Fe (III) in the sample solution is first reduced to Fe (II) in HCl medium using stannous chloride. E — Organic Chemistry Chemistry Question

Fe (III) in the sample solution is first reduced to Fe (II) in HCl medium using stannous chloride. Excess of stannous chloride is oxidized by addition of mercury (II) chloride. The Fe(II) is then titrated with standard potassium dichromate solution.

Chemicals and solutions:
* Sample solution
* K₂Cr₂O₇ aqueous solution, c = 0.0167 mol dm^-3
* Equimolar H₂SO₄ and H₃PO₄ acid mixture
* Conc. HCl
* 5% HgCl₂ solution
* 3% SnCl₂ solution
* Diphenylamine indicator
Note: NH₄Fe(SO₄)₂.12 H₂O is used for the preparation of the sample solution

Preparation of K₂Cr₂O₇ aqueous solution, c = 0.0167 mol dm^-3:
Weigh accurately 1.226 g of pure K₂Cr₂O₇ and transfer it to a 250 cm₃ volumetric flask. Prepare the solution using distilled water.

Procedure:
Dilute the given Fe(III) sample solution to 100 cm₃ using the standard volumetric flask. Take 10 cm₃ of the diluted sample solution in a clean conical flask. Add 2 cm₃ of concentrated HCl and boil the solution. To the hot solution, add SnCl₂ solution dropwise till the reaction mixture becomes colourless. Add 2 - 3 drops of SnCl₂ solution in excess.
Cool the solution under tap water. Add 2 to 3 cm₃ of HgCl₂ solution at once. A white precipitate is obtained at this stage. (If a grey precipitate is obtained, reject the sample and start again.)
Add 2 to 3 cm₃ of the acid mixture and 1 drop of the diphenylamine indicator and titrate it against K₂Cr₂O₇ solution. Continue the titration until a colour change from colourless to permanent blue or violet is observed.