Borgmann Aquaponics Hydroponics
Molybdenum occurs in nutrient solutions mainly as molybdate ion (MoO₄²⁻) .
There are various methods for determining molybdenum:
- Atomic absorption spectroscopy (AAS): High-precision determination of Mo.
- Thiocyanate spectrophotometry: formation of a red Mo-thiocyanate complex.
- Redox titration with iron(II) sulfate: reduction of molybdenum(VI) to molybdenum(III).
Detailed titration of molybdenum with iron(II) sulfate
1. Principle of the method
Molybdate ions (MoO₄²⁻) are reduced in acidic solution with iron(II) sulfate (Fe²⁺):
The end point of the titration is identified by the color change from blue to colorless .
2. Chemicals
- 0.01 mol/L iron(II) sulfate solution (FeSO₄)
- 1 mol/L sulfuric acid (H₂SO₄)
- Distilled water
3. Experimental setup
Required equipment:
- Burette (25 mL, division 0.1 mL)
- Erlenmeyer flask (250 mL)
- Pipette (10 mL)
- Magnetic stirrer
4. Implementation
- Pour 10 mL of the nutrient solution into a 250 mL Erlenmeyer flask.
- Add 10 mL of 1 mol/L sulfuric acid.
- Titrate with 0.01 mol/L FeSO₄ until the blue color disappears.
5. Calculation of the molybdenum concentration
The concentration of Mo is calculated using the formula:
6. Example calculation:
- Iron(II) sulfate concentration: 0.01 mol/L
- Consumed volume: 6.8 mL (0.0068 L)
- Sample volume: 50 mL (0.050 L)
Conclusion
Redox titration with iron(II) sulfate is a very reliable method for the quantitative determination of molybdenum in nutrient solutions.
The quantitative analysis of molybdenumplaces special demands on measurement accuracy due to the extremely low concentrations required. As an essential trace element, molybdenumtypically required in hydroponic nutrient solutions at concentrations of 0.01 to 0.05 mg/l, since even in the smallest amounts it acts as a cofactor for the nitrate reductase enzyme and thus controls the nitrogen utilization of the plant. Highly sensitive methods such as graphite furnace AAS or ICP-MS have been established for the reliable detection of these low concentrations, reaching detection limits in the ng/l range. In hydroponic practice, photometric methods are often used to determine the formation of an orange molybdenumcomplex with thiocyanate in acidic environment. It should be noted that molybdenumhas a high mobility in the pH range of 5.5 to 6.5 compared to other trace elements and rarely causes precipitation problems. The analytical results are to be evaluated particularly critically if the nutrient solution contains elevated sulfate concentrations, since sulfate is the molybdenumcan inhibit uptake via antagonistic mechanisms without the measured molybdenum-Concentration itself is changed - Despite sufficient nutrient solution concentration, deficiency often occurs at low pH values below 5.0, where the absorption capacity of the roots is limited.
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