Mohr's method

Introduction to Mohr's Method:

• Mohr's method is a precipitation titration used to determine chloride ion concentrations in a solution.

• It uses silver nitrate (AgNO₃) as the titrant, forming a white precipitate of silver chloride (AgCl).

• Potassium chromate (K₂CrO₄) is used as the indicator, which forms a red precipitate of silver chromate (Ag₂CrO₄) at the endpoint.

Illustration of Mohr's method for determining chloride ion concentrations with silver nitrate as the titrant

(image may contain incorrect data, it is just for illustration purpose)

Principle:

• The method is based on the selective precipitation of chloride ions before chromate ions.

Reactions:

1. Ag⁺ + Cl⁻ → AgCl (s)

2. After all chloride has precipitated, excess Ag⁺ reacts with chromate ions:

2Ag⁺ + CrO₄²⁻ → Ag₂CrO₄ (s, red)

• The appearance of a red precipitate of silver chromate indicates the endpoint.

2 AgNO3 + K2CrO4 → Ag2CrO4(s, red) + 2 KNO3

Procedure:

1. Pipette a known volume of the chloride-containing solution into an Erlenmeyer flask.

2. Add distilled water to dilute the sample.

3. Add 2-3 drops of potassium chromate indicator (solution turns yellow).

4. Titrate with standardized AgNO₃ solution, forming a white AgCl precipitate.

5. Continue titrating until a faint reddish-brown precipitate (Ag₂CrO₄) appears, indicating the endpoint.

Theory:

• Silver chloride precipitates first due to its lower solubility product (Ksp) compared to silver chromate.

• Once all chloride is consumed, excess silver reacts with chromate to form Ag₂CrO₄, signaling the endpoint.

Calculation:

C_Cl- = (C_AgNO3 × V_AgNO3) / V_sample

Where:

• C_Cl- = concentration of chloride ions,

• C_AgNO3 is the concentration of the silver nitrate titrant.

• V_AgNO3 is the volume of silver nitrate titrant used at the endpoint.

• V_sample is the volume of the chloride-containing sample.