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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.
Principle:
The method is based on the selective precipitation of chloride ions before chromate ions.
Reactions:
Ag⁺ + Cl⁻ → AgCl (s)
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:
Pipette a known volume of the chloride-containing solution into an Erlenmeyer flask.
Add distilled water to dilute the sample.
Add 2-3 drops of potassium chromate indicator (solution turns yellow).
Titrate with standardized AgNO₃ solution, forming a white AgCl precipitate.
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.
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