Lab Page

Synthesis of Potassium tris(oxalato)ferrate

1 lab period; work in pairs. Complete the Preparation page before laboratory.


  • To improve your skill in observation
  • To synthesize a chemical substance and purify it by recrystallization
  • To discover the effect of light on the substance

Chemical synthesis--the art and practice of the design and creation of molecules--is the essence of chemistry. In no other field of science does the practitioner have the ability to create the objects of his/her interest. It is this single fact that sets chemistry apart from physics, geology, and biology, in which the scientist is restricted to examining what s/he finds, rather than what s/he creates. Chemistry has an extra dimension that combines vision and art in the process of creating new substances with potentially new and exciting uses.
This experiment provides you with an opportunity to experience chemical synthesis first hand, and to spectroscopically characterize the molecule (i.e., to discover what types and energies of light the molecule interacts with). The reaction that you will carry out is shown in equation (1).
(1): Fe3+ + 3 C2O42- ---> Fe(C2O4)33-
The reaction will be carried out in water solvent. This reaction is yet another example of a class of reaction that permeates chemistry: the donation of an electron pair by a Lewis base to a Lewis acid to form an adduct in which acid and base are joined by a covalent bond. As we have seen and will continue to see, the donation/acceptance of electron pairs is involved in a huge number of chemical reactions ranging from simple laboratory processes to the complex reactions in biological systems.
The adduct of Fe3+ with oxalate ion is called a transition metal complex to indicate that it is a complex molecule involving an electron pair donor and a transition metal ion (Fe3+).

Focus Questions
  1. In what molar ratio are the two reactants, iron chloride hexahydrate and oxalic acid, initially mixed?
  2. Propose a 3-dimensional structure for the adduct, keeping in mind that the oxalate ions tend to arrange themselves symmetrically around the iron(III) ion.
  3. In narrative form, discuss the infrared spectrum of the adduct. Attempt to assign observed bands to particular structural features of the adduct.

Equipment and Materials
  • 5.75" Pasteur pipets, 6 per group
  • 1 Pasteur pipet bulb
  • Kimwipes
  • aspirator with trap
  • 3 25-mL Erlenmeyer flasks
  • Rubber stoppers for Erlenmeyer flasks
  • ramrod
  • 100-mL beaker
  • Bunsen burner
  • Ring stand/ring/wire gauze
  • plastic wash bottle containing distilled water
  • Buchner funnel
  • suction flask
  • filter paper
  • glass stirring rod
  • FeCl3.6H2O
  • oxalic acid, H2C2O4
  • potassium hydroxide, KOH

Safety goggles must be worn at all times in the laboratory. Be very careful around open burner flames.

Record your observations in your notebook.

Synthesis. Obtain the required equipment. If necessary, wash the glassware using brushes and Alconox detergent, then thoroughly rinse with water to remove all traces of Alconox.
Place 6.5 millimoles (mmole) of oxalic acid, H2C2O4, and 12 mmole potassium hydroxide in a 25-mL Erlenmeyer flask, and add 5 mL of distilled water. Gently heat the mixture until the solids dissolve. To the hot solution, add 2 mmole of FeCl3.6H2O and swirl to dissolve. What do you observe? Filter the hot solution through a Kimwipe-plugged Pasteur pipet into a 25-mL Erlenmeyer flask. Stopper the flask, label it, and place it in an ice bath in the dark for 30 minutes. During this time, the product should crystallize. If no crystals are evident, scratch the inside wall of the Erlenmeyer with a glass stirring rod and return the solution to the ice bath. When crystallization is complete, collect the product by suction filtration. DO NOT WASH THE SOLID WITH WATER. Suction the product dry.
Purification. Place the product in a clean 25 Erlenmeyer. Add 3 mL of water and heat the mixture until the solid is dissolved. Once again place the hot solution in the ice bath in the dark until the product crystallizes. Quickly suction filter the mixture to isolate the product. Wash once with 1 mL of ice cold water, and suction dry. Vial the solid.
Characterization. If desired, you may characterize your product using infrared and uv-visible spectroscopies. See your instructor for assistance.
Light Response.
Clean-up. When you have finished all of your work,
  • Clean all glassware with Alconox and water, shake dry.
  • Rinse Pasteur pipets and graduated pipets with distilled water by inserting the nozzle of your wash bottle in the top end and squirting water through. Aspirate dry.
  • Return all components to the labkit.
  • Return all borrowed equipment to the instructor before leaving lab.

Disposal Methods
When finished with the experiment, hand in the vial containing your potassium tris(oxalato)ferrate to the instructor before leaving lab.

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