Iron is one of the most important transition metals used in industry. The ability of iron to readily — Organic Chemistry Chemistry Question

Iron is one of the most important transition metals used in industry. The ability of iron to readily change its oxidation state accounts for numerous applications of this metal in chemical and biochemical redox processes. The most common oxidation states of iron are II and III; in both of these oxidation states, the metal can bind to several (usually up to six) donor atoms, such as the nitrogen atoms in various amines or organic heterocycles, the oxygen atoms in water or hydroxide ion, and carboxylates and other similar molecules or anions. In this experiment, an iron(III) oxalate complex will be prepared in two steps from an iron(II) precursor. Iron(III) oxalate complex is an interesting compound, in particular, because it is photosensitive. This compound is used in chemical actinometry for determining the number of photons that passes through the system. Upon exposure to visible or UV light, green crystals of iron(III) oxalate complex gradually decompose into a yellow-orange product.

Upon reacting an iron(II) salt with an oxalate, followed by oxidation in the presence of excess oxalate, one of three possible iron(III) oxalate complexes could be produced:

[VISUAL]

K3[Fe(C₂O₄)₃] .3H₂O K[Fe(C₂O₄)₂(H₂O)₂].3H₂O
[Fe(C₂O₄)(H₂O)₄](HSO₄).3H₂O
oxalate (ox^2-)

The number of oxalate ligands in the iron(III) oxalate complex synthesized in the present experiment will be determined by titration with potassium permanganate solution.

Chemicals and Reagents
* Ferrous ammonium sulfate hexahydrate, (NH₄)2[Fe(H₂O)₂(SO₄)₂] · 6 H₂O
* H₂SO₄(aq), c = 6 mol dm–3
* Oxalic acid (H₂C₂O₄), solution (c = 1 mol dm–3 ) or solid, H₂C₂O₄ · 2 H₂O
* Potassium oxalate, K₂C₂O₄ solution (c = 2 mol dm–3) or sodium oxalate, Na₂C₂O₄ solution (c = 2 mol dm–3).
* Aqueous hydrogen peroxide, H₂O₂, 6% solution
* Ethanol, C₂H₅OH
* KMnO₄ solution (~0.02 mol dm–3)

Equipment and Glassware
* Erlenmeyer flasks, 125 cm₃ (2), 50 cm₃ (1), 25 cm₃ (3)
* Pasteur pipettes and rubber pipette bulbs
* Hot plate
* Graduated cylinder, 25 cm₃
* Hot water bath
* Ice water bath
* Conical funnel, paper filters
* Fritted funnel for vacuum filtration
* Setup for vacuum filtration (stand, clamps to secure flasks, aspirator, filtering flask, conical rubber adaptor).
* Burette, 10 cm₃, with burette stand
* Small funnel to fill the burette

Procedure

A. Preparation of iron(III) oxalate complex.

Step 1
1. In a 25 cm₃ Erlenmeyer flask, dissolve 1.0 g of ferrous ammonium sulfate hexahydrate, (NH₄)²Fe(SO₄)₂ · 6 H₂O, in 3 cm₃ of H₂O to which has been added 3 drops of H₂SO₄ solution (6 mol dm–3).
2. While continuously swirling the flask, add 5.0 cm₃ of oxalic acid (H₂C₂O₄, 1 mol dm–3), and carefully heat the mixture to boiling (it is important to continuously swirl the flask while heating). Remove the flask from the hot plate and let the solid settle to the bottom of the flask.
3. Separate the solid product from the liquid by decantation: Do not disturb the solid product on the bottom of the flask. (Transfer the liquid to an Erlenmeyer flask and label as Liquid Waste). To wash the solid product, add ~3 cm₃ of hot water to the flask (heat water in the Erlenmeyer flask up to about 80 oC on a hot plate), swirl the mixture, allow the mixture to settle and pipette off the liquid layer without disturbing the solid product (transfer liquid to the Liquid Waste container). Repeat the washing step one more time.

Step 2
1. To the wet solid, add 2 cm₃ of potassium oxalate, K₂C₂O₄, solution (2 mol dm–3).
2. With the flask in a 40 oC water bath, carefully add 2 cm₃ of 6% H₂O₂ (continuously swirl the flask).
3. Transfer the flask to a hot plate, add 1.5 cm₃ of 1 mol dm–3 oxalic acid H₂C₂O₄, and bring the mixture to a boil. Let the mixture boil for 1 min.
4. Remove the flask from the heat and cool to room temperature.
5. Separate the solid from the liquid using gravity filtration (collect the filtrate in a clean, 50- cm₃ Erlenmeyer flask).
6. Cool the filtrate in an ice-water bath. To precipitate the product from the solution, add 8 cm₃ of ethanol to the filtrate and swirl the flask.
7. Collect the solid product by vacuum filtration.
8. Air-dry the crystals (alternatively, dry the crystals between two sheets of filter paper).
9. Transfer the dry crystals to a clean, dry pre-weighed vial. Determine the mass of crystalline iron(III) oxalate complex produced.

Part B. Analysis of Iron (III) Oxalate Complex

Step 1 Standardization of the ~0.02 mol dm–3 KMnO₄ solution.
1. Place ~0.02 mol dm–3 KMnO₄ solution into a 10 cm₃ burette. Into a 125-cm₃ Erlenmeyer flask, add about 0.020 g of precisely weighed sodium oxalate. To this Erlenmeyer flask, add 20 cm₃ of water and 5 cm₃ of sulfuric acid solution (c = 6 mol dm–3). Warm up the content of the flask in a hot water bath (maintained at ~80 °C).
2. Titrate the sodium oxalate solution using the ~0.02 mol dm–3 KMnO₄ solution; stop the titration when addition of the last drop of KMnO₄ changes the color of the titrated solution to light-pink, and the color persists for ca. 1 minute. Record the volume of KMnO₄ used for this titration, and determine the molarity of KMnO₄ solution.

Step 2
1. Add ~0.020 g of the precisely weighed iron(III) oxalate product obtained in Part A into a 125 cm₃ Erlenmeyer flask,. To this Erlenmeyer flask add 20 cm₃ of water and 5 cm₃ of 6 mol dm–3 sulfuric acid. Warm the content of the flask in a hot water bath (maintained at ~ 80 °C).
2. Titrate the hot solution in the flask with potassium permanganate of known concentration until a slight pink color that persists for ~30 sec. (Use the solution standardized in Part B, Step 1.) Record the volume of permanganate used for titration.

Data Treatment