🌿 Nitrogen monoxide and nitrogen dioxide in the air at workplaces — the spectrophotometric method with diazotisation of sulfanilic acid and coupling with N-(1-naphthyl)ethylenediamine

Environment PN-Z-04009-11

Overview

Nitrogen dioxide (NO₂, CAS 10102-44-0) is a red-brown gas with a sharp, suffocating odour, of a molecular mass of 46.0, a boiling point of 21.1 °C and a density relative to air of 1.58; it dissolves very readily in water, reacting with it. In the working environment it arises during the combustion of organic substances containing nitrogen, the detonation of explosives, the electrochemical working of metals and the operation of diesel engines. The critical organ — in humans and in laboratory animals — is the respiratory system; acute poisoning manifests itself in pulmonary oedema leading even to a fatal outcome, and the compound is classified as toxic. The NDS value is 0.7 mg/m³ (0.35 ppm) and the NDSCh 1.5 mg/m³ (0.79 ppm) — item 201 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 195, and the values themselves have not changed (act read on 23.09.2026); the basis of the NDS was the results of industrial studies demonstrating the pneumotoxic action of the compound, calculated from a LOAEL value of 2.95 mg/m³ and two uncertainty factors.

The standard is double by design — it covers nitrogen monoxide and nitrogen dioxide in one procedure, because NO is determined by oxidation to NO₂ and measuring it by the same colour reaction. The scope given by PKN reads: „metoda polegająca na utlenianiu tlenku azotu w kwaśnym roztworze manganianu(VII) potasu, pochłanianiu powstałego ditlenku azotu w alkalicznym roztworze arsenianu(III) sodu i kwasu sulfanilowego, sprzężeniu otrzymanego związku z N-(1-naftylo)-etylenodiaminą i analizie spektrofotometrycznej" [a method consisting in the oxidation of nitrogen monoxide in an acid solution of potassium manganate(VII), the absorption of the nitrogen dioxide formed in an alkaline solution of sodium arsenate(III) and sulfanilic acid, the coupling of the compound obtained with N-(1-naphthyl)ethylenediamine and spectrophotometric analysis]. The smallest concentration of nitrogen monoxide that can be determined by the method given is about 0.7 mg/m³, and of nitrogen dioxide 0.2 mg/m³. This last figure is critical in practice: the NDS for nitrogen dioxide is 0.7 mg/m³, so the limit of determination of the method lies in the region of 0.3 NDS — the standard PN-EN 482 requires of a method a range of 0.1–2.0 of the limit value, which for NO₂ means the necessity of a correspondingly long sampling and of checking whether the laboratory really goes that low (accredited ranges begin for example at 0.044 mg/m³).

A note on the naming: this method is colloquially called „the Saltzman method", because the final chemistry — the diazotisation of sulfanilic acid by the nitrates(III) formed from NO₂ and the coupling with N-(1-naphthyl)ethylenediamine to a pink azo dye — is exactly the Saltzman reaction. The absorbing solution here is however different from that of the classical Saltzman reagent: it is alkaline, with sodium arsenate(III), that is in the variant known as the Jacobs–Hochheiser method. The Saltzman reagent expressly, by name, appears in another standard of the series — PN-Z-04009-9:1997 „Ochrona czystości powietrza — Badania zawartości azotu i jego związków — Oznaczanie dwutlenku azotu w powietrzu atmosferycznym (imisja) metodą spektrofotometryczną z odczynnikiem Saltzmana" [Protection of the purity of air — Tests for the content of nitrogen and its compounds — Determination of nitrogen dioxide in atmospheric air (immission, i.e. ambient air) by the spectrophotometric method with the Saltzman reagent], which concerns ambient air and not workplaces. If someone writes „Saltzman" in an order, it has to be established whether a measurement at a workplace is meant (PN-Z-04009-11) or atmospheric air (PN-Z-04009-9).

What this description does not contain: we have not seen the full text of the standard — below, the principle and the limits of determination are given word for word after the PKN catalogue card, and the chemistry of the colour reaction and the typical conditions of measurement after a laboratory instruction describing the same reaction system (2 NO₂ + H₂O → HNO₂ + HNO₃, diazotisation of sulfanilic acid, coupling with N-(1-naphthyl)ethylenediamine hydrochloride, measurement at 550 nm). The concentrations of the reagents, the number and the filling of the impingers, the volume and the time of sampling for workplaces and the manner of introducing the manganate(VII) oxidant have to be taken directly from the standard — we do not guess them.

Method principle

The air sample is drawn through an absorbing assembly. Nitrogen monoxide is oxidised in an acid solution of potassium manganate(VII) to nitrogen dioxide; the nitrogen dioxide (that of the sample itself and that formed from the oxidation of NO) is absorbed in an alkaline solution of sodium arsenate(III) and sulfanilic acid, where the NO₂ is retained as nitrate(III) — the reaction with water proceeds according to 2 NO₂ + H₂O → HNO₂ + HNO₃ — and the sulfanilic acid undergoes diazotisation. The diazonium compound obtained is coupled with N-(1-naphthyl)ethylenediamine (used in the form of the hydrochloride), giving a pink azo dye. The intensity of the colour, proportional to the amount of nitrogen dioxide absorbed, is measured spectrophotometrically; in the instructions describing the same reaction the measurement is carried out at a wavelength of 550 nm against a blank, in cells with a layer thickness of 1 cm, after waiting a few minutes for the colour to develop. The content of the component determined is read off from a calibration curve prepared from sodium nitrate(III), and the concentration in the air is calculated for the volume of air reduced to normal conditions, taking into account the temperature and the pressure at the place of sampling. The smallest concentration determinable by this method is about 0.7 mg/m³ for nitrogen monoxide and 0.2 mg/m³ for nitrogen dioxide.

Applications

• Welding and thermal cutting workplaces — nitrogen oxides arise in the arc and in the flame
• Halls with movement of combustion-engine vehicles: workshops, vehicle testing stations, garages, mines and tunnels (diesel engines)
• Blasting work and working after the detonation of explosives
• Electroplating and pickling shops — electrochemical working of metals and baths with nitric(V) acid
• Boiler houses and processes of combustion of organic substances containing nitrogen
• Assessment of the conformity of the working conditions with the NDS of 0.7 mg/m³ and the NDSCh of 1.5 mg/m³ within the obligatory tests and measurements

Key parameters

Standard

Standard number

PN-Z-04009-11:2008

Title (PL)

Ochrona czystości powietrza — Badania zawartości azotu i jego związków — Część 11: Oznaczanie tlenku azotu i ditlenku azotu na stanowiskach pracy metodą spektrofotometryczną

Step-by-step procedure

1. Planning of the measurement

   Establish the workplaces, the time and the number of samples in accordance with the strategy from PN-EN 689+AC:2019-06 and PN-Z-04008-7. Choose the sampling volume so that the expected concentration falls within the range 0.1–2.0 NDS (for NO₂ the NDS is 0.7 mg/m³ and the limit of determination of the method 0.2 mg/m³).

2. Preparation of the absorbing assembly

   Fill the impingers with the absorbing solution (alkaline sodium arsenate(III) with sulfanilic acid) in the amounts laid down in the standard; if nitrogen monoxide is also to be determined, include in the line a vessel with the acid solution of potassium manganate(VII). Check the tightness of the assembly.

3. Taking the air sample

   Draw through the assembly the established volume of air from the workplace, maintaining a constant flow rate. Record the volume, the time, the temperature and the atmospheric pressure at the place of sampling.

4. Transfer of the solutions

   After taking the sample transfer the contents of the impingers into graduated vessels, rinse each impinger with water and add the rinsings to the sample, preserving the quantitative character of the transfer.

5. Development of the colour

   Add the coupling reagent — N-(1-naphthyl)ethylenediamine hydrochloride — make up to the established volume, mix and wait for the pink colour to develop (in the instructions describing this reaction about 5 minutes).

   ⏱ Time: about 5 min

6. Series of standards

   In parallel prepare a series of standards from the solution of sodium nitrate(III), with the same absorbing solution and the same amount of coupling reagent, and a blank without the standard.

7. Spectrophotometric measurement

   Measure the absorbance of the samples and of the standards against the blank at a wavelength of 550 nm, in cells of a layer thickness of 1 cm. Plot the calibration curve of the dependence of the absorbance on the mass of nitrate(III) in the sample.

8. Calculation of the concentration

   Add up the masses determined in the successive impingers and divide by the volume of the air taken, reduced to normal conditions (the conversion from the temperature and the pressure at the place of sampling). Give the result separately for nitrogen dioxide and, if it was determined, for nitrogen monoxide.

9. Assessment of the exposure and the report

   Compare the exposure index with the NDS of 0.7 mg/m³, and the results of the short-term measurements with the NDSCh of 1.5 mg/m³. In the report give the number and the edition of the standard, the conditions of sampling, the limit of determination and the expanded uncertainty.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Nitrogen dioxide is toxic and insidious: it acts as an irritant on the respiratory tract, and pulmonary oedema may occur with a delay after the exposure — even in the absence of complaints at the time of the measurement the person exposed requires medical observation.
• The sodium arsenate(III) in the absorbing solution is an arsenic compound — highly toxic; prepare the solutions under a fume hood, in gloves, collect the used solutions as hazardous waste.
• Potassium manganate(VII) in an acid medium is a strong oxidising agent — do not mix it with organic substances, avoid contact with the skin and the eyes.
• N-(1-naphthyl)ethylenediamine and sulfanilic acid — aromatic amines, work in gloves, avoid inhaling the dust when weighing out.
• A measurement at welding workplaces and in garages means simultaneous exposure to welding fumes, carbon monoxide and the exhaust gases of diesel engines — choose the protective measures for the whole set of agents.

Frequently asked questions