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.

Principle to Mohr’s method:

• 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.

Procedure:

1. Pipette a known volume of the chloride-containing solution into a 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:

• $C_{\mathrm{Cl^-}} = \frac{C_{\mathrm{AgNO_3}} \times V_{\mathrm{AgNO_3}}}{V_{\mathrm{sample}}}$

• ​​ Where:
• $C_{\mathrm{Cl^-}} = \text{concentration of chloride ions}
  C_{\mathrm{AgNO_3}} = \text{concentration of } \mathrm{AgNO_3} \\
  V_{\mathrm{AgNO_3}} = \text{volume of } \mathrm{AgNO_3} \text{ at the end point} \\
  V_{\mathrm{sample}} = \text{volume of chloride-containing sample}$
• This arrangement simplifies the content while maintaining clarity.

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