ESTIMATION OF FERRIC IRON (Redox Titrations)

Aim: To estimate the amount of Ferric iron in the whole of the given solution using a standard solution of Potassium Dichromate.

Apparatus: 100 ml standard flask, Burette, Conical Flask, 20 ml Pipette, Simple balance with weights

Chemicals Required: K2Cr2O7, SnCl2, HgCl2, HCl, Ferric ion solution, H2SO4, Ortho phosphoric acid, Diphenylamine & distilled water.

Principle: Ferric Iron is reduced to ferrous iron by stannous chloride in the present of Hydrochloric acid at a temperature of 70-900C.

The excess stannous chloride is removed by the addition of mercuric chloride. The ferrous iron formed is titrated with standard Potassium dichromate solution in the presence of Sulphuric acid and Phosphoric acid using diphenylamine as indicator.

Chemical reactions:

2FeCl3 + SnCl2 -> 2FeCl2 + SnCl4

SnCl2 + 2HgCl2 -> SnCl4 + Hg2Cl2

K2Cr2O7 + 4H2SO4 -> K2SO4 + Cr2(SO4)3 + 4H2O + 3(O)

2FeSO4 + H2SO4 + (O) -> Fe2(SO4)3 + H2O

K2Cr2O7 = 3(O) = 6Fe

Procedure:

1. Preparation of standard potassium dichromate: Weigh out accurately the given pure crystalline sample of potassium dichromate and transfer into 100 ml standard (volumetric) flask provided with a funnel. Dissolve the dichromate in a small quantity of distilled water, and make upto the mark. The contents in the flask are shaken well for uniform concentration. Calculate the normality of potassium dichromate.

2. Estimation of ferric iron: Make up the given ferric iron solution upto the mark of the 100ml standard flask with distilled water and shake well for uniform concentration. Pipette out 20ml of the ferric solution into a clean conical flask and 10ml of conc. HCl and heat the solution to boiling. The colour of the solution changes to clear yellow. Add stannous chloride solution drop by drop into the hot iron solution from a burette till the yellow colour disappears. Cool the solution under a tap and add 10 ml of saturated solution of mercuric chloride in one portion silky white precipitate of mercurous chloride is obtained,

(if the solution turns black, due to formation of finely divided mercury, discard the solution and take fresh solution of ferric iron and reduce) add 20 ml of the acid mixture and 3 to 4 drops of diphenylamine indicator and titrate against standard solution of potassium dichromate till the green colour changes to blue violet. Repeat the titration for concurrent value and calculate the normality of iron solution.

Preparation of Standard solution:

W1 = Weight of bottle + substance = ____________ gms

W2 = Weight of bottle = ____________ gms

Weight of substance = (W1-W2) = ____________ gms.

Normality of the solution (K2Cr2O7) =((W1-W2) X 10)/Equivalent Weight =

                               =( (W1-W2) X 10)/49

N1 = Normality of K2Cr2O7 solution =

N2 = Normality of Ferric iron solution =

V1 = Volume of K2Cr2O7 solution =

V2 = Volume of Ferric iron solution = 20 ml

N1 V1 = N2 V2

N2 = (N1 V₁)/V2

Normality of Ferric iron solution = N2 = (N1 V₁)/V2

Amount of Ferric iron present in the whole of the given solution (100 ml) = (N2 X 55.85)/10

=

Result: Amount of Ferric iron present in the whole of the given solution (100 ml) = _______ gm.