Experiment 52

Objective

To identify the presence of formic acid in a sample by observing its reaction with mercuric chloride.

Introduction

Formic acid reacts with mercuric chloride (HgCl₂) to produce a white precipitate of mercurous chloride, followed by a gray precipitate of mercury. This reaction is specific to formic acid and helps in its identification.

Reactions:

1.    Formation of mercurous chloride:

HgCl2​ + HCOOH → CO2​ + HCl + Hg2​Cl2​ (white ppt)

2.    Formation of mercury

HCOOH + Hg2​Cl2​ → CO2 ​+ HCl + Hg (gray ppt)

Materials

Chemicals

●      Formic acid

●      Mercuric chloride (HgCl₂)

●      Acetic acid (optional for reagent preparation)

Apparatus

●      Test tube

●      Test tube holder

●      Dropper

●      Bunsen burner

Reagents

●      5–6 ml of formic acid in a test tube

●      2–3 ml of mercuric chloride solution, prepared with acetic acid if desired

Procedure

1.    Place the formic acid sample in a test tube.

2.    Secure the test tube in a test tube holder.

3.    Add the mercuric chloride solution to the formic acid in the test tube.

4.    Wait for 1–2 minutes and observe any color changes.

Observation

The initial reaction produces a white precipitate of mercurous chloride, followed by a gray precipitate of elemental mercury, confirming the presence of formic acid.

Questions

1.    Why is HgCl₂ used in this test?

2.    What color changes are observed during the reaction?

3.    How does the formic acid identification test work?