🌿 Ammonia in the air at workplaces — the spectrophotometric method with Nessler’s reagent

Environment PN-71/Z-04041

Overview

Ammonia (NH₃, CAS 7664-41-7) is a colourless gas with a sharp, characteristic odour, lighter than air, very readily soluble in water — at a temperature of 20 °C the solubility reaches 42.8% by mass. At workplaces it appears in industrial refrigeration, in animal houses and farms, in the production of fertilisers, in sewage treatment plants and in flue gas denitrification installations. The admissible values are 14 mg/m³ (NDS) and 28 mg/m³ (NDSCh) — item 23 of the list from the regulation of the Minister of Family, Labour and Social Policy of 12 June 2018, Annex No 1 AS WORDED BY the regulation of 26 March 2026 (Dz.U. 2026 item 447, in force since 2 April 2026). The values have not changed; what has changed is the ITEM NUMBER — in the original 2018 wording ammonia stood under number 24 (act read on 23.09.2026). Ammonia is classified as a flammable gas of category 2 (H221), toxic if inhaled (H331) and corrosive to the skin (H314); the odour threshold is given in a wide range of 0.4–40 mg/m³, and its upper value lies above the NDSCh (28 mg/m³) — the odour is therefore not a reliable warning of an exceedance of the admissible values.

The method is a classical colorimetric determination: a coloured complex of the component determined with an inorganic reagent, and then the measurement of the intensity of the radiation after passing through the coloured substance. The complexing agent is Nessler’s reagent, by means of which the ammonium ion is converted into a coloured, yellow-orange compound. The method is applied to the determination of the content of ammonia in the air surrounding workplaces when carrying out checks of the sanitary and hygienic conditions.

The status of the standard has to be known before it is entered into a procedure. PN-Z-04041:1971 „Oznaczanie zawartości amoniaku w powietrzu" [Determination of the content of ammonia in air] was published on 27.01.1971 (it replaced PN-Z-04041:1955), and the Polish Committee for Standardization WITHDREW it on 10.04.2006 — the catalogue card does not indicate a replacing standard. Despite the withdrawal the standard lives on in practice: in the PCA scopes of accreditation it figures as the reference document for the determination of the concentration of ammonia in the air of the working environment in the range (0.75–30) mg/m³ and (0.00750–0.30) mg in the sample, and the reports of accredited occupational hygiene laboratories from 2020 give the agent expressly as „Amoniak wg PN-71/Z-04041" [Ammonia according to PN-71/Z-04041]. The archival list of CIOP-PIB names two standards for ammonia: PN-71/Z-04041 and PN-90/Z-04009/02 (the indophenol method); the CHEMPYŁ database today gives in the column of methods of determination PN-Z-04009-03:1990 with the annotation „(w)", that is likewise withdrawn. In other words: the whole national standardisation output for ammonia at workplaces is formally withdrawn, and the laboratories work on withdrawn standards — this has to be called by its name in the documentation and justified (PKN permits the use of withdrawn Polish Standards provided the user decides so knowingly).

What this description does not contain: we have not seen the text of the standard itself — PKN makes it available for a fee, and the catalogue card of PN-Z-04041:1971 (2 pages) contains no summary of the scope. The reagents, volumes, rate of drawing the air and course of the determination given below come from a laboratory instruction of the Faculty of Chemistry of the Rzeszów University of Technology, whose only item of literature is precisely the standard PN-71/Z-04041 — they are therefore a faithful rendering of the method, but not a quotation from the standard. Before entering the procedure into the quality system, check the parameters in the original of the standard.

THE SCALE OF CITATIONS AND A CORRECTION OF THE KEY (23.09.2026). The key "PN-Z-04041" gave 6 entries at 6 laboratories. In the scopes the old scheme dominates: "PN-71/Z-04041" 43 times (plus three records with an added note about an excluded clause or with the number of an in-house procedure), and the new "PN-Z-04041:1971" — 6. After the key was changed to "PN-71/Z-04041" the tab shows 46 entries at 40 laboratories. In the list of seven forms there are also two distorted records that no prefix will cover: "PN-71-Z-04041" with a hyphen instead of a slash and "PN-Z-71-Z-04041" with a doubled letter element.

Method principle

The air tested is drawn through a bubbler impinger containing 25 cm³ of a solution of sulfuric(VI) acid of a concentration of 0.01 mol/dm³; the ammonia is retained in it as the ammonium ion. To a portion of the absorbing solution Nessler’s reagent is added — an alkaline solution of potassium tetraiodomercurate(II) — with which the ammonium ion forms a coloured, yellow-orange compound according to the reaction NH₄⁺ + 2 [HgI₄]²⁻ + 4 OH⁻ → coloured product + 8 I⁻ + 3 H₂O. The absorbance is measured with a spectrophotometer in the range of visible light at a wavelength of 400 nm (or with a colorimeter with a violet-blue filter of maximum transmittance 400 nm) in cells with an absorbing layer thickness of 10 mm, against a reference, before an hour has elapsed from the addition of the reagent. The content of ammonia is read off from a calibration curve prepared from ammonium chloride, and the concentration in the air is calculated from the volume of the absorbing solution, the volume taken for the determination and the volume of the air drawn through.

Applications

• Measurements of occupational exposure in animal houses, piggeries and poultry houses — ammonia from droppings and litter
• Checking of the tightness of ammonia refrigeration installations (food industry, dairies, cold stores)
• Sewage treatment plants, pumping stations and sludge dewatering stations
• Production and packaging of nitrogen fertilisers and of ammoniacal solutions
• Assessment of the conformity of the working conditions with the NDS and the NDSCh within the obligatory tests and measurements of harmful agents

Key parameters

Standard

Standard number

PN-Z-04041:1971 (PN-71/Z-04041)

Title (PL)

Oznaczanie zawartości amoniaku w powietrzu

Step-by-step procedure

1. Preparation of the water and the reagents

   Prepare distilled water free of ammonia (evaporation of 1/3 of the volume during boiling) and prepare all the solutions from it. Prepare Nessler’s reagent at least a day in advance and decant it from above the precipitate.

2. Taking the air sample

   Through a bubbler impinger containing 25 cm³ of a solution of sulfuric(VI) acid 0.01 mol/dm³ draw 5 dm³ of air at a rate not greater than 60 dm³/h. Note the volume of the air, the temperature and the pressure and the time of sampling.

3. Series of standards

   Into 6 volumetric flasks of 50 cm³ measure out 20 cm³ of distilled water and 10 cm³ of the solution of sulfuric(VI) acid each, then in turn 0.30; 0.60; 1.00; 1.40; 1.80 and 2.00 cm³ of the standard ammonia solution, which corresponds to a content of 0.03–0.2 mg of ammonia.

4. Development of the colour in the standards

   Mix the solutions, add 1 cm³ of Nessler’s reagent to each, mixing the contents of the flask gently, make up with distilled water to 50 cm³ and mix thoroughly. Prepare the reference: 20 cm³ of water, 10 cm³ of sulfuric(VI) acid and 1 cm³ of Nessler’s reagent, made up to the mark.

5. Measurement of the standards and the calibration curve

   Before an hour has elapsed measure the absorbances of the standards against the reference at 400 nm (or with the violet-blue filter). Plot the calibration curve, and where the relation is rectilinear calculate the mean absorption coefficient. Check the curve for every new batch of reagents, especially for a new Nessler’s reagent.

   ⏱ Time: measurement within 60 min of the addition of the reagent

6. Determination in the sample

   Into a volumetric flask of 50 cm³ transfer 15 cm³ of the solution from the impinger, add 20 cm³ of distilled water and 1 cm³ of Nessler’s reagent, mixing gently during the addition. Make up to the mark with water and mix thoroughly.

7. Measurement of the absorbance of the sample

   Before an hour has elapsed measure the absorbance of the sample in the same way as that of the standards. If the absorbance exceeds the range of the calibration curve, repeat the determination with a smaller volume of the solution from the impinger.

   ⏱ Time: within 60 min

8. Calculation of the concentration

   Calculate the concentration of ammonia in mg/m³ from the formula taking into account: 25 cm³ of the total volume of the absorbing solution, the absorbance read off or the content read off from the calibration curve, the volume of the air drawn through the impinger in dm³ and the volume of the absorbing solution taken for the determination.

9. Assessment of the exposure

   Determine the exposure index for the work shift and compare it with the NDS of 14 mg/m³, and the results of the short-term measurements with the NDSCh of 28 mg/m³. Carry out the measurements and the strategy in accordance with PN-Z-04008-7 and PN-EN 689+AC:2019-06.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Nessler’s reagent contains mercury(II) compounds and a strong base — it is highly toxic and corrosive; prepare and dispense it under a fume hood, in gloves and goggles, never pipette by mouth.
• Used solutions with Nessler’s reagent are mercury waste — collect them separately and hand them over to an authorised receiver, do not pour them into the drains.
• Ammonia in the form of gas and vapour causes pain and watering of the eyes, coughing, sore throat, breathlessness and, at higher concentrations, oedema of the larynx and of the lungs; when measuring at the place of emission use respiratory protection.
• Contact of the skin with the gas, the mist or a solution of ammonia causes a chemical burn with deep ulceration, and liquid ammonia — frostbite; contamination of the eyes threatens ulceration of the cornea.
• Ammonia is a flammable gas (explosive limits in air 15–28% by volume) — in cold stores and ammonia machine rooms the rules of work in an explosion hazard zone apply.
• Carry the casualty out into the fresh air, place them in a half-lying position and ensure immobility (exertion threatens pulmonary oedema), administer oxygen and call a doctor; rinse the eyes with cool water for at least 15 minutes.

Frequently asked questions