Borgmann Aquaponics Hydroponics
Phosphorus is present in nutrient solutions mainly as phosphate ion (PO₄³⁻) .
There are various methods for determining phosphorus:
- Spectrophotometry with ammonium molybdate: formation of the blue phosphomolybdenum complex.
- Gravimetric precipitation with ammonium molybdate: precipitation of magnesium ammonium phosphate.
- Titration with lanthanum(III) chloride: precipitation of lanthanum phosphate.
Detailed titration of phosphorus with lanthanum(III) chloride
1. Principle of the method
Phosphate ions (PO₄³⁻) react with lanthanum(III) ions (La³⁺) to form poorly soluble lanthanum phosphate:
The end point of the titration is indicated by the sodium rhodizonate indicator .
2. Chemicals
- 0.01 mol/L lanthanum(III) chloride solution (LaCl₃)
- 1 mol/L nitric acid (HNO₃)
- Sodium rhodizonate as an indicator
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 5 mL of 1 mol/L nitric acid.
- Add 1-2 drops of sodium rhodizonate indicator.
- Titrate with 0.01 mol/L LaCl₃ until the color changes from red to colorless .
5. Calculation of phosphorus concentration
The concentration of P is calculated using the formula:
6. Example calculation:
- Lanthanum (III) chloride concentration: 0.01 mol/L
- Consumed volume: 7.2 mL (0.0072 L)
- Sample volume: 50 mL (0.050 L)
Conclusion
Titration with lanthanum(III) chloride is a reliable method for the quantitative determination of phosphorus in nutrient solutions.
In hydroponic systems, the quantitative phosphorus-analysis should be viewed in a particularly differentiated manner, since phosphoruscan be present in various inorganic and organic forms that differ in their bioavailability. Take plants phosphorusexclusively as orthophosphate ions (H₂PO₄⁻ and HPO₄²⁻), whose ratio is pH-dependent and is about 1:1 at pH 6.5. The most common analytical method is photometric determination as a molybdenum blue complex, in which ammonium molybdate with phosphate in acidic solution to molybdenumphosphorussäure reacts, which is then reduced to a blue color complex. This method primarily detects plant-available orthophosphate, but not organically bound phosphorusconnections. For complete process control in closed hydroponic systems, regular measurement of the totalphosphorussafter oxidative digestion to enrich organic phosphorusconnections to be detected early. The target concentrations for phosphorusin nutrient solutions are typically between 30 and 100 ppm/l, with values above 60 mg/l leading to precipitation with calcium, especially at high pH values. A specific problem of the phosphorus-Analytics in hydroponics is the possible falsification of the results by silicate ions, which can cause similar color reactions and therefore must be switched off by suitable masking reagents during sample preparation.
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