Estimation of water of crystallization in Mohr’s salt by titrating with standardized KMnO4

1. SHREE MALLIKARJUN COLLEGE CLASS:FYBSC
SAFETY: Refer to MSDS of Mohr’s salt, potassium permanganate, sulphuric acid, sodium oxalate
Dr. Mithil S. Fal Desai
Aim: Estimation of water of crystallization in Mohr’s salt by titrating with standardized KMnO4.
Chemicals: Mohr’s salt, potassium permanganate, sulphuric acid, sodium oxalate.
Apparatus: Standard volumetric flask, pipette, burette, conical flask, beakers, funnel and glass rod.
Theory: The aqueous solution of KMnO4 needs to be standardized using sodium oxalate. The amount of
Fe is quantitatively measured in the fixed amount of Mohr’s salt. The Mohr’s salt molecular formula is
(NH4)2Fe(SO4)2.’X’H2O, thus X can be determined by comparing the percentage of Fe with different X
values as shown in Table 1.
Table 1.
Molecular formula
(NH4)2Fe(SO4)2.’X’H2O
Molecular weight
of (NH4)2Fe(SO4)2
‘A’ in g
Molecular
mass of
XH2O
‘B’ in g
A + B = ‘C’ Atomic weight
of Fe
% of Fe
=(55.85/C)*100
(NH4)2Fe(SO4)2.0H2O
283.93
0*18.02 283.93
55.85 g
19.67
(NH4)2Fe(SO4)2.1H2O 1*18.02 301.95 18.49
(NH4)2Fe(SO4)2.2H2O 2*18.02
(NH4)2Fe(SO4)2.3H2O 3*18.02
(NH4)2Fe(SO4)2.4H2O 4*18.02
(NH4)2Fe(SO4)2.5H2O 5*18.02
(NH4)2Fe(SO4)2.6H2O 6*18.02
(NH4)2Fe(SO4)2.7H2O 7*18.02
(NH4)2Fe(SO4)2.8H2O 8*18.02
(NH4)2Fe(SO4)2.9H2O 9*18.02
Reaction:
2KMnO4 + 5H2C2O4 + 3H2SO4  2MnSO4 + K2SO4 + 10CO2 + 8H2O
2KMnO4 + (NH4)2Fe(SO4)2 + 8H2SO4  5Fe2(SO4)3 + K2SO4 + 2MnSO4 + 10(NH4)2SO4 + 8H2O
Procedure:
i) Preparation of 0.01N KMnO4 (500 mL) [molecular weight check on the chemical container], calculate
ii) Preparation of 2N H2SO4 (250 mL) [Laboratory normality 36 N],calculate
iii) Preparation of 0.01N Na2C2O4 (500 mL) [molecular weight check on the chemical container], calculate
iv) Preparation of Mohr’s salt solution (100 mL)
Dissolve 0.4 g of Mohr’s salt (note the exact weight) in ~5 mL distilled water and add 2-3 drops of
concentrated sulphuric acid. Quantitatively transfer this solution in a 100 mL standard flask and dilute up
to the mark.
v) Standardization of KMnO4
Rinse the clean burette with a minimum quantity of KMnO4 and fill it appropriately. Pipette out 10 mL of
0.01 N sodium oxalate solution in a clean conical flask. Add about 20 mL of 2N H2SO4. Heat the solution
in flask to about 60 °C. Titrate it against KMnO4, until light pink colour permanently develops in the flask.
Repeat the titration to get constant readings.

2. SHREE MALLIKARJUN COLLEGE CLASS:FYBSC
SAFETY: Refer to MSDS of Mohr’s salt, potassium permanganate, sulphuric acid, sodium oxalate
Dr. Mithil S. Fal Desai
vi) Estimation of Fe(II) in Mohr’s salt
Rinse the burette with a minimum quantity of standardized KMnO4 and fill it appropriately. Pipette out 10
mL of given Mohr’s salt solution in a clean conical flask. Titrate it against KMnO4,until light pink colour
permanently develops in the flask. Repeat the titration to get constant readings.
Results:
i) Experimental percentage of Fe in ___(~0.4)g of Mohr’s salt is____% which corresponds to ___ moles
of water of crystallization.
ii)Water of crystallization in Mohr’s salt is X=____.
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.
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?