🧪 Anionic detergents in water (MBAS)
In short
Determination of anionic surfactants in water as methylene blue active substances (MBAS). The test is performed according to PN-EN ISO 7875-1:2002; Measurement range: 0.05–5 mg/L MBAS. The procedure comprises 7 steps (about 1 h 50 min in total); it is used for: Monitoring of municipal wastewater (detergent concentration), Control of drinking water (detergents as pollution indicator), Monitoring of surface waters (wastewater impact).
At a glance
- Standard: PN-EN ISO 7875-1:2002
- Category: Physicochemistry
- Procedure steps: 7
- Total time of stages: 1 h 50 min
- Measurement range: 0.05–5 mg/L MBAS
- Limit of quantification: ~0.05 mg/L
- Wavelength: 650 nm (MB complex absorption maximum)
Overview
Anionic surfactants constitute the largest group of detergents used in cleaning products, cosmetics, textile, and metallurgical industries. The most widespread anionic surfactant is sodium lauryl sulfate (SLS) and linear alkylbenzene sulfonates (LAS).
The MBAS (Methylene Blue Active Substances) determination is the standard method for assessing water contamination with anionic surfactants. The method is not specific — all compounds forming ion pairs with cationic methylene blue react, including besides detergents also some organic sulfates, sulfonates, and phosphates.
The permissible MBAS limit in drinking water is not directly specified in Polish regulations, but the indicator is routinely monitored. In wastewater discharged to waters, surfactants are subject to control. MBAS is also important in assessing detergent biodegradability (EU Regulation 648/2004).
Method principle
The method is based on forming ion pairs between anionic surfactants and the cationic dye — methylene blue (MB) — in alkaline environment, and their extraction into organic phase (chloroform).
In buffer environment (pH 10, borate buffer), anionic surfactant (R-SO₃⁻) forms with methylene blue (MB⁺) an electrically neutral ion pair:
R-SO₃⁻ + MB⁺ → [R-SO₃ · MB] (ion pair)
The ion pair dissolves in chloroform (CHCl₃) — the organic phase turns blue. Color intensity is measured spectrophotometrically at wavelength 650 nm.
Calibration is performed using sodium salt of dodecylbenzenesulfonic acid (MBAS standard). The result is expressed as mg MBAS/L, calculated for this standard.
Applications
- Monitoring of municipal wastewater (detergent concentration)
- Control of drinking water (detergents as pollution indicator)
- Monitoring of surface waters (wastewater impact)
- Control of wastewater treatment plant efficiency (LAS biodegradation)
- Studies of surfactant biodegradability (Regulation 648/2004/EC)
- Monitoring of stormwater and groundwater (detection of sewer leaks)
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 0.05–5 mg/L MBAS |
| Limit of quantification | ~0.05 mg/L |
| Wavelength | 650 nm (MB complex absorption maximum) |
| Calibration standard | Sodium salt of dodecylbenzenesulfonic acid |
| Repeatability | RSD < 5–10% |
| Interferences | Organic sulfates, proteins, organic phosphates, amines |
Standard
- Standard number
- PN-EN ISO 7875-1:2002
- Title (PL)
- Jakość wody — Oznaczanie surfaktantów — Część 1: Oznaczanie surfaktantów anionowych przez pomiar indeksu metylenobłękitowego (MBAS)
- Title (EN)
- Water quality — Determination of surfactants — Part 1: Determination of anionic surfactants by measurement of the methylene blue index (MBAS)
Step-by-step procedure
-
Sample preparation
Collect sample in glass bottle (avoid PE — adsorbs surfactants). Do not filter. Analysis as soon as possible, max 48h at 4°C.
-
Alkaline extraction
To 100 mL sample in separatory funnel add borate buffer (pH 10) and 5 mL methylene blue solution. Add 15 mL CHCl₃. Shake vigorously 30 s. Separate phases. Collect CHCl₃ phase (lower).
-
Extraction repetition
Repeat CHCl₃ extraction 2 more times with 10 mL. Combine chloroform extracts.
-
Extract purification
Transfer CHCl₃ extract to new funnel with 50 mL H₂SO₄ 0.5 mol/L and 5 mL MB. Shake. Collect CHCl₃ phase. Repeat 2×.
-
Absorbance measurement
Filter CHCl₃ extract through filter paper with anhydrous Na₂SO₄ (water removal). Measure absorbance at 650 nm in 1 cm cuvette against pure CHCl₃.
-
Calibration
Standards: 0; 0.1; 0.5; 1.0; 2.0; 5.0 mg/L MBAS. Procedure identical as for samples. Calibration curve A = f(c). R² > 0.998.
-
Calculations
Read MBAS concentration from calibration curve. Result in mg/L MBAS (as dodecylbenzenesulfonic acid). Account for dilution factor.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| UV-Vis spectrophotometer | Hach DR6000, Shimadzu UV-1900, Thermo Genesys 150 | 15 000–80 000 PLN |
| 250 mL separatory funnels | Borosilicate glass with PTFE stopcock, min. 3 pieces per sample | 150–400 PLN/pc |
| Separatory funnel stand | Metal stand for 4–6 funnels | 500–1 500 PLN |
| Spectrophotometric cuvettes | Glass cuvettes 1 cm, compatible with CHCl₃ | 100–300 PLN/pc |
| Automatic pipettes | Eppendorf, Brand — ranges 1–10 mL | 800–2 000 PLN/pc |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Methylene blue | 61-73-4 | 0.035% solution in water. Cationic dye forming ion pairs with surfactants |
| Chloroform (CHCl₃) | 67-66-3 | Analytical grade purity, solvent for ion pair extraction. ~50 mL/sample (multiple extractions) |
| Borate buffer pH 10 | — | Boric acid + NaOH to set reaction pH |
| Diluted sulfuric acid | 7664-93-9 | 0.5 mol/L H₂SO₄, for acidification in extract purification stage |
| Sodium salt of dodecylbenzenesulfonic acid | — | MBAS calibration standard, 1000 mg/L |
| Deionized water | — | For solution preparation and glassware rinsing |
Health and safety (OHS)
- Chloroform — harmful, possible carcinogen (IARC Group 2B). Work only in fume hood
- Methylene blue — stains skin and clothing. Nitrile gloves
- Sulfuric acid — corrosive. Acid-resistant gloves, goggles
- Chloroform waste — collect separately as halogenated organic waste
- Do not allow CHCl₃ contact with open flame (phosgene!)
- Lab coat, goggles, gloves mandatory. Fume hood ventilation functional
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 7875-1:2002 — “Water quality — Determination of surfactants — Part 1: Determination of anionic surfactants by measurement of the methylene blue index (MBAS)”.
How does this method work?
The method is based on forming ion pairs between anionic surfactants and the cationic dye — methylene blue (MB) — in alkaline environment, and their extraction into organic phase (chloroform). In buffer environment (pH 10, borate buffer), anionic surfactant (R-SO₃⁻) forms with methylene blue (MB⁺) an electrically neutral ion pair: R-SO₃⁻ + MB⁺ → [R-SO₃ · MB] (ion pair) The ion pair dissolves in chloroform (CHCl₃) — the organic phase turns blue.
What is the measuring range and accuracy?
Measurement range: 0.05–5 mg/L MBAS; Limit of quantification: ~0.05 mg/L; Wavelength: 650 nm (MB complex absorption maximum); Calibration standard: Sodium salt of dodecylbenzenesulfonic acid.
How long does the test take?
The times given for the individual stages add up to approximately 1 h 50 min. The procedure comprises 7 steps.
What equipment is required?
UV-Vis spectrophotometer, 250 mL separatory funnels, Separatory funnel stand, Spectrophotometric cuvettes, Automatic pipettes. Indicative cost of the core instrument (UV-Vis spectrophotometer): 15 000–80 000 PLN.
Where is this test used?
Monitoring of municipal wastewater (detergent concentration); Control of drinking water (detergents as pollution indicator); Monitoring of surface waters (wastewater impact); Control of wastewater treatment plant efficiency (LAS biodegradation); Studies of surfactant biodegradability (Regulation 648/2004/EC); Monitoring of stormwater and groundwater (detection of sewer leaks).
What safety precautions apply?
Chloroform — harmful, possible carcinogen (IARC Group 2B). Work only in fume hood; Methylene blue — stains skin and clothing. Nitrile gloves; Sulfuric acid — corrosive. Acid-resistant gloves, goggles; Chloroform waste — collect separately as halogenated organic waste; Do not allow CHCl₃ contact with open flame (phosgene!); Lab coat, goggles, gloves mandatory. Fume hood ventilation functional.
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 7875-1.