Synthesis of potassium dioxalatodiaquachromate(III) K[Cr(C₂O₄)₂(H₂O)₂]

Synthesis of Potassium Dioxalatodiaquachromate(III) K[Cr(C₂O₄)₂(H₂O)₂]

Aim

To synthesize potassium dioxalatodiaquachromate(III) complex, K[Cr(C₂O₄)₂(H₂O)₂], using chromium(III) salt and oxalic acid.

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Principle

Oxalate ions $(C_2O_4^{2-})$ act as bidentate ligands and coordinate with Cr³⁺ ions to form a stable octahedral complex. Two oxalate ligands and two water molecules coordinate with chromium(III) forming the complex ion [Cr(C₂O₄)₂(H₂O)₂]⁻.

Potassium ions combine with this complex ion to form the salt K[Cr(C₂O₄)₂(H₂O)₂], which crystallizes from the solution.

Reaction

$$C{r}^{3+}+2C_2{O}_4^{2-}+2H_{2}O\to [Cr(C_2{O}_4{)}_2(H_{2}O{)}_2{]}^{-}$$$$K^{+}+[Cr(C_2{O}_4{)}_2(H_{2}O{)}_2{]}^{-}\to K[Cr(C_2{O}_4{)}_2(H_{2}O{)}_2]$$

Requirements

Chemicals 

Chromium(III) chloride (CrCl₃) or chromium(III) nitrate, Oxalic acid / potassium oxalate, Potassium chloride, Distilled water, Ethanol

Apparatus 

Beaker, Glass rod, Water bath, Funnel, Filter paper, Measuring cylinder

Procedure

1. Dissolve about 2 g of chromium(III) chloride in 20 mL distilled water in a beaker.

2. Add oxalic acid or potassium oxalate solution slowly with constant stirring.

3. Heat the mixture gently on a water bath for about 15 minutes to ensure complete reaction.

4. Add a small quantity of potassium chloride solution to provide potassium ions.

5. Continue heating until a clear green solution forms.

6. Allow the solution to cool slowly.

7. Green crystals of potassium dioxalatodiaquachromate(III) separate out.

8. Filter the crystals using filter paper.

9. Wash the crystals with cold water followed by ethanol and dry between filter papers.

Observation

Observation	Result
Colour of chromium salt solution	Green
Colour of reaction mixture	Dark green
Colour of crystals obtained	Green crystals

Result

Potassium dioxalatodiaquachromate(III) complex K[Cr(C₂O₄)₂(H₂O)₂] was successfully synthesized as green crystals.