☠️ Arsenic in water
In short
Determination of arsenic (including organically bound arsenic) in drinking, ground and surface waters by the hydride AAS technique — reduction to arsenic(III), generation of arsine with sodium tetrahydroborate and measurement of the absorbance. The test is performed according to PN-EN ISO 11969:1999; Range: 1–10 µg/l; higher concentrations after dilution (clause 1). The procedure comprises 7 steps; it is used for: Water intended for human consumption — arsenic 10 µg/l (Journal of Laws 2026, item 748, Annex 1 Part B), Monitoring of surface and ground waters — the hydride technique listed for the arsenic indicator (Journal of Laws 2021, item 1576, Annex 8), Drinking, ground and surface waters in the range 1–10 µg/l, higher concentrations after dilution (clause 1).
At a glance
- Standard: PN-EN ISO 11969:1999
- Category: Physicochemistry
- Procedure steps: 7
- Range: 1–10 µg/l; higher concentrations after dilution (clause 1)
- Wavelength: 193.7 nm (clause 3, 8.4)
- Pre-reduction: 20 ml HCl and 4 ml of KI–ascorbic acid solution, 15 min at 50 °C (8.3.2)
Overview
STATUS (as of 29.10.2026). PN-EN ISO 11969:1999 — Polish version, published 24.08.1999, implements EN ISO 11969:1996 and ISO 11969:1996 [IDT]. The standard was WITHDRAWN on 01.07.2016; the PKN card does not name a replacing standard. ISO 11969:1996 and EN ISO 11969:1996 are withdrawn, and ISO 11969:1996 was replaced by ISO 17378-2:2014 "Water quality — Determination of arsenic and antimony — Part 2: Method using hydride generation atomic absorption spectrometry (HG-AAS)" — its foreword states that the first edition of ISO 17378-2 "cancels and replaces ISO 11969:1996". There is no Polish standard implementing ISO 17378-2: the PKN search for "ISO 17378" returns only PKN-CEN/TS 17378:2023-10 on intelligent transport systems (a different document). The description below presents the method according to the text of the withdrawn ISO 11969:1996; the differences in the replacing standard — see SUCCESSOR.
SCOPE (clause 1). Arsenic, including organically bound arsenic, in drinking, ground and surface waters in the range 1–10 µg/l; higher concentrations — after suitable dilution of the sample.
INTERFERENCES (clause 7). Most organic materials interfere with the determination — they are removed by digestion (8.3.1). Samples that foam when tetrahydroborate is added have to be pretreated (e.g. with an anti-foaming agent or by complete digestion); the anti-foaming agent is then also added to the blank and calibration solutions. According to the tests in Annex A (Laboratory of the Government Chemist, United Kingdom), of the substances tested, at 1.0 µg/l arsenic only copper above 2.0 mg/l, antimony above 0.2 mg/l, selenium above 0.05 mg/l and nitrate above 100 mg/l interfered; noble metals, e.g. platinum and palladium, may suppress the hydride signal.
PRECISION (clause 11). The data come from an interlaboratory trial carried out in 1982 with a method based on the same principle on drinking water samples topped up with water of known arsenic content (Annex B — outside the available text).
SUCCESSOR — ISO 17378-2:2014. It covers arsenic and antimony in drinking, surface, ground and rain water, in an approximate linear application range of 0.5–20 µg/l; sea water is generally outside its scope. The standard notes that the method is unlikely to detect organo-arsenic compounds (in ISO 11969 they are covered by the digestion). Procedure: acidification with hydrochloric acid, reduction of arsenic(V) to arsenic(III) with potassium iodide–ascorbic acid solution, generation of arsine with sodium tetrahydroborate, transfer to a heated silica cell and measurement at 193.7 nm. In flow systems Cr, Fe and Cu up to 500 mg/l, Ni up to 250 mg/l, Pb up to 100 mg/l, Sb, Se and Te up to 1 mg/l, and Sn and Hg up to 0.1 mg/l do not interfere.
LEGAL REQUIREMENTS. Water intended for human consumption: arsenic 10 µg/l (Regulation of the Minister of Health of 22 May 2026, Journal of Laws 2026, item 748, Annex 1 Part B item 4); limit of quantification not greater than 30 % of the parametric value (3 µg/l) and measurement uncertainty up to 30 % (Annex 4 Part B). Monitoring of surface and ground waters: atomic absorption spectrometry with hydride generation — the standard transposing ISO 11969 or a laboratory procedure, alongside ISO 11885, PN-EN ISO 15586 and ISO 17294-2 (Journal of Laws 2021, item 1576, Annex 8, indicator 3.6.2). Waste water: for arsenic, Annex 12 of the regulation in Journal of Laws 2019, item 1311 (item 5) lists PN-EN ISO 15586, PN-EN 26595, PN-EN ISO 11885 and PN-EN ISO 17294-2 — PN-EN ISO 11969 is not referenced there.
WHAT WE DO NOT GIVE HERE. Precision data from Annex B (outside the available text) or the procedure according to ISO 17378-2 beyond the sentences of the SUCCESSOR paragraph — the available text of that standard ends with the reagents (5.10).
Method principle
The method is based on measuring the atomic absorption of arsenic generated by thermal decomposition of arsenic(III) hydride (clause 3). Under the conditions of the method only arsenic(III) is quantitatively converted to the hydride, so other oxidation states are reduced to arsenic(III) before the determination — with potassium iodide and ascorbic acid solution, with heating (8.3.2). Arsenic(III) is reduced to gaseous arsenic(III) hydride, arsine (AsH₃), with sodium tetrahydroborate in a hydrochloric acid medium; the absorbance is measured at 193.7 nm.
Most organoarsenic compounds are decomposed by digestion with sulfuric acid and hydrogen peroxide (8.3.1); if it is known that the sample contains no organically bound arsenic, the digestion may be omitted.
Applications
- Water intended for human consumption — arsenic 10 µg/l (Journal of Laws 2026, item 748, Annex 1 Part B)
- Monitoring of surface and ground waters — the hydride technique listed for the arsenic indicator (Journal of Laws 2021, item 1576, Annex 8)
- Drinking, ground and surface waters in the range 1–10 µg/l, higher concentrations after dilution (clause 1)
Key parameters
| Parameter | Value |
|---|---|
| Range | 1–10 µg/l; higher concentrations after dilution (clause 1) |
| Wavelength | 193.7 nm (clause 3, 8.4) |
| Pre-reduction | 20 ml HCl and 4 ml of KI–ascorbic acid solution, 15 min at 50 °C (8.3.2) |
| Hydride generation | 5 ml ± 0.1 ml of sodium tetrahydroborate solution per 20 ml of solution (8.4) |
| Calibration | at least 5 solutions, e.g. 1, 3, 5, 8 and 10 µg/l (8.2) |
| Result | in µg/l, to two significant figures and one decimal place (clause 10) |
Standard
- Standard number
- PN-EN ISO 11969:1999
- Title (PL)
- Jakość wody — Oznaczanie arsenu — Metoda absorpcyjnej spektrometrii atomowej (technika wodorkowa)
- Title (EN)
- Water quality — Determination of arsenic — Atomic absorption spectrometric method (hydride technique)
Step-by-step procedure
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Sampling and preservation
According to ISO 5667-1 and ISO 5667-2, into cleaned polyethylene or borosilicate glass containers; on site add 20 ml of hydrochloric acid per 1000 ml of sample, and if the pH is still above 2 — more acid until the pH is 2 or less; storage — ISO 5667-3 (clause 6).
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Digestion — when the sample may contain organically bound arsenic
50 ml of sample in a round-bottomed flask with 5 ml of sulfuric acid and 5 ml of hydrogen peroxide (in a fume cupboard), with anti-bumping beads; heat to boiling, collecting the condensate, until sulfuric acid fumes appear. If the sample is turbid and almost colourless — add a further 5 ml of hydrogen peroxide and continue heating; when it is colourless and clear — cool and return the condensate to the flask. Never evaporate to dryness (8.3.1).
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Reduction of arsenic(V) to arsenic(III)
To the flask with the digested sample (or with 50 ml of undigested sample) add 20 ml of hydrochloric acid and 4 ml of potassium iodide–ascorbic acid solution, heat gently for 15 min at 50 °C, cool, transfer quantitatively to a 100 ml flask and make up with water (8.3.2).
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Blank and calibration solutions
Blank: 2 ml of hydrochloric acid in a 100 ml flask, made up with water. Calibration: at least five solutions from standard solution 2, e.g. 1, 3, 5, 8 and 10 ml into 100 ml flasks with 2 ml of hydrochloric acid (1, 3, 5, 8 and 10 µg/l). The blank and calibration solutions are treated exactly like the sample (8.1–8.2).
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Instrument set-up
According to the manufacturer's instructions (193.7 nm); position the absorption cell for maximum transmission of the beam; pass argon or nitrogen and set the instrument to zero. With a different hydride system larger or smaller volumes are permitted, but the quantity ratios must be maintained (8.4).
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Measurement
Order: blank, calibration solutions, samples. Transfer an appropriate volume of solution to the reaction vessel, connect it to the hydride system and pass gas until the signal returns to zero; add 5 ml ± 0.1 ml of tetrahydroborate solution per 20 ml of solution and record the signal. Repeat with separate portions and use the mean; the calibration curve is established from the means (8.4).
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Checks and calculation
It is good practice to check the blank and calibration points from time to time; with unknown samples the method should be checked by adding a known amount of arsenic, and if recovery is unsatisfactory — the standard addition method should be used. The concentration is calculated from the calibration curve, taking all dilutions into account; result in µg/l to two significant figures and one decimal place (8.4, clauses 9–10).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Atomic absorption spectrometer with a hydride system | radiation source for arsenic, e.g. an electrodeless discharge lamp or a hollow cathode lamp, with background correction if necessary (5.1) | — |
| Gas supply | argon or nitrogen (5.2) | — |
| Digestion apparatus | 250 ml round-bottomed flask with reflux condenser, condensate reservoir and PTFE stopcock (example — Figure 1); anti-bumping beads (8.3.1) | — |
| Glassware | cleaned immediately before use with warm dilute nitric acid 10 % (V/V) and rinsed with water (5.3); 100 ml and 1000 ml volumetric flasks | — |
| Sample containers | polyethylene or borosilicate glass, cleaned with nitric acid (e.g. 10 % V/V) and rinsed with water (clause 6) | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Sulfuric acid, ρ = 1.84 g/ml | 7664-93-9 | for digestion (4.1) |
| Hydrochloric acid, ρ = 1.15 g/ml | 7647-01-0 | for preserving samples, standard solutions, the blank and pre-reduction (4.2) |
| Hydrogen peroxide, w = 30 % (m/m) | 7722-84-1 | for digestion (4.3) |
| Sodium tetrahydroborate solution | — | dissolve 1 g of sodium hydroxide in about 20 ml of water, add 3 g of NaBH₄ and dilute to 100 ml; prepare on the day of use. For flow-through systems — according to the manufacturer; a solution of 0.5 % sodium tetrahydroborate and 0.5 % sodium hydroxide is suitable, stable for at least one week (4.5) |
| Potassium iodide–ascorbic acid solution | — | dissolve 3 g of KI and 5 g of L(+)-ascorbic acid in 100 ml of water; prepare on the day of use; ascorbic acid is unnecessary if a 20 % potassium iodide solution is used (4.6) |
| Arsenic stock solution, 1000 mg/l | — | dissolve 1.320 g of arsenic(III) oxide (As₂O₃) with 2 g of sodium hydroxide in a small quantity of water in a 1000 ml flask and make up; commercial solutions are available — if they contain arsenic(V), the standards are reduced in the same way as the sample (4.7) |
| Arsenic standard solutions 1 (10 mg/l) and 2 (0.1 mg/l) | — | 10 ml of the more concentrated solution and 20 ml of hydrochloric acid each to 1000 ml; solution 1 stable for about 1 month (4.8–4.9) |
| Water and reagents | — | of analytical grade, with an arsenic content negligible compared with the lowest concentration to be determined (clause 4) |
Health and safety (OHS)
- Arsenic and arsenic compounds are toxic and recognized as human carcinogens — avoid inhalation and use personal protection wherever exposure is possible (warning of the standard)
- Fumes of hot concentrated sulfuric acid are irritant — digestion is carried out in a fume cupboard (8.3.1)
- Sodium tetrahydroborate solution is not kept in a closed container (hydrogen evolves and pressure builds up) and must not come into contact with acid during disposal (ISO 17378-2, 5.7)
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 11969:1999 — “Water quality — Determination of arsenic — Atomic absorption spectrometric method (hydride technique)”.
How does this method work?
The method is based on measuring the atomic absorption of arsenic generated by thermal decomposition of arsenic(III) hydride (clause 3). Under the conditions of the method only arsenic(III) is quantitatively converted to the hydride, so other oxidation states are reduced to arsenic(III) before the determination — with potassium iodide and ascorbic acid solution, with heating (8.
What is the measuring range and accuracy?
Range: 1–10 µg/l; higher concentrations after dilution (clause 1); Wavelength: 193.7 nm (clause 3, 8.4); Pre-reduction: 20 ml HCl and 4 ml of KI–ascorbic acid solution, 15 min at 50 °C (8.3.2); Hydride generation: 5 ml ± 0.1 ml of sodium tetrahydroborate solution per 20 ml of solution (8.4).
How long does the test take?
The procedure comprises 7 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Atomic absorption spectrometer with a hydride system, Gas supply, Digestion apparatus, Glassware, Sample containers.
Where is this test used?
Water intended for human consumption — arsenic 10 µg/l (Journal of Laws 2026, item 748, Annex 1 Part B); Monitoring of surface and ground waters — the hydride technique listed for the arsenic indicator (Journal of Laws 2021, item 1576, Annex 8); Drinking, ground and surface waters in the range 1–10 µg/l, higher concentrations after dilution (clause 1).
What safety precautions apply?
Arsenic and arsenic compounds are toxic and recognized as human carcinogens — avoid inhalation and use personal protection wherever exposure is possible (warning of the standard); Fumes of hot concentrated sulfuric acid are irritant — digestion is carried out in a fume cupboard (8.3.1); Sodium tetrahydroborate solution is not kept in a closed container (hydrogen evolves and pressure builds up) and must not come into contact with acid during disposal (ISO 17378-2, 5.7).
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 11969.