Estimating Chloride (Cl-) in Water: Principles and Methods

 

Estimating Chloride (Cl-) in Water: Principles and Methods

Chloride (Cl-) is a common ion found in water. Estimating its concentration is crucial for water quality assessment. Here's a breakdown of the principles and methods involved:

Principles:

The primary method for Cl- estimation is titration, specifically the Mohr method. This relies on a precipitation reaction:

• Silver nitrate (AgNO3) reacts with chloride ions (Cl-) to form insoluble silver chloride (AgCl) precipitate.

  AgNO3 (aq) + Cl- (aq) → AgCl (s) + NO3- (aq)

However, visually detecting the white AgCl precipitate is imprecise. Here's where the magic happens:

• Potassium chromate (K2CrO4) acts as an indicator.
• As titration nears completion (all Cl- reacted), any excess silver ions (Ag+) from AgNO3 will react with chromate ions (CrO4²-) from the indicator.
• This forms a reddish-brown precipitate of silver chromate (Ag2CrO4), signifying the endpoint.

Methods:

Mohr Titration involves these steps:

1. Preparation:

   • Standardize the AgNO3 solution using a standard NaCl solution.
   • Measure a known volume of the water sample.
   • Add a small amount of K2CrO4 indicator to the sample.

2. Titration:

   • Slowly add standardized AgNO3 solution from a burette to the sample.
   • Continuously stir the solution during addition.

3. Endpoint Detection:

   • Observe for a color change. The initial yellow (K2CrO4) turns faint reddish-brown when all Cl- has reacted, and excess Ag+ reacts with CrO4²-.
   • This is the endpoint.
   • Record the volume of AgNO3 solution used.

4. Calculation:

   • Use the volume and concentration (from standardization) of AgNO3 solution to calculate the Cl- concentration in the water sample.

Important Points:

• This method works well for Cl- concentrations above 1 mg/L.
• The solution's pH needs to be between 6.3 and 10.5 for accurate results.
• This is a laboratory technique requiring proper training and equipment.

Safety Precautions:

• Silver nitrate is corrosive. Wear gloves and eye protection while handling it.
• Dispose of waste according to recommended guidelines.