Mercury (Hg) is a highly toxic heavy metal that is normally not found in drinking water. However, it can get into the environment through industrial emissions, mining activities or fossil fuels. Due to its toxicity and enrichment in organisms, the detection of mercury in drinking water is of great importance.

Limit values for mercury in drinking water

• EU limit: approx. 1 µg / L (0.001 mg / L) for inorganic mercury
• WHO guideline: approx. 6 µg / L (0.006 mg / L) (depending on the shape of the mercury)

Alternative analytical methods for mercury

• Cold vapor atomic absorption spectrometry (CVAAS): Very sensitive, suitable for trace analysis.
• Inductively coupled plasma mass spectrometry (ICP-MS): Extremely sensitive, detection in the ng / L range.
• Anodic Stripping Voltammetrie (ASV): Electrochemical method with high sensitivity.
• Spectrophotometry with dithizone: Color reaction that enables qualitative proof.

Qualitative detection reaction using dithizone

The method can be used with Dithizone for a qualitative analysis of mercury in drinking water. Dithizon reacts with Hg ²⁺ to form an intense red colored chelate complex.

1. Principle of method

In the reaction, Dithizone is converted to a stable Hg-Dithizone complex with Hg ²⁺:

Hg²⁺+Dithizon→[Hg (Dithizon)]

The resulting complex shows an intense red-violet color that is visible even at very low concentrations.

2. Chemicals

• 0,01 mol/L Dithizon-Lösung (C₁₃H₁₂N₄S)
• Diluted sulfuric acid (H ₂ SO ₄) – to adjust a suitable acidic environment
• Chloroform (CHCl ₃) – optional, for extracting the colored complex to improve visibility
• Pufferlösung (from. B. Essigsäure / Natriumacetate, pH 4 – 5)

3. Experimental setup

Required devices:

• Bürette (25 mL, division 0.1 mL)
• Erlenmeyer flask (250 mL)
• Pipettes (10 mL and 50 mL)
• Magnetic stirrer
• Separating funnel (for chloroform extraction, if used)

4. Implementation

1. Transfer 10 mL of the drinking water sample into a 250 mL Erlenmeyer flask.
2. Add 5 mL of the buffer solution (pH 4 – 5) to adjust the pH.
3. Add 5 mL to the 0.01 mol / L Dithizone solution and mix the sample thoroughly.
4. Optional: perform a chloroform extraction in a separating funnel to isolate the colored complex.
5. Watch the color change. An intense red-violet hue indicates the presence of Hg ²⁺ ions.

Conclusion

The Dithizone test is a qualitative method that, due to its high sensitivity, is suitable for the detection of mercury in drinking water. It can already make Hg ²⁺ concentrations in the µg / L range visible, which is sufficient compared to the legal limit values (approx. 1 – 6 µg / L). However, instrumental methods such as CVAAS, ICP-MS or ASV should be used for a precise quantitative determination.

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