Experiment 74

Objective

To identify the presence of sulphide ions in a sample by observing the formation of a black precipitate with lead acetate.

Introduction

Sulphide salts, such as sodium sulphide (Na₂S), react with lead acetate (Pb(CH₃COO)₂) to form a black precipitate of lead sulphide (PbS). This color change confirms the presence of sulphide ions in the sample.

Reaction:

Na2S + Pb(CH3COO)2 → 2Na(CH3COO) + PbS

Materials

Chemicals

·      Sodium sulphide (Na₂S)

·      Lead acetate (Pb(CH₃COO)₂)

·      Distilled water

Apparatus

·      Test tube

·      Test tube holder

·      Glass rod

Reagents

·      3–4 ml of the sample solution in a test tube

·      5% lead acetate solution prepared in a 250 ml beaker

Procedure

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

2.    Add lead acetate to the test tube.

3.    Mix the solution with a glass rod.

4.    Shake well to ensure thorough mixing.

5.    Observe and record any changes in the solution.

Observation

A black precipitate forms upon the addition of lead acetate, indicating the presence of sulphide ions in the sample.

Questions

1.    What reagent is used in the sulphide test?

2.    What happens during the sulphide test?

3.    What color precipitate forms when lead acetate is added to the sulphide solution?