ISO/TS 20917:2023

TECHNICAL ISO/TS
SPECIFICATION 20917
First edition
2023-07
Fertilizers, soil conditioners
and beneficial substances —
Determination of ammonium citrate,
disodium-EDTA soluble phosphorus
and potassium by ICP-OES in inorganic
fertilizers
Engrais, amendements et substances bénéfiques – Détermination
de la teneur en phosphore et potassium solubles dans le citrate
d’ammonium et l’EDTA disodique par ICP-OES dans les engrais
inorganiques
Reference number
© ISO 2023
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Reagents . 2
6 Apparatus . 3
7 Sampling and sample preparation . 4
8 Procedure .4
8.1 Extraction of P and K . 4
8.2 Instrument conditions . 4
8.3 Preparation of standards . 5
8.3.1 Standards from reagent salts . 5
8.3.2 Standards from commercial stock solutions . 5
8.3.3 Calibration and wavelength splitting . 7
8.3.4 Analysis . 7
8.4 Quality control . 7
8.4.1 Reference material . 7
8.4.2 Duplicates . 7
9 Calculations and expression of results . 7
9.1 Calculations . 7
9.2 Precision . . . 8
9.2.1 General . 8
9.2.2 Repeatability, r . 9
9.2.3 Reproducibility, R . 9
10 Test report .10
Annex A (informative) Ring test background and results .11
Annex B (informative) Critical factors .24
Bibliography .26
iii
Foreword
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beneficial substances.
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iv
Introduction
Inductively coupled plasma-optical emission spectrometer (ICP-OES) instrumentation is becoming
increasingly popular in fertilizer testing laboratories (see References [1], [2] and [3]). Because these
instruments are automated and can determine multiple elements simultaneously, they offer significant
productivity gains. Also, the method does not generate hazardous wastes. The ammonium citrate
disodium etheylenediamine tetraacetic acid (EDTA) organic solvent and samples containing high
phosphorus and/or potassium concentrations pose some analytical challenges for the instrumentation,
which must be addressed. Due to the increasing use of this technology, this document was created.
For organic fertilizers and for liquid fertilizers containing phosphorous acid (H PO or phosphite), an
3 3
alternative method that specifically measures PO is preferred since the ICP-OES measures all elemental
phosphorus, which can result in a high bias for these types of fertilizer materials (see Reference [1]).
Due to the inability of the ICP-OES to differentiate between phosphorus species, under normal
operating conditions, this method is primarily intended for fertilizer materials in which the source(s)
of phosphorus is known. While this document can be used with minimal bias, the repeatability and
reproducibility is not consistent with other standards routinely used for inspection at this time, such as
[4] [5] [6]
EN 15959 , EN 15477 or ISO 22018 .
As a result, this document is intended for screening purposes, where a large number of fertilizer
phosphorus and potassium results must be obtained as efficiently as possible.
v
TECHNICAL SPECIFICATION ISO/TS 20917:2023(E)
Fertilizers, soil conditioners and beneficial substances —
Determination of ammonium citrate, disodium-EDTA
soluble phosphorus and potassium by ICP-OES in inorganic
fertilizers
1 Scope
This document specifies an inductively coupled plasma-optical emission spectrometer (ICP-OES)
method for the determination of the ammonium citrate, disodium-etheylenediamine tetraacetic acid
(EDTA) soluble phosphorus and potassium content in fertilizer extract solutions.
This document is applicable to all inorganic fertilizer products, whether compound or complex fertilizer
products or blends thereof, and straight concentrated fertilizer products.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 8157, Fertilizers, soil conditioners and beneficial substances — Vocabulary
ISO 14820-1, Fertilizers and liming materials — Sampling and sample preparation — Part 1: Sampling
ISO 14820-2, Fertilizers and liming materials — Sampling and sample preparation — Part 2: Sample
preparation
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 8157 apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Principle
Phosphorus and potassium contained in inorganic fertilizers that is soluble in a heated, dilute
ammonium citrate and disodium-EDTA solvent is determined. The neutral ammonium citrate is the
primary solvent for phosphate, while the EDTA is mainly included to chelate calcium, magnesium and
other soluble metallic cations to minimize their interaction with phosphate and eliminate the water-
wash step (see Reference [7]). Also, since this solvent is a relatively weak solution, it extracts primarily
the highly soluble potassium compounds and has been used for fertilizer potassium determination
since 1993. It compares very favourably with other soluble potash solvents such as ammonium oxalate.
The user of this document is not obligated to determine both phosphorus and potassium; however, one
of the main advantages of this approach is to measure both simultaneously in a single extract in a more
productive, cost-effective way.
The resulting extract solution is tested by ICP-OES to quantify the levels of phosphorus and potassium,
which can be converted to their equivalent P O and K O concentrations. The extract solution is
2 5 2
nebulized to produce a fine aerosol, which is introduced to a plasma or high energy source to ionize
or excite electrons to higher energy orbitals. As these electrons return to ground state, they emit
characteristic ultraviolet or vi
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