💧 Determination of non-ionic surfactants (bismuth active substances, BiAS) in waste water and waters — stripping from the salted sample with a stream of nitrogen or air into ethyl acetate, precipitation with Dragendorff reagent (KBiI4 + BaCl2 + acetic acid), dissolution of the precipitate in ammonium tartrate and determination of bismuth by potentiometric titration with sodium pyrrolidine dithiocarbamate or, according to the annex, by AAS or UV spectrometry at 263.5 nm; result in mg/l as nonylphenol with 10 ethylene oxide groups (NP 10), detection limit 0.05 mg/l, PN-ISO 7875-2

Physicochemistry PN-ISO 7875-2

Overview

WHAT THE STANDARD COVERS. Scope from the PKN catalogue card of PN-ISO 7875-2:2002 (Polish version), in full (our translation): “A description is given of a method intended for the determination of low concentrations of bismuth active substances, used for municipal waste water entering and leaving treatment plants and for other waste waters. When surface waters are examined, it may be necessary to use large sample volumes”. We read the text of the standard in the iTeh sample of ISO 7875-2:1984 (French version) — in full: from the foreword through Clauses 0–11 to the figure of the apparatus and the annex (determination of bismuth: A.1 — AAS, A.2 — UV spectrometry). The scan has distorted characters in places; we give numbers only where they can be read unambiguously.

ACCORDING TO THE TEXT OF ISO 7875-2:1984. The standard was prepared by ISO/TC 147 “Water quality”; ISO 7875 has two parts — part 1 on anionic surfactants (methylene blue) and part 2 on non-ionic ones. Scope (2): determination of low concentrations of bismuth active substances (BiAS), i.e. non-ionic surfactants of the alkylene oxide derivative type with alkylphenols and alcohols — insofar as they can be stripped and precipitated with Dragendorff reagent (e.g. ethoxylates with 5–30 ethylene oxide groups per molecule); the method is suitable for waste water entering and leaving treatment plants and other waste waters, and for surface waters may require samples of up to 5000 ml; detection limit 0.05 mg/l for a 1 l sample, optimum range of application 250–800 µg of surfactant. Principle (4): stripping of the surfactants from the sample by an air stream into ethyl acetate, removal of the ethyl acetate, precipitation with Dragendorff reagent (KBiI4 + BaCl2 + CH3COOH), isolation and dissolution of the precipitate, determination of bismuth — equivalent to the surfactant — by potentiometric titration with a sodium pyrrolidine dithiocarbamate solution; other ways of determining bismuth include AAS and spectrometry. Reagents (5): among others sodium chloride, sodium hydrogen carbonate, freshly distilled ethyl acetate and methanol, glacial acetic acid (ρ = 1.05 g/ml), solution A (basic bismuth(III) nitrate in acetic acid with potassium iodide), solution B (barium chloride dihydrate), precipitating reagent — 2 volumes of solution A and 1 volume of solution B, stable about one week in a dark glass bottle; ammonium tartrate solution (12.40 g tartaric acid and 12.40 g ammonia solution to 1000 ml); acetate buffer; sodium pyrrolidine dithiocarbamate 0.5 mmol/l (103.0 mg of the dihydrate salt, 10.0 ml amyl alcohol and 0.50 g sodium hydrogen carbonate to 1000 ml); copper(II) sulfate standard solution (1.249 g pentahydrate in 1000 ml) for standardising the titrant; bromocresol purple; cation exchanger in SO3H+ form 0.15–0.30 mm. Ethyl acetate is flammable and toxic. Apparatus (6): gas-stripping apparatus (commercially available; frit diameter equal to the tube diameter), ion exchange column 16 mm × 200 mm, recording potentiometer with platinum–calomel or platinum–silver chloride electrodes, range 250 mV, with an automatic burette of 20–25 ml; glassware washed with water, ethanolic HCl 10 % (m/m) and rinsed with water. Sampling (7): according to ISO 5667-2 and ISO 5667-3; the sample is not taken through a layer of foam; glass bottles rinsed with methanol; cooling to 4 °C for short storage; 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, but surfactant adsorbed on the suspended matter will then not be determined. Procedure (8): apparatus in a fume hood; with suspended matter above 0.3 g/l the sample is centrifuged; sample volume containing 200–1000 µg of non-ionic surfactant, 100 g sodium chloride and 5 g sodium hydrogen carbonate; 100 ml ethyl acetate, nitrogen or air flow 20–50 l/h for 5 min, without disturbing the phase boundary; if more than 20 % of the organic phase dissolves in the water, the sample is discarded; a second stripping with fresh 100 ml ethyl acetate; the ethyl acetate is evaporated on a water bath in a hood. Blank (8.2): 5 ml methanol and 40 ml water instead of the sample; titrant consumption below 1 ml, otherwise heavy metals in the reagents are checked. Precipitation (8.3): residue in 5 ml methanol, 40 ml water, 0.5 ml HCl solution and 30 ml precipitating reagent, stirring 10 min, standing at least 5 min, filtration through a sintered glass crucible (porosity 4); the precipitate is dissolved with hot (about 80 °C) ammonium tartrate solution (8.4). Standardisation of the titrant (8.5): before each use or once a day, on 10.0 ml copper sulfate standard solution. Titration (8.6): after neutralisation with ammonia against bromocresol purple and addition of 10 ml acetate buffer, at a rate of 2 ml/min; end point — intersection of the tangents of the potential curve; a flattened inflection is avoided by cleaning the platinum electrode with emery paper. Calculation (9.1): each non-ionic surfactant has its own conversion factor depending on the length of the ethylene oxide chain, so the result refers to a reference substance — NP 10; 1 ml titrant corresponds to 54 µg NP 10, factor f = 54 mg/l; result in mg NP 10 per litre of sample. Reproducibility (9.2): relative standard deviation ± 10 % at concentrations of about 0.5–1.0 mg/l. Interferences (10): anionic surfactants do not interfere up to a tenfold excess; cationic ones react with the precipitating reagent like non-ionic ones and are determined with them — if necessary they are removed on a cation exchanger (residue in about 20 ml methanol, column with 10 ml resin, washing with 50–60 ml methanol; with ethoxylates of more than 25 ethylene oxide groups — a mixture of 4 volumes methanol and 1 volume methylene chloride); gas stripping removes polyethylene glycols and most non-surface-active substances, but little is known about other interfering substances; with much suspended matter recovery is incomplete. Annex: A.1 — bismuth by AAS (standard solution of 0.500 g bismuth in 500 ml, calibration 1–5 mg Bi/l); A.2 — UV spectrometry: precipitate dissolved in ammonium tartrate with 4 ml EDTA 0.02 mol/l, 20 mm cell, absorbance at 263.5 nm against water, calibration 0, 200, 400, 600 and 1000 µg of standard surfactant precipitated like the sample; the absorbance of the bismuth–EDTA complex is constant in the pH range 2–9, and tartrate and EDTA give a blank absorbance of about 0.03–0.04.

STATUS (card as of 03.10.2026). The card of PN-ISO 7875-2:2002 (Polish version, published 03-12-2002, 11 pages, price group F) has no “Withdrawn” header: Health, Environment and Medicine Sector, KT 122 Water Quality — Chemical Testing — Organic Substances, ICS 13.060.50, “Introduces: ISO 7875-2:1984 [IDT]”, “Replaces: PN-C-04550-11:1988 — Polish version”. In the iTeh catalogue (read on 03.10.2026) ISO 7875-2:1984 has the status “Published” and stage 90.93 “International Standard confirmed” (completed 29.08.2026). Anionic surfactants are described in the Knowledge Base in the entries “Anionic detergents in water (MBAS)” (ISO 7875-1) and “Anionic surfactants in water and wastewater — the methylene blue index (MBAS): extraction with chloroform and measurement of the absorbance at 650 nm” (PN-EN 903).

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database, data up to 17.09.2026, read on 03.10.2026). The number 7875-2 appears in 5 records at 5 laboratories: AB 412, AB 418, AB 667, AB 849, AB 1188 — everywhere “PN-ISO 7875-2:2002”, at AB 849 with the note “Metoda (A2)”. In the current PCA documents (read on 03.10.2026, issues from 14.11.2025 to 26.06.2026) the number appears in 5 items of the same 5 laboratories. The “Laboratories” tab (“PN-ISO 7875-2”) shows the same 5 laboratories — checked with the tab query on the production server labcoda.pl on 03.10.2026.

WHAT THE LABORATORIES TEST (items in PCA). All five laboratories give the spectrophotometric method: AB 412 — “non-ionic detergents” in water and waste water, (0.2–10.0) mg/l; AB 418 — (0.20–50) mg/l; AB 667 — (0.20–100) mg/l; AB 849 — “non-ionic detergents”, (0.20–30) mg/l, “Metoda (A2)”; AB 1188 — (0.4–70) mg/l. The note at AB 849 points to the variant of Annex A.2 — UV spectrometry of the bismuth–EDTA complex at 263.5 nm; the main text of the standard determines bismuth by potentiometric titration, which none of these laboratories lists in its scope. AB 418 has a second item for the same determination according to its in-house procedure PB-34/08.2019, (0.10–50) mg/l. The lower limits of the ranges (0.2–0.4 mg/l) lie above the detection limit of Clause 2 (0.05 mg/l for a 1 l sample); upper limits of 10 to 100 mg/l mean, with 200–1000 µg in the portion (8.1), small sample volumes.

WHERE A SEEMINGLY CORRECT RESULT IS EASY. In sampling: the standard forbids taking the sample through a layer of foam and requires glass bottles rinsed with methanol (7); for storage longer than 24 h it requires a preservative to be considered — formaldehyde (up to 4 days) or chloroform (up to 8 days) — and a sample stored longer without one has no basis in the standard. With suspended matter: after centrifugation surfactant adsorbed on suspended matter is not determined, and with much suspended matter recovery is incomplete (7, 10) — the result refers to the dissolved part. With cationic surfactants: they precipitate like non-ionic ones and raise the result if they are not removed on a cation exchanger (10). With the result: each non-ionic surfactant has its own conversion factor, and the result is expressed as NP 10 (9.1) — the number in mg/l is not the mass of the ethoxylates actually present and cannot be compared directly with a result calculated against another standard. With the blank: titrant consumption above 1 ml indicates heavy metals in the reagents (8.2). In stripping: loss of more than 20 % of the ethyl acetate to the aqueous phase invalidates the determination (8.1).

WHAT WE DO NOT GIVE. We do not copy individual numbers that the scan does not allow to be read unambiguously (e.g. some concentrations in the recipe of solutions A and B). We do not give limit values for surfactants in waste water or water — we have not read the texts of the regulations for this entry — nor the content of the in-house procedure PB-34/08.2019.

Method principle

Sodium chloride and sodium hydrogen carbonate are added to the sample, and the non-ionic surfactants are transferred by a stream of nitrogen or air (20–50 l/h) into a layer of ethyl acetate above the sample. After evaporation of the solvent the surfactants are precipitated with Dragendorff reagent (potassium iodobismuthate with barium chloride in acetic acid), the precipitate is dissolved in hot ammonium tartrate and bismuth is determined — by potentiometric titration with sodium pyrrolidine dithiocarbamate (main text), by AAS (A.1) or by UV spectrometry of the bismuth–EDTA complex at 263.5 nm (A.2). The amount of bismuth is converted to nonylphenol with 10 ethylene oxide groups (NP 10).

Applications

• Waste water entering and leaving treatment plants and other waste waters — low concentrations of non-ionic surfactants, detection limit 0.05 mg/l for a 1 l sample (2)
• Surface waters — with samples of up to 5000 ml (2)
• Non-ionic surfactants of the alkylphenol and alcohol ethoxylate type, e.g. with 5–30 ethylene oxide groups (2)
• Water and waste water — “non-ionic detergents”, spectrophotometric method, in the accreditation scopes of 5 laboratories in PCA

Key parameters

Standard

Standard number

PN-ISO 7875-2:2002

Title (PL)

Jakość wody — Oznaczanie surfaktantów — Część 2: Oznaczanie surfaktantów niejonowych z zastosowaniem odczynnika Dragendorffa

Title (EN)

Water quality — Determination of surfactants — Part 2: Determination of non-ionic surfactants using Dragendorff reagent

Step-by-step procedure

1. Sampling and preservation

   Glass bottles rinsed with methanol, not through a foam layer; 4 °C; over 24 h — formaldehyde (up to 4 days) or chloroform (up to 8 days). PN-ISO 7875-2:2002 current (PKN card as of 03.10.2026).

2. Gas stripping

   Sample with 200–1000 µg surfactant, 100 g NaCl, 5 g NaHCO3, 100 ml ethyl acetate, nitrogen or air 20–50 l/h for 5 min; repetition with fresh ethyl acetate; evaporation of the solvent.

3. Precipitation

   Residue in 5 ml methanol and 40 ml water, 0.5 ml HCl, 30 ml precipitating reagent; 10 min stirring, at least 5 min standing; filtration through a sintered crucible.

4. Determination of bismuth

   Precipitate in hot ammonium tartrate; potentiometric titration with sodium pyrrolidine dithiocarbamate (2 ml/min) or AAS (A.1) or UV at 263.5 nm (A.2).

5. Result

   In mg/l as NP 10 (1 ml titrant = 54 µg NP 10), after subtracting the blank; cationic surfactants — removed beforehand on a cation exchanger.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Ethyl acetate is flammable and toxic — stripping and evaporation in an efficient fume hood (5.3, 8.1)
• For preserving samples the standard names formaldehyde and chloroform — both require work in a fume hood and disposal as hazardous waste (7)
• Barium chloride and bismuth compounds are toxic; methylene chloride used with the cation exchanger is volatile and harmful (5.12, 10)

Frequently asked questions