Precipitation Titration: Mohr's Method

• Precipitation titration involves reactions resulting in the formation of a colored, insoluble precipitate.
• Standard solutions and reaction substances react to form a precipitate in this type of titration.
• Precipitation is the formation of a solid mass (precipitate) in a liquid.
• An example equation is: Sodium chloride + Silver Nitrate -> Silver chloride ppt + Sodium nitrate
• Sodium chloride reacts with silver nitrate, precipitating silver chloride

Types of Precipitation Titration

• Includes Mohr's method, Volhard's method and Modified Volhard's method, and Fajan's method

Mohr's Method

• An important precipitation titration method first developed by Mohr in 1856
• Potassium chromate (K2CrO4) is utilized as the indicator.
• It involves titrating silver nitrate against halides in a neutral solution using a 2% potassium chromate solution as an indicator
• The endpoint is signaled by the appearance of a brick red precipitate with silver chromate and silver chloride formation
• During titration, AgNO3 reacts with NaCl to form AgCl (silver chloride) precipitate.
• When all chloride ions are exhausted, AgNO3 reacts with K2CrO4 indicator to form Ag2CrO4 precipitate, marking the endpoint.
• The calculation used is: N1V1 = N2V2 or M1V1 = M2V2, where V1 is the volume of AgNO3 solution (from the burette reading).

Limitations of Mohr's Method

• It is not feasible to perform titration in basic solutions, as it would produce silver hydroxide.
• It is not feasible to perform titration in the presence of ammonia ions and several anions.

Pharmaceutical Applications of Mohr's Method

• Sodium chloride and dextrose injection
• Determination of chloride, bromide, and thiocyanate

Volhard's Method

• Designed by German scientist Jacob Volhard in 1874
• Used for the silver estimation by titrating against using a standard thiocyanate solution in the presence of ferric salt indicators
• It is a type of back titration.
• Silver chloride is first titrated against AgNO3 (silver nitrate) in excess.
• Then, this AgNO3 is titrated against a thiocyanate solution until the endpoint is reached (reddish brown).
• Standard 0.1 N AgNO3, 0.1 N NaCl, and 0.1 N KSCN are prepared
• A (Fe3+) indicator is prepared using purified nitric acid
• Excess AgNO3 is used for NaCl's titration
• After complete precipitation of Silver chloride (AgCl), excess AgNO3 is added.
• The equation is AgNO3 + NaCl -> AgCl + NaNO3 + AgNO3 (unreacted)

Volhard's Method Continued

• The appearance of reddish brown indicates the end point OR
• fe3+ + SCN- -> [FeSCN]2+

Pharmaceutical Applications of Volhard's Method

• Aminophylline, chlorophenothane, ethionamide and sodium chloride injection can be determined suing this method

Modified Volhard's Method

• Chloroform or other wetting agents are included after adding excessive silver nitrate to solubilize AgCl

Fajan's Method

• Fajan introduced this method in 1923–24.
• This uses an absorption indicator device for end point detection.
• This method relies on the adsorption indicator, where the indicator bonds to the precipitate's surface, changing the precipitate's color.
• Used indicators: Fluorescein, Dichlorofluorescein, Eosin
• Chloride ion is first titrated against silver nitrate to create the silver chloride precipitate.
• When the silver chromate adsorbs exess silver ions, it will indicate its endpoint

Application of Precipitation Titrations

• Help to standardize unknown NaCl solutions
• Standardize AgNO3 solutions
• Determine complexes
• Used to eliminate salts from water and various substances.
• Used in metallurgy.