🌿 Sulfur dioxide (SO₂) in waste gases — manual reference method with absorption in H₂O₂
In short
Sampling of the gas with a heated probe through a filter into gas-washing bottles with a solution of hydrogen peroxide (0. The test is performed according to PN-EN 14791:2017-04 (EN 14791:2017); Range of validation: 0.5–2000 mg/m³ SO₂ (ion chromatography); 5–2000 mg/m³ (thorin); sampling 30 min. The procedure comprises 6 steps; it is used for: Periodic measurements of SO₂ emissions from power boilers, waste incineration and co-incineration plants and industrial installations, Calibration and control (QAL2, AST) of automated SO₂ analysers on stacks — the reference method under the regulation on measurements of emissions, Periodic determination of the content of SO₂ in waste gases (at least once a quarter) in the control of the degree of desulfurisation.
At a glance
- Standard: PN-EN 14791:2017-04 (EN 14791:2017)
- Category: Environment
- Procedure steps: 6
- Range of validation: 0.5–2000 mg/m³ SO₂ (ion chromatography); 5–2000 mg/m³ (thorin); sampling 30 min
- Absorption solution: H₂O₂ 0.3 % (< 1000 mg/m³) or 3.0 % (1000–2000 mg/m³); from 30 % H₂O₂ (ρ 1.11 kg/l), stable for 1 week in the dark
- Water: ultrapure, conductivity < 10 µS/m
Overview
EN 14791 specifies the standard reference method (SRM) for the determination of sulfur dioxide in waste gases from ducts and stacks, based on a sampling system and two analytical techniques: ion chromatography and the thorin method. The standard gives the performance characteristics and the criteria for measuring systems, it is applied to periodic monitoring and to the calibration and control of automated measuring systems (AMS) installed on the stack, and it also lays down criteria for the equivalence of alternative methods (EN 14793). It was validated in field tests at incineration plants, co-incineration plants and large combustion plants for sampling periods of 30 min in the range 0.5–2000 mg/m³ SO₂ (ion chromatography) and 5–2000 mg/m³ (thorin); the limit values of the directives are expressed in mg/m³ SO₂ for dry gas at 273 K and 101.3 kPa. The 2017 edition replaced EN 14791:2005. In Poland the regulation on the measurement of the quantity of emissions indicates PN-EN 14791 (or an equivalent instrumental method in accordance with PN-ISO 7935) as the reference method for SO₂ in the control of continuous measurements and for the periodic determination of the content of SO₂ in the gases.
TWO NAMES FOR THE SAME SUBSTANCE (23.09.2026). This entry uses the systematic name "ditlenek siarki" [sulfur dioxide]; in the accreditation scopes the traditional name "dwutlenek siarki" is more common — measurement on the copy of the scopes, test name or parameter, counting inflected forms: "dwutlenek siarki" 79 laboratories / 154 entries, "ditlenek siarki" 40 / 76. When searching for laboratories, it is worth entering both names.
Method principle
The gas sample is taken with a heated probe of controlled temperature, filtered and passed for a set time at a controlled flow through absorption solutions of hydrogen peroxide; the sulfur dioxide is absorbed and oxidised to the sulfate ion. The mass concentration of sulfates in the solutions is determined by ion chromatography or by titration with a solution of barium perchlorate against thorin as the indicator (1 ml of a solution of 0.005 mol/l Ba(ClO₄)₂ corresponds to 0.320 mg SO₂). SO₃ is also absorbed and converted into sulfate, so it constitutes an interferent. The method was validated with a solution of 0.3 % H₂O₂ for concentrations below 1000 mg/m³ and 3.0 % H₂O₂ for concentrations of 1000–2000 mg/m³; where the effectiveness of the absorption is poor, the flow is reduced or the concentration of the solution increased. The effectiveness of the absorption ε = q1/(q1 + q2) is checked from the share of the second gas-washing bottle, and the result is corrected for the chemical blank (the content of sulfates in the unexposed solution) and for the field blank.
Applications
- Periodic measurements of SO₂ emissions from power boilers, waste incineration and co-incineration plants and industrial installations
- Calibration and control (QAL2, AST) of automated SO₂ analysers on stacks — the reference method under the regulation on measurements of emissions
- Periodic determination of the content of SO₂ in waste gases (at least once a quarter) in the control of the degree of desulfurisation
- Parallel sampling with water vapour (EN 14790) in the same system
- Demonstration of the equivalence of instrumental methods with respect to the SRM (EN 14793)
Key parameters
| Parameter | Value |
|---|---|
| Range of validation | 0.5–2000 mg/m³ SO₂ (ion chromatography); 5–2000 mg/m³ (thorin); sampling 30 min |
| Absorption solution | H₂O₂ 0.3 % (< 1000 mg/m³) or 3.0 % (1000–2000 mg/m³); from 30 % H₂O₂ (ρ 1.11 kg/l), stable for 1 week in the dark |
| Water | ultrapure, conductivity < 10 µS/m |
| Titration (thorin) | Ba(ClO₄)₂ 0.005 mol/l in 80 % 2-propanol; fa = 0.320 mg SO₂/ml; standardisation against H₂SO₄ 0.005 mol/l |
| Ion chromatography | eluent as recommended by the manufacturer (e.g. 1.7 mmol/l NaHCO₃ + 1.8 mmol/l Na₂CO₃ with a suppressor); sulfate standard 1000 mg/l (1.814 g K₂SO₄/l) |
| Reference conditions | 273 K, 101.3 kPa, dry gas; expanded uncertainty k = 2 (95 %) |
Standard
- Standard number
- PN-EN 14791:2017-04 (EN 14791:2017)
- Title (PL)
- Emisja ze źródeł stacjonarnych — Oznaczanie stężenia masowego tlenków siarki — Standardowa metoda odniesienia
- Title (EN)
- Stationary source emissions — Determination of mass concentration of sulphur oxides — Standard reference method
Step-by-step procedure
-
Planning and the place of measurement
Plan the measurement in accordance with EN 15259 (the sampling plane, the points, the homogeneity); establish the expected concentration of SO₂ and choose the concentration of the absorption solution (0.3 % or 3 %).
-
Absorption solution
Prepare a solution of 0.3 % H₂O₂ (about 10 ml of 30 % H₂O₂ with 500 ml of water, made up to 1000 ml), store it in glass or PE in the dark for no longer than a week, with the cap loosely tightened or with a safety stopper (decomposition and bursting of the bottle are possible). Fill the gas-washing bottles.
-
Sampling
Introduce the heated probe into the measurement point, pass the gas through the filter and the series of gas-washing bottles for a set time (e.g. 30 min) at a controlled flow; record the volume, the temperature and the pressure at the gas meter. Prepare a chemical blank and a field blank.
-
Analysis — ion chromatography
Prepare the sulfate standards by diluting the standard solution with the absorption solution, determine the sulfates in the solutions from each gas-washing bottle and in the blanks; check the resolution of the peaks.
-
Analysis — the thorin method
Alternatively: titrate the prepared portion of the solution (with neutralisation with KOH or HClO₄ where necessary) with barium perchlorate 0.005 mol/l in 2-propanol against thorin; take into account the volume used for the chemical blank.
-
Calculations
Calculate the mass of SO₂ in the solution (for thorin fa = 0.320 mg/ml), the effectiveness of the absorption ε = q1/(q1 + q2), the volume of dry gas under standard conditions and the mass concentration in mg/m³; convert it to reference O₂ and draw up an uncertainty budget (k = 2).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Heated probe of controlled temperature with a particulate filter | inert materials; temperature above the acid dew point | — |
| Gas-washing bottles (absorbers) in series | at least two, for the determination of the effectiveness of the absorption ε | — |
| Pump, flow controller, gas meter with a thermometer and a manometer | the volume converted to standard conditions with a correction for the water vapour pressure at a wet gas meter | — |
| Ion chromatograph with a suppressor and a conductivity detector | anion column; the composition of the regenerating solution of the suppressor as given in the manufacturer's instructions | — |
| Titration set (burette) and glass or PE vessels | for the thorin variant; glass or PE bottles for the H₂O₂ solution with protection against overpressure | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Hydrogen peroxide 30 % (ρ = 1.11 kg/l), analytical grade | — | preparation of the absorption solution of 0.3 % (about 10 ml per 1000 ml) or 3 % |
| Ultrapure water (< 10 µS/m) | — | absorption and standard solutions |
| Sulfate standard solution 1000 mg/l (or 1.814 g of analytical grade K₂SO₄/l) | — | standards for ion chromatography (diluted with the absorption solution) |
| Eluent for ion chromatography (e.g. NaHCO₃ 1.7 mmol/l + Na₂CO₃ 1.8 mmol/l) and the regenerating solution of the suppressor | — | IC analysis — the composition as given by the manufacturer of the column |
| 2-propanol ≥ 99.8 % and barium perchlorate 0.005 mol/l (1.7 g of anhydrous Ba(ClO₄)₂ in 200 ml of water, made up to 1000 ml with 2-propanol) | — | titration in the thorin method |
| Sulfuric acid 0.005 mol/l (standardised solution) | — | standardisation of the barium perchlorate |
| Potassium hydroxide 0.1 mol/l; perchloric acid about 1 % (16 ml of 60 % HClO₄ made up to 1000 ml) | — | correction of the samples where there are high concentrations of acidic (SO₂ > 100, HCl > 200, NO₂ > 400 mg/m³) or alkaline (NH₃ > 50 mg/m³) components |
Health and safety (OHS)
- Hydrogen peroxide 30 % is corrosive and oxidising; the absorption solution may decompose — do not close the bottles tightly, use safety stoppers
- Barium perchlorate and perchloric acid: barium compounds are toxic, perchlorates — oxidising; 2-propanol is flammable
- Work at the hot probe and on the stack platform — heat-resistant gloves, protection against falling
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN 14791:2017-04 (EN 14791:2017) — “Stationary source emissions — Determination of mass concentration of sulphur oxides — Standard reference method”.
How does this method work?
The gas sample is taken with a heated probe of controlled temperature, filtered and passed for a set time at a controlled flow through absorption solutions of hydrogen peroxide; the sulfur dioxide is absorbed and oxidised to the sulfate ion. The mass concentration of sulfates in the solutions is determined by ion chromatography or by titration with a solution of barium perchlorate against thorin as the indicator (1 ml of a solution of 0.
What is the measuring range and accuracy?
Range of validation: 0.5–2000 mg/m³ SO₂ (ion chromatography); 5–2000 mg/m³ (thorin); sampling 30 min; Absorption solution: H₂O₂ 0.3 % (< 1000 mg/m³) or 3.0 % (1000–2000 mg/m³); from 30 % H₂O₂ (ρ 1.11 kg/l), stable for 1 week in the dark; Water: ultrapure, conductivity < 10 µS/m; Titration (thorin): Ba(ClO₄)₂ 0.005 mol/l in 80 % 2-propanol; fa = 0.320 mg SO₂/ml; standardisation against H₂SO₄ 0.005 mol/l.
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?
Heated probe of controlled temperature with a particulate filter, Gas-washing bottles (absorbers) in series, Pump, flow controller, gas meter with a thermometer and a manometer, Ion chromatograph with a suppressor and a conductivity detector, Titration set (burette) and glass or PE vessels.
Where is this test used?
Periodic measurements of SO₂ emissions from power boilers, waste incineration and co-incineration plants and industrial installations; Calibration and control (QAL2, AST) of automated SO₂ analysers on stacks — the reference method under the regulation on measurements of emissions; Periodic determination of the content of SO₂ in waste gases (at least once a quarter) in the control of the degree of desulfurisation; Parallel sampling with water vapour (EN 14790) in the same system; Demonstration of the equivalence of instrumental methods with respect to the SRM (EN 14793).
What safety precautions apply?
Hydrogen peroxide 30 % is corrosive and oxidising; the absorption solution may decompose — do not close the bottles tightly, use safety stoppers; Barium perchlorate and perchloric acid: barium compounds are toxic, perchlorates — oxidising; 2-propanol is flammable; Work at the hot probe and on the stack platform — heat-resistant gloves, protection against falling.
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 14791.