⚗️ Mercury in water — digestion with bromine/bromine chloride, reduction with tin(II) chloride and cold-vapour atomic fluorescence spectrometry (CV-AFS)
In short
The sample (15 ml of HCl of 120 g/kg and 2 ml of the bromide/bromate reagent per 100 ml) is digested with chemically generated bromine and BrCl, the excess bromine is removed with ascorbic acid, and the mercury(II) is reduced with tin(II) chloride to mercury vapour, which is purged with argon through a membrane dryer into an atomic fluorescence spectrometer. The test is performed according to PN-EN ISO 17852:2009 (ISO 17852:2006); Range: linearity about 1 ng/l–100 µg/l; practical working range 10 ng/l–10 µg/l; higher concentrations after dilution. The procedure comprises 6 steps; it is used for: Drinking water, surface water, groundwater and rainwater — mercury at the ng/l level (monitoring, assessment of the chemical status), Industrial and municipal wastewater after an additional digestion — the reference method under Dz.U. 2019 poz. 1311, Mineral and medicinal waters, mine waters and brines, leachate and aqueous extracts (environmental laboratories with AFS).
At a glance
- Standard: PN-EN ISO 17852:2009 (ISO 17852:2006)
- Category: Physicochemistry
- Procedure steps: 6
- Range: linearity about 1 ng/l–100 µg/l; practical working range 10 ng/l–10 µg/l; higher concentrations after dilution
- Limit of detection: < 1 ng/l with high-purity reagents; RSD typically < 5 % above 20 × the limit
- Digestion: bromine/BrCl generated from KBr/KBrO₃ (2 ml/100 ml) in HCl (15 ml/100 ml); the excess bromine is removed by ascorbic acid (100 µl/10 ml)
Overview
ISO 17852:2006 specifies a method for determining mercury in drinking water, surface water, groundwater and rainwater by atomic fluorescence spectrometry; it can be used for industrial and municipal wastewater after an additional digestion step under suitable conditions. In natural waters the mercury compounds are usually present below 0.1 µg/l (higher concentrations occur for example in industrial wastewater), in inorganic and organic forms; mercury also accumulates in sediments. Digestion is necessary for the complete decomposition of all the mercury compounds — it can be omitted only when it is certain that the mercury can be measured without this treatment. The potential linear dynamic range is about 1 ng/l–100 µg/l, and in practice the working range is often 10 ng/l–10 µg/l; samples of higher concentration are analysed after dilution. The limit of detection depends on the working conditions and on the calibration range — with high-purity reagents less than 1 ng/l is achieved; the relative standard deviation is usually below 5 % for concentrations above twenty times the limit of detection. In Poland it is in force as PN-EN ISO 17852:2009 (Polish version). The 2019 regulation (Dz.U. 2019 poz. 1311 [Journal of Laws 2019, item 1311]) names PN-EN ISO 17852 (fluorescence spectroscopy) as the reference method for mercury in wastewater; environmental laboratories run procedures for Hg under it (including with preconcentration).
The cold-vapour technique with fluorescence detection (CV-AFS) is increasingly used instead of CV-AAS — its advantage is greater sensitivity (a review of methods for determining mercury, Nafta-Gaz 11/2020). In the commercial analysers based on this principle, bromine chloride digests the organic matter, its excess is masked with hydroxylamine hydrochloride (or — according to the standard — with ascorbic acid), after which tin(II) chloride solution is added: the Hg²⁺ ions are reduced to atomic mercury of low solubility in water, which is carried into the gas phase by a stream of argon; atomic mercury irradiated with UV radiation in the flowing argon fluoresces, and the intensity of the fluorescence is proportional to the amount of mercury. Laboratories with AFS analysers using double amalgamation determine mercury in waters, wastewater and extracts in the range from 0.05 ng/l to 1000 ng/l directly (above that after dilution).
Method principle
Atomic fluorescence is emission: atoms excited by the absorption of a beam of electromagnetic radiation return to the ground state, giving up the excess energy as photons whose intensity is measured. A portion of the sample is digested with chemically generated bromine and bromine chloride (BrCl), which decomposes all the commonly occurring organic mercury compounds to mercury(II); immediately before the analysis the excess bromine is removed with ascorbic acid. Elemental mercury vapour is generated from the digest by reduction with tin(II) chloride and is purged out of the solution by a stream of argon; the moisture is continuously removed from the gas stream (a hygroscopic membrane) and the mercury vapour is detected by an atomic fluorescence spectrometer (AFS). The procedure is usually automated (autosampler, continuous-flow vapour generator, gas–liquid separator, dryer, spectrometer, controlling computer). Interferences: exchange reactions (adsorption/desorption) on the walls of the vessels; the diffusion of mercury vapour through plastics (glass or FEP tubing; silicone is unsuitable); the quenching of the fluorescence by water vapour or aerosol in the cell (drying with a membrane) and by dissolved gases (removed during the digestion); anions that strongly complex mercury (sulfide, iodide, bromide — the KBr/KBrO₃ reagent used correctly does not quench); noble metals (Au, Ag, Pt) that amalgamate with the mercury vapour; volatile organic compounds do not interfere. Reagents: grade 1 water (ISO 3696); KBrO₃ 0.0333 mol/l (1.39 g/250 ml; stable for about a week; purified at 250 °C overnight) and KBr 0.2 mol/l (5.95 g/250 ml; about a month; 300 °C) mixed in equal volumes on the day of use (200 ml is enough for 100 samples; ampoules are available); L-ascorbic acid 100 g/l (a week); HNO₃ 1.4 g/ml; HCl 120 g/kg (167 ml of HCl of 360 g/kg made up to 500 ml); SnCl₂·2H₂O 20 g/l in HCl (10.0 g in 150 ml of HCl, dissolved while hot, made up to 500 ml; purging with argon, nitrogen or air at 2 l/min for 15 min removes traces of mercury); reagent blank: 15 ml of HCl and 2 ml of the KBr/KBrO₃ reagent per 100 ml + 100 µl of ascorbic acid per 10 ml (treated like a sample; in a flow system it is the background for automatic subtraction). Standards: solution A 1000 mg/l (commercial or 0.1354 g of HgCl₂ in 20 ml of water + 5 ml of HNO₃ made up to 100 ml; do not dry the salt — it is highly toxic; at least 6 months), B 10 mg/l (weekly, with 2 ml of KBr/KBrO₃), C 100 µg/l and D 1 µg/l (on the day of use, in the reagent blank, in borosilicate flasks); at least five calibration standards, e.g. 10, 30, 50, 70, 100 ng/l (1–10 ml of solution D made up to 100 ml) or 2–20 ng/l from a working solution of 100 ng/l — each with 15 ml of HCl and 2 ml of KBr/KBrO₃ per 100 ml, the matrix being identical with that of the blank. Apparatus: an AFS system (autosampler, vapour generator, gas–liquid separator, dryer, spectrometer); argon 99.99 % with a two-stage regulator and an activated carbon purifier (nitrogen is allowed, with lower sensitivity); a hygroscopic membrane; vessels soaked for at least 48 h in HNO₃ (1+1), rinsed 3 times with water, then for 24 h in the KBr/KBrO₃ cleaning mixture, with an excess of ascorbic acid, and stored in double bags; a clean workstation with air filtration.
Applications
- Drinking water, surface water, groundwater and rainwater — mercury at the ng/l level (monitoring, assessment of the chemical status)
- Industrial and municipal wastewater after an additional digestion — the reference method under Dz.U. 2019 poz. 1311
- Mineral and medicinal waters, mine waters and brines, leachate and aqueous extracts (environmental laboratories with AFS)
- Determination of total mercury (organic and inorganic) thanks to the BrCl digestion
- Laboratories working at the ultra-trace level — a clean workstation, high-purity reagents, automation
Key parameters
| Parameter | Value |
|---|---|
| Range | linearity about 1 ng/l–100 µg/l; practical working range 10 ng/l–10 µg/l; higher concentrations after dilution |
| Limit of detection | < 1 ng/l with high-purity reagents; RSD typically < 5 % above 20 × the limit |
| Digestion | bromine/BrCl generated from KBr/KBrO₃ (2 ml/100 ml) in HCl (15 ml/100 ml); the excess bromine is removed by ascorbic acid (100 µl/10 ml) |
| Reduction and carrier gas | SnCl₂·2H₂O 20 g/l in HCl; argon 99.99 % (nitrogen — lower sensitivity); drying with a hygroscopic membrane |
| Calibration | at least 5 standards, e.g. 10–100 ng/l or 2–20 ng/l, in the matrix of the reagent blank, in borosilicate flasks, on the day of use |
| Vessels | HNO₃ (1+1) for at least 48 h, 3 × water, KBr/KBrO₃ for 24 h + ascorbic acid; glass or FEP tubing (not silicone) |
Standard
- Standard number
- PN-EN ISO 17852:2009 (ISO 17852:2006)
- Title (PL)
- Jakość wody — Oznaczanie rtęci — Metoda atomowej spektrometrii fluorescencyjnej
- Title (EN)
- Water quality — Determination of mercury — Method using atomic fluorescence spectrometry
Step-by-step procedure
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Preparation of the vessels
Soak reusable vessels for at least 48 h in HNO₃ (1+1), rinse them 3 times with water, fill them with the KBr/KBrO₃ cleaning mixture for 24 h, add an excess of ascorbic acid (it removes the free bromine), empty them and rinse 3 times with water; store them in double bags in a clean place. Single-use vessels — once their mercury content has been shown to be negligible.
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Reagents
On the day of use mix equal volumes of KBrO₃ 0.0333 mol/l and KBr 0.2 mol/l (200 ml per 100 samples). Prepare SnCl₂ 20 g/l in HCl and purge it with argon at 2 l/min for 15 min. Prepare the reagent blank (15 ml of HCl + 2 ml of KBr/KBrO₃ per 100 ml) from the same reagents as the samples and the standards.
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Digestion of the samples
To the portion of the sample add HCl (15 ml/100 ml) and the KBr/KBrO₃ reagent (2 ml/100 ml) — the chemically generated bromine and BrCl decompose the organic mercury compounds to Hg(II); wastewater requires an additional digestion step. Immediately before the analysis remove the excess bromine with ascorbic acid (100 µl per 10 ml).
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Calibration standards
From solution D (1 µg/l, freshly prepared) prepare at least 5 standards in the matrix of the reagent blank, e.g. 10, 30, 50, 70, 100 ng/l (1–10 ml made up to 100 ml) or 2, 5, 10, 15, 20 ng/l from a working solution of 100 ng/l; in borosilicate flasks, on the day of use.
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Measurement
In the flow system (autosampler) the solution is mixed with SnCl₂; the mercury vapour purged out by the argon passes through the gas–liquid separator and the drying membrane into the AFS cell; the reagent blank serves as the background for automatic subtraction. Measure the standards, the blanks and the samples; dilute samples that are above the range.
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Calculation and test report
Establish the mercury concentration from the calibration function (ng/l or µg/l) after subtracting the background; report the limit of detection of the method for the conditions adopted, the digestion used (for wastewater) and any deviations.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Atomic fluorescence spectrometer (AFS) with a continuous-flow cold-vapour generator, a gas–liquid separator and an autosampler | e.g. CV-AFS analysers with SnCl₂ reduction; versions with double amalgamation down to 0.05 ng/l | — |
| Dryer for the gas stream — a hygroscopic membrane | removal of the water vapour before the fluorescence cell | — |
| Argon 99.99 % with a two-stage regulator and an activated carbon purifier | carrier and drying gas | — |
| Clean workstation with air filtration (clean air station) for trace work | limiting the contamination and the losses of mercury | — |
| Borosilicate volumetric flasks of 100 ml and 500 ml, glass or FEP tubing | standards and samples; no plastics permeable to Hg vapour | — |
| Muffle furnace (250–300 °C) | purification of the KBrO₃ and the KBr from mercury | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Potassium bromate 0.0333 mol/l and potassium bromide 0.2 mol/l (the KBr/KBrO₃ reagent, equal volumes) | — | generation of bromine and BrCl — digestion of the mercury compounds |
| L-ascorbic acid 100 g/l | — | removal of the excess bromine before the measurement |
| Hydrochloric acid 120 g/kg (from HCl of 360 g/kg) and nitric acid 1.4 g/ml | — | matrix of the samples and the standards; cleaning of the vessels |
| Tin(II) chloride dihydrate 20 g/l in HCl (purged with argon) | — | reduction of Hg(II) to elemental mercury vapour |
| Mercury standards: A 1000 mg/l (commercial or HgCl₂), B 10 mg/l, C 100 µg/l, D 1 µg/l | — | calibration in the ng/l range |
| Grade 1 water according to ISO 3696 | — | all dilutions and preparations |
Health and safety (OHS)
- Mercury compounds (HgCl₂, the standards) are highly toxic — do not dry the salt, wear gloves, work in a fume hood; dispose of the mercury waste
- Bromine and bromine chloride generated in the solution — irritant and corrosive vapours; use a fume hood; neutralise the excess with ascorbic acid
- Concentrated hydrochloric and nitric acids — goggles, gloves, protective clothing
- Potassium bromate is an oxidant and a carcinogenic substance — avoid the dust, keep it away from combustible substances
- Argon under pressure — secure the cylinder; vent the vapours from the separator into the fume hood
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 17852:2009 (ISO 17852:2006) — “Water quality — Determination of mercury — Method using atomic fluorescence spectrometry”.
How does this method work?
Atomic fluorescence is emission: atoms excited by the absorption of a beam of electromagnetic radiation return to the ground state, giving up the excess energy as photons whose intensity is measured. A portion of the sample is digested with chemically generated bromine and bromine chloride (BrCl), which decomposes all the commonly occurring organic mercury compounds to mercury(II); immediately before the analysis the excess bromine is removed with ascorbic acid.
What is the measuring range and accuracy?
Range: linearity about 1 ng/l–100 µg/l; practical working range 10 ng/l–10 µg/l; higher concentrations after dilution; Limit of detection: < 1 ng/l with high-purity reagents; RSD typically < 5 % above 20 × the limit; Digestion: bromine/BrCl generated from KBr/KBrO₃ (2 ml/100 ml) in HCl (15 ml/100 ml); the excess bromine is removed by ascorbic acid (100 µl/10 ml); Reduction and carrier gas: SnCl₂·2H₂O 20 g/l in HCl; argon 99.99 % (nitrogen — lower sensitivity); drying with a hygroscopic membrane.
How long does the test take?
The procedure comprises 6 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Atomic fluorescence spectrometer (AFS) with a continuous-flow cold-vapour generator, a gas–liquid separator and an autosampler, Dryer for the gas stream — a hygroscopic membrane, Argon 99.99 % with a two-stage regulator and an activated carbon purifier, Clean workstation with air filtration (clean air station) for trace work, Borosilicate volumetric flasks of 100 ml and 500 ml, glass or FEP tubing, Muffle furnace (250–300 °C).
Where is this test used?
Drinking water, surface water, groundwater and rainwater — mercury at the ng/l level (monitoring, assessment of the chemical status); Industrial and municipal wastewater after an additional digestion — the reference method under Dz.U. 2019 poz. 1311; Mineral and medicinal waters, mine waters and brines, leachate and aqueous extracts (environmental laboratories with AFS); Determination of total mercury (organic and inorganic) thanks to the BrCl digestion; Laboratories working at the ultra-trace level — a clean workstation, high-purity reagents, automation.
What safety precautions apply?
Mercury compounds (HgCl₂, the standards) are highly toxic — do not dry the salt, wear gloves, work in a fume hood; dispose of the mercury waste; Bromine and bromine chloride generated in the solution — irritant and corrosive vapours; use a fume hood; neutralise the excess with ascorbic acid; Concentrated hydrochloric and nitric acids — goggles, gloves, protective clothing; Potassium bromate is an oxidant and a carcinogenic substance — avoid the dust, keep it away from combustible substances; Argon under pressure — secure the cylinder; vent the vapours from the separator into the fume hood.
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 17852.