🌿 Hydrogen sulfide in the air at workplaces — the spectrophotometric method with methylene blue

Environment PN-Z-04015-13

Overview

Hydrogen sulfide (H₂S, CAS 7783-06-4) is a colourless gas, heavier than air, with the odour of rotten eggs, which dissolves readily in water, forming hydrogen sulfide water or, at higher concentrations, hydrosulfuric acid. The molecular mass is 34.09, the boiling point −60.7 °C at 101.3 kPa, the explosive limits in air 4.3–45.5% by volume, and the odour threshold 0.18 mg/m³ (0.13 ppm). It arises among the products of the putrefaction of proteins, occurs in volcanic exhalations and in some mineral waters, and is used for the production of sulfuric acid and as a laboratory reagent. According to the data of the Chief Sanitary Inspector, above-limit exposure was recorded in agriculture and hunting, in construction and in health care and social assistance, and also in the abstraction, treatment and supply of water.

The toxicity of hydrogen sulfide consists in blocking the activity of enzymes containing metals in the prosthetic group — in the cells it blocks the active iron of cytochrome oxidase; the most sensitive are the mucous membranes, and the main target organs in acute poisoning are the central nervous system and the lungs. The values in force are an NDS of 7 mg/m³ and an NDSCh of 14 mg/m³ — item 493 of Annex 1 as worded by the regulation of 26 March 2026 (Dz.U. 2026 item 447); in the original 2018 wording it was item 467, values unchanged (act read on 23.09.2026) — lowered with respect to the earlier 10 and 20 mg/m³ on the basis of documentation adopted by the Commission for NDS and NDN on 3.03.2010. A treacherous feature of hydrogen sulfide is that at high concentrations the sense of smell is paralysed — the absence of odour must not be treated as proof of the absence of a hazard.

The method of determining the concentration of hydrogen sulfide in the air at workplaces is contained in the standard PN-Z-04015-13:1996 „Ochrona czystości powietrza — Badania zawartości siarki i jej związków — Oznaczanie siarkowodoru na stanowiskach pracy metodą spektrofotometryczną" [Protection of the purity of air — Tests for the content of sulfur and its compounds — Determination of hydrogen sulfide at workplaces by the spectrophotometric method] — so the documentation of the admissible values of occupational exposure published by CIOP-PIB expressly indicates. The standard was published on 21.03.1996 and replaced PN-Z-04042:1970; the PKN catalogue card does not mark it as withdrawn. The smallest concentration of hydrogen sulfide that can be determined by this method is according to PKN 2 mg/m³ — that is barely under 0.3 NDS, so for measurements intended to demonstrate that 0.1 NDS is met, the real determinability of the laboratory’s validated procedure has to be checked (in the scope of accreditation AB 746 that was checked, the lower limit is 0.70 mg/m³).

What this description does not contain: we have not seen the full text of the standard. The principle, the reagents used and the limit of determination come word for word from the PKN catalogue card; the accredited range — from the PCA scope of accreditation; the data on the substance, the NDS and the toxicology — from the documentation of CIOP-PIB and from the regulation. The concentrations of the solutions, the volume of the absorbing solution, the volume and the time of sampling, the wavelength of the measurement and the manner of preparing the calibration curve have to be taken directly from the standard — we do not guess them (for the methylene blue reaction itself, different wavelengths are met with in the literature, among others 625 and 670 nm, so all the more none is to be assumed „by eye").

THE SCALE OF CITATIONS (read on 23.09.2026). Matching by the BEGINNING of the string: **24 entries at 22 laboratories**; by all forms of the core — 25 at 23. The ratio of entries to laboratories is 1.1, i.e. practically every laboratory has this standard in one line of its scope: the determination of hydrogen sulfide at workplaces is a single entry, not a family of tests. For a comparison of scale in the same field: the method for the determination of ammonia described in a separate entry of this knowledge base (PN-Z-04041) has 46 entries at 40 laboratories, i.e. a reach nearly twice as wide with the same shape of dispersion.

Method principle

The method consists in drawing the air tested through an impinger with a solution of zinc acetate, in which the hydrogen sulfide is retained in the form of sparingly soluble zinc sulfide. To the solution obtained, N,N-dimethyl-p-phenylenediamine and iron(III) chloride are added. In the mixture obtained a colour reaction takes place, as a result of which methylene blue is formed. The methylene blue formed is the basis of the spectrophotometric determination: the intensity of the colour is proportional to the amount of hydrogen sulfide absorbed from the air sample. The smallest concentration of hydrogen sulfide that can be determined by this method is 2 mg/m³. A note on the quotation: the PKN catalogue card writes at this point „błękit metylowy" [methyl blue] — that is an obvious slip of the card, the product of the reaction is methylene blue.

Applications

• Sewage treatment plants, pumping stations, sewer manholes and closed tanks — the classic places of acute poisoning by hydrogen sulfide
• Agriculture and animal husbandry: slurry tanks, silos, the stirring of slurry in cowsheds and piggeries
• Water intakes and treatment stations and water supply networks (the Chief Sanitary Inspector recorded above-limit exposure there)
• The refining, coking and paper industries — hydrogen sulfide gas as a by-product of the processes
• Work in construction in excavations and confined spaces, where hydrogen sulfide, as a gas heavier than air, accumulates at the bottom
• Assessment of the conformity of the working conditions with the NDS of 7 mg/m³ and the NDSCh of 14 mg/m³ within the obligatory tests and measurements

Key parameters

Standard

Standard number

PN-Z-04015-13:1996

Title (PL)

Ochrona czystości powietrza — Badania zawartości siarki i jej związków — Oznaczanie siarkowodoru na stanowiskach pracy metodą spektrofotometryczną

Step-by-step procedure

1. Planning of the measurement

   Choose the workplaces and the time of measurement in accordance with the strategy from PN-EN 689+AC:2019-06 and PN-Z-04008-7. Remember that hydrogen sulfide is heavier than air and accumulates at the floor, in excavations and in manholes — choose the place of sampling according to the real position of the breathing zone of the worker.

2. Preparation of the impinger

   Fill the impinger with the solution of zinc acetate in the amount given in the standard; check the tightness of the measuring line before starting the sampling.

3. Taking the air sample

   Draw through the impinger the established volume of the air tested at a constant rate. Record the volume, the time of sampling, the temperature and the pressure at the place of sampling.

4. Securing the sample

   After the end of the sampling transfer the solution from the impinger quantitatively into the measuring vessel, rinsing the impinger. Sulfides oxidise in air — work up the sample without unnecessary delay, in accordance with the time limit given in the standard.

5. Colour reaction

   To the solution add N,N-dimethyl-p-phenylenediamine and iron(III) chloride in the order and the amounts laid down in the standard; mix and wait for the methylene blue colour to develop.

6. Series of standards and blank

   In parallel prepare a series of standards from the sulfide solution and a blank carried through the whole course of the procedure (absorbing solution, reagents, the same impinger), so as to take the background of the reagents into account.

7. Spectrophotometric measurement

   Measure the absorbance of the samples and of the standards against the blank at the wavelength given in the standard and read off the content of hydrogen sulfide from the calibration curve.

8. Calculation of the concentration

   Convert the mass of hydrogen sulfide in the sample into a concentration in mg/m³, taking into account the volume of the air taken, reduced to the reference conditions, and the volume of the solution taken for the determination.

9. Assessment of the exposure and the report

   Compare the exposure index with the NDS of 7 mg/m³, and the results of the short-term measurements with the NDSCh of 14 mg/m³. In the report give the number and the edition of the standard, the limit of determination (2 mg/m³ according to the standard or a lower one, confirmed by validation), the conditions of sampling and the expanded uncertainty.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Hydrogen sulfide is a gas of high acute toxicity: at high concentrations the sense of smell is paralysed, and loss of consciousness and arrest of breathing follow within seconds — never enter a manhole, a tank or an excavation without a measurement with a detector, without a standby person and without self-contained breathing apparatus.
• The absence of the odour of rotten eggs does not mean the absence of the gas — the odour threshold is 0.18 mg/m³, but at dangerous concentrations the sense of smell fails.
• Hydrogen sulfide is heavier than air and accumulates at the bottom of confined spaces; before the work ventilate and measure at the height at which the worker actually is.
• The gas is explosive in a mixture with air in the range 4.3–45.5% by volume — in hazard zones use equipment of explosion-proof design.
• N,N-dimethyl-p-phenylenediamine is an aromatic amine of sensitising and irritant action — work in gloves and under a fume hood; collect the zinc and iron salts and the solutions after the analysis as chemical waste.
• Carry the casualty out of the zone only with respiratory protective equipment — a rescuer without protection becomes the next victim; ensure oxygen and immediate medical assistance.

Frequently asked questions