To prepare a standard iron solution, dissolve ferrous ammonium sulfate hexahydrate in purified water with sulfuric acid. Here's a detailed breakdown of the process:
Preparing a Standard Iron Solution
This method describes how to create a stock iron solution. From this stock, you can dilute to desired concentrations.
Materials Needed:
- Ferrous ammonium sulfate hexahydrate [Fe(NH4)2(SO4)2•6H2O] (reagent grade) - 0.7022 g
- Purified water - 100 mL
- Sulfuric acid solution
- Beaker
- Graduated cylinder
- Hot plate or water bath
- Stirring rod
- Volumetric flask
Step-by-Step Instructions:
- Weigh the Ferrous Ammonium Sulfate: Accurately weigh 0.7022 g of reagent grade ferrous ammonium sulfate hexahydrate.
- Dissolve in Water: Transfer the weighed ferrous ammonium sulfate hexahydrate to a 100 mL beaker. Add 100 mL of purified water.
- Add Sulfuric Acid: Add 10 mL of sulfuric acid solution to the beaker. The sulfuric acid helps to prevent the oxidation of ferrous iron (Fe2+) to ferric iron (Fe3+).
- Heat the Solution: Heat the solution to 60-65 °C. You can use a hot plate or a water bath for this purpose. Gentle heating helps to dissolve the salt completely. Do not boil.
- Stir the Solution: Use a stirring rod to stir the solution until the ferrous ammonium sulfate hexahydrate is completely dissolved. The solution should be clear and free of any undissolved particles.
- Cool and Transfer: Allow the solution to cool to room temperature.
- Transfer to Volumetric Flask (Optional): Quantitatively transfer the solution to a volumetric flask of appropriate size (e.g. 100 ml or 250 ml) and dilute to the mark with purified water. This ensures an accurate concentration.
- Storage: Store the iron solution in a tightly sealed container, preferably in a dark and cool place, to minimize oxidation.
Important Notes:
- Water Purity: Use purified water to avoid introducing contaminants that may interfere with the iron solution.
- Temperature Control: Maintaining the temperature within the specified range (60-65 °C) is crucial for efficient dissolution without causing decomposition.
- Sulfuric Acid: The addition of sulfuric acid is essential for stabilizing the iron solution and preventing oxidation.
- Concentration: This procedure creates a stock solution. Dilute this stock solution as needed to create solutions of lower concentrations.
- Safety Precautions: Wear appropriate personal protective equipment (PPE), such as gloves and eye protection, when handling chemicals, especially sulfuric acid.
Dilution Calculations
To prepare solutions with lower concentrations, use the formula: C1V1 = C2V2, where:
- C1 = Concentration of the stock solution
- V1 = Volume of the stock solution needed
- C2 = Desired concentration of the diluted solution
- V2 = Desired final volume of the diluted solution
Example: If you have a stock solution and need 100 mL of a 10 ppm solution, use the formula to calculate the volume of stock solution needed. Convert units to be consistent (e.g., ppm to mg/L or vice versa).
