Precipitation Titration (Mohr Method)

Determining chloride concentration using silver nitrate

Titrations Advanced (University) 45 min ~$20.00

Objective

Determine the chloride ion concentration in a solution using the Mohr method, where silver nitrate is the titrant and potassium chromate is the indicator.

Background

In the Mohr method, silver nitrate reacts with chloride ions to form white silver chloride precipitate. When all chloride is consumed, the first excess silver reacts with the chromate indicator to form brick-red silver chromate, signaling the endpoint. This method works at near-neutral pH.

Safety Warnings

  • Silver nitrate stains skin and clothing permanently
  • Potassium chromate is toxic and a suspected carcinogen — handle with extreme care
  • Wear nitrile gloves and safety goggles
  • Dispose of silver and chromate waste in designated containers

Required PPE

goggles gloves lab_coat

Materials

  • Sodium chloride solution (100 mL)
    Unknown concentration
  • Silver nitrate (AgNO₃) (100 mL)
    0.1M standardized
  • Potassium chromate (K₂CrO₄) (10 mL)
    5% indicator solution
  • Distilled water (200 mL)

Equipment

50 mL burette Burette stand and clamp 250 mL Erlenmeyer flask 25 mL pipette White tile

Procedure

1

Pipette 25.0 mL of the chloride solution into the Erlenmeyer flask.

2 min
2

Add 1 mL of 5% potassium chromate indicator. The solution turns pale yellow.

1 min Chromate is toxic — use minimal amount
3

Fill the burette with 0.1M AgNO₃. Record the initial volume.

3 min AgNO₃ stains permanently
4

Titrate while swirling vigorously. A white precipitate of AgCl forms.

10 min
5

Near the endpoint, the precipitate becomes more curd-like. Add AgNO₃ drop by drop until a permanent brick-red color of Ag₂CrO₄ appears.

5 min
6

Record the volume. Repeat for concordant results.

15 min
7

Calculate chloride concentration: C(Cl⁻) = C(Ag+) x V(AgNO₃) / V(sample).

5 min

Expected Results

The endpoint is marked by the first permanent appearance of a brick-red color due to Ag₂CrO₄ formation. This color should persist for at least 30 seconds after swirling.

Cleanup

Collect all silver-containing waste in the designated heavy metal waste container. Collect chromate waste separately. Never dispose down the drain. Wash hands thoroughly.

Details

Category
Titrations
Difficulty
Advanced (University)
Duration
45 min
Est. Cost
$20.00
Steps
7
Materials
4

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Frequently Asked Questions

What is the objective of Precipitation Titration (Mohr Method)?
Determine the chloride ion concentration in a solution using the Mohr method, where silver nitrate is the titrant and potassium chromate is the indicator.
How difficult is Precipitation Titration (Mohr Method)?
This experiment is rated as Advanced (University). It takes approximately 45 minutes to complete.
What safety precautions are needed for Precipitation Titration (Mohr Method)?
Key safety precautions include: Silver nitrate stains skin and clothing permanently; Potassium chromate is toxic and a suspected carcinogen — handle with extreme care; Wear nitrile gloves and safety goggles.
What materials are needed for Precipitation Titration (Mohr Method)?
The main materials required are: Sodium chloride solution, Silver nitrate (AgNO₃), Potassium chromate (K₂CrO₄), Distilled water.
What results should I expect from Precipitation Titration (Mohr Method)?
The endpoint is marked by the first permanent appearance of a brick-red color due to Ag₂CrO₄ formation. This color should persist for at least 30 seconds after swirling.