Potassium tris(oxalato) ferrate (III), also known as potassiumferrioxalate, is a green crystalline- compound of octahedralgeometry. It is a metal complex of iron with three oxalate (C2O42-)ligands bonded to the central metal atom (Fe). These ligands arebidentate, meaning that each of them binds to the metal atom at 2different places.
Potassium ferrioxalate is a very interesting photosensitivematerial. In solution, it reacts with
2− light photons and undergoes a reduction reaction anddecomposes to give Fe(C2O4)2 .
Experimental Procedure
I. Preparation of Potassium tris(oxalato) ferrate(III)trihydrate K3[Fe(C2O4)3].3H2O
a. In a beaker place 5 g of ferrous ammonium sulfate in 20 mLof warm water containing 1 mL of dilute sulfuric acid.
b. Stir the solution well.
c. Add to the beaker a solution of 2.5 g of oxalic aciddihydrate in 25 mL of water.
d. Slowly heat the mixture to boiling (beware of bumping) thenallow the yellow precipitate to settle.
e. Decant the supernatant using a Buchner funnel making sureit has a properly fitted filter paper.
f. Add 15 mL of hot water to any solid remaining in thebeaker, stir and filter.
g. Drain well and then transfer all the precipitate from thefilter paper back into the beaker with 10 mL
hot water.
h. Add to the beaker 3.5 g solid potassium oxalate monohydrateand heat to approximately 40°C.
i. Add slowly, using a dropper, 9 mL of \"20 vol\" hydrogenperoxide.
j. If the precipitate looks yellowish, not brown and settlesreadily, decant the supernatant, add a
solution of 0.2 - 0.4 g potassium oxalate monohydrate in 1 - 2mL water and then hydrogen peroxide
dropwise until the precipitate dissolves. Then add thepreviously decanted supernatant.
k. Heat to boiling, and add a solution of 2 g of oxalic aciddihydrate in 30 mL of water in portions, add 20 mL initially, thenif the brown precipitate still remains, add more solution little bylittle until it all dissolves.
l. Boil the clear solution down to a volume of 40 to 50mL.
m. Add 95% ethanol slowly until a precipitate starts to form(~ 30 mL).
n. Cool and leave to crystallize.
o. Filter and wash the crystals on the Buchner funnel with a1:1 ethanol/water mixture and finally with
acetone.
p. Dry in the air and weigh.
The complex is photosensitive and should not be exposed tolight unnecessarily. Store in a sample bottle wrapped in foil. Thebright green crystals on exposure soon become covered with a yellowpowder of ferrous oxalate.
2K3[Fe(C2O4)3].3H2O ï‚®2FeC2O4.2H2O + 2CO2 + 3K2C2O4 + 2H2O
II. Determination of the oxalate content of Potassiumtrisoxalatoferrate(III) trihydrate.
The iron(III) complex is first decomposed in hot acid solutionand the free oxalic acid is titrated against standard (0.02 M)potassium permanganate solution. No indicator isrequired.
a. Weigh accurately about 0.2 g of the potassiumtrisoxalatoferrate(III) complex previously prepared.
b. Boil the sample with 50 mL of 1 M sulfuric acid in aconical flask.
c. Allow the solution to cool to about 60°C.
d. Titrate slowly with a standard potassium permanganatesolution.
e. Continue titrating until the warm solution retains a slightpink color after standing for about 30 sec.
III. Photochemical reactions of Potassiumtrisoxalatoferrate(III) trihydrate.
a. Prepare a solution containing 0.2 g of your sample in 15 mLof dilute sulfuric acid.
b. Dilute the solution to 50 mL with distilled water andexpose it to sunlight for one hour (note
carefully what happens).
c. Titrate with the standardized potassium permanganatesolution to determine the amount of
reducing agent present.
Postlab Assignment
I. Preparation of Potassium tris(oxalato) ferrate(III)trihydrate K3[Fe(C2O4)3]
- Calculate the theoretical yield. Show all yourcalculations.
- Calculate the percent yield
II. Determination of the oxalate content of Potassiumtrisoxalatoferrate(III) trihydrate.
2. Calculate the percentage by weight of oxalate in thecomplex; compare this with the theoretical
value and then calculate the percentage purity of thecomplex.
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