1. To Prevent Hydrolysis and Oxidation:
* Hydrolysis: Mohr salt, which is ferrous ammonium sulfate hexahydrate [(NH₄)₂Fe(SO₄)₂·6H₂O], can undergo hydrolysis in water, leading to the formation of iron hydroxide (Fe(OH)₂) and a decrease in the concentration of ferrous ions (Fe²⁺). This can happen because the Fe²⁺ ion can react with water molecules to form Fe(OH)₂ and hydrogen ions (H⁺), which can then shift the equilibrium towards hydrolysis.
* Oxidation: The ferrous ions (Fe²⁺) in Mohr salt are susceptible to oxidation in the presence of air, leading to the formation of ferric ions (Fe³⁺). This oxidation can occur at a faster rate in neutral or alkaline solutions.
Adding dilute sulfuric acid helps to prevent both hydrolysis and oxidation by:
* Shifting the equilibrium: The H⁺ ions from the acid react with the hydroxide ions (OH⁻) produced by hydrolysis, thus pushing the equilibrium back towards the formation of Fe²⁺ ions.
* Providing an acidic environment: The acidic environment suppresses the oxidation of Fe²⁺ ions.
2. To Enhance Stability and Shelf Life:
The addition of dilute sulfuric acid creates a more stable solution by:
* Minimizing hydrolysis: As mentioned above, the acidic environment reduces the rate of hydrolysis, thus maintaining the concentration of Fe²⁺ ions for a longer period.
* Preventing precipitation: The acid helps to prevent the precipitation of iron hydroxide, which can occur in neutral or alkaline solutions.
In summary, the addition of dilute sulfuric acid is crucial for preparing a stable and accurate standard Mohr salt solution. It prevents unwanted reactions that can alter the concentration of ferrous ions and ensure the solution's stability over time.
