🧪 Anionic detergents in water (MBAS)
In short
Spectrometric determination of anionic surfactants as the methylene blue index (MBAS) in drinking water, surface water and waste water — methylene blue salts of the surfactants are extracted with chloroform from an alkaline solution, the extract is washed with acidic methylene blue solution and measured at 650 nm. The test is performed according to ISO 7875-1:1996; Range: 0.1–5.0 mg/l; limit of detection about 0.05 mg/l (clause 1). The procedure comprises 8 steps; it is used for: Drinking water, surface waters and waste water (clause 1), Primary biodegradation of surfactants in test systems with natural or synthetic waste water — laboratory scale and technical treatment plants (clause 1), Effluents of municipal treatment plants — the index also covers natural anionic substances (clause 1).
At a glance
- Standard: ISO 7875-1:1996
- Category: Physicochemistry
- Procedure steps: 8
- Range: 0.1–5.0 mg/l; limit of detection about 0.05 mg/l (clause 1)
- Measurement: 650 nm, chloroform phase against chloroform (clause 3); cells 10–50 mm (according to the 1984 edition)
- Result: as sodium dodecylbenzene sulfonate (clause 3)
Overview
STATUS (as of 29.10.2026). ISO 7875-1:1996 (second edition, 15.12.1996; it replaced, after technical revision, the first edition of 1984) together with the technical corrigendum ISO 7875-1:1996/Cor 1:2003 is published in the ISO catalogue, with no newer edition. The standard has not been adopted into the set of Polish Standards under this number: the PKN search for "ISO 7875" returns only PN-ISO 7875-2:2002 (non-ionic surfactants). The query "7875-1" gives "No search results". In the set of Polish Standards the MBAS method is contained in PN-EN 903:2002 "Water quality — Determination of anionic surfactants by measurement of the methylene blue index MBAS" (Polish version, published 03.12.2002, implements EN 903:1993 [IDT] and ISO 7875-1:1984 [MOD], replaced PN-C-04550-02:1985; range on the card 0.1–5.0 mg/l, limit of detection about 0.05 mg/l). PN-EN 903 is thus based on the first edition of ISO 7875-1 in a modified form — the details of the two documents are not interchangeable; the method according to PN-EN 903 has a separate description in the Knowledge Base.
SCOPE (clause 1). A spectrometric method for determining anionic surfactants by measuring the methylene blue index (MBAS) in aqueous media: in drinking water, surface water and waste water, e.g. for determining the primary degradation of surfactants under investigation in test systems with natural or synthetic waste water, at laboratory scale and in technical waste-water treatment plants. In effluents of municipal treatment plants the MBAS index comprises not only synthetic but, to a considerable extent, also natural anionic surface-active substances. Range 0.1–5.0 mg/l, limit of detection about 0.05 mg/l for solutions of standard surfactants in distilled water. Under the conditions of the method mainly sulfonates and sulfates are measured; positive and negative interferences may occur.
MBAS INDEX (introduction). Natural and synthetic anionic surface-active substances are determined as methylene blue active substances — a summary parameter called the MBAS index.
INTERFERENCES (according to the 1984 edition, clause 10). Low results are caused by cationic substances, e.g. quaternary ammonium compounds and proteins, which bind anionic surfactants — equal amounts of anionic and cationic surfactants may form stable complexes that do not react with methylene blue. High results are caused by substances other than surfactants that form chloroform-soluble compounds with methylene blue; this is minimized by stripping the surfactants into ethyl acetate (sublation). Methylene blue active may be, among others, organic sulfates, sulfonates, carboxylates, phenols and inorganic anions, e.g. cyanate, thiocyanate and sulfide; urea and ammonia as well as the preservatives formaldehyde and mercury(II) chloride gave no interference. Not all natural interferents can be eliminated, hence the name "methylene blue active substances".
LEGAL REQUIREMENTS. Waste water: the reference method for anionic surfactants is PN-EN 903 — molecular absorption spectrophotometry (Journal of Laws 2019, item 1311, Annex 12 item 58); ISO 7875-1 is not referenced there. The regulation on water intended for human consumption (Journal of Laws 2026, item 748) does not include anionic surfactants.
WHAT WE DO NOT GIVE HERE AND WHERE THE PROCEDURE DETAILS COME FROM. The available text of the 1996 edition covers clauses 1–4 (up to the beginning of 4.11 — the standard). Apparatus, sampling, sublation, determination, calibration and calculation are given according to the full text of the 1984 edition, confirmed by a description of the procedure according to PN-EN 903. In the common part the two editions differ, among other things, in the field of application and in the requirement for the blank of the neutral methylene blue solution (1984: absorbance not greater than 0.02, 1996: not greater than 0.2 per 10 mm at 650 nm) — figures marked "according to the 1984 edition" must be checked in the current text of the standard. We do not give precision data (they are not in the available text of the 1996 edition).
Method principle
In an alkaline medium methylene blue forms salts with anionic surfactants, which are extracted with chloroform; the chloroform solution is given an acid treatment. Interferences are eliminated by extracting the surfactant–methylene blue complex from alkaline solution and shaking it with acidic methylene blue solution. The absorbance of the separated organic phase is measured at the maximum absorption wavelength of 650 nm, and the result is read from a calibration curve (clause 3).
For reasons of purity and stability the preferred standard is dodecylbenzene sulfonic acid methyl ester (tetrapropylene type, relative molecular mass 340); the calibration standard is prepared from it after saponification to the sodium salt, and the MBAS index is calculated as sodium dodecylbenzene sulfonate. Other standards may be used; the conversion factors are given in Corrigendum 1:2003: sodium dodecylbenzene sulfonate 1.0000, dodecane-1-sulfonic acid sodium salt 1.2794, dodecane-1-sulfuric acid sodium salt 1.2084, dioctyl sulfosuccinic acid sodium salt 0.7839, dodecylbenzene sulfonic acid methyl ester 1.0235.
Applications
- Drinking water, surface waters and waste water (clause 1)
- Primary biodegradation of surfactants in test systems with natural or synthetic waste water — laboratory scale and technical treatment plants (clause 1)
- Effluents of municipal treatment plants — the index also covers natural anionic substances (clause 1)
- Waters of unknown composition or with interferences — with stripping of the surfactants into ethyl acetate (according to the 1984 edition, 8.1)
Key parameters
| Parameter | Value |
|---|---|
| Range | 0.1–5.0 mg/l; limit of detection about 0.05 mg/l (clause 1) |
| Measurement | 650 nm, chloroform phase against chloroform (clause 3); cells 10–50 mm (according to the 1984 edition) |
| Result | as sodium dodecylbenzene sulfonate (clause 3) |
| Test portion | 20–200 µg MBAS, up to 100 ml (according to the 1984 edition, 8.4) |
| Buffer pH 10 | 24 g of NaHCO₃ and 27 g of Na₂CO₃ per litre; for very hard water — alkaline borate solution (4.10) |
| Calibration | 0; 1.0; 2.0; 4.0; 6.0; 8.0 ml of working solution, recommended 1–2 times a month and with new batches of reagents (according to the 1984 edition, 8.3) |
Standard
- Standard number
- ISO 7875-1:1996
- 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
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Sampling and storage
According to ISO 5667-2 and ISO 5667-3; do not sample through a foam layer; clean glass bottles washed with methanol. For short periods — cooling to 4 °C; for storage longer than 24 h a preservative: 1 % (V/V) of 40 % formaldehyde solution (up to 4 days) or saturation with chloroform (up to 8 days). Suspended matter is separated by centrifugation — surfactant adsorbed on suspended matter will not be determined (according to the 1984 edition, clause 7).
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Stripping into ethyl acetate — when needed
For waters of unknown composition or with interfering substances and for concentrating small amounts: up to 1000 ml of sample in the apparatus, 100 g of sodium chloride, 100 ml of ethyl acetate; gas (nitrogen or air) 20–50 l/h for 5 min without mixing the phases; if more than 20 % of the organic phase is lost, discard the sample. Repeat with 100 ml of ethyl acetate, filter the combined phases, evaporate the ethyl acetate on a water bath in a hood, dissolve the residue in about 5 ml of methanol and 50 ml of water and make up to 100 ml (according to the 1984 edition, 8.1).
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Extraction from alkaline solution
Into a separating funnel a portion with 20–200 µg MBAS (less than 100 ml — make up with water to 100 ml), 5.0 ml of neutral methylene blue solution, 10 ml of buffer (not necessary with pre-extracted methylene blue) and 15 ml of chloroform; shake evenly and gently, about twice a second for 1 min, preferably in a horizontal plane; let the phases separate (according to the 1984 edition, 8.4).
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Washing with acidic methylene blue
After 2 min run as much as possible of the chloroform layer into a second separating funnel with 110 ml of water and 5.0 ml of acidic methylene blue solution, shake for 1 min; filter the chloroform phase through a filter wetted with chloroform into a 50 ml volumetric flask (according to the 1984 edition, 8.4).
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Repeated extraction
Repeat the extraction of the alkaline and acid solutions twice with 10 ml portions of chloroform, filtering into the same flask; make up with chloroform and mix. For each batch carry out the complete extraction for a blank of 100 ml of water and for one calibration solution (according to the 1984 edition, 8.4).
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Measurement
Absorbance at 650 nm in 10–50 mm cells against chloroform; rinse the cells with chloroform; if the absorbance in a 10 mm cell is less than 0.1 — repeat the readings in 40 or 50 mm cells. The absorbance of the blank should not exceed 0.02 per 10 mm (according to the 1984 edition, 8.2, 8.4).
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Calibration
Working solution: 25 ml of standard solution into a 500 ml flask; into separating funnels 0; 1.0; 2.0; 4.0; 6.0 and 8.0 ml, make up with water to 100 ml and proceed as with the sample; graph of absorbance against mass of surfactant (µg) for 10 and 50 mm cells (according to the 1984 edition, 8.3).
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Calculation
Concentration of anionic surfactants as the sodium salt of dodecylbenzene sulfonic acid, µg/ml = (A₀ − A₁) · f₂/V₀, where A₀ is the absorbance of the sample, A₁ of the blank, f₂ the mass (µg) giving an absorbance of 1.000, V₀ the volume of the portion in ml (according to the 1984 edition, 9.1).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Spectrometer | for measurements at 650 nm, with 10–50 mm cells (according to the 1984 edition, 6.2) | — |
| pH meter | with glass electrodes (according to the 1984 edition, 6.1) | — |
| 250 ml separating funnels and 50 ml volumetric flasks | extraction, washing of the extract, collection of the chloroform phase (according to the 1984 edition, 8.3–8.4) | — |
| Cotton or glass wool filter | wetted with chloroform, for filtering the chloroform phase; some sorption of surfactants may occur on cotton wool, glass wool may not retain all the water (according to the 1984 edition, 8.4) | — |
| Gas-stripping (sublation) apparatus | commercially available, with a sintered disc of the same diameter as the inner diameter of the cylinder; operated in an efficient fume hood (according to the 1984 edition, 6.3, 8.1) | — |
| Glassware | washed with water, then with ethanolic hydrochloric acid 10 % (m/m) and rinsed with water (according to the 1984 edition, note to clause 6) | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Chloroform | 67-66-3 | if blank results are high, purify by filtration through Al₂O₃ (neutral, W 200) (4.3) |
| Methylene blue — neutral solution | 61-73-4 | dissolve 0.350 g of methylene blue (the purest available) in water and dilute to 1000 ml; prepare at least 24 h before use, stable for at least 2 weeks; absorbance of the chloroform phase of the blank not greater than 0.2 per 10 mm at 650 nm — otherwise another batch of methylene blue or purification by extraction with buffer and chloroform (4.8) |
| Methylene blue — acidic solution | — | dissolve 0.350 g of methylene blue in 500 ml of water, add 6.50 ml of sulfuric acid (ρ = 1.84 g/ml) and after mixing dilute to 1000 ml; at least 24 h before use; blank not greater than 0.02 per 10 mm (4.9) |
| Buffer pH 10 | — | dissolve 24 g of sodium hydrogen carbonate and 27 g of anhydrous sodium carbonate in water and dilute to 1000 ml; alternatively, especially for very hard water, alkaline borate solution: equal volumes of 0.05 mol/l disodium tetraborate (19 g of the decahydrate in 1000 ml) and 0.1 mol/l sodium hydroxide (4 g in 1000 ml), stable for at least one week (4.10) |
| Dodecylbenzene sulfonic acid methyl ester — standard solution | — | weigh 400–450 mg to the nearest 0.1 mg into a round-bottomed flask, add 50 ml of 0.1 mol/l ethanolic sodium hydroxide (4 g of NaOH in 1000 ml of 95 % ethanol) and anti-bumping granules, boil for 1 h under reflux, rinse the condenser with about 30 ml of ethanol (4.11); further according to the 1984 edition: neutralize with 0.5 mol/l sulfuric acid against phenolphthalein, transfer to a 1000 ml flask and make up with water; stable for at least 6 months |
| For sublation and others | — | sodium chloride, freshly distilled ethyl acetate, freshly distilled methanol (stored in a glass bottle), ethanol 95 % (V/V), 0.5 mol/l sulfuric acid (4.1–4.6); distilled water or water of equivalent purity |
Health and safety (OHS)
- Chloroform is a suspected carcinogen; the standard notes that because of its toxicity it would be desirable to replace it with another solvent (4.3)
- Ethyl acetate is flammable and toxic (4.2); the stripping apparatus is installed in an efficient fume hood (according to the 1984 edition, 8.1)
- Handle formaldehyde, sulfuric acid and ethanolic sodium hydroxide according to the safety data sheets
Frequently asked questions
Which standard describes this test?
The test is performed according to ISO 7875-1:1996 — “Water quality — Determination of surfactants — Part 1: Determination of anionic surfactants by measurement of the methylene blue index (MBAS)”.
How does this method work?
In an alkaline medium methylene blue forms salts with anionic surfactants, which are extracted with chloroform; the chloroform solution is given an acid treatment. Interferences are eliminated by extracting the surfactant–methylene blue complex from alkaline solution and shaking it with acidic methylene blue solution.
What is the measuring range and accuracy?
Range: 0.1–5.0 mg/l; limit of detection about 0.05 mg/l (clause 1); Measurement: 650 nm, chloroform phase against chloroform (clause 3); cells 10–50 mm (according to the 1984 edition); Result: as sodium dodecylbenzene sulfonate (clause 3); Test portion: 20–200 µg MBAS, up to 100 ml (according to the 1984 edition, 8.4).
How long does the test take?
The procedure comprises 8 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Spectrometer, pH meter, 250 ml separating funnels and 50 ml volumetric flasks, Cotton or glass wool filter, Gas-stripping (sublation) apparatus, Glassware.
Where is this test used?
Drinking water, surface waters and waste water (clause 1); Primary biodegradation of surfactants in test systems with natural or synthetic waste water — laboratory scale and technical treatment plants (clause 1); Effluents of municipal treatment plants — the index also covers natural anionic substances (clause 1); Waters of unknown composition or with interferences — with stripping of the surfactants into ethyl acetate (according to the 1984 edition, 8.1).
What safety precautions apply?
Chloroform is a suspected carcinogen; the standard notes that because of its toxicity it would be desirable to replace it with another solvent (4.3); Ethyl acetate is flammable and toxic (4.2); the stripping apparatus is installed in an efficient fume hood (according to the 1984 edition, 8.1); Handle formaldehyde, sulfuric acid and ethanolic sodium hydroxide according to the safety data sheets.
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 ISO 7875-1.