Mohr Titration Method

Introduction

• Mohr titration is a volumetric method for determining the concentration of a substance (analyte) using a solution of known concentration (titrant).
• Silver nitrate (AgNO₃) is the common titrant.
• The endpoint is visually observed by the formation of a precipitate (light-colored against a darker background).

Principle

• The titration relies on a precipitation reaction between the analyte and the silver nitrate titrant.
• Frequently, the analyte is a halide ion (Cl⁻, Br⁻, I⁻).
• The result is a slightly soluble silver salt (AgCl, AgBr, or AgI).
• The endpoint is marked by a color change (appearance or disappearance).
• Silver ions (Ag⁺) react with halide ions (X⁻) to form a silver halide precipitate (AgX).
• A chromate indicator (CrO₄²⁻) is essential.

Indicator

• Potassium chromate (K₂CrO₄) is the indicator for Mohr titration.
• Potassium chromate gives the solution a yellow color.
• When all halide ions are reacted, extra silver ions react with chromate ions, forming a reddish-brown precipitate of silver chromate (Ag₂CrO₄).
• This color change (yellow to reddish-brown) signifies the endpoint.

Procedure

• Prepare a standard silver nitrate (AgNO₃) solution with a known concentration.
• Prepare a solution of the unknown analyte (halide ion).
• Add a few drops of potassium chromate indicator to the analyte solution.
• Gradually add the standard silver nitrate solution to the analyte solution, while stirring continuously.
• Observe the color change.
• The endpoint is marked by the first permanent reddish-brown color.
• Note the volume of silver nitrate solution used to reach the endpoint.

Applications

• Used to determine halide ion concentrations (chloride, bromide, iodide) in various samples.
• Implemented in industrial settings and chemistry labs for quality control.
• Useful for analysis of chlorine and chloride in water samples.
• Used to measure chloride content in different materials and substances.

Factors Affecting Accuracy

• Interfering ions can influence the results.
• Solution pH can affect the reaction.
• Temperature changes impact the reaction.
• Proper stirring ensures uniform reactant mixing.
• Indicator concentration plays a role in accuracy.
• The quality of the standard silver nitrate solution is vital.
• Oxidizing or reducing agents present can affect the results.
• Presence of other ions can interfere with the reaction.

Precautions

• Prevent chemical spills.
• Use appropriate personal protective equipment (PPE).
• Manage chemical waste properly.
• Handle glassware appropriately.
• Accurate standardization of the silver nitrate titrant is crucial for reliable results.
• Conduct titrations in a well-lit space.

Limitations

• Mohr titration isn't suitable for determining bromide or iodide in the presence of substantial amounts of chloride.
• It can't reliably analyze mixtures of different halide ions.
• Colored ions can interfere with endpoint detection.
• The reaction is sensitive to pH fluctuations.
• The presence of other metal ions can influence the findings and lead to inaccurate data.