Mercury (Hg) does not occur naturally in plant nutrient solutions. It is a heavy metal that is highly toxic to plants, animals, and humans.

Nevertheless, mercury can enter plants from contaminated soil, water sources, or the air. It is usually absorbed in the form of Hg²⁺ ions or organic mercury compounds such as methylmercury (CH₃Hg⁺).

There are various methods for determining mercury:

• Atomic absorption spectroscopy (AAS): Highly sensitive method for determining traces of mercury.
• Cold vapor atomic absorption spectroscopy (CVAAS): A special form of AAS that is particularly suitable for mercury.
• Inductively coupled plasma mass spectrometry (ICP-MS): High-precision method for determining extremely low mercury concentrations.
• Voltammetry: Electrochemical method for the determination of Hg²⁺ in solutions.
• Spectrophotometry with dithizone: Color reaction of Hg²⁺ with dithizone results in a quantifiable color change.
• Potentiometric titration with dithizone: A rare method for Hg²⁺ detection.

Detailed titration of mercury with dithizone

1. Principle of the method

Mercury ions (Hg²⁺) react with dithizone (C₁₃H₁₂N₄S) in a complexometric titration, forming a stable, colored Hg dithizonate:

The end point of the titration is identified by a clear color change from red to violet .

2. Chemicals

• 0.01 mol/L dithizone solution (C₁₃H₁₂N₄S)
• Sulfuric acid (H₂SO₄, diluted)
• Chloroform (CHCl₃, for extraction)
• Buffer solution (pH 4-5)

3. Experimental setup

Required equipment:

• Burette (25 mL, division 0.1 mL)
• Erlenmeyer flask (250 mL)
• Pipette (10 mL)
• Magnetic stirrer
• Separatory funnel for chloroform extraction

4. Implementation

1. Pour 10 mL of the nutrient solution into a 250 mL Erlenmeyer flask.
2. Add 5 mL of buffer solution (pH 4-5).
3. Add 5 mL of dithizone solution and shake vigorously.
4. Titrate with 0.01 mol/L Hg²⁺ standard until the color changes from red to violet .
5. For better sensitivity, chloroform extraction can be performed.

5. Calculation of the mercury concentration

The concentration of Hg is calculated using the formula:

6. Example calculation:

• Dithizone concentration: 0.01 mol/L
• Consumed volume: 7.2 mL (0.0072 L)
• Sample volume: 50 mL (0.050 L)

Conclusion

Titration with dithizone is a possible method for determining mercury in solutions, but is rarely used because more sensitive techniques such as CVAAS or ICP-MS are more accurate.