Question

Potassium tris(oxalato) ferrate (III), also known as potassium ferrioxalate, is a green crystalline- compound of octahedral geometry. It is a metal complex of iron with three oxalate (C2O42-) ligands bonded to the central metal atom (Fe). These ligands are bidentate, meaning that each of them binds to the metal atom at 2 different places.

Potassium ferrioxalate is a very interesting photosensitive material. In solution, it reacts with

2− light photons and undergoes a reduction reaction and decomposes 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 mL of 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 acid dihydrate in 25 mL of water.

d. Slowly heat the mixture to boiling (beware of bumping) then allow the yellow precipitate to settle.

e. Decant the supernatant using a Buchner funnel making sure it has a properly fitted filter paper.

f. Add 15 mL of hot water to any solid remaining in the beaker, stir and filter.

g. Drain well and then transfer all the precipitate from the filter paper back into the beaker with 10 mL

hot water.

h. Add to the beaker 3.5 g solid potassium oxalate monohydrate and heat to approximately 40°C.

i. Add slowly, using a dropper, 9 mL of "20 vol" hydrogen peroxide.

j. If the precipitate looks yellowish, not brown and settles readily, decant the supernatant, add a

solution of 0.2 - 0.4 g potassium oxalate monohydrate in 1 - 2 mL water and then hydrogen peroxide

dropwise until the precipitate dissolves. Then add the previously decanted supernatant.

k. Heat to boiling, and add a solution of 2 g of oxalic acid dihydrate in 30 mL of water in portions, add 20 mL initially, then if the brown precipitate still remains, add more solution little by little until it all dissolves.

l. Boil the clear solution down to a volume of 40 to 50 mL.

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 a 1:1 ethanol/water mixture and finally with

acetone.

p. Dry in the air and weigh.

The complex is photosensitive and should not be exposed to light unnecessarily. Store in a sample bottle wrapped in foil. The bright green crystals on exposure soon become covered with a yellow powder of ferrous oxalate.

2K3[Fe(C2O4)3].3H2O  2FeC2O4.2H2O + 2CO2 + 3K2C2O4 + 2H2O

II. Determination of the oxalate content of Potassium trisoxalatoferrate(III) trihydrate.

The iron(III) complex is first decomposed in hot acid solution and the free oxalic acid is titrated against standard (0.02 M) potassium permanganate solution. No indicator is required.

a. Weigh accurately about 0.2 g of the potassium trisoxalatoferrate(III) complex previously prepared.

b. Boil the sample with 50 mL of 1 M sulfuric acid in a conical flask.

c. Allow the solution to cool to about 60°C.

d. Titrate slowly with a standard potassium permanganate solution.

e. Continue titrating until the warm solution retains a slight pink color after standing for about 30 sec.

III. Photochemical reactions of Potassium trisoxalatoferrate(III) trihydrate.

a. Prepare a solution containing 0.2 g of your sample in 15 mL of dilute sulfuric acid.

b. Dilute the solution to 50 mL with distilled water and expose it to sunlight for one hour (note

carefully what happens).

c. Titrate with the standardized potassium permanganate solution to determine the amount of

reducing agent present.

Postlab Assignment

I. Preparation of Potassium tris(oxalato) ferrate(III) trihydrate K3[Fe(C2O4)3]

1. Calculate the theoretical yield. Show all your calculations.
2. Calculate the percent yield

II. Determination of the oxalate content of Potassium trisoxalatoferrate(III) trihydrate.

2. Calculate the percentage by weight of oxalate in the complex; compare this with the theoretical

value and then calculate the percentage purity of the complex.

  

Answer 1

Calculations:

A. Theoretical yield :

Starting Compound End Product

FeSO4.(NH4)2SO4.6H2O K3[Fe(C2O4)3].3H2O

392 g 491 g

392g is at starting will be 491 g of complex

5 g of starting compound will be = 5 x 491 / 392

= 6.267 g of complex

B. Practical yield of the complex = W g

C. Percent practical yield

6.267 g of complex = 100%

therfore W gram of complex = (W x100)/6.267

= Z % yield

generally yield is around 78-82% i.e. value for W is range between 4.9 to 5.2 gram of complex for good chemist.

Answer for Que 2.)

A. Theoretical yield :

C2O4(2−) or C2O4-2 is of 88.02 g per mol

KMNO4 is of 158.034 g per mol

KMNO4 is in equivalence with oxalate content of Potassium trisoxalatoferrate(III) trihydrate.

N1V1 = N2V2

N1 is Normality of standardized KMNO4,

V1 is volume of KMNO4 used in titration (F.B.R> final burette reading)

V2 is sample volume taken for titration

by this we can calculate N2 = Normality of oxalate in present titrated sample

B. To find out strength of oxalate =   W g x 88.02 = Zgper L

here W = weight of  0.2 g of the potassium trisoxalatoferrate(III) complex

C. Equivalence weight if oxalate(W2) = Strength of oxalate / N2

Percent practical yield = w2 x 0.03578 / 100 = Y %