Experiment M03
Chemicals:
Preparation of an iron complex
Iron(II) ammonium sulphate, FeSO4  (NH4)2SO4 6H2O, (7.5g)
Oxalic acid (10 g in 100 cm3 water), (45 cm3)
Potassium oxalate, (5 g in 15 cm3 water)
H2O2 (1.8 M), (12.5 cm3)
Ethanol. (25 cm3)
Apparatus:
Burette,
Funnel,
Filter paper,
Burner,
Thermometer.
Principle:
In this experiment you will prepare a complex compound of iron, potassium trioxalatoferrate(III),
K3Fe(C2O4)3. In this compound each oxalate ligand has two bonds to the iron(III) ion, and hence
this compound is an example of a chelate. You will aim at obtaining the maximum yield of
product of the highest purity permitted by this method. Your final product should be in the form
of bright green crystals.
The compound will be prepared by first preparing iron(II) oxalate, FeC2O42H2O, and then
oxidizing this with hydrogen peroxide, in the presence of potassium oxalate, to convet it to the
ferric-oxalate ion, Fe(C2O4)33-.
C2O42H2O2, K2C2O4
H2C2O4
Fe  FeC2O4 2H2O  Fe(OH)3  K3Fe(C2O4)3 3H2O
2H2O
+ K3Fe(C2O4)3
2+
Thus all the iron in the iron(II) ammonium sulphate is converted to ferrioxalate, one third of the
ferrioxalate being formed from the iron(III) hydroxide. Both H2O2 and Fe(OH)3 are unstable to
heat. Potassium trioxalatoferrate(III) is photosensitive, i.e. it decomposes when exposed to light,
reforming iron(II) oxalate.
P.1
Experiment M03
Preparation of an iron complex
Procedure:
a.
Dissolve 7.5 g of iron(II) ammonium sulphate, FeSO4  (NH4)2SO4 6H2O in 25 cm3 of
warm water(40oC). Add to this warm solution, with stirring, 32.5 cm3 of an oxalic acid
solution made by dissolving 10 g of the acid in 100 cm3 of water.
b.
Cautiously heat the mixture to boiling, and then allow the yellow granular ppt to settle.
c.
Decant the supernatant liquid and wash the ppt by stirring it with 50 cm3 of hot water
(~80oC). Allow the ppt to settle and again decant the clear liquid. Repeat with a further 50
cm3 of hot water.
d.
Suspend the washed ppt in a warm solution of 5 g of potassium oxalate in 15 cm3 of water.
Add slowly from a burette 12.5 cm3 of 1.8 M H2O2, stirring continuously and keeping the
temperature at about 40 oC.
e.
Heat the mixture to boiling.
f.
Add the boiling mixture, in one portion, 10 cm3 of a solution of oxalic acid containing 10 g
of the acid in 100 cm3 of water. If the mixture has not turned to a clear green, add oxalic
acid solution, drop by drop, until it becomes clear (up to 2.5 cm3 may be required).
g.
The solution should now be green due to the presence of the ferrioxalate ion. Filter the hot
solution if necessary and add 15 cm3 of ethanol.
h.
Set aside in the dark to crystallize. After crystallization, decant off the supernatant liquid,
washed by decantation with a little water, and finally with 10 cm3 ethanol.
i.
Spread the crystals on a fresh filter paper and allow them to dry in the dark. Weigh the dry
crystal and record the result.
P.2
Experiment M03
Preparation of an iron complex
Name:
Seat No.:
Date:
Grade:
Calculation:
1.
Calculate the theoretical yield, i.e. the mass of potassium trioxalatoferrate(III) expected from
complete conversion of the mass of iron(II) ammonium sulphate used.
2.
Calculate the percentage yield in this exercise, i.e. the percentage of the theoretical yield that
has actually been achieved.
Questions for discussion:
1.
What is the name of the compound precipitated in procedure a?
Write an equation for the reaction.
2.
In procedure d, why should the temperature be kept at about 40 oC?
3.
In procedure e, what do you observe?
Write an equation for the reaction.
4.
Why excess oxalic acid should be avoided in procedure f?
Write an equation for the reaction.
5.
In procedure g, what is the action of the ethanol?
Why is it effective?
6.
In procedure h, why is it necessary to exclude light?
Why are the crystals finally washed with ethanol?
P.3