Why H2SO4 Preferably Replaces HCl in Mohr Salt Titrations

Mohr's salt, a double salt (FeSO?.NH??SO?.6H?O), has long been a cornerstone in chemical analysis for its ability to form complexes that aid in quantitative determinations. However, when it comes to the choice of titrant, opting for H?SO? over HCl is more effective and justifiable for certain reasons.

The Use of HCl in Titration

When considering the traditional titration procedures using HCl, it is often questioned why this acid is not the default choice. The primary concern lies in the stability and reactivity of FeSO? under acidic conditions with HCl.

Issues with Using HCl

Fe Oxidation: Conventional sulfuric acid (H?SO?) has a higher pKa value (1.99), making it a weaker acid compared to hydrochloric acid (HCl). This reduces the risk of Fe ions being oxidized to Fe3? in the presence of strong oxidizing agents such as KMnO? or K?CrO?. The stronger oxidizing properties of HCl with its lower pKa (approximately -7.0) could cause unwanted oxidation of metallic Fe to Fe?? or Fe3?, complicating the titration process. Cl? Interference: Although HCl is a common titrant, the chloride ions (Cl?) in HCl may react with KMnO? or K?CrO?, a strong oxidizing agent in many titrimetric procedures. This could lead to incorrect or unpredictable results as Cl? ions could be oxidized, potentially causing a halogen effect that may affect the endpoint detection in the titration. Sulfuric acid, on the other hand, does not contain chloride ions, thus avoiding this interference.

The Benefits of Using H?SO?

Switching to H?SO? as the titrant in Mohr salt titrations offers a number of advantages over HCl:

Safety and Stability

First and foremost, H?SO? is less likely to cause unwanted side reactions such as hydrolysis and oxidation. FeSO? tends to hydrolyze in the presence of excess acidic conditions, particularly with HCl, where an acid-base reaction may produce Fe(OH)? and NH?OH. This can be detrimental to the titration process, as it can form precipitates or alter the solution's pH, complicating the endpoint. However, when using H?SO?, these hydrolysis reactions are significantly suppressed, leading to a more stable and predictable environment.

Elimination of Chloride Interference

In many analytical procedures, KMnO? or K?CrO? are utilized as the oxidizing reagents. The presence of chloride ions in HCl can react with these reagents, leading to a false indication of titration, as chloride ions may be oxidized. Sulfuric acid, not containing chloride ions, ensures that there is no such interference, providing more accurate results.

Improved Endpoint Detection

The most critical advantage of using H?SO? is its ability to provide a clearer and more reliable endpoint. The interaction between Mohr's salt and the oxidizing agents in the presence of H?SO? is more controlled compared to that with HCl. This results in a more precise and consistent titration, reducing the likelihood of errors and improving the overall accuracy of the analysis.

Conclusion

Given the considerations of oxidation, chloridometric interference, and stability, H?SO? emerges as the preferred titrant for Mohr salt titrations. Its ability to suppress unwanted side reactions while maintaining a stable environment significantly enhances the accuracy and reliability of the analysis. By choosing H?SO?, laboratories and analytical chemists can conduct titrations with greater confidence and precision.