🧮 Total nitrogen (calculated) — PN-73/C-04576/14 (standard WITHDRAWN; a calculated quantity, not a determined one)

Environment PN-73/C-04576

Overview

STATUS OF THE DOCUMENT: STANDARD WITHDRAWN. The PKN catalogue card gives: "Number: PN-C-04576-14:1973 - Polish version", "Title: Water and waste water -- Tests for content of nitrogen compounds -- Calculation of total nitrogen", "Publication date: 06-12-1973", "Withdrawal date: 16-01-2006". The card has the heading "Withdrawn" and does not contain the row "Replaced by". The very title of the standard says "CALCULATION", not "determination" — and this is the key to the whole entry.

SCALE OF CITATIONS. From the measurement on 62 896 entries of accreditation scopes: the standard is cited by 33 laboratories in 36 entries (whole value of the field; when matching by the beginning of the notation — 35 laboratories and 38 entries), with the methodology described in the scopes explicitly as "Concentration of total nitrogen (calculated)", for the waste water matrix.

WHY YOU WILL NOT FIND A METHOD FOR IT. Total nitrogen is not measured — it is the sum of the results of other determinations. The Regulation of the Minister of Maritime Economy and Inland Navigation of 12 July 2019 in force (Journal of Laws 2019 item 1311) defines this parameter in its very name, in the table of the highest permissible values (our translation of the Polish text): "Total nitrogen (sum of Kjeldahl nitrogen (N_Norg + N_NH4), nitrite nitrogen and nitrate nitrogen)". There are therefore THREE components of the sum: Kjeldahl nitrogen, nitrite nitrogen and nitrate nitrogen — the first of which itself covers two forms of nitrogen, organic and ammonium.

THE LOCATION OF THIS DEFINITION IS ITSELF THE ANSWER. The definition of total nitrogen is NOT in Annex No. 12 with the reference methodologies — because the annex lists DETERMINATIONS, i.e. what is measured with what. The parameter stands instead in the table of permissible values, i.e. where it is stated WHAT IS LIMITED. A reader looking for a "method for determining total nitrogen" will not find it anywhere, and this is not a gap in the regulations: there is no such method, there is a formula. The reference methodologies must therefore be checked for each of the components separately — in the same annex they appear as separate entries (ammonium nitrogen, nitrate nitrogen, nitrite nitrogen).

WHAT FOLLOWS FROM THIS FOR THE UNCERTAINTY OF THE RESULT. For a calculated quantity the uncertainty is also calculated — it is inherited from the uncertainty of the components, not from its own measurement, because there is no own measurement. For total nitrogen this means an uncertainty combined from three determinations, each of which has its own. The standard does not give this method — and we did not have its text, so we do not even know whether it addressed it at all.

HOW THE ACCREDITATION SCOPES RECORD IT. The annotation "(z obliczeń)" [calculated] is an established notation in PCA scopes and occurs with many parameters of this kind — for example "sum of PAHs (calculated)" or "calorific value (calculated)". A laboratory that has "Concentration of total nitrogen (calculated)" in its scope therefore has the information entered explicitly that this is not a measurement result.

WHAT WE DO NOT GIVE. We did not have the text of PN-73/C-04576/14 — the standard is withdrawn and is not on sale. We do not know the exact form of the formula it gave, whether it provided for rounding the result, or how it treated cases in which one of the components is below the limit of quantification — and for a sum this is a practical question. The definition quoted above comes from the regulation in force, not from the withdrawn standard; we have not checked whether the two definitions are identical.

WIDENING OF THE KEY TO INCLUDE SHEET SEPARATORS (23.09.2026). The key had the form "PN-73/C-04576/14" — with a slash before the sheet number — and covered 38 entries at 35 laboratories. The scopes record the same sheet in three ways: with a slash 36 times (plus two notations with an addition), with a dot 14 times, with a hyphen 4 times. After shortening the key to "PN-73/C-04576" the tab covers all three and shows 57 entries at 53 laboratories.

Checked before the change, because shortening a key always carries the risk of pulling in an adjacent sheet: under the prefix "PN-73/C-04576" there is ONLY sheet 14 — other sheets of this series bear the year 1982 ("PN-82/C-04576…", sheet 08) or are recorded in the new convention. The year in the prefix therefore acts here as an anchor of the sheet.

Method principle

Total nitrogen is determined by calculation as the sum of three components determined separately: Kjeldahl nitrogen (covering organic and ammonium nitrogen), nitrite nitrogen and nitrate nitrogen. The result does not come from its own measurement, but from adding the results of three determinations, each of which has its own reference methodology and its own uncertainty.

The entry concerns the STATUS of the reference document and the nature of the quantity (calculated, not measured); it does not replace the calculation procedure of the laboratory.

Applications

• Identifying an outdated citation of PN-73/C-04576/14 in the accreditation scope of a laboratory
• Explaining why there is neither a reference methodology nor a measurement method for total nitrogen
• Establishing the appropriate methodologies for the three components of the sum, determined separately
• Reading reports and documentation dated before 16-01-2006 and issued later citing this standard
• Distinguishing a calculated parameter from a measured one when reading an accreditation scope — the annotation "(z obliczeń)" [calculated]

Key parameters

Standard

Standard number

PN-73/C-04576/14 (PN-C-04576-14:1973, wersja polska)

Title (PL)

Woda i ścieki — Badania zawartości związków azotu — Obliczanie azotu ogólnego

Title (EN)

Water and waste water — Tests for content of nitrogen compounds — Calculation of total nitrogen

Step-by-step procedure

1. Identifying the citation

   Find in the accreditation scope or in the report the number PN-73/C-04576/14 (also found as PN-C-04576-14:1973). In scopes it is usually accompanied by the notation "Stężenie azotu ogólnego (z obliczeń)" [Concentration of total nitrogen (calculated)].

2. Checking the date of the document in which the citation stands

   A citation in a document dated before 16-01-2006 was correct at the time of issue; a later one is outdated, although the card does not indicate what to replace it with.

3. Accepting that there is and can be no reference methodology

   Do not look for a standard for "determination of total nitrogen" — the parameter is a sum, not a determination. The definition in force stands in the table of permissible values of the regulation, not in the annex with the methodologies.

4. Checking the methodologies for the three components separately

   For ammonium, nitrate and nitrite nitrogen, Annex No. 12 lists their own methodologies and standards. Reference documents are established for each component separately — it is they, not the withdrawn calculation standard, that are the basis of the result.

5. Establishing how the uncertainty is determined

   Assume that the uncertainty of total nitrogen is an uncertainty combined from the three component determinations. Describe the way of determining it in the own procedure — the regulation does not give it, and we cannot state this about the withdrawn standard, because we did not have its text.

6. Deciding how to handle a component below the limit of quantification

   Establish in the procedure how a component of the sum with a result below the limit of quantification is treated — for a sum of three determinations this is a practical, not a formal question, and none of the available sources gives an answer to it.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• The entry concerns the status of the reference document and the nature of the quantity, not a laboratory activity — safety rules concern the component determinations and are described by their own methodologies
• Testing must not be carried out according to a withdrawn standard with the result described as compliant with a reference document in force
• Total nitrogen must not be presented as a measurement result — it is a calculated quantity, and omitting the annotation "(z obliczeń)" [calculated] changes the meaning of the result in the report

Frequently asked questions