Estimation of Zinc as Zinc Ammonium Phosphate

 Estimation of Zinc as Zinc Ammonium Phosphate

Aim

To estimate the amount of zinc present in the given solution by gravimetric method by precipitating it as zinc ammonium phosphate.

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Principle

Zinc ions react with ammonium phosphate in the presence of ammonia solution to form a white crystalline precipitate of zinc ammonium phosphate $(ZnNH_4PO_4)$.

The precipitate is filtered, washed, dried and then ignited, during which it converts into zinc pyrophosphate (Zn₂P₂O₇). The weight of Zn₂P₂O₇ obtained is used to calculate the amount of zinc present in the solution.

Reactions

Precipitation reaction

$$Zn^{2+} + NH_4^+ + PO_4^{3-} \rightarrow ZnNH_4PO_4 \downarrow$$

Ignition reaction

$$2ZnNH_4PO_4 \rightarrow Zn_2P_2O_7 + 2NH_3 + H_2O$$

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Requirements

Chemicals

Given zinc solution

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

Apparatus

Beaker

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

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Procedure

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

2. Add a small quantity of ammonium chloride to the solution.

3. Make the solution slightly alkaline by adding ammonium hydroxide.

4. Add ammonium phosphate solution slowly with stirring.

5. A white crystalline precipitate of zinc ammonium phosphate forms.

6. Heat the mixture gently to allow the precipitate to settle completely.

7. Filter the precipitate through filter paper.

8. Wash the precipitate with warm distilled water.

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

10. Ignite strongly until the precipitate converts into zinc pyrophosphate (Zn₂P₂O₇).

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 + Zn₂P₂O₇	W₂ g
3	Weight of Zn₂P₂O₇ obtained	(W₂ − W₁) g

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Calculation

Molecular weight of Zn₂P₂O₇ = 304.7 g/mol
Atomic weight of Zn = 65.38 g/mol

Amount of zinc in Zn₂P₂O₇

$$\frac{2 \times 65.38}{304.7} = \frac{130.76}{304.7}$$ $$\text{Weight of Zn} = \frac{130.76}{304.7} \times (W_2 - W_1)$$

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Result

The amount of zinc present in the given solution was determined gravimetrically as zinc ammonium phosphate (converted to Zn₂P₂O₇ on ignition).