☣️ Chromium VI in Water
In short
Determination of hexavalent chromium Cr(VI) by spectrophotometric method with 1,5-diphenylcarbazide. The test is performed according to PN-EN ISO 18412:2007; Measurement range: 2–50 µg/L Cr(VI). The procedure comprises 10 steps (about 1 h 30 min in total); it is used for: Drinking water quality control (Cr total ≤50 µg/L), Monitoring of wastewater from electroplating plants and tanneries, Testing of groundwater in areas of waste disposal sites.
At a glance
- Standard: PN-EN ISO 18412:2007
- Category: Physicochemistry
- Procedure steps: 10
- Total time of stages: 1 h 30 min
- Measurement range: 2–50 µg/L Cr(VI)
- Detection limit (LOD): ~2 µg/L
- Wavelength: 540 nm
Overview
Hexavalent chromium Cr(VI) is one of the most dangerous water pollutants — it is a strong oxidizer, carcinogenic (group 1 according to IARC), mutagenic and teratogenic. Sources of Cr(VI) in waters are electroplating plants, tanneries, chemical plants, power plants, and natural leaching from chromium-bearing minerals. The permissible concentration of total chromium in drinking water is 50 µg/L (according to EU Directive 2020/2184 and MH Regulation).
The method with 1,5-diphenylcarbazide (DPC) is selective for Cr(VI) — in an acidic environment (pH 1–2), chromate ions react with DPC forming a red-violet Cr(III)-diphenylcarbazone complex, whose absorbance is measured at 540 nm. The reaction is highly sensitive — the detection limit is about 2 µg/L, and the linear range 2–50 µg/L.
The PN-EN ISO 18412 standard is dedicated to lightly contaminated waters (drinking water, groundwater, surface water). For heavily contaminated waters (industrial wastewater), additional dilution or alternative methods (IC, ICP-MS) are used. It is important that the method determines only Cr(VI) — it does not react with Cr(III), which allows chromium speciation.
Samples for Cr(VI) determination require special storage conditions — they must be analyzed as soon as possible (preferably within 24 h), stored at 4°C, and the container must not contain reducing substances that could reduce Cr(VI) to Cr(III).
Method principle
In an acidic environment (pH 1–2, after adding sulfuric acid), chromate ions CrO₄²⁻ react with 1,5-diphenylcarbazide (DPC) forming a red-violet Cr(III)-diphenylcarbazone complex. The reaction proceeds in two stages: first Cr(VI) oxidizes DPC to diphenylcarbazone, simultaneously reducing to Cr(III), then Cr(III) forms a colored chelate with diphenylcarbazone. The absorbance of the complex is measured spectrophotometrically at a wavelength of 540 nm. The color intensity is proportional to the Cr(VI) concentration according to the Beer-Lambert law.
Applications
- Drinking water quality control (Cr total ≤50 µg/L)
- Monitoring of wastewater from electroplating plants and tanneries
- Testing of groundwater in areas of waste disposal sites
- Industrial wastewater quality control
- Monitoring of surface waters (Water Framework Directive)
- Chromium speciation — separate determination of Cr(VI) and Cr(III)
- Control of chromium wastewater treatment processes
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 2–50 µg/L Cr(VI) |
| Detection limit (LOD) | ~2 µg/L |
| Wavelength | 540 nm |
| Cuvette optical path | 10–50 mm |
| Repeatability | CV <5% at 20 µg/L |
| Color reaction time | 5–10 minutes |
Standard
- Standard number
- PN-EN ISO 18412:2007
- Title (PL)
- Jakość wody — Oznaczanie chromu(VI) — Metoda fotometryczna dla wód słabo zanieczyszczonych
- Title (EN)
- Water quality — Determination of chromium(VI) — Photometric method for weakly contaminated water
Step-by-step procedure
-
Preparation of Cr(VI) standard solution
Dissolve 2.829 g K₂Cr₂O₇ (dried at 102°C, 2h) in water and make up to 1 L. Solution 1000 mg/L Cr(VI). Prepare working dilutions from it.
-
Preparation of DPC solution
Dissolve 250 mg 1,5-diphenylcarbazide in 50 mL acetone. Store in dark bottle in refrigerator (4°C). Solution loses validity after color change.
-
Calibration series
Prepare standards: 0, 5, 10, 20, 30, 40, 50 µg/L Cr(VI) in 50 mL volumetric flasks from working solution.
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Acidification of samples and standards
To each sample (and standard) add 1 mL H₂SO₄ 0.2 mol/L per 50 mL solution. Mix. pH should be 1–2.
-
Addition of DPC reagent
Add 1 mL DPC solution to each sample. Mix and leave for 5 minutes for full development of red-violet color.
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Absorbance measurement
Measure absorbance at 540 nm in 10 mm cuvette (or 50 mm for low concentrations) against blank (water + reagents).
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Plotting calibration curve
Plot absorbance vs Cr(VI) concentration. Correlation coefficient R² ≥0.999. Read sample concentrations from curve.
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Measurement of test samples
Measure absorbance of acidified samples with DPC. Read concentrations from calibration curve. For concentrations >50 µg/L — dilute sample.
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Quality control
Measure control sample (standard of known concentration), duplicate and blank. Recovery of control sample: 90–110%.
-
Calculation and reporting
Calculate Cr(VI) concentration from calibration curve. Consider any dilution. Report result in µg/L with accuracy to 1 µg/L.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| UV-Vis Spectrophotometer | Hach DR6000, Shimadzu UV-1900i, JASCO V-770 | 15 000–60 000 PLN |
| Glass/quartz cuvettes | Hellma 100-QS 10 mm and 50 mm | 150–600 PLN/pc. |
| Automatic pipettes | Eppendorf Research Plus 0.5–5 mL, 100–1000 µL | 800–1 800 PLN |
| Volumetric flasks class A | DURAN / Brand 50, 100, 250 mL | 40–100 PLN/pc. |
| Water bath (optional) | For accelerating color reaction | 3 000–8 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| 1,5-Diphenylcarbazide (DPC) | 140-22-7 | 0.5% (w/v) solution in acetone; 250 mg in 50 mL acetone. Store in dark glass bottle, valid 1–2 weeks. |
| Sulfuric acid 0.2 mol/L | 7664-93-9 | H₂SO₄ for acidifying sample — ensures pH 1–2 necessary for reaction |
| Acetone analytical grade | 67-64-1 | Solvent for DPC — analytical quality |
| Potassium dichromate (standard) | 7778-50-9 | K₂Cr₂O₇ — for preparing Cr(VI) 1000 mg/L standard solution; dried at 102°C |
| Distilled water free from chromium | — | Quality at least grade 2 according to PN-EN ISO 3696, verified for Cr presence |
Health and safety (OHS)
- Potassium dichromate K₂Cr₂O₇ — carcinogenic (H350), mutagenic, toxic — absolutely use gloves, goggles and fume hood
- Cr(VI) solutions — collect as hazardous waste, do not pour into sewage
- 1,5-Diphenylcarbazide — irritating, avoid skin contact
- Acetone — flammable, work away from fire sources, under fume hood
- Sulfuric acid — corrosive, use gloves and safety goggles
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 18412:2007 — “Water quality — Determination of chromium(VI) — Photometric method for weakly contaminated water”.
How does this method work?
In an acidic environment (pH 1–2, after adding sulfuric acid), chromate ions CrO₄²⁻ react with 1,5-diphenylcarbazide (DPC) forming a red-violet Cr(III)-diphenylcarbazone complex. The reaction proceeds in two stages: first Cr(VI) oxidizes DPC to diphenylcarbazone, simultaneously reducing to Cr(III), then Cr(III) forms a colored chelate with diphenylcarbazone.
What is the measuring range and accuracy?
Measurement range: 2–50 µg/L Cr(VI); Detection limit (LOD): ~2 µg/L; Wavelength: 540 nm; Cuvette optical path: 10–50 mm.
How long does the test take?
The times given for the individual stages add up to approximately 1 h 30 min. The procedure comprises 10 steps.
What equipment is required?
UV-Vis Spectrophotometer, Glass/quartz cuvettes, Automatic pipettes, Volumetric flasks class A, Water bath (optional). Indicative cost of the core instrument (UV-Vis Spectrophotometer): 15 000–60 000 PLN.
Where is this test used?
Drinking water quality control (Cr total ≤50 µg/L); Monitoring of wastewater from electroplating plants and tanneries; Testing of groundwater in areas of waste disposal sites; Industrial wastewater quality control; Monitoring of surface waters (Water Framework Directive); Chromium speciation — separate determination of Cr(VI) and Cr(III); Control of chromium wastewater treatment processes.
What safety precautions apply?
Potassium dichromate K₂Cr₂O₇ — carcinogenic (H350), mutagenic, toxic — absolutely use gloves, goggles and fume hood; Cr(VI) solutions — collect as hazardous waste, do not pour into sewage; 1,5-Diphenylcarbazide — irritating, avoid skin contact; Acetone — flammable, work away from fire sources, under fume hood; Sulfuric acid — corrosive, use gloves and safety goggles.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN ISO 18412.