Lead (Pb) is a toxic heavy metal that can enter drinking water through water pipes, old pipes, or industrial pollution. It has harmful effects on the nervous system, especially in children.

Legal limits for lead in drinking water

• EU limit (Drinking Water Regulation): 10 µg/L (0.01 mg/L), from 2028 the limit is 5 µg/L (0.005 mg/L)
• WHO guideline value: 10 µg/L
• EPA (USA) Action Limit: 15 µg/L

• Current limit value (since 2013): 10 µg/L (0.01 mg/L) – Source
• Planned limit value (from 12 January 2028): 5 µg/L (0.005 mg/L) – Source

Detection limits of qualitative detection methods

1. Potassium iodide (KI) test

• Principle: Formation of a yellow precipitate of lead iodide (PbI₂) upon reaction of Pb²⁺ with iodide ions.
• Detection limit: Typically in the range of mg/L (e.g. 1 mg/L or 1,000 µg/L).
• Assessment: Due to the high detection limit, this test is not sensitive enough to detect lead in drinking water at the legally permitted concentrations.

2. Sulfuric acid (H₂SO₄) test

• Principle: Formation of a white precipitate of lead sulfate (PbSO₄) when sulfuric acid is added to a solution containing Pb²⁺.
• Detection limit: Similar to the KI test in the range of mg/L.
• Assessment: Not sensitive enough to detect lead in drinking water below legal limits.

3. Dithizone test

• Principle: Formation of a colored complex between dithizone and Pb²⁺, which shows an intense coloration.
• Detection limit: Approximately 40 ng (0.00004 mg) in the sample – Source
• Rating: High sensitivity, suitable for the qualitative detection of lead in drinking water.

Most conventional qualitative detection methods, such as the potassium iodide or sulfuric acid test, are unsuitable for detecting lead in drinking water due to their high detection limits. The dithizone test, on the other hand, is sufficiently sensitive to detect lead in drinking water. However, precise quantitative determination requires instrumental methods such as atomic absorption spectrometry (AAS) or inductively coupled plasma mass spectrometry (ICP-MS).

Depending on your motivation, the following methods may be sufficient.

Qualitative detection reactions for lead

Since lead can occur in very low concentrations in drinking water, sensitive detection methods are necessary. Here are some qualitative methods:

1. Detection with potassium iodide (KI test)

Principle: Lead ions (Pb²⁺) react with potassium iodide (KI) to form lead iodide (PbI₂), which precipitates as a yellow precipitate.

Reaction equation:

Pb²⁺+2I⁻→PbI2↓

Procedure: Add a few drops of potassium iodide solution to the sample. A yellow color or a yellow precipitate indicates the presence of lead.

2. Detection with sulfuric acid (PbSO₄ precipitation)

Principle: Lead ions react with sulfuric acid (H₂SO₄) or sulfates to form lead sulfate (PbSO₄), which precipitates as a white precipitate.

Reaction equation:

Pb²⁺+SO₄²⁻→PbSO₄↓

Procedure: Add a few drops of sulfuric acid to the sample. A white precipitate indicates lead.

3. Detection with Dithizone (sensitive for drinking water)

Principle: Dithizone forms an intensely red chelate complex with lead, which is visible even at very low concentrations (below 10 µg/L).

Procedure: Mix a dithizone solution with the water sample. A reddish color indicates the presence of lead.

More sensitive methods for trace analysis

For a more precise determination of lead in drinking water, instrumental methods are used:

• Atomic absorption spectroscopy (AAS) – Very precise method for traces of lead.
• ICP-MS (Inductively Coupled Plasma Mass Spectrometry) – Extremely sensitive, can detect lead in the ng/L range.
• Anodic stripping voltammetry (ASV) – electrochemical method for trace analysis.

Conclusion

Qualitative analysis of lead can be performed using precipitation reactions or color reactions. For drinking water, which often contains very low levels of lead (<10 µg/L), sensitive methods such as the Dithizone test or instrumental analyses such as AAS and ICP-MS are recommended.

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