![SHREE MALLIKARJUN COLLEGE CLASS:FYBSC
SAFETY: Refer to MSDS of Mohr’s salt, potassium dichromate, sulphuric acid, phosphoric acid, diphenylamine
r. Mithil S. FalDesai
Aim: Estimation of Fe(II) ions by titrating against K2Cr2O7 using internal indicator.
Chemicals: Mohr’s salt, potassium dichromate, sulphuric acid, phosphoric acid and diphenylamine
Apparatus: Standard volumetric flask, pipette, burette, conical flask, beakers, funnel and glass rod.
Theory: K2Cr2O7 is an excellent oxidizing agent in acidic medium. Unlike KMnO4, it can be used as
primary standard. Ferrous ions can be titrated against a standard solution of K2Cr2O7 in acidic medium.
However, the use of external redox indicator like diphenylamine is desirable as it is difficult to observe a
sharp colour change with the addition of the dichromate solution. The solution appear green progressively
with the addition of dichromate due to the increase in Cr(III) ions. The endpoint is observed violet-blue as
an extra drop reacts with diphenylamine to form diphenylbenzidine.
Reaction:
K2Cr2O7 + 6(NH4)2Fe(SO4)2+ 7H2SO4à Cr2(SO4)3+ 3Fe2(SO4)3 + K2SO4 + 6(NH4)2SO4 + 7H2O
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 Mohr’s salt solution (100 mL)
Dissolve about 1.9 g of Mohr’s salt (note the exact weight) in ~5 mL distilled water and add 1-2 drops of
concentrated sulphuric acid. Quantitatively transfer this solution in 100 mL standard flask and dilute up to
the mark.
iv) Estimation of Fe(II)
Rinse the burette with a minimum quantity of standard K2Cr2O7 and fill it appropriately. Pipette out 10
mL of given Mohr’s salt solution in a clean conical flask. Add about 20 mL of 2N H2SO4 and 1-2 mL 2N
H3PO4. Add 2-3 drops of diphenylamine indicator and titrate against a standard solution of dichromate.
Repeat the titration to get constant readings.
Result: i) Amount of Fe in 100 mL solution = ___ g
ii) Experimental percentage of Fe in Mohr’s salt =___%
iii) Theoretical percentage of Fe in Mohr’s salt =___%
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, 6th
Edition.
Questions.
1) Which is 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) Is it possible to quantitatively determine ferric ions by this method?](https://image.slidesharecdn.com/estimationoffeiiionsbytitratingagainstk2cr2o7usinginternalindicator-191023062746/85/Estimation-of-fe-ii-ions-by-titrating-against-k2-cr2o7-using-internal-indicator-1-320.jpg)
Estimation of fe(ii) ions by titrating against k2 cr2o7 using internal indicator
- 1. SHREE MALLIKARJUN COLLEGE CLASS:FYBSC SAFETY: Refer to MSDS of Mohr’s salt, potassium dichromate, sulphuric acid, phosphoric acid, diphenylamine r. Mithil S. FalDesai Aim: Estimation of Fe(II) ions by titrating against K2Cr2O7 using internal indicator. Chemicals: Mohr’s salt, potassium dichromate, sulphuric acid, phosphoric acid and diphenylamine Apparatus: Standard volumetric flask, pipette, burette, conical flask, beakers, funnel and glass rod. Theory: K2Cr2O7 is an excellent oxidizing agent in acidic medium. Unlike KMnO4, it can be used as primary standard. Ferrous ions can be titrated against a standard solution of K2Cr2O7 in acidic medium. However, the use of external redox indicator like diphenylamine is desirable as it is difficult to observe a sharp colour change with the addition of the dichromate solution. The solution appear green progressively with the addition of dichromate due to the increase in Cr(III) ions. The endpoint is observed violet-blue as an extra drop reacts with diphenylamine to form diphenylbenzidine. Reaction: K2Cr2O7 + 6(NH4)2Fe(SO4)2+ 7H2SO4à Cr2(SO4)3+ 3Fe2(SO4)3 + K2SO4 + 6(NH4)2SO4 + 7H2O 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 Mohr’s salt solution (100 mL) Dissolve about 1.9 g of Mohr’s salt (note the exact weight) in ~5 mL distilled water and add 1-2 drops of concentrated sulphuric acid. Quantitatively transfer this solution in 100 mL standard flask and dilute up to the mark. iv) Estimation of Fe(II) Rinse the burette with a minimum quantity of standard K2Cr2O7 and fill it appropriately. Pipette out 10 mL of given Mohr’s salt solution in a clean conical flask. Add about 20 mL of 2N H2SO4 and 1-2 mL 2N H3PO4. Add 2-3 drops of diphenylamine indicator and titrate against a standard solution of dichromate. Repeat the titration to get constant readings. Result: i) Amount of Fe in 100 mL solution = ___ g ii) Experimental percentage of Fe in Mohr’s salt =___% iii) Theoretical percentage of Fe in Mohr’s salt =___% 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, 6th Edition. Questions. 1) Which is 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) Is it possible to quantitatively determine ferric ions by this method?
- 2. SHREE MALLIKARJUN COLLEGE CLASS:FYBSC SAFETY: Refer to MSDS of Mohr’s salt, potassium dichromate, sulphuric acid, phosphoric acid, diphenylamine r. Mithil S. FalDesai