🧪 Anionic detergents in water (MBAS)

Physicochemistry ISO 7875-1

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).

DESIGNATION OF THE DOCUMENT — CORRECTED ON 11.09.2026. Earlier versions of this entry gave "PN-EN ISO 7875-1:2002" in the header. NO SUCH DOCUMENT EXISTS. In the PKN catalogue the query "ISO 7875" returns ONE item, and it is PN-ISO 7875-2:2002 (Polish version) "Water quality -- Determination of surfactants -- Part 2: Determination of non-ionic surfactants using Dragendorff reagent" — that is, PART 2, and moreover with the prefix PN-ISO, not PN-EN ISO. The query "7875-1" returns nothing (checked on 11.09.2026; the channel was controlled with a query for a number present in the catalogue, which returned a hit). Part 1 has therefore not been adopted into the PN collection under this number. WHAT A LABORATORY SHOULD USE INSTEAD. In the PN collection the MBAS method is carried by PN-EN 903:2002 (Polish version) "Water quality -- Determination of anionic surfactants by measurement of the methylene blue index MBAS"; the card has no "Date of withdrawal" or "Superseded by" field, it implements EN 903:1993 [IDT] and ISO 7875-1:1984 [MOD] and supersedes PN-C-04550-02:1985. The range given on the card: concentrations from 0.1 mg/l to 5.0 mg/l, detection limit about 0.05 mg/l. This is the same measurement as the one described below, which is why the entry stays — only the designation of the document has been changed to one that exists. A separate entry on PN-EN 903 is in the knowledge base, and that is the one to read when a Polish standard is needed for an accreditation scope. A NOTE ON THE WORD "MOD": PN-EN 903 adopts ISO 7875-1 as MODIFIED, not identical — the two documents must therefore not be treated as interchangeable as regards the details of the procedure.

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

Standard

Standard number

ISO 7875-1

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

1. Sample preparation

   Collect sample in glass bottle (avoid PE — adsorbs surfactants). Do not filter. Analysis as soon as possible, max 48h at 4°C.

   ⏱ Time: 5 min

2. 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).

   ⏱ Time: 10 min

3. Extraction repetition

   Repeat CHCl₃ extraction 2 more times with 10 mL. Combine chloroform extracts.

   ⏱ Time: 10 min

4. 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×.

   ⏱ Time: 15 min

5. 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.

   ⏱ Time: 5 min

6. 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.

   ⏱ Time: 60 min

7. Calculations

   Read MBAS concentration from calibration curve. Result in mg/L MBAS (as dodecylbenzenesulfonic acid). Account for dilution factor.

   ⏱ Time: 5 min

Required equipment and apparatus

Reagents, media and consumables

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