Transition elements are known to form variety of co-ordination complexes. These complexes are often highly colored and depending on the ligand, the color of a given complex varies for a given transition element.

For example, Fe(III) shows a variety of colors with different ligands like, oxalate, thiocyanate, cyanide, chloride, water etc. Some of these complexes are shown below.

In this experiment we will prepare a co-ordination complex of Fe (III) and oxalate in part A. In part B of the experiment, we will be analyzing the given complex for its iron and oxalate contents using titration technique. The molecular formula of the prepared complex is K_aFe_bOx_c.dH₂O.

In part C, we will be performing some qualitative tests with solutions of iron (III) nitrate and the complex. The qualitative tests are conducted for comparing the strength of different ligands that bind with iron.

Procedure:

PART A: Synthesis of the complex

You are supplied with 3.00 g of Fe(NO₃)₃.9H₂O in a vial and 8.00g of K₂C₂O₄.H₂O in another vial. Transfer the Fe(III) nitrate to a beaker and dissolve it in 3 mL of water. In another beaker, dissolve the potassium oxalate in 8 mL of hot water. Add the Fe(III) nitrate solution slowly to the oxalate solution with constant stirring. After the entire addition is done, heat the contents for 5 minutes on the hot plate. Remove the beaker from the hot plate and allow it to cool for 2-3 minutes. Then transfer the beaker to a prepared ice bath and keep it for 15 to 20 minutes. Filter the product obtained by suction filtration and wash with ice-cold water. Allow the product to dry for a half an hour in air or 15 to 20 minutes in a preheated oven (Temperature = 70-75°C). At the end of this time interval, carefully transfer the product to the pre-weighed butter paper. Take the product for weighing to nearest weighing balance.

PART B: Analysis of the synthesized co-ordination complex of iron

Determination of the oxalate content:

A sample of 0.250 g will be weighed in duplicate for analysis. Transfer the contents of one vial completely to a clean conical flask. Add 25 mL of 4 M H₂SO₄. Heat the solution on a hot plate to 70 – 80°C. Remove the flask from the hot plate and titrate the hot solution against standardized KMnO₄ till it is light pink in colour.

Do not discard the contents after the titration, as you will be estimating iron from the same solution.

Determination of the iron content:

After titration of oxalate, to the same solution, carefully add one vial of zinc powder. After 1 or 2 minutes, keep the solution on the hot plate. Boil the solution for 10-15 minutes. Carefully remove the flask from the hot plate and allow the solution to cool. If necessary, filter the solution using a filter paper. Titrate the solution/filtrate agaia nst standardized KMnO₄ solution. Perform both the titrations with another sample. Enter your results in the answer sheet.

PART C: Comparison of binding strength of different ligands

You are given 0.1 M solution of Fe(NO₃)₃ (that is, Solution 1) and of the complex (that is, Solution 2). In solution 1, iron exists as [Fe(H₂O)₆]³⁺.

Carry out the following tests in the supplied cavity plates and report your observations. If required some tests can also be performed in test tubes.

Based on the observations, arrange the ligands, that is, H₂O, OH-, Cl-, SCN- and C₂O₄^‾2 on the basis of their binding strength with iron. Explain your answer in brief.