Preparation of Pure Sample of Potassium trioxalatoferrate
(III) Potassium trioxalatoferrate (III) is an octahedral
transition metal complex with the formula K3[Fe(C2O4)3], in which three bidentate oxalate
ions are bound to an iron center. The central iron atom is in the +3 oxidation state. This
complex can be prepared by dissolving freshly prepared ferric hydroxide in a solution of
potassium oxalate and oxalic acid. Our aim here is to prepare a pure sample of
Potassium trioxalatoferrate (III) Materials Required:
3.5g anhydrous ferric chloride, 4g potassium hydroxide, 4g hydrated oxalic acid, 5.5g hydrated
potassium oxalate, Ethyl alcohol, distilled water, cold water, china dish, Buchner funnel,
spatula, glass rod, beakers, measuring jars, trough, watch glass, filter papers, black
paper and sand bath. Procedure: Take about 3.5g anhydrous ferric chloride
in a 250 ml beaker. To this add about 50 ml of distilled water
using a measuring jar. Stir the solution using a glass rod to dissolve
ferric chloride. In another beaker take 4g of potassium hydroxide.
Pour another 50 ml of distilled water into this beaker.
Stir the solution using a glass rod to dissolve potassium hydroxide.
Add potassium hydroxide solution to ferric chloride solution in small proportions with
constant stirring using a glass rod. Ferric chloride reacts with potassium hydroxide
to form a brown precipitate of ferric hydroxide. Filter the precipitate of ferric hydroxide
so formed through a Buchner funnel. Wash the precipitate with distilled water
and transfer it into a watch glass. In another beaker take 4g of hydrated oxalic
acid and 5.5g of hydrated potassium oxalate. To this add about 100 ml of distilled water.
Stir the solution thoroughly using a glass rod to get a clear solution.
Using a spatula, add freshly prepared ferric hydroxide precipitate in small proportions
to the solution of potassium oxalate and oxalic acid with constant stirring to dissolve the
precipitate. If the precipitate does not dissolve, then
warm it and leave the contents for some time. Then, filter the solution into a china dish
using a funnel and filter paper. The stem of the funnel should touch the wall
of the china dish to avoid the solution splashing out.
After filtration the insoluble impurities are left behind on the filter paper.
Heat the solution in the china dish over the sand bath.
To avoid crust formation on the side of the dish, stir the solution continuously with
a glass rod. When the volume of the solution is reduced
to about one-half of its original volume, take out a drop of the concentrated solution
at the end of the glass rod and cool it by blowing. Formation of thin crust indicates
that crystallisation point has been reached. Now place the china dish in a trough containing
cold water. Cover the dish with a black paper and allow
it to cool slowly for some time. On cooling, emerald green crystals of potassium
trioxalatoferrate (III) separate out. Decant the mother liquor carefully into a
beaker. Wash the crystals 2 or 3 times with small
amount of ethyl alcohol. Using a spatula, remove the crystals to a
filter paper which soaks up the solution. Transfer the crystals to another filter paper
and dry them by pressing gently between the folds of the filter paper.
Now, transfer the crystals into a watch glass. The crystals of pure potassium trioxalatoferrate
(III) are emerald green in colour and are octahedral in shape. Precautions: The solution should be evaporated slowly by
gently heating during concentration. Cool the solution slowly to get good crystals.