🌱 Phosphorus soluble in neutral ammonium citrate — extraction at 65 °C — PN-EN 15957

Physicochemistry PN-EN 15957

Overview

WHAT THIS TEST IS — SCOPE FROM THE PKN CARD (Polish version), IN FULL: "W niniejszym dokumencie określono metodę ekstrakcji fosforu rozpuszczalnego w obojętnym cytrynianie amonu. Metoda ma zastosowanie do wszystkich nawozów, dla których w Rozporządzeniu (WE) 2003/2003, Załącznik I jest określona rozpuszczalność w obojętnym cytrynianie amonu" [This document specifies a method for the extraction of phosphorus soluble in neutral ammonium citrate. The method is applicable to all fertilizers for which solubility in neutral ammonium citrate is specified in Regulation (EC) 2003/2003, Annex I]. The same scope stands in clause 1 of the text of EN 15957:2011. Regulation (EC) No 2003/2003 is repealed with effect from 16 July 2022 (Article 51 of Regulation (EU) 2019/1009, which requires references to it to be construed as references to Regulation 2019/1009).

PRINCIPLE (clause 4): extraction of phosphorus at a temperature of 65 °C with a neutral ammonium citrate solution of pH = 7, under specified conditions. The phosphorus itself is determined separately in the extract — in 16 of the 16 entries of accreditation scopes PN-EN 15959 stands next to this standard (a separate entry in this database).

THE REAGENT DECIDES THE RESULT (6.2). The neutral ammonium citrate solution of pH = 7 contains 185 g of crystalline citric acid per litre and has a relative density of 1.09 at 20 °C. Preparation: dissolve 370 g of crystalline citric acid (C₆H₈O₇·H₂O) in about 1.5 l of water and neutralize with approximately 345 ml of ammonia solution containing 28 % to 29 % NH₃ (with weaker ammonia, correspondingly more solution and less water). Cool, then bring EXACTLY to pH 7 at 20 °C by adding ammonia dropwise with continuous mixing by a mechanical stirrer and with the pH meter electrodes immersed in the solution. Make up to 2 l and check the pH again. The solution is stored in a closed vessel, and its pH is checked at regular intervals.

PROCEDURE (clause 8): • 1 g or 3 g of the laboratory sample (for the size of the test portion the standard refers to Annex I A and B of the regulation) is poured into a 200 ml or 250 ml conical flask containing 100 ml of the ammonium citrate solution previously heated to 65 °C; • the flask is closed and shaken so as to suspend the sample without lumps; the stopper is removed for a moment to equalize the pressure, and the flask is closed again; • the flask is placed in a water bath keeping the contents at EXACTLY 65 °C and connected to the stirrer; the level of the suspension must remain below the water level in the bath at all times; without a mechanical stirrer, the flask may be shaken by hand every 5 min; • after EXACTLY 1 h the flask is removed, immediately cooled under running water to ambient temperature and at once transferred quantitatively with a jet of water from a wash bottle to a 500 ml volumetric flask; it is made up to the mark, mixed and filtered through a dry pleated filter (medium-speed, phosphate-free), discarding the first approx. 50 ml; about 100 ml of clear filtrate is collected.

WATER BATH WITH A STIRRER (7.5, Figure 1): thermostatted at 65 °C; in the figure — a contact thermometer, an arm moved by an eccentric, a geared motor and a belt drive, a copper bath. The test report has to state, among other things, whether the repeatability limit requirement was met.

EDITIONS (state of the PKN card on 25.09.2026). PN-EN 15957:2011 — Polish version, publication 11-06-2015, 10 pages, KT 156 Fertilizers, introduces EN 15957:2011 [IDT], without the "Wycofana" [Withdrawn] header; the catalogue also has the English version. EN 15957:2011 replaced CEN/TS 15957:2009. At the publisher (the CEN catalogue run by iTeh, read on 25.09.2026): published 28-09-2011; stage 90.93 — decision to confirm after the systematic review (enquiry from 20-06-2023, closing 23-09-2026).

HOW ACCREDITATION SCOPES CITE THE STANDARD. Measurement of 25.09.2026 on the database of PL accreditation scopes (1310 laboratories with entries): 16 entries at 16 laboratories, all with the notation "PN-EN 15957:2011". The subject of the test is "Nawozy mineralne (nieorganiczne)" [Mineral (inorganic) fertilizers] (14); the methodology most often "Zawartość fosforu rozpuszczalnego w obojętnym cytrynianie amonu" [Content of phosphorus soluble in neutral ammonium citrate] (10); the technique "Metoda wagowa" [Gravimetric method] (15) and once "Metoda wagowa po ekstrakcji w obojętnym roztworze cytrynianu amonu" [Gravimetric method after extraction in neutral ammonium citrate solution]. In the same entries: PN-EN 15959:2024-05 (12), PN-EN 15959:2011 (4), PN-EN 15956:2011 and PN-EN 15958:2011 (2 each), PN-C-87015:1988 clause 6 (1). The declared ranges (in % P₂O₅ or % P) have lower limits from 0.2 % to 5.0 %, and upper ones from 23.6 % to 60 %.

THE KEY OF THE "LABORATORIES" TAB: "PN-EN 15957" — 16 laboratories and 16 entries, without foreign entries.

WHAT WE DO NOT GIVE. We had a fragment of the text of EN 15957:2011 made available by the publisher: the foreword and clauses 1–9. We do not have the bibliography or the content of Annex I A and B of the regulation, to which the standard refers for the choice of a 1 g or 3 g test portion — which is why we do not write when each one is used.

Method principle

Phosphorus is extracted with a neutral ammonium citrate solution (pH 7 at 20 °C, 185 g of citric acid per litre) at 65 °C: 1 g or 3 g of fertilizer in 100 ml of solution, exactly 1 h of stirring in a water bath, immediate cooling, dilution to 500 ml and filtration. The phosphorus in the filtrate is determined according to PN-EN 15959.

Applications

• Fertilizers for which phosphorus soluble in neutral ammonium citrate is declared (among others phosphate and compound fertilizers)
• One of the three forms of phosphorus declared in fertilizers — next to the water-soluble form (PN-EN 15958) and the form soluble in mineral acids (PN-EN 15956)
• The first stage of the "extraction + determination" pair: PN-EN 15957 with PN-EN 15959

Key parameters

Standard

Standard number

PN-EN 15957:2011 (wersja polska z 2015 r.; w katalogu PKN także wersja angielska; wprowadza EN 15957:2011 [IDT])

Title (PL)

Nawozy — Ekstrakcja fosforu rozpuszczalnego w obojętnym cytrynianie amonu

Title (EN)

Fertilizers — Extraction of phosphorus soluble in neutral ammonium citrate

Step-by-step procedure

1. Ammonium citrate solution

   370 g of citric acid in about 1.5 l of water, about 345 ml of ammonia 28–29 %; cool, bring exactly to pH 7 at 20 °C, make up to 2 l, check the pH again.

   🌡 Temperature: 20 °C when adjusting the pH

2. Test portion

   1 g or 3 g of sample into a conical flask containing 100 ml of the solution heated to 65 °C; close, shake without lumps, equalize the pressure.

3. Extraction

   Water bath — the contents of the flask at exactly 65 °C, mechanical stirring (or by hand every 5 min); the suspension below the water level.

   ⏱ Time: exactly 1 h • 🌡 Temperature: 65 °C

4. Cooling and dilution

   Cool immediately under running water, at once transfer quantitatively to a 500 ml flask, make up to the mark, mix.

5. Filtration

   Through a dry pleated filter into a dry vessel; discard about 50 ml, collect about 100 ml of clear filtrate.

6. Determination

   Phosphorus in the filtrate — according to PN-EN 15959; the test report includes, among other things, information on the repeatability limit.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Ammonia solution 28–29 % — irritant and corrosive; neutralization in a fume cupboard, with cooling
• Flasks with hot solution — equalize the pressure by briefly opening the stopper
• Water bath with a mechanical drive — guard over the moving parts

Frequently asked questions