Aluminum occurs in nutrient solutions primarily as the aluminum ion (Al³⁺) or as hydroxo complexes. Essential for some plants (e.g., peas, corn, sunflowers, and cereals). Can be toxic to some plants at concentrations above 10 ppm. Sometimes used to produce flower pigments (e.g., hydrangeas). Variable micronutrient.

There are different methods for determining aluminum:

• Complexometric titration with EDTA: Formation of a stable Al-EDTA complex.
• Spectrophotometry with eriochrome cyanine R: color development by complex formation.
• Atomic absorption spectroscopy (AAS): High-precision determination of aluminum.

Detailed titration of aluminum with EDTA

1. Principle of the method

Aluminum ions (Al³⁺) react with ethylenediaminetetraacetic acid (EDTA, C₁₀H₁₆N₂O₈) to form a stable chelate complex:

The endpoint of the titration is detected using the xylenol orange indicator. The color changes from yellow to red .

2. Chemicals

• 0.01 mol/L EDTA solution (C₁₀H₁₆N₂O₈)
• Buffer solution (pH 5, acetate buffer)
• Xylenol orange (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 acetate buffer solution (pH 5).
3. Add 2-3 drops of xylenol orange indicator.
4. Titrate with 0.01 mol/L EDTA until the color changes from yellow to red.

5. Calculation of the aluminum concentration

The concentration of Al³⁺ is calculated using the formula:

6. Example calculation:

• EDTA concentration: 0.01 mol/L
• Consumed volume: 7.8 mL (0.0078 L)
• Sample volume: 50 mL (0.050 L)

Conclusion

Complexometric titration with EDTA is a reliable method for the quantitative determination of aluminum in nutrient solutions.

Other names for  xylenol orange:

• • C₃₁H₃₂N₂O₁₃S
  • C₃₁H₂₈N₂Na₄O₁₃S (Tetranatriumsalz)
  • 3,3-Bis(N,N-bis(carboxymethyl)amino­methyl)kresolsulfonphthalein