🌱 Phosphorus in fertilizer extracts — gravimetric method (quinoline phosphomolybdate) — PN-EN 15959

Physicochemistry PN-EN 15959

Overview

WHAT THIS TEST IS — SCOPE FROM THE PKN CARD (PN-EN 15959:2024-05, Polish version), IN FULL: "W niniejszym dokumencie określono metodę oznaczania fosforu w ekstraktach nawozowych. Metodę stosuje się dla wszystkich ekstraktów nawozowych do oznaczania różnych form fosforu jako fosforu rozpuszczalnego w kwasach mineralnych, fosforu rozpuszczalnego w wodzie, fosforu rozpuszczalnego w roztworach cytrynianu amonu, fosforu rozpuszczalnego w 2 % kwasie cytrynowym i fosforu rozpuszczalnego w kwasie mrówkowym o stężeniu masowym 2 %. Metoda została zwalidowana tylko dla nawozów nieorganicznych, ale może być stosowana do wszystkich wyekstrahowanych form fosforu, jeśli stosowane są odpowiednie metody ekstrakcji" [This document specifies a method for the determination of phosphorus in fertilizer extracts. The method is used for all fertilizer extracts for the determination of the various forms of phosphorus as phosphorus soluble in mineral acids, water-soluble phosphorus, phosphorus soluble in ammonium citrate solutions, phosphorus soluble in 2 % citric acid and phosphorus soluble in formic acid with a mass concentration of 2 %. The method has been validated only for inorganic fertilizers, but it may be used for all extracted forms of phosphorus if appropriate extraction methods are used]. That is why the title of the new edition has "inorganic fertilizers".

THE SECOND HALF OF THE PAIR. This standard does not extract — it determines phosphorus in an extract prepared according to another standard. In accreditation scopes it stands next to: PN-EN 15957 (ammonium citrate, 16 entries), PN-EN 15958 (water, 16), PN-EN 15956 (mineral acids, 14) — each of them has a separate entry in this database — and also PN-C-87015:1988 clause 6 (7 entries, in various notations), PN-EN 15919:2011 (2) and PN-EN 15920:2011 (1).

PRINCIPLE (clause 4 of EN 15959:2023). After hydrolysis, if any, the phosphorus is precipitated in an acid medium as quinoline phosphomolybdate. The precipitate is filtered off, washed, dried at 250 °C and weighed. Under these conditions, compounds that may be present in the solution (mineral and organic acids, ammonium ions, soluble silicates, etc.) do not interfere — provided that a reagent based on sodium molybdate or ammonium molybdate is used for the precipitation.

SEQUENCE OF OPERATIONS (from the table of contents): preparation of the solution (8.1), hydrolysis (8.2), weighing of the crucible (8.3), precipitation (8.4), filtration and washing (8.5), drying and weighing (8.6), blank test (8.7), check (8.8); calculation of the result (clause 9), precision — interlaboratory tests, repeatability, reproducibility (clause 10), statistics of the interlaboratory tests (Annex A).

WHAT THE 2023 EDITION CHANGED (foreword of EN 15959:2023): precision data for phosphorus soluble in formic acid (P₂O₅) were added to Table 2 and Annex A. The determination method has not changed — the change concerns the precision data.

EDITIONS (state of the PKN cards on 25.09.2026). PN-EN 15959:2024-05 — English version: publication 27-05-2024, 15 pages; Polish version: publication 11-12-2025, 16 pages; KT 156 Fertilizers; introduces EN 15959:2023 [IDT]; replaces PN-EN 15959:2011 in the Polish and English versions. PN-EN 15959:2011 (Polish version of 04-11-2013, 14 pages, title "Nawozy -- Oznaczanie wyekstrahowanego fosforu" [Fertilizers -- Determination of extracted phosphorus]) — "Wycofana" [Withdrawn], date of withdrawal 27-05-2024. At the publisher (the CEN catalogue run by iTeh, read on 25.09.2026): EN 15959:2023 published 13-12-2023, stage 60.60 (definitive text made available), replaced EN 15959:2011.

HOW ACCREDITATION SCOPES CITE THE STANDARD. Measurement of 25.09.2026 on the database of PL accreditation scopes (1310 laboratories with entries): 51 entries at 16 laboratories. The 2024-05 edition — 39 entries at 12 laboratories. The WITHDRAWN 2011 edition — 12 entries at 4 laboratories (AB 284, AB 787, AB 789, AB 930), which do not have the new edition in their scope. The subject of the test is "Nawozy mineralne (nieorganiczne)" [Mineral (inorganic) fertilizers] (41) or "Nawozy nieorganiczne (nawozy mineralne)" [Inorganic fertilizers (mineral fertilizers)] (6); the methodology: phosphorus soluble in water (11), in neutral ammonium citrate (10), in mineral acids (7), in 2 % formic acid (2), in 2 % citric acid (1) and other formulations; the technique "Metoda wagowa" [Gravimetric method] (48 of 51). The declared ranges — in % P₂O₅ or in % P — have lower limits from 0.2 % to 5.0 %, and upper ones from 5.0 % to 60 %.

THE KEY OF THE "LABORATORIES" TAB: "PN-EN 15959" — 16 laboratories and 51 entries, without foreign entries; the tab shows both editions.

WHAT WE DO NOT GIVE. We had a fragment of the text of EN 15959:2023 made available by the publisher: the table of contents, the foreword, clauses 1–4 and the beginning of clause 5 (water, nitric acid of ρ = 1.40 g/ml). We do not have the composition of the molybdate reagent with quinoline, the conditions of hydrolysis and precipitation, the type of crucible, the formula for converting the mass of the precipitate to P₂O₅ or the values of repeatability and reproducibility. We do not give these numbers.

Method principle

In the fertilizer extract (after hydrolysis, if any), phosphorus is precipitated in an acid medium as quinoline phosphomolybdate with a reagent based on sodium or ammonium molybdate; the precipitate is filtered off, washed, dried at 250 °C and weighed. The result is expressed as P₂O₅.

Applications

• Phosphorus in extracts: from mineral acids (PN-EN 15956), from water (PN-EN 15958), from neutral ammonium citrate (PN-EN 15957)
• Phosphorus soluble in 2 % citric acid and in 2 % formic acid — with the appropriate extraction standards
• Method validated for inorganic fertilizers; for others — with an appropriate extraction method

Key parameters

Standard

Standard number

PN-EN 15959:2024-05 (wersja angielska z 27-05-2024 i polska z 11-12-2025; wprowadza EN 15959:2023 [IDT]; zastąpiła PN-EN 15959:2011)

Title (PL)

Nawozy nieorganiczne — Oznaczanie wyekstrahowanego fosforu P₂O₅

Title (EN)

Inorganic fertilizers — Determination of extracted phosphorus P₂O₅

Step-by-step procedure

1. Extract

   Prepare the extract according to the appropriate extraction standard (e.g. PN-EN 15956, 15957, 15958).

2. Preparation of the solution and hydrolysis

   8.1 and 8.2 — WE DO NOT GIVE the details.

3. Crucible

   Weigh the crucible (8.3).

4. Precipitation

   Precipitate quinoline phosphomolybdate in an acid medium with the molybdate reagent (8.4).

5. Filtration, drying, weighing

   Filter and wash the precipitate (8.5), dry at 250 °C and weigh (8.6).

   🌡 Temperature: 250 °C

6. Blank test and check

   Blank test (8.7) and check (8.8); calculation of P₂O₅ (clause 9) — WE DO NOT GIVE the formula.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Quinoline and molybdate reagents — toxic; work in a fume cupboard, gloves
• Concentrated nitric acid — corrosive and oxidizing
• Drying oven 250 °C — transfer hot crucibles with tongs to a desiccator

Frequently asked questions