Introduction
Potassium Ferric Oxalate crystal is an inorganic complex that is light green in color and is prepared in the presence of oxalic acid from ferric sulfate and barium oxalate. This is one of the examples of an Oxidation-Reduction reaction.
Potassium Ferric Oxalate is also called Potassium ferrioxalate, Potassium Trioxalatoferrate (III), Potassium tris (oxalato) ferrate(III), and Potassium iron(3+) oxalate.
Aim
To prepare the pure Potassium Ferric Oxalate from three chemicals such as potassium monohydrate, oxalic acid, and ferric chloride dihydrate.
Theory
When a freshly prepared ferric hydroxide is treated with an oxalic acid solution, Ferrous oxalate in the form of a yellow precipitate will be formed. When dissolved further with potassium oxalate monohydrate, Potassium Ferric Oxalate is formed in the form of a green precipitate. Potassium ferric oxalate is considered a complex compound with the chemical formula of K3 [Fe(C₂O₄)3].3H₂O.
The chemical reactions,
FeCl₃ + 3KOH → Fe(OH)₃ + 3KCl
2Fe(OH)₃ + 3(COOH)2.2H₂O → Fe2(C₂O₄)3 + 12H₂O
Fe₂(C₂O₄)3 + 3(COOK)2.H₂O → 2K₃[Fe(C₂O₄)₃].3H₂O
(Potassium Ferric Oxalate)
Ferric hydroxide gets dissolved and the soluble complex is formed when the oxalic acid is added excessively. To precipitate the complex iron salt alcohol is added to the solution, as it is less soluble in alcohol than water.
Materials Required
Oxalic acid hydrate
Ferric chloride
Potassium oxalate
Potassium hydroxide
Beaker
Conical flask
Tripod stand
Glass rod
Wire gauze
Filter paper
China dish
Funnel
Apparatus Setup
Procedure
Dissolve 3.5g of freshly prepared ferric chloride in 10 ml of water, in a beaker.
Dissolve 4g of potassium hydroxide in another beaker with 50ml of water.
With constant stirring, slowly add the potassium hydroxide solution to the ferric chloride solution to form a brown color ferric hydroxide precipitate.
Through the funnel, filter the ferric hydroxide precipitate and wash it with hot water.
Take 4g of oxalic acid and 5.5g of potassium oxalate in another beaker. Add 100ml of water and stir it well to get a clear potassium oxalate solution.
To potassium oxalate solution, gradually add the freshly prepared ferric hydroxide precipitate by stirring it constantly so that precipitate dissolves completely and the green color solution is formed.
Remove the insoluble impurities by filtering the solution.
Take a china dish and transfer the green-coloured solution into it and concentrate the solution till the crystallization point is reached.
In cold water, place the china dish and let it cool for an hour.
Green crystals of potassium ferric oxalate are formed. From the mother, liquor removes all the crystals.
With ethyl alcohol, wash the crystals and dry them between the folds of filter paper.
To know the yield, weigh the crystals.
Observations
Results and Discussion:
The Potassium ferric oxalate yield is _______gm.
Precautions
Do not concentrate too much on the solution.
To get big crystals during crystallization, do not disturb the solution.
To dissolve unwanted salt, wash the crystals with hot water.
FAQs on Preparation of Potassium Ferric Oxalate
1. What exactly is potassium ferric oxalate?
Potassium ferric oxalate is a coordination compound, also known as a complex salt, with the chemical formula K₃[Fe(C₂O₄)₃]·3H₂O. It appears as striking emerald green crystals. Its official IUPAC name is Potassium tris(oxalato)ferrate(III) trihydrate. In this compound, an iron atom in the +3 oxidation state is centrally located and bonded to three bidentate oxalate ions.
2. What is the step-by-step procedure to prepare potassium ferric oxalate crystals in the lab?
The preparation of potassium ferric oxalate crystals is a multi-step process typically performed in a school laboratory. The key steps are:
- Preparation of Ferric Hydroxide: A solution of ferric chloride is treated with a potassium hydroxide solution to produce a reddish-brown precipitate of ferric hydroxide, Fe(OH)₃.
- Formation of the Complex: The freshly prepared ferric hydroxide precipitate is then added to a hot solution containing potassium oxalate and oxalic acid. The precipitate dissolves upon stirring, forming a green-coloured solution of the potassium ferric oxalate complex.
- Crystallisation: The green solution is carefully heated to concentrate it until the crystallisation point is reached. It is then cooled slowly and left undisturbed.
- Isolation of Crystals: As the solution cools, beautiful emerald green crystals of potassium ferric oxalate separate out. These crystals are then filtered, washed with a small amount of cold water, and dried.
3. What are the chemical equations for the preparation of potassium ferric oxalate?
The synthesis of potassium ferric oxalate involves two main chemical reactions:
1. Formation of Ferric Hydroxide: A precipitation reaction between ferric chloride and potassium hydroxide.
FeCl₃(aq) + 3KOH(aq) → Fe(OH)₃(s) + 3KCl(aq)
2. Formation of the Complex Salt: The freshly prepared ferric hydroxide is reacted with a mixture of potassium oxalate and oxalic acid to form the soluble green complex.
Fe(OH)₃(s) + 3K₂C₂O₄(aq) + 3H₂C₂O₄(aq) → 2K₃[Fe(C₂O₄)₃](aq) + 6H₂O(l)
4. What apparatus and chemicals are needed to prepare potassium ferric oxalate?
To carry out the experiment as per the CBSE syllabus for the 2025-26 session, you will need the following:
- Chemicals: Ferric chloride (FeCl₃), potassium hydroxide (KOH), oxalic acid dihydrate (H₂C₂O₄·2H₂O), potassium oxalate monohydrate (K₂C₂O₄·H₂O), and distilled water.
- Apparatus: Beakers, a glass rod for stirring, a funnel, filter paper, a china dish for crystallisation, a tripod stand, and a wire gauze for heating.
5. Why must the ferric hydroxide be freshly prepared for this synthesis?
Ferric hydroxide must be freshly prepared because its reactivity changes over time. When first precipitated, it is in an amorphous, highly reactive state that readily dissolves in the oxalic acid solution to form the desired complex. If it is allowed to stand for a long time, it 'ages' by converting into a more stable, less soluble polymeric form, which will not react properly to form the green potassium ferric oxalate complex.
6. Why does the solution turn green during the preparation of potassium ferric oxalate?
The distinct green colour is a direct result of the formation of the tris(oxalato)ferrate(III) complex ion, [Fe(C₂O₄)₃]³⁻. The initial reddish-brown precipitate of ferric hydroxide reacts and dissolves, forming this new coordination entity. The arrangement of the oxalate ligands around the central iron(III) ion alters the splitting of its d-orbitals, causing it to absorb light in the red part of the visible spectrum and transmit light that appears as a vibrant emerald green.
7. What is the role of potassium ferric oxalate in blueprinting?
Potassium ferric oxalate is highly photosensitive, meaning it decomposes when exposed to light, particularly UV light. This property is fundamental to the cyanotype process used for making blueprints. When coated on paper and exposed to light through a drawing, the iron(III) in the complex is reduced to iron(II). This light-exposed area then reacts with another chemical, potassium ferricyanide, to form a stable, insoluble blue pigment called Prussian blue. The unexposed complex is simply washed away, leaving a white-on-blue copy of the original drawing.
8. What are the most important precautions to take during this experiment?
For a successful preparation and to ensure safety, students should follow these precautions:
- Handle oxalic acid and its salts with care, as they are toxic if ingested.
- Do not overheat or boil the green solution while concentrating it, as this can cause the complex to decompose.
- To grow large, well-defined crystals, the concentrated solution must be cooled slowly and without disturbance.
- Wash the final crystals with a very small amount of cold distilled water to remove impurities without dissolving the product.
9. What is the key difference between potassium ferric oxalate and potassium ferrous oxalate?
The primary difference lies in the oxidation state of the central iron atom. In potassium ferric oxalate (K₃[Fe(C₂O₄)₃]), iron is in the +3 oxidation state. In potassium ferrous oxalate, the iron would be in the +2 oxidation state. This difference leads to distinct properties, including their chemical formula, colour (ferric is green, ferrous compounds are typically pale yellow or colourless), and stability.
