🌱 Phosphorus soluble in mineral acids — extraction from fertilizers — PN-EN 15956

Physicochemistry PN-EN 15956

Overview

WHAT THIS TEST IS — SCOPE FROM THE PKN CARD (Polish version), IN FULL: "W niniejszym dokumencie określono metodę oznaczania fosforu rozpuszczalnego w kwasach mineralnych. Metoda ma zastosowanie wyłącznie do nawozów fosforowych wymienionych w Rozporządzeniu (WE) 2003/2003, Załącznik I" [This document specifies a method for the determination of phosphorus soluble in mineral acids. The method is applicable exclusively to the phosphate fertilizers listed in Regulation (EC) 2003/2003, Annex I]. The same scope stands in clause 1 of the text of EN 15956:2011.

BEWARE OF THE WORD "OZNACZANIA" [DETERMINATION]. The title speaks of extraction, and extraction is all that the standard describes: clause 8 ends with the filtration, and the test report follows right after it. The phosphorus in the extract is determined separately — in accreditation scopes PN-EN 15959 stands next to this standard in all 15 entries (determination of extracted phosphorus, a separate entry in this database). The draft of the new edition (prEN 15956, state in the CEN catalogue on 25.09.2026) already has the scope "method for the extraction of phosphorus soluble in mineral acids in inorganic fertilizers".

THE REGULATION IN THE SCOPE IS NO LONGER IN FORCE. The scope of the 2011 edition refers to Annex I of Regulation (EC) No 2003/2003. Under Article 51 of Regulation (EU) 2019/1009, Regulation (EC) No 2003/2003 "traci moc ze skutkiem od dnia 16 lipca 2022 r." [is repealed with effect from 16 July 2022], and references to it are construed as references to Regulation 2019/1009. The bibliography of the standard points to Annex IV, method 3.1.1, of Regulation 2003/2003 — the standard is the counterpart of that method. The draft of the new edition defines the scope by the type of product (inorganic fertilizers), without referring to the regulation.

PRINCIPLE (clause 4): extraction of phosphorus from the fertilizer with a mixture of nitric acid and sulfuric acid under specified conditions.

PROCEDURE — THE WHOLE EXTRACTION (clauses 5–8): • sample according to EN 1482-2; the standard recommends grinding the laboratory sample for the sake of homogeneity; • 2.5 g of sample, weighed to the nearest 0.001 g, into a dry Kjeldahl flask (at least 500 ml) or a 250 ml round-bottomed flask with a glass tube as a reflux condenser; • 15 ml of water — suspend the sample; 20 ml of nitric acid (ρ = 1.40 g/ml); carefully 30 ml of sulfuric acid (ρ = 1.84 g/ml); • after the violent reaction has ceased, slowly bring to the boil and boil for 30 min; cool, carefully add about 150 ml of water while mixing and boil for another 15 min; • cool completely, transfer quantitatively to a 500 ml volumetric flask, make up to the mark, mix and filter through a dry phosphate-free pleated filter, discarding the first portion of the filtrate. The test report has to state, among other things, whether the repeatability limit requirement was met (clause 9, item f).

EDITIONS AND A NEW EDITION ON THE WAY (state on 25.09.2026). PKN: PN-EN 15956:2011 — Polish version, publication 11-06-2015, 10 pages, KT 156 Fertilizers, introduces EN 15956:2011 [IDT], without the "Wycofana" [Withdrawn] header; the catalogue also has the English version. EN 15956:2011 replaced the specification CEN/TS 15956:2009 (the specification adopted as a standard, editorial changes). CEN (iTeh catalogue): the draft FprEN 15956 "Inorganic fertilizers — Extraction of phosphorus soluble in mineral acids" is at stage 45.99 — forwarding of the draft to the formal vote (since 16-07-2026), with publication planned for 14-01-2027; the Slovenian version of the draft for enquiry is dated 01-02-2026.

HOW ACCREDITATION SCOPES CITE THE STANDARD. Measurement of 25.09.2026 on the database of PL accreditation scopes (1310 laboratories with entries): 15 entries at 15 laboratories; notation "PN-EN 15956:2011" (14) and once in one field together with "PN-EN 15959:2011". The subject of the test is "Nawozy mineralne (nieorganiczne)" [Mineral (inorganic) fertilizers] (13); the methodology most often "Zawartość fosforu rozpuszczalnego w kwasach mineralnych" [Content of phosphorus soluble in mineral acids] (7); the technique "Metoda wagowa" [Gravimetric method] (13) — gravimetric is the determination method of PN-EN 15959, not the extraction itself. In the same entries: PN-EN 15959 in all 15 (11 × the 2024-05 edition, 4 × the withdrawn 2011 edition, including once in one field together with PN-EN 15956), PN-EN 15957:2011 and PN-EN 15958:2011 (2 each) and PN-C-87015:1988 clause 6 (1). The declared ranges — in % P₂O₅ or in % P — have lower limits from 0.2 % to 2.5 %, and upper ones from 23.6 % to 60 %.

THE KEY OF THE "LABORATORIES" TAB: "PN-EN 15956" — 15 laboratories and 15 entries, without foreign entries.

WHAT WE DO NOT GIVE. We had a fragment of the text of EN 15956:2011 made available by the publisher — the foreword, clauses 1–9 and the bibliography, i.e. the whole content of the standard. We do not have the text of the draft of the new edition beyond the title page, so we do not describe whether the course of the extraction will change.

Method principle

Phosphorus is extracted from 2.5 g of fertilizer with a mixture of nitric acid (20 ml, ρ = 1.40 g/ml) and sulfuric acid (30 ml, ρ = 1.84 g/ml): 30 min of boiling, dilution with about 150 ml of water, a further 15 min of boiling, making up to 500 ml and filtration. The phosphorus in the extract is determined according to PN-EN 15959.

Applications

• Phosphate fertilizers — declaration of phosphorus soluble in mineral acids (the so-called total phosphorus in phosphate fertilizers)
• The first stage of the "extraction + determination" pair: PN-EN 15956 with PN-EN 15959
• Comparison of the forms of phosphorus in one fertilizer together with PN-EN 15957 (ammonium citrate) and PN-EN 15958 (water)

Key parameters

Standard

Standard number

PN-EN 15956:2011 (wersja polska z 2015 r.; w katalogu PKN także wersja angielska; wprowadza EN 15956:2011 [IDT])

Title (PL)

Nawozy — Ekstrakcja fosforu rozpuszczalnego w kwasach mineralnych

Title (EN)

Fertilizers — Extraction of phosphorus soluble in mineral acids

Step-by-step procedure

1. Sample

   According to EN 1482-2; grind the laboratory sample (recommendation for the sake of homogeneity).

2. Test portion

   2.5 g ± 0.001 g into a dry Kjeldahl flask.

3. Acids

   15 ml of water and suspend the sample; 20 ml of HNO₃; carefully 30 ml of H₂SO₄.

4. Boiling

   After the violent reaction has ceased, slowly bring to the boil and boil for 30 min; cool, carefully add about 150 ml of water, boil for 15 min.

   ⏱ Time: 30 min + 15 min • 🌡 Temperature: boiling

5. Extract

   Cool completely, transfer to a 500 ml flask, make up to the mark, mix, filter through a dry pleated filter, discarding the first portion.

6. Determination and test report

   Phosphorus in the filtrate — according to PN-EN 15959. The test report includes, among other things, information on whether the repeatability limit requirement was met.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Concentrated nitric and sulfuric acids — fume cupboard, goggles, acid-resistant gloves; add the sulfuric acid carefully
• Violent reaction after adding the acids and evolution of nitrogen oxides — heat only after it has ceased, under an extraction hood
• Diluting the hot acid mixture with water — in small portions, while mixing

Frequently asked questions