Sulfur (S) is usually present in nutrient solutions as the sulfate ion (SO₄²⁻) . Various methods are available for quantitative analysis:
- Gravimetry: Precipitation as barium sulfate (BaSO₄) and weighing.
- Turbidimetry: Photometric determination of the turbidity of BaSO₄.
- Ion chromatography (IC): separation and detection of sulfate ions.
- ICP-OES / ICP-MS: High-precision determination of sulfur concentration.
- Titration: Determination by complex formation with thorium(IV).
Titration of sulfate with thorium(IV)
Principle:
Sulfate ions ( SO₄²⁻ ) react with thorium(IV) ions ( Th⁴⁺ ) to form an insoluble thorium sulfate complex:
Excess thorium(IV) binds the indicator thoron, resulting in a color change from red to yellow .
Implementation:
Reagents:
- Thorium(IV) nitrate solution (0.01 M, Th(NO₃)₄)
- Thoron indicator (color change from red to yellow)
- Ethanol/water mixture as medium
- Diluted hydrochloric acid (HCl) for pH adjustment (pH 1-2)
- Sample solution with sulfate ions
Titration steps:
- Pour 50 mL of the nutrient solution into an Erlenmeyer flask.
- If necessary, adjust pH to 1-2 with HCl.
- Add 1-2 drops of thoron indicator (solution turns red).
- Slowly add 0.01 M thorium(IV) solution while stirring.
- The end point is reached when the color changes from red to yellow.
Calculation of sulfate concentration
Since sulfate reacts with thorium(IV) in a 1:1 molar ratio , the following applies:
with:
- V Th = consumed thorium(IV) solution (L)
- c Th = concentration of the thorium(IV) solution (mol/L)
- M SO₄²⁻ = 96.06 g/mol (molar mass of sulfate)
- V sample = volume of sample solution (L)
Example calculation:
- Thorium(IV) solution: 0.01 mol/L
- Consumed volume: 5.5 mL (0.0055 L)
- Sample volume: 50 mL (0.050 L)
Comparison of methods
| method | sensitivity | Equipment costs | Disturbances |
|---|---|---|---|
| Thorium(IV) titration | Medium (from 5 mg/L) | Low (burette, indicator) | Phosphate interferes |
| Iodometric titration | High (< 1 mg/L) | Complex (reduction necessary) | Oxidizing agents interfere |
| Gravimetric BaSO₄ precipitation | Medium (from 10 mg/L) | Scales, drying cabinet | Time-consuming |
| Ion chromatography (IC) | Very high (< 0.1 mg/L) | Expensive devices | High-precision |
Conclusion
Thorium (IV) titration is a precise and simple method for the quantitative determination of sulfate in nutrient solutions with medium to high concentrations (5-500 mg/L). Alternatively, methods such as ICP-OES, IC, or gravimetry are suitable for higher precision.
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