Mohr's method, also known as the argentometric method, is a widely used analytical technique in pharmaceutical analysis for the determination of halide ions, particularly chloride ions (Cl⁻). The principle of Mohr's method is based on the precipitation of the halide ion with a standardized silver nitrate (AgNO₃) solution to form a sparingly soluble silver halide precipitate.
The principle of Mohr's method can be summarized as follows:
Standardization of the Silver Nitrate Solution: A standardized silver nitrate solution of known concentration is prepared. This is typically done by titrating a solution containing a known amount of chloride ions with the silver nitrate solution until the equivalence point is reached. The equivalence point is the point at which the stoichiometrically equivalent amounts of chloride ions and silver ions react to form the silver chloride precipitate.
Precipitation of the Halide Ion: The pharmaceutical sample, which may contain chloride ions along with other interfering substances, is dissolved in a suitable solvent. A few drops of an indicator, such as potassium chromate (K₂CrO₄), are added to the solution. The indicator forms a red-colored chromate ion when all chloride ions have reacted with silver ions.
Titration with Silver Nitrate Solution: The standardized silver nitrate solution is then slowly added to the sample solution until the appearance of a persistent reddish-brown color, indicating the excess silver ions reacting with the chromate ions. At this point, all the chloride ions have precipitated as silver chloride, and any additional silver ions react with the indicator.
Endpoint Determination: The endpoint of the titration is determined by the disappearance of the reddish-brown color, indicating that all chromate ions have reacted with the excess silver ions. This marks the completion of the reaction between the chloride ions and silver nitrate solution.
Calculation of Chloride Ion Concentration: The volume of the silver nitrate solution required to reach the endpoint is recorded. From the known concentration of the silver nitrate solution and the volume used, the concentration of chloride ions in the pharmaceutical sample can be calculated using stoichiometry.
