Silicon occurs in nutrient solutions primarily as silicic acid (H₄SiO₄) and silicate ions (SiO₄⁴⁻) . It is present in most plants and abundant in cereals, grasses, and tree bark. SiO ²⁻ ₃ is thought   to enhance plant disease resistance.

There are various methods for determining silicon:

• Molybdenum blue spectrophotometry: formation of a molybdenum silicate complex with color change.
• Gravimetric determination: precipitation of silicon dioxide (SiO₂) and weighing.
• Complexometric titration with fluoride: formation of a SiF₆²⁻ complex, back titration with lanthanum.

Detailed titration of silicon with fluoride

1. Principle of the method

Silicon reacts with fluoride ions (F⁻) to form a stable hexafluorosilicate complex:

The excess fluoride ions are back-titrated with lanthanum(III) chloride (LaCl₃) . The endpoint is determined using alizarin complexone as an indicator. Color change: pink to violet .

2. Chemicals

• 0.01 mol/L sodium fluoride (NaF) solution
• 0.01 mol/L lanthanum (III) chloride (LaCl₃) solution
• Buffer solution (pH 3, acetic acid/sodium acetate buffer)
• Alizarin complexone (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 0.01 mol/L NaF solution and let it react for 5 minutes.
3. Add 5 mL of buffer solution (pH 3).
4. Add 2-3 drops of alizarin complexone indicator.
5. Titrate with 0.01 mol/L LaCl₃ until the color changes from pink to violet .

5. Calculation of the silicon concentration

The concentration of Si is calculated using the formula:

6. Example calculation:

• Lanthanum(III) chloride concentration: 0.01 mol/L
• Consumed volume: 9.2 mL (0.0092 L)
• Sample volume: 50 mL (0.050 L)

Conclusion

Back titration with lanthanum is a precise method for the quantitative determination of silicon in nutrient solutions.