Difference Between Mohr Volhard and Fajans Method

The key difference between Mohr Volhard and Fajans method is that Mohr method refers to the reaction between silver ion and halide ion in the presence of chromate indicator, but Volhard method refers to the reaction between excess silver ions and halide ions. Meanwhile, Fajans method refers to the adsorption reaction between silver halide and fluorescein.

Mohr method, Volhard method and Fajans method are important analytical techniques that can be used as precipitation reactions to determine the halide concentration in a given sample. These methods are named after the scientists who developed the method.

CONTENTS

1. Overview and Key Difference
2. What is Mohr Method
3. What is Volhard Method
4. What is Fajans Method
5. Side by Side Comparison – Mohr Volhard vs Fajans Method in Tabular Form
6. Summary

What is Mohr Method?

Mohr method is an analytical technique in which we can determine the halide concentration through direct titration. The method uses silver nitrate and the sample containing halide ions. Usually, this method determines the amount of chloride ions. Here, we use an indicator to detect the endpoint of the titration; potassium chromate is the indicator.

In the Mohr method, we have to add silver nitrate from the burette to the sample. The indicator is also added to the sample before starting the titration. Then the chloride ions in the sample react with the added silver cations, forming silver chloride precipitate. When all the chloride ions are precipitated, adding one more drop of silver nitrate will change the colour of potassium chromate indicator, indicating the endpoint of the titration. The colour change is due to the formation of silver chromate red precipitate. But, this red precipitate does not form at the beginning because the solubility of silver chloride is very low compared to the solubility of silver chromate.

Moreover, this method requires a neutral medium; if we use an alkaline solution, then the silver ions react with hydroxide ions before forming the silver chloride precipitate. Also, we cannot use acidic media because chromate ions here tend to convert into dichromate ions. Therefore, we have to keep the solution pH at about 7. Besides, since it is a direct titration method, there will also be an error in detecting the endpoint. For example, to get an intense colour, we have to use more indicators. Then the amount of silver ions required for the precipitation of these chromate ions is high. Thus, this gives a slightly large value than the actual value.

What is Volhard Method?

Volhard method is an analytical technique in which we can determine the halide concentration through a back titration. In this method, we can first titrate the chloride solution with silver ions by adding an excess amount of silver, followed by the determination of the excess silver ion content in the sample. In this experiment, the indicator is a solution containing ferric ion, which can give a red colour with thiocyanate ions. The excess amount of silver ions is titrated using a thiocyanate ion solution. Here, the thiocyanate tends to react with silver ions rather than with ferric ions. However, after all the silver ions are used, the thiocyanate will react with ferric ions.

In this experiment, the indicator system is very sensitive, and it usually gives better results. However, we have to keep the solution acidic because the ferric ions tend to form ferric hydroxide in the presence of the basic medium.

What is Fajans Method

Fajans method is an analytical technique in which we can determine the halide concentration through adsorption. In this method, the fluorescein and its derivatives are adsorbed on to the surface of colloidal silver chloride. After these adsorbed ions occupy all the chloride ions, the addition of another drop of fluorescein reacts with the silver ions, forming a red colour precipitate.

What is the Difference Between Mohr Volhard and Fajans Method?

Mohr method, Volhard method and Fajans method are important analytical techniques that can be used as precipitation reactions to determine the halide concentration in a given sample. The key difference between Mohr Volhard and Fajans method is that Mohr method refers to the reaction between silver ion and halide ion in the presence of chromate indicator, but Volhard method refers to the reaction between excess silver ions and halide ions. Whereas, Fajans method refers to the adsorption reaction between silver halide and fluorescein.

Below infographic summarizes the difference between Mohr Volhard and Fajans method.

Summary – Mohr Volhard vs Fajans Method

Mohr method, Volhard method, and Fajans method are important analytical techniques that can be used as precipitation reactions to determine the halide concentration in a given sample. Mohr method is the reaction between silver ion and halide ion in the presence of chromate indicator, while Volhard method refers to the reaction between excess silver ions and halide ions. Fajans method, on the other hand, refers to the adsorption reaction between silver halide and fluorescein. So, this is the key difference between Mohr Volhard and Fajans methods.

Reference:

1. Rahman, Ashikur. “Precipitation Titration.” LinkedIn SlideShare, 15 Oct. 2014, Available here.
2. Jahid, Mehedi Hassan. “Mohr Method.” LinkedIn SlideShare, 10 Dec. 2014, Available here.
3. “Argentometry.” Wikipedia, Wikimedia Foundation, 4 July 2019, Available here.

Image Courtesy:

1. “Common Silver Halide Precipitates” By Cychr – (CC BY 3.0) via Commons Wikimedia
2. “Argentometry Titration mohr method” By Anhella – Own work (CC BY-SA 3.0) via Commons Wikimedia