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

1. Pour 10 mL of the nutrient solution into a 250 mL Erlenmeyer flask.
2. Add 5 mL of 1 mol/L nitric acid.
3. Add 1-2 drops of sodium rhodizonate indicator.
4. 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)