what is double titration and back titration
Double titration means two titrations are done in a process. In this process:
- The first titration is done to standardize the titrant (usually taken in burette).
- The second titration is used to find the molarity of the titrand (unknown solution and taken in conical flask).
For example, we have to calculate molarity of HCl acid by titrating it with NaOH base. But for that NaOH must also be standardized.So for the whole process, we do double titration. First we standardize NaOH by titrating it with a standard oxalic acid solution. And then, we find out the molarity of HCl by titrating it with standardized NaOH solution.
Back Titration is the titration done in the reverse direction. In this titration, a reactant of known concentration is added in excess, to the reactant of unknown concentration. The leftover reactant ok known concentration is then titrated with another titrant, to calculate the amount left in the reaction mixture. It is done when the end point of direct titration cannot be observed, or if the reaction is very slow.
For example, consider that, the concentration of A is not known, to this solution we add excess amount of reactant B of known concentration. A and B will react and form a product C. The amount of B left unreacted can be calculated, by titrating it against a solution D. This will gives the amount of B left unreacted and then we can calculate the concentration of A. The necessary condition for back titration is that B must react with A and D, but B should not react with the product C, of the reaction between A and B.