Experiment 174

Objective

To test the purity of chloroform by detecting the presence of contaminants, specifically hydrochloric acid (HCl), through a reaction with silver nitrate.

Introduction

Chloroform can become contaminated by hydrochloric acid (HCl) and phosgene gas (COCl₂) when exposed to oxygen and light. This reaction forms harmful byproducts:

2CHCl3 ​+ O2 → COCl2 ​+ 2HCl

To ensure chloroform is safe for use, its purity can be tested by adding silver nitrate (AgNO₃). If HCl is present, it reacts with AgNO₃ to form a white precipitate of silver chloride (AgCl):

HCl + AgNO3 ​→ AgCl + HNO3

Materials

Chemicals

·       Chloroform (CHCl₃)

·       Silver nitrate (AgNO₃)

Apparatus

·       Test tube

·       Test tube holder

·       Cotton or tissue

·       Protective mask

Reagent

·       Use 1-2 mL of chloroform and add 1 drop of silver nitrate.

Procedure

1.    Preparation of Test Solution

o   Place 1-2 mL of chloroform in a test tube.

2.    Addition of Silver Nitrate

o   Add 1-2 drops of silver nitrate (AgNO₃) to the test tube.

3.    Observation

o   Shake the mixture gently and allow it to rest for one minute.

Observation

The appearance of a white precipitate (AgCl) indicates the presence of HCl, suggesting that the chloroform is contaminated.

Questions

1.    What is phosgene gas?

2.    What happens to chloroform in the presence of light?

3.    Why is chromatic glass used to preserve chloroform?

4.    Why is it essential to perform a chloroform purity test?