Mohr’s Method | Determination of Concentration of Silver Nitrate

Theoretical Background / Context

• Precipitimetry: It is a volumetric analysis method that involves the formation of a practically insoluble salt using a precipitating agent.
• Solubility product (Ksp): In a saturated solution of sparingly soluble electrolytes, the product of the molar concentration of ions each raised to a power equal to the number of ions produced is constant at room temperature and pressure.

Ksp of AnBm =[A]n[B]m

N.B: Substance with low Ksp precipitate first.

Conditions required for a precipitimetry reactions:

1. The precipitate must be practically insoluble.
2. Rapid precipitation.
3. Ease of detection of the endpoint.

Principle Work of Mohr’s Method

• Mohr’s method is a direct argentometric titration (silver nitrate titration) used for quantitative chemical analysis.
• In a neutral or slightly alkaline medium, sodium chloride is titrated with silver nitrate (AgNO3) forming silver chloride precipitate using n-chromate as a Mohr method indicator.

NaCl + AgNO3NaNO3 +AgCl

• After precipitation of all chloride, the first drop excess of AgNO3 will react with n-chromate forming:

Ag2CrO4 reddish-brown precipitate in Mohr’s titration. (endpoint