To estimate amount of ferrous(fe2+) and ferric(fe3+) ions in the given solution containing ferric chloride and ferrous sulphate by using potassium dichromate.

To estimate amount of ferrous (Fe2+) and ferric (Fe3+) ions in the given solution containing ferric chloride and ferrous sulphate by using potassium dichromate

1. SHREE MALLIKARJUN COLLEGE CLASS: TYBSC
SAFETY: Refer to MSDS of ferric chloride, ferrous sulphate, potassium dichromate, stannous chloride, mercuric chloride sulphuric
acid, phosphoric acid and diphenylamine
Dr. Mithil S. Fal Desai

Aim: To estimate amount of ferrous (Fe
2+
) and ferric (Fe
3+
) ions in the given solution containing ferric
chloride and ferrous sulphate by using potassium dichromate.
Chemicals: Ferric chloride, ferrous sulphate, potassium dichromate, stannous chloride, mercuric chloride
sulphuric acid, phosphoric acid and diphenylamine
Apparatus: Standard volumetric flask, pipette, burette, conical flask, beakers, funnel and glass rod.
Theory: The solution of K2Cr2O7 is oxidizing in the acidic medium that is used as a primary standard by
careful preparation. The amount of ferrous ions in the mixture can be determined directly by titrating it
against a standard solution of K2Cr2O7 using external redox indicators like diphenylamine. The solution
appears green progressively with the addition of dichromate due to the increase in the concentration of Cr
3+

ions. The endpoint is observed violet-blue as an extra drop develops the oxidizing potential that oxidises
diphenylamine to diphenylbenzidine.
After the estimation of Fe
2+
ion in a mixture of Fe
2+
and Fe
3+
, the Fe
3+
ions need to be reduced quantitatively
to Fe
2+
ions using stannous chloride. The excess of stannous chloride is oxidized using mercuric chloride.
The solution now has an excess of Fe
2+
ion formed by reduction of Fe
3+
, which are then titrated against the
standard solution of dichromate.
Reaction:
2Fe
3+
(aq) + SnCl2(aq)  2Fe
2+
(aq) + SnCl4(aq)
SnCl2(aq) + 2HgCl2(aq)  Hg2Cl2(s) + SnCl4(aq)
6Fe
2+
(aq) + (Cr2O7)
2-
(aq) + 14H
+
(aq)  2Cr
3+
(aq) + 6Fe
3+
(aq) + 7H2O(l)
Procedure:
i) Preparation of 0.05N K2Cr2O7 (100 mL) [molecular weight check on the chemical container], calculate
ii) Preparation of 2N H2SO4 (250 mL)[ Laboratory normality 36 N], calculate
iii) Preparation of 3% (w/v) stannous chloride (100 mL) calculate
iv) Preparation of saturated HgCl2 (100 mL)
v) Estimation of Fe
2+

Rinse the burette with a minimum quantity of standard K2Cr2O7 and fill it appropriately. Pipette out 10 mL
of a given solution containing ferric chloride and ferrous sulphate in a clean conical flask. Add about 25
mL of 2N H2SO4 and 0.5-1 mL concentrated H3PO4. Add 2-3 drops of diphenylamine indicator and titrate
against a standard solution of dichromate. Repeat the titration to get constant readings (Vx). The constant
burette reading ‘Vx’ precisely corresponds to the amount of Fe
2+
in 10 mL of the given mixture.
vi) Estimation of Fe
3+
Pipette out 10 mL of a given solution containing ferric chloride and ferrous sulphate in a clean conical flask.
Add 1 mL of concentrated HCl and heat the solution (~80 °C). Add dropwise 3% (w/v) stannous chloride
to the hot solution until the solution becomes colorless. Add 1-2 drops excess of stannous chloride to ensure
the complete reduction of Fe
3+
. Cool the solution and add 3 mL of a saturated solution of mercuric chloride
until a precipitate of mercurous chloride is formed. Add about 25 mL of 2N H2SO4 and 1mL concentrated

2. SHREE MALLIKARJUN COLLEGE CLASS: TYBSC
SAFETY: Refer to MSDS of ferric chloride, ferrous sulphate, potassium dichromate, stannous chloride, mercuric chloride sulphuric
acid, phosphoric acid and diphenylamine
Dr. Mithil S. Fal Desai

H3PO4. Add 2-3 drops of diphenylamine indicator and titrate against a standard solution of dichromate.
Repeat the titration to get constant readings (Vy). The constant burette reading ‘Vy’ corresponds to the total
amount of iron (Fe
2+
and Fe
3+
) in 10 mL of the given mixture. Thus, the amount of dichromate that
quantitatively precisely relates to Fe
3+
(Vz) is equal to (Vy-Vz) mL.

Result: i) Amount of Fe
2+
in 100 mL solution = ___ g
ii) Amount of Fe
3+
in 100 mL solution = ___ g
iii) Amount in of FeSO4100 mL solution = ___ g
iv) Amount of of FeCl3 in 100 mL solution = ___ g

Further reading (available in library)
1) Experiments in Applied Chemistry, S. Rattan, S. K Kataria and Sons.
2) Vogel’s textbook of quantitative chemical analysis, J. Mendham, R. C. Denney, J. D. Barns, M. Thomas,
B. Sivasankar, Pearson India Education, 6
th
Edition.
Questions.
1) Which is the stronger oxidizing agent potassium dichromate or potassium permanganate? Justify.
2) Write the structure of diphenylamine and di-phenylbenzidine.
3) Why sulphuric acid is usually used in redox titration? Why not HCl or HNO3?
4) Why of stannous chloride and mercuric chloride are added in this redox titration?