Experiment 61

Objective

To identify nitrate ions in a sample using the ring test.

Introduction

The ring test for nitrates involves reacting a nitrate salt with ferrous sulfate (FeSO₄) and concentrated sulfuric acid (H₂SO₄) to form a brown ring of nitrosoferrous sulfate (FeSO₄.NO). Sulfuric acid is added carefully to create a separate layer in the solution where the brown ring forms, indicating the presence of nitrate ions.

Reactions:

1.    Formation of nitric acid:

NaNO3 + H2SO4 → NaHSO4  + HNO3

2.    Formation of nitrosoferrous sulfate:

HNO3 + FeSO4 + HSO4 −→ Fe(SO4)3 + NO + H2O

3.    Formation of brown ring:

FeSO4 + NO → FeSO.NO (brown ring)

Materials

Chemicals

·       Ammonium chloride (sample)

·       Concentrated sulfuric acid (H₂SO₄)

·       Ferrous sulfate (FeSO₄) solution

Apparatus

·       Test tube

·       Test tube holder

·       Dropper

Reagents

·       Dissolve 3.269 g of ammonium chloride in 250 ml of distilled water.

·       Dissolve 3.659 g of ferrous sulfate in 100 ml of distilled water.

Procedure

1.    Place 3–5 ml of the sample solution in a test tube.

2.    Add 2–4 ml of ferrous sulfate solution to the sample.

3.    Heat the solution for 1–2 minutes.

4.    Cool the sample by placing the test tube in water.

5.    Carefully add concentrated sulfuric acid along the side of the test tube to create a separate layer.

6.    Observe the reaction for the formation of a brown ring.

Observation

A brown ring forms at the interface between the sulfuric acid and sample solution layers, indicating the presence of nitrate ions.

Questions

1.    What is a nitrate salt?

2.    Why is ferrous sulfate added to the nitrate solution?

3.    What color forms in the nitrate test?

4.    What is the ring test?

5.    What compound forms during the ring test?