🧫 Sodium hydroxide in workplace air — FAAS according to PN-Z-04435

Environment PN-Z-04435

Overview

STATUS (state of the PKN card on 22.09.2026). PN-Z-04435:2011 is CURRENT — the card contains neither a withdrawal note nor a "Replaced by" row. The standard was published on 12-09-2011, has 7 pages, is maintained by KT 159 (Chemical and Dust Hazards in the Working Environment), ICS 13.040.30. The card states that the standard REPLACES PN-Z-04005-06:1988. We give the date of reading because the status is the only field of the entry that can change without our involvement and without a trace.

WHAT IT MEASURES AND WHAT IT DETERMINES. The distinction is more important here than usual: the instrument measures SODIUM, and the result is given as SODIUM HYDROXIDE. The step from one quantity to the other is multiplication by the factor 1.74 (the ratio of the molar mass of NaOH to the molar mass of Na). This means that the method does not distinguish sodium hydroxide from other sodium compounds retained on the filter — all the sodium that went through the digestion is counted in the result as hydroxide. With dust containing sodium salts the result is therefore an UPPER value, not a selective determination. The standard is entitled "Determination of sodium hydroxide", but it determines sodium, and this is also how the source procedure describes it.

WHAT THE RESULT REFERS TO. Sodium hydroxide [1310-73-2] has in the list an NDS value of 0.5 mg/m³ and an NDSCh of 1 mg/m³ (maximum admissible short-term concentration); the legal norm does not set an NDSP value for this substance, and it is not marked with the "skin" notation. The measurement result refers to these values and in the report it should decide whether the concentration DOES NOT EXCEED the limit value — a mere "amounts to" does not say what the measurement was carried out for.

A NOTE ON WHERE THE LIMIT VALUE IS FOUND — THE SAME NUMBER, A DIFFERENT PLACE. The list of NDS values is contained in the regulation of the Minister of Family, Labour and Social Policy of 12 June 2018 (Journal of Laws 2018 item 1286). The amending regulation of 26 March 2026 (Journal of Laws 2026 item 447, in force since 2 April 2026) gave Annex No. 1 an ENTIRELY NEW WORDING. We checked both versions of the list: sodium hydroxide has the same values of 0.5 and 1 mg/m³ in them, but stands under a DIFFERENT ITEM — item 548 in the original wording and item 576 in the wording in force. A test procedure that refers to the item number instead of the substance name therefore has an outdated reference since April 2026, even though the number written in it is correct. This is a case in which checking the value alone does not detect the change.

WHERE THE LIMIT OF QUANTIFICATION OF 0.024 mg/m³ COMES FROM — AND WHY LABORATORIES GIVE OTHER NUMBERS. The value given in the standard is not a property of the determination itself, but the product of two things: the sensitivity of the determination and the VOLUME OF AIR DRAWN. The source procedure provides for drawing up to 720 l (0.72 m³) of air at a flow rate of up to 2 l/min in personal dosimetry or up to 20 l/min in stationary measurement. The previous method (PN-88/Z-04005.06) captured the hydroxide in a sintered-glass impinger containing water and made it possible to determine 1/5 of the NDS value, and after increasing the sample volume 1/10. Replacing the impinger with a NITROCELLULOSE FILTER made it possible to collect more air, and this is precisely what raised the limit of quantification to 1/20 of the NDS. WHY THE OLD METHOD WAS NO LONGER SUFFICIENT — THE REASON IS GIVEN DIRECTLY BY THE SOURCE, NOT BY US. The procedure on which this standard is based says that the development of the new method was forced by the introduction into the Polish collection of the European standard PN-EN 482 "Workplace atmospheres — General requirements for the performance of procedures for the measurement of chemical agents". This is a standard superior to individual methods: it does not say how to determine sodium hydroxide, but what requirements EVERY measurement procedure must meet, including the ability to determine a sufficiently low fraction of the limit value. A method meeting these requirements could not be built by amending the old one — the way the sample is taken had to be changed. For a reader with an outdated citation in the accreditation scope this means something practical: the difference between PN-88/Z-04005.06 and PN-Z-04435:2011 is not one of tidying up or editing, but results from a superior requirement that the old method did not meet. PN-EN 482 has its own entry in this database. So it was not the chemistry of the determination itself that changed, but the way of sampling — and the limit figure followed. This can be seen in the accreditation scopes: next to the range in mg/m³, laboratories give a second range in "mg w próbce" [mg in the sample], and only both numbers together make sense. From our measurement on the database of scopes (22-09-2026): (0.032–4.400) mg/m³ with (0.023–3.170) mg in the sample, (0.012–2.0) mg/m³ with (0.0086–1.44) mg in the sample and (0.017–8.7) mg/m³ with (0.009–4.5) mg in the sample. The quotient of the two lower limits gives the volume of air: in two cases 0.72 m³, i.e. exactly the volume from the procedure, in the third about 0.53 m³. COMPARING THE LIMITS IN mg/m³ ALONE BETWEEN LABORATORIES IS THEREFORE A MISTAKE — a lower number may mean better sensitivity or simply a larger sample volume.

WHAT THE SOURCE DOES NOT EXPLAIN. The procedure gives a limit of QUANTITATIVE determination of 0.03 µg/ml in the solution and a limit of quantification of the method of 0.024 mg/m³ in air. These two numbers do not convert into each other directly via the volume of the test solution (50 ml) and the volume of air (720 l) — the conversion gives a value several times lower. The source does not explain this difference, and we do not know its reason, so we give both numbers as they stand and warn against converting one into the other on one's own. A POSSIBLE EXPLANATION — WE GIVE IT AS UNCONFIRMED, BECAUSE WE HAVE NOTHING TO CHECK IT WITH. These two numbers may come from two different WAYS OF ESTABLISHING them, not from one calculation: the limit of quantitative determination in the solution is an ANALYTICAL quantity and says what the instrument will measure; the limit of quantification of the method expressed in mg/m³, on the other hand, may be established against the requirement of the SUPERIOR STANDARD, i.e. say what the method is approved for. This is supported by the fact that the values of successive methods line up as fractions of the limit value — 1/5, then 1/10, then 1/20 of the NDS — rather than as numbers resulting from the sensitivity of the instrument. If this were so, both numbers would be true and SHOULD NOT BE CONVERTED into each other, because they answer different questions. WE DO NOT CONFIRM THIS: we do not know the numerical requirements of PN-EN 482 from the text of the standard, and the entry on it in this database warns directly that values given in studies are sometimes copied from earlier editions. We leave this as a possible explanation, not as a finding.

CITATION OF THE SAMPLING SHEET UNDER A DESIGNATION THAT NO LONGER EXISTS. The source procedure refers, as regards sampling, to "PN-Z-04008-07". The standard with this designation (PN-Z-04008-07:1989) was replaced by PN-Z-04008-7:2002 "Principles of sampling air in the working environment and of interpreting the results" — and it is this standard, with the amendment Az1:2004, that is current (state of the PKN card on 22.09.2026). A laboratory that copies the citation from the article directly into its procedure will write a designation that is not in the catalogue as valid. WE CHECKED THIS PREDICTION BY A MEASUREMENT IN THE ACCREDITATION SCOPES (23.09.2026) AND IT IS ONLY HALF FULFILLED: the notation with a two-digit sheet ("PN-Z-04008-07:2002+Az1:2004" and variants with a slash) has 16 entries at 6 laboratories — but ALL of them indicate the year 2002, i.e. the CURRENT edition, merely written in the old sheet-numbering scheme. Not a single laboratory cites the 1989 edition. The most frequent form is "PN-Z-04008-7:2002+Az1:2004" — 253 entries at 111 laboratories. The sentence about the catalogue remains true; the prediction about practice was premature. PREDICTION REPLACED BY MEASUREMENT (23.09.2026). The sentence above was a GUESS about what laboratories do — and this can be checked in the database of accreditation scopes. Measurement of the PN-Z-04008 family (23.09, copy of the scopes, pattern with a two-sided boundary: no digit may stand before or after the number): the notation with a TWO-DIGIT sheet, in the old scheme — "PN-Z-04008-07:2002+Az1:2004" and variants with a slash — occurs **16 times at 6 laboratories**, against 477 entries at 202 laboratories in the one-digit scheme ("PN-Z-04008-7:2002"). AS TO FORM, THE PREDICTION CAME TRUE: the old sheet-numbering scheme does indeed find its way into the scopes. AS TO CONTENT — NO. All six laboratories indicate the year **2002**, i.e. the CURRENT edition, merely written in the old scheme. **Not a single laboratory** cites the year 1989 with this number (measurement: zero). The guess said that a laboratory would copy from the article a designation that is NOT IN FORCE; in reality it copies the one in force, only in an outdated notation. INCIDENTALLY TO THE MEASUREMENT — A FALSE HIT OF OUR OWN PATTERN, REPORTED BECAUSE IT CHANGES THE NUMBERS: searching for the string "04008" without a boundary gives 628 entries at 231 laboratories, with a two-sided boundary 578 at 228. The difference of 50 notations is made by the EAD FAMILY — European Assessment Documents, not PN standards — in four forms: "2013 EAD 040083-00-0404" 24 notations, "2013 EAD 040083-01-0404" 13, "EAD 040083-XX-0404" 11, "EAD 040083-xx-0404" 2; altogether at 3 laboratories. OWN CORRECTION (23.09.2026): earlier it said here that these 50 entries are the single form "EAD 040083-00-0404" — that one has 24 notations at ONE laboratory; the number 50/3 describes the whole EAD family, not this form. SECOND CORRECTION: the increase from 16/6 to 19/8 does NOT come from EAD (after the element 04008 there is the digit 3, not a separator), but from SHEET 02, i.e. A DIFFERENT STANDARD of the same family — "PN-84/Z-04008-02" 2 notations at 1 laboratory and "PN-Z-04008.02:1984" 1 notation at 1. This came out only when each separator was counted separately. LIST OF THE FORMS OF SHEET 07, SO THAT THE NUMBER 16/6 CAN BE CHECKED WITHOUT REPEATING THE MEASUREMENT: "PN-Z-04008-07:2002+Az1:2004" 10 notations at 3 laboratories, "PN-Z-04008/07:2002" 3 at 1, "PN-Z-04008-07:2002+" 2 at 1, "PN-Z-04008/07:2002+Az1:2004" 1 at 1 — altogether 16 notations at 6 laboratories. A SUM WITHOUT A LIST OF FORMS DOES NOT SAY WHAT WENT INTO IT — and that is precisely why the first count here gave 19/8, before the forms were listed.

WHAT CAESIUM CHLORIDE DOES. An equal amount of caesium chloride solution is added to the test solution, to the standards and to the blank sample. Sodium ionises easily in the flame, which lowers the number of free atoms and therefore the absorbance; caesium, which ionises even more easily, added in excess takes over this role and suppresses the ionisation of sodium. It is therefore a condition of the correctness of the measurement, not a technical addition — and that is why it must be added in the same way to all solutions, including the blank.

THE BLANK FROM A CLEAN FILTER IS MANDATORY. In parallel with the filter on which the sample was collected, a clean, unused filter is digested in an identical way and the blank solution is prepared from it. With sodium this is not a formality: sodium is a ubiquitous element, so the filter, the reagents and the glassware introduce it by themselves. Hence also the conditioning of the filters before use — soaking for 24 h in 80 ml of water and drying.

WHAT WE DO NOT GIVE. We did not have the text of PN-Z-04435:2011 — the standard is sold for a fee. The description comes from the PKN catalogue card (scope, title, publication data) and from the FULL analytical procedure published by CIOP-PIB in "Podstawy i Metody Oceny Środowiska Pracy" 2009, No. 1(59), pp. 189–194, which the CIOP-PIB database of chemical agents indicates next to PN-Z-04435:2011 as the method for determining this substance. We did not check whether the text of the 2011 standard differs from the 2009 procedure in numerical details — this is a gap, not a feature of the entry.

Method principle

The air under test is drawn through a cellulose ester membrane filter with a pore size of 0.85 µm, on which the aerosol of caustic soda is retained. The filter is digested hot with concentrated nitric(V) acid, evaporated to a dry residue, and the residue is dissolved in dilute nitric acid and made up with water to a known volume with the addition of caesium chloride as an ionisation buffer. Sodium in the resulting solution is determined by flame atomic absorption spectrometry at a wavelength of 589 nm in an air-acetylene flame, and the sodium content is converted to sodium hydroxide with the factor 1.74. A blank from a clean filter is run in parallel.

Applications

• Assessment of occupational exposure to caustic soda aerosol — personal dosimetry and stationary measurements
• The soap, textile, paper, printing, refinery and metallurgical industries and electroplating processes, where sodium hydroxide occurs as an aerosol
• Monitoring of sanitary and hygienic conditions at workplaces with organic synthesis
• Reading an accreditation scope in which two ranges are given — in mg/m³ and in mg in the sample

Key parameters

Standard

Standard number

PN-Z-04435:2011 (wersja polska)

Title (PL)

Ochrona czystości powietrza — Oznaczanie wodorotlenku sodu na stanowiskach pracy metodą płomieniową absorpcyjnej spektrometrii atomowej

Title (EN)

Air purity protection — Determination of sodium hydroxide at workplaces by flame atomic absorption spectrometry

Step-by-step procedure

1. Preparing the filters

   Place the membrane filter in a 100 ml borosilicate glass beaker or polyethylene vessel, cover with 80 ml of water and leave covered for 24 h. Decant the water and dry the filter. Transfer the filters only with tweezers.

   ⏱ Time: 24 h

2. Collecting the air sample

   At the sampling point, draw up to 720 l of air through the filter in its holder at a volume flow of up to 2 l/min for personal dosimetry or up to 20 l/min for stationary measurement. The principles of sampling and of distributing the samples over the work shift — according to the sheet on the principles of sampling air in the working environment. Record the volume: without it the result cannot be converted into a concentration.

3. Storing the sample

   Store the filter with the collected sample in a closed plastic container for no longer than two weeks.

4. Digestion

   Place the filter in a 50 ml beaker, add 2 ml of concentrated nitric(V) acid and heat slowly on a hot plate at a temperature of about 140 °C until the nitrogen oxides have evaporated and a dry residue is obtained.

   🌡 Temperature: approx. 140 °C

5. Preparing the test solution

   Add 4 ml of nitric(V) acid with a concentration of 1 mol/l to the beaker and leave for about 30 min. Transfer the contents of the beaker with 5–6 portions of water into a 50 ml flask, add 5 ml of caesium chloride solution, make up to the mark with water and mix.

   ⏱ Time: approx. 30 min

6. Blank from a clean filter

   In parallel, digest a clean, unused filter in an identical way and prepare the blank solution from it. With sodium this step is a condition of the correctness of the result, not a formality — sodium is introduced by the filter, the reagents and the glassware.

7. Calibration curve

   Measure the absorbance of the five working sodium standard solutions three times under the set operating conditions of the spectrometer, zeroing the instrument with the blank sample solution. Prepare the curve immediately before carrying out the determination.

8. Measurement

   Measure the absorbance of the test solution and the blank solution under the same conditions as for the calibration curve. Read the sodium concentrations from the curve.

9. Calculating the result

   Calculate the concentration of sodium hydroxide in air from the formula X = (c − c₀) · V₁ · 1.74 / V, where c and c₀ are the sodium concentrations in the test solution and in the blank in µg/ml, V₁ — the volume of the test solution in ml (50 ml), V — the volume of air drawn through the filter in litres, and 1.74 — the factor converting sodium to sodium hydroxide.

10. Relating to the limit value

    Relate the result to the NDS of 0.5 mg/m³ and the NDSCh of 1 mg/m³ and record it so that it decides whether the concentration DOES NOT EXCEED the limit value. State the volume of air drawn — without it it is impossible to assess to which limit of quantification a negative result refers.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Sodium hydroxide causes severe skin burns and eye damage (Skin Corr. 1A, H314) — it is particularly dangerous to the eyes; sampling at the workplace requires eye and skin protection
• Carry out all operations with concentrated nitric(V) acid in protective clothing under an efficient laboratory fume hood — digestion on the hot plate releases nitrogen oxides
• Acetylene is a flammable gas — operate the cylinder and the burner installation according to the instrument instructions; do not leave the flame unattended
• Collect the reagent and standard solutions remaining after the analysis in designated containers and hand them over for disposal to authorised bodies
• Report the result with the volume of air drawn and with the decision against the NDS — a mere "the concentration amounts to" does not answer the question of what the measurement was carried out for

Frequently asked questions