What is the Difference Between Mohr Volhard and Fajans Method

The main difference between Mohr Volhard and Fajans method is that the Mohr method is based on the formation of a precipitate using a chromate ion indicator, while the Volhard method relies on the formation of a complex with thiocyanate ions using an iron(III) ion indicator, and Fajans method involves the adsorption of a colored indicator onto the surface of a precipitate.

Mohr, Volhard, and Fajans methods are fundamental techniques in analytical chemistry for the determination of halide ions. 

Key Areas Covered

1. What is Mohr Method
     – Definition, Technique, Applications
2. What is Volhard Method
     – Definition, Technique, Applications
3. What is Fajans Method
    – Definition, Technique, Applications
4. Similarities Between Mohr Volhard and Fajans Method
     – Outline of Common Features
5. Difference Between Mohr Volhard and Fajans Method
     – Comparison of Key Differences

Key Terms

Mohr Method, Volhard Method, Fajans Method

What is Mohr Method

The Mohr method is a classic volumetric titration technique used for the determination of chloride ions (Cl⁻) in a solution. The Mohr method is based on the principle that chloride ions react with silver ions to form a white precipitate of silver chloride (AgCl). The reaction is represented as follows:

Ag⁺ + Cl⁻ → AgCl

The endpoint of the titration is indicated by the formation of a slight excess of silver ions that react with the chloride ions. To detect this endpoint, a chromate ion (CrO₄²⁻) is commonly used as an indicator. The chromate ion reacts with the excess silver ions to form a red precipitate of silver chromate (Ag₂CrO₄). Moreover, the appearance of the red precipitate indicates the completion of the titration.

Titration Using Mohr Method

1. Preparation of Solutions:

• • Prepare a standardized silver nitrate (AgNO₃) solution of known concentration.
  • Prepare a potassium chromate (K₂CrO₄) indicator solution.

2. Titration:

• • Pipette a known volume of the chloride-containing solution into a titration flask.
  • Next, add a few drops of the potassium chromate indicator solution.
  • Begin titrating the chloride solution by slowly adding the standardized silver nitrate solution from a burette.
  • Now, swirl the titration flask after each addition to ensure thorough mixing.
  • As silver ions react with chloride ions, a white precipitate of silver chloride starts forming.
  • Continue the titration until a reddish-brown color appears, indicating the excess of silver ions reacting with the chromate ions to form silver chromate.
  • Note the volume of silver nitrate solution added.

3. Calculate the amount of chloride ions in the original solution using the volume of the silver nitrate solution and its known concentration.

• • Generally, one mole of silver nitrate (AgNO₃) reacts with one mole of chloride ions (Cl⁻) to form one mole of silver chloride (AgCl).

Applications of the Mohr Method

The Mohr method is widely used in analytical chemistry, particularly for determining chloride ion concentrations. Its applications include water analysis for assessing water salinity, pharmaceutical analysis for quality control in drugs, monitoring chloride in food and beverages for safety, the petrochemical industry to evaluate corrosion risks, and as a teaching tool in research and educational institutions for introducing students to chemical analysis principles.

What is Volhard Method

The Volhard method is a classical volumetric titration technique used for the determination of halide ions, particularly chloride (Cl⁻), bromide (Br⁻), and iodide (I⁻) ions, in a solution. The Volhard method is based on the reaction between an excess of silver nitrate (AgNO₃) and the halide ions, forming a precipitate of silver halide. The excess silver ions are then titrated with a standard solution of a thiocyanate ion (SCN⁻), and the formation of a reddish-brown complex of silver thiocyanate indicates the endpoint.

The Volhard method is based on the principle that halide ions react with silver ions to form a precipitate of silver halide. The reaction is represented as follows, taking chloride ions as an example:

Ag⁺ + Cl⁻ → AgCl

In the Volhard method, an excess of silver ions is added to the solution containing the halide ions. After the complete reaction between the halide ions and the excess silver ions, the unreacted silver ions are titrated with a standard solution of a thiocyanate ion. The thiocyanate ions react with the excess silver ions to form a reddish-brown complex of silver thiocyanate, indicating the endpoint of the titration.

Titration Using Volhard Method

The following steps outline the procedure for the Volhard method:

1. Preparation of Solutions:

• • Prepare a standardized silver nitrate (AgNO₃) solution of known concentration.
  • Prepare a standardized solution of a thiocyanate ion (usually potassium thiocyanate, KSCN).

2. Titration: Pipette a known volume of the sample solution containing the halide ions into a titration flask.

• • First, add a few drops of a suitable indicator solution. Common indicators include ferric ammonium sulfate or ferric chloride, which form a reddish-brown complex with thiocyanate ions.
  • Next, begin titrating the solution by adding the standardized silver nitrate solution from a burette. The silver ions react with the halide ions to form the corresponding silver halide precipitate.
  • Continue the titration until a slight excess of silver ions is present, indicating the complete reaction with the halide ions.
  • Then, add a few drops of the indicator solution, and start titrating the excess silver ions by adding the standardized thiocyanate ion solution from a second burette.
  • The formation of a reddish-brown coloration indicates the endpoint of the titration.

Note the volume of thiocyanate ion solution added.

Applications of the Volhard Method

The Volhard method is widely used in analytical chemistry for determining halide ions. Its applications include pharmaceutical analysis for drug content, environmental analysis for water quality, food industry for product safety, various chemical analyses, and as a teaching tool in research and educational institutions.

What is Fajans Method

Fajans method is a classical analytical technique developed by the Polish chemist Kazimierz Fajans. It is used for the determination of halide ions, particularly chloride (Cl⁻), bromide (Br⁻), and iodide (I⁻) ions, in a solution. Fajans method relies on the adsorption of a colored organic indicator onto the surface of a precipitate, which undergoes a color change upon adsorption. This color change indicates the endpoint of the titration.

Fajans method is based on the principle of adsorption indicator titration. In this method, a small amount of a suitable adsorption indicator is added to the solution containing the halide ions. The adsorption indicator is a colored organic compound that can be adsorbed onto the surface of a precipitate formed during titration. The adsorption of the indicator onto the surface of the precipitate alters its color, resulting in a noticeable color change. This color change indicates the endpoint of the titration.

Titration Using Fajans Method

The following steps outline the procedure for the Fajans method:

1. Preparation of Solutions:

• • Prepare a standardized solution of a silver ion source, usually silver nitrate (AgNO₃).
  • Prepare a solution of a suitable adsorption indicator that undergoes a color change upon adsorption onto the surface of the precipitate. Common indicators include dyes such as ferric indicator or xylenol orange.

2. Titration:

• • Pipette a known volume of the sample solution containing the halide ions into a titration flask.
  • Add a few drops of the adsorption indicator solution to the flask.
  • Next, begin titrating the solution by slowly adding the standardized silver ion solution from a burette. The silver ions react with the halide ions to form the corresponding silver halide precipitate.
  • As the precipitate begins to form, the adsorption indicator is adsorbed onto its surface, causing a color change.
  • Continue the titration until the noticeable color change appears, indicating the endpoint of the titration.

Note the volume of the silver ion solution added.

3. Calculation:

• • Calculate the amount of halide ions in the original sample solution using the volume and concentration of the silver ion solution.
  • The stoichiometry of the reaction between the halide ions and the silver ions is one mole of halide ions to one mole of silver ions.

Applications of the Fajans Method

Fajans’ method is helpful in analytical chemistry for determining halide ions. Its applications include water analysis for environmental monitoring, pharmaceutical analysis for drug quality control, food industry for product safety, various chemical analyses, and as a teaching tool in research and educational institutions.

Similarities Between Mohr Volhard and Fajans Method

• All three methods help in the determination of halide ions, including chloride (Cl⁻), bromide (Br⁻), and iodide (I⁻) ions.
• Furthermore, Mohr, Volhard, and Fajans methods involve precipitation reactions as part of the titration process.
• In addition, in all three methods, the endpoint of the titration is determined by observing a visible change that indicates the completion of the reaction.

Difference Between Mohr Volhard and Fajans Method

Definition

The Mohr method is based on the formation of a precipitate when a chloride ion reacts with a silver ion. Meanwhile, the Volhard method is based on the reaction between an excess of silver nitrate and the substance being determined, forming a silver complex. The Fajans method is based on the adsorption of a colored organic indicator on the surface of a precipitate.

Indicators

In Mohr’s method, a chromate ion (usually potassium chromate) acts as an indicator, reacting with silver ions to form a red precipitate of silver chromate, indicating the endpoint of the titration. Volhard’s method, on the other hand, employs an iron(III) ion (ferric ion) as an indicator, reacting with excess silver ions to form a reddish-brown complex of silver thiocyanate at the endpoint. Fajans’ method uses an adsorption indicator, such as a colored organic compound, that changes color when adsorbed onto the surface of a precipitate formed during the titration.

Procedure

The Mohr method involves titrating a chloride-containing solution with a standardized silver nitrate solution until the formation of a red precipitate of silver chromate indicates the endpoint. However, in the Volhard method, an excess of silver nitrate solution is added to the sample solution, and the excess silver ions are titrated with a standardized thiocyanate ion solution until the formation of a reddish-brown complex of silver thiocyanate indicates the endpoint. On the other hand, Fajans’ method includes the addition of a small amount of an adsorption indicator to the sample solution. The color change of the indicator upon adsorption onto the surface of the precipitate indicates the endpoint.

Conclusion

In brief, Mohr, Volhard, and Fajans methods are fundamental techniques in analytical chemistry for the determination of halide ions. Mohr method is based on the formation of a precipitate using a chromate ion indicator. Moreover, the Volhard method relies on the formation of a complex with thiocyanate ions using an iron(III) ion indicator. Meanwhile, the Fajans method involves the adsorption of a colored indicator onto the surface of a precipitate. Thus, this is the main difference between Mohr Volhard and Fajans method.

Reference:

1. Rappleye, W.C.“A Simple Application of the Volhard Principle for Blood Plasma Chlorides.” Science Direct.
2. “Fajan’s Rule.” Byju’s.

Image Courtesy:

1. “AgCl-neerslag” By Dr.T – Own work (CC BY-SA 3.0) via Commons Wikimedia