
• In a titration you determines exactly how much substitute was left over after the first reaction.
• Then you can calculate how much of the original substance there was.

We also know special titrations:
For example, if direct determination of a concenctration is not possible. The substance to investigate could be a gas, or an insoluble solid, or it might be unstable. What to do then?
It is possible to execute an indirect titration.
The substance to determine must first completely react with a "substitute".
• You know exactly how much substitute you first had.
• In a titration you determines exactly how much substitute was left over after the first reaction.
• Then you can calculate how much of the original substance there was.

An example:
Marble contains a high amount of Calcium carbonate. that is unsoluble in water and cannot as such be titrated directly.
We can do now the following: you weigh an amount of marble very precisely. Later, after calculating how much calcium carbonate was in the marbel, you know the percentage. But how to do this in practice?
You bring the weighed amount of marble in an exactly known amount (excess, more then enough) strong acid solution of which you know well the concentration.
All calcium carbonate in the marble will react with the acid until finishing. A certain amount of the acid will be left over.
With a titration you determine now how much acid was left over. That you subtract from the original amount of acid, and you know the amount of acid that reacted with the calcium carbonate.
You know the reaction equation of the acid with calcium carbonate, and thus the mol proportion.
So now you can execute the calculation, find the amount of Calcium carbonate and the percentage of it in the marble.