- back titration

i have given given a task to determine the concentration of a saturated aqueous solution of chlorine. The question is as follows:

The concentration of a saturated aqueous solution of chlorine (approximately 7g dm3) can be determined by a back titration method.

In the first stage of the procedure, the solution of chlorine is added to an excess suitable reducing agent such as iron(II) compound.

In the second stage the amount of reducing agent left over is determined by titration.

Plan and describe a procedure that could be used safely to determine the exact concentration of saturated aqueous solution of chlorine by back titration. Can anyone please help me with this task. i dont know how to write up the procedure.
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