Adding Sulfuric Acid to Mohr's Salt Solution: A Comprehensive Guide

When preparing a standard solution of Mohr's salt (FeSO4middot;(NH4)2SO4middot;6H2O), it is a common practice to add concentrated sulfuric acid (H2SO4) to the solution. This addition serves two primary purposes: preventing the hydrolysis of ferrous ions and inhibiting the oxidation of these ions to ferric ions. This article delves into the reasons behind this practice and the importance of these steps in the preparation of a reliable titration solution.

1. Preventing Hydrolysis of Ferrous Ions

Mohr's salt, due to its ferrous ions (Fe2 ), is susceptible to hydrolysis in an aqueous solution. The hydrolysis of these ions can be represented by the following reaction:

Fe2 H2O ? Fe(OH)2 2H

This reaction leads to the formation of insoluble ferrous hydroxide and the release of hydrogen ions, which can alter the pH of the solution and affect the stability of the Mohr's salt. To combat this, a small amount of concentrated sulfuric acid (H2SO4) is added to the solution. The sulfuric acid dissociates into hydronium ions (H3O ) and sulfate ions (SO42-), but the primary function of the added sulfuric acid is to maintain a slightly acidic pH. Increasing the acidity of the solution shifts the equilibrium of the hydrolysis reaction to the left, thus preventing the hydrolysis of ferrous ions and ensuring the stability of the Mohr's salt solution.

2. Preventing Oxidation of Ferrous Ions to Ferric Ions

Another critical reason for adding concentrated sulfuric acid to the Mohr's salt solution is to prevent the oxidation of the ferrous ions (Fe2 ) to ferric ions (Fe3 ). This oxidation can be initiated by strong oxidizing agents such as potassium permanganate (KMnO4) or potassium dichromate (K2Cr2O7), which are commonly used in titration processes. The oxidation of ferrous ions to ferric ions can be expressed as:

2Fe2 1/2O2 2H → 2Fe3 H2O

The presence of sulfuric acid in the solution effectively reduces the pH, which helps to suppress the potential oxidation of Fe2 to Fe3 . The lower pH environment makes it less favorable for the oxidation reaction to occur, thereby maintaining the stability of the Fe2 ions and ensuring an accurate titration process.

3. Practical Considerations and Applications

The addition of concentrated sulfuric acid to Mohr's salt solutions is not only a theoretical necessity but also a practical one. In the context of analytical chemistry and chemical engineering, the reliability of Mohr's salt solutions is paramount. Any deviation from an ideal condition, such as hydrolysis or oxidation of the Fe2 ions, can lead to inaccuracies in titration results. Proper management of these ions ensures that the titration process is both accurate and reliable.

4. Conclusion

In summary, the addition of concentrated sulfuric acid to Mohr's salt solutions is essential for both hydrolysis prevention and the suppression of ferric ion formation. This practice is crucial for maintaining the stability and reliability of Mohr's salt as a titrant in various chemical analyses and experiments.

Keywords: Sulfuric Acid, Mohr's Salt, Hydrolysis Prevention