Experiment 45

Objective

To identify the presence of copper ions in a sample by observing color changes with ammonium hydroxide.

Introduction

When ammonium hydroxide is added to a copper sulfate solution, it first forms a light blue precipitate. On adding excess ammonium hydroxide, the precipitate dissolves, forming a deep blue solution of tetraamminecopper(II) sulfate, indicating the presence of copper ions.

Reactions:

1.    Initial formation of copper hydroxide:

CuSO4 + NH4OH → CuSO4 ⋅ Cu(OH)2 + (NH4)2SO4

2.    Formation of deep blue tetraamminecopper(II) complex:

CuSO4 ⋅ Cu(OH)2 + (NH4)2SO4 + NH4OH → [Cu(NH3)4]SO4 + H2O

Materials

Chemicals

·       Copper sulfate (CuSO₄)

·       Ammonium hydroxide (NH₄OH)

·       Distilled water

Apparatus

·       Test tube

·       Test tube holder

·       Bunsen burner

Reagents

·       Dissolve 3.657 g of the copper sulfate sample in 250 ml of distilled water.

·       Ammonium hydroxide solution prepared in a beaker.

Procedure

1.    Place 5–6 ml of the copper sulfate sample solution in a test tube.

2.    Add 3–4 ml of ammonium hydroxide to the test tube.

3.    Observe the initial color change to light blue.

4.    Add an additional twofold quantity of ammonium hydroxide and observe the color change to deep blue.

Observation

Upon initial addition of ammonium hydroxide, a light blue precipitate forms. With excess ammonium hydroxide, the solution turns deep blue, indicating the formation of a tetraamminecopper(II) complex.

Questions

1.    What color change is observed during the copper test?

2.    What is the role of ammonium hydroxide in this test?

3.    What complex forms with excess ammonium hydroxide?