Estimation of Iron as Fe₂O₃ by Precipitating Iron as Fe(OH)₃

 Estimation of Iron as Fe₂O₃ by Precipitating Iron as Fe(OH)₃

Aim

To estimate the amount of iron present in the given solution by gravimetric method by precipitating iron as ferric hydroxide Fe(OH)₃ and weighing it as ferric oxide Fe₂O₃.

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Principle

Iron(III) ions in the given solution are precipitated as ferric hydroxide Fe(OH)₃ by adding ammonium hydroxide in hot solution.

The precipitate is filtered, washed and then ignited, during which ferric hydroxide decomposes to form ferric oxide Fe₂O₃.

The weight of Fe₂O₃ obtained is used to calculate the amount of iron present in the sample.

Reactions

Precipitation reaction

$$F{e}^{3+}+3O{H}^{-}\to Fe(OH{)}_3\downarrow$$

Ignition reaction

$$2Fe(OH{)}_3\to F{e}_2{O}_3+3H_{2}O$$

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Requirements

Chemicals

Given iron solution

Ammonium hydroxide (NH₄OH)
Ammonium chloride (NH₄Cl)
Distilled water

Apparatus

Beaker,Glass rod

Filter paper
Funnel
Porcelain crucible
Bunsen burner / muffle furnace
Desiccator
Weighing balance

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Procedure

1. Take 25 mL of the given iron solution in a beaker.

2. Add a small amount of ammonium chloride.

3. Heat the solution nearly to boiling.

4. Add ammonium hydroxide slowly with constant stirring until precipitation is complete.

5. A reddish brown precipitate of Fe(OH)₃ is formed.

6. Heat the solution for a few minutes to allow the precipitate to settle.

7. Filter the precipitate through filter paper.

8. Wash the precipitate with hot distilled water until free from impurities.

9. Transfer the precipitate along with filter paper into a pre-weighed porcelain crucible.

10. Ignite strongly using a Bunsen burner until the precipitate converts into Fe₂O₃.

11. Cool the crucible in a desiccator and weigh.

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Observation Table

Sr. No.	Observation	Value
1	Weight of empty crucible	W₁ g
2	Weight of crucible + Fe₂O₃	W₂ g
3	Weight of Fe₂O₃ obtained	(W₂ − W₁) g

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Calculation

Molecular weight of Fe₂O₃ = 159.7 g/mol
Atomic weight of Fe = 55.85 g/mol

Iron present in Fe₂O₃

$$\frac{2 \times 55.85}{159.7} = \frac{111.7}{159.7}$$ $$\text{Weight of Fe} = \frac{111.7}{159.7} \times (W_2 - W_1)$$

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Result

The amount of iron present in the given solution was determined gravimetrically as ferric oxide (Fe₂O₃).