Vanadium occurs in nutrient solutions primarily as the vanadate ion (VO₄³⁻) or the vanadyl ion (VO²⁺) . It is beneficial for rhizobial N2 fixation.

There are various methods for determining vanadium:

• Atomic absorption spectroscopy (AAS): High-precision method for trace analysis.
• Spectrophotometry with peroxovanadate complexes: color development with hydrogen peroxide.
• Redox titration with iron(II) sulfate: A chemical method for the quantitative determination of vanadium.

Detailed redox titration of vanadium with iron(II) sulfate

1. Principle of the method

Vanadium in the oxidation state +5 (VO₂⁺) is reduced with iron(II) ions (Fe²⁺):

The reduced vanadium(IV) can then be determined by back titration with potassium permanganate (KMnO₄).

2. Chemicals

• 0.01 mol/L iron(II) sulfate solution (FeSO₄)
• 0.01 mol/L potassium permanganate solution (KMnO₄)
• 1 mol/L sulfuric acid (H₂SO₄) as acid medium
• Diphenylamine sulfonate as a redox indicator

3. Experimental setup

Required equipment:

• Burette (25 mL, division 0.1 mL)
• Erlenmeyer flask (250 mL)
• Pipette (10 mL)
• Magnetic stirrer

4. Implementation

1. Pour 10 mL of the nutrient solution into a 250 mL Erlenmeyer flask.
2. Add 10 mL of 1 mol/L sulfuric acid (H₂SO₄).
3. Add 10 mL of 0.01 mol/L iron(II) sulfate solution.
4. Titrate with 0.01 mol/L potassium permanganate until the color changes from colorless to light pink .

5. Calculation of the vanadium concentration

The concentration of vanadium is calculated using the formula:

6. Example calculation:

• Potassium permanganate concentration: 0.01 mol/L
• Consumed volume: 9.2 mL (0.0092 L)
• Sample volume: 50 mL (0.050 L)

Conclusion

Redox titration with iron(II) sulfate and potassium permanganate is a reliable method for the quantitative determination of vanadium in nutrient solutions.