Vanadium, quantitative analysis

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)

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

Vanadium analysis in hydroponic systems requires exceptionally low detection limits due to the minimal concentrations required for plant growth. Early research with Scenedesmus obliquus demonstrated vanadium essentiality at 10 μg/L, with no substitution possible by titanium, chromium, tungsten, aluminum, arsenic, cadmium, or 13 other tested elements. Vanadium deficiency in algae shows less pronounced chlorophyll decline compared to molybdenum deficiency, and increasing growth rates occur with concentrations up to 100 μg/L. For higher plants such as lettuce and tomato, adequate tissue levels may be below 2 ng/g dry weight, derived from nutrient solutions containing less than 0.04 ng/mL vanadium. Plants grown with 50 ng/mL vanadium showed tissue levels of 117–418 ng/g with comparable growth to controls. Analytical methods must achieve ng/L sensitivity, with ICP-MS being the preferred technique. Sample handling requires ultra-clean protocols to avoid contamination, as vanadium is widely distributed in laboratory reagents and dust.

Quellen: Arnon, D. I., & Wessel, G. (1953). Vanadium as an essential element for green plants. Kaplan, D. I., et al. (1990). Vanadium uptake and effects in lettuce and tomato.