ISO 22018:2021

INTERNATIONAL ISO
STANDARD 22018
First edition
2021-07
Fertilizers, soil conditioners
and beneficial substances —
Determination of EDTA soluble
phosphorus content in inorganic
fertilizers
Engrais, amendements et substances bénéfiques — Détermination
de la teneur en phosphore soluble dans l'EDTA dans les engrais
inorganiques
Reference number
©
ISO 2021
© ISO 2021
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ii © ISO 2021 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Reagents . 1
6 Apparatus and materials. 2
7 Test procedure . 3
7.1 General . 3
7.2 Preparation of test sample . 3
7.3 Weigh of test portion . 3
7.4 Extraction of EDTA soluble phosphorus . 3
7.5 Determination of the EDTA soluble phosphorus content . 3
7.6 Blank test . 3
8 Calculation and expression of results . 4
8.1 General . 4
8.2 Precision . 4
8.2.1 Ring test . 4
8.2.2 Repeatability, r .4
8.2.3 Reproducibility, R .4
9 Test report . 4
Annex A (informative) Ring test report . 6
Bibliography .14
Foreword
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iv © ISO 2021 – All rights reserved

Introduction
Owing to the variation of basic phosphate component, character and processing technology, traditionally,
different extraction solutions and methods have been utilized for the determination of the phosphorus
content of phosphate fertilizers. Extraction solutions currently employed for phosphorus content
determination include alkaline ammonium citrate (i.e. Petermann’s solution), neutral ammonium
citrate, citric acid, ethylene diamine tetraacetic acid (EDTA) and citric acid, or just EDTA. Each of these
extractants is designed to target specific phosphate components of phosphorus-based fertilizers.
Due to rapid developments in the modern fertilizer industry, especially with the formulation of
compound/complex fertilizers, many phosphate fertilizers may have multiple phosphorus-containing
components. The co-existence of these various phosphorus sources in a compound/complex fertilizer
can complicate the effective extraction and determination of the phosphorus content.
INTERNATIONAL STANDARD ISO 22018:2021(E)
Fertilizers, soil conditioners and beneficial substances —
Determination of EDTA soluble phosphorus content in
inorganic fertilizers
1 Scope
This document specifies the method for the determination of the EDTA soluble phosphorus content
of inorganic fertilizers. The method is applicable for fertilizers composed of or blended from multiple
sources such as superphosphate, ammonium phosphate, triple superphosphate, and/or nitrophosphate.
It is not suitable for fertilizers containing calcium magnesium phosphates.
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 proposed standard. 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 and soil conditioners — Vocabulary
ISO 8358, Solid fertilizers — Preparation of samples for chemical and physical analysis
ISO 14820-2, Fertilizers and liming materials — Sampling and sample preparation — Part 2: Sample
preparation
ISO 5725-2, Accuracy (trueness and precision) of measurement methods and results — Part 2: Basic method
for the determination of repeatability and reproducibility of a standard measurement method
3 Terms and definitions
For the purposes of this proposed standard, the terms and definitions given in ISO 8157 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at https:// www .iso .org/ obp
4 Principle
The EDTA soluble phosphorus content in inorganic fertilizers (calcium magnesium phosphate excluded)
is extracted by ethylene diamine tetraacetic acid disodium salt solution. Phosphorus content in the
extracting solution in the form of orthophosphate reacts with quimociac reagent in the acid medium
to form yellow quinolinium molybdophosphate precipitate. The EDTA soluble phosphorus content is
determined by gravimetric quinolinium molybdophosphate method.
5 Reagents
WARNING — Nitric acid is both corrosive and toxic. Refer to the applicable safety data sheet
(SDS). Quinoline is irritating to the eyes, skin and respiratory system. Refer to the applicable
SDS. The related operations shall be performed in the fume hood. This document does not point
out all possible safety problems, and the user shall bear the responsibility to take proper safety
and health measures.
NOTE Related laws and regulations of corresponding countries and/or regions can apply to these operations.
Analytical grade reagent (AR) chemicals shall be used in all tests, unless otherwise indicated. The
purity of water used throughout shall be understood to mean reagent water with electrical resistivity
≥ 18,2 MΩ·cm.
5.1 Ethylene diamine tetraacetic acid disodium salt dihydrate (C H N Na O ·2H O) solution
10 14 2 2 8 2
(37,5 g/l, eq. 0,1 mol/l).
Dissolve 37,5 g of EDTA with a certain amount of deionized water in a 1 000 ml beaker. Dilute by adding
deionized water to the 1 000 ml marking. Mix well.
5.2 Quimociac reagent.
The reagents used in the preparation of quimociac reagent:
— sodium molyddate (Na MoO 2H O, CAS: 10102-40-6);
2 4· 2
— citric acid (C H O H O, CAS: 5949-29-1);
6 8 7· 2
— nitric acid (HNO , 65 – 68 %, CAS: 7697-37-2);
— quinoline (2-azabicyclo[4.4.0]deca-1(6),2,4,7,9-pentaene, C H N, CAS: 91-22-5);
9 7
— acetone (C H O, CAS: 67-64-1).
3 6
To make solution a, add 70g sodium molybdate (Na MoO ·2H O) into a 400 ml beaker. Add 100 ml water
2 4 2
and stir to dissolve.
To make solution b, add 60 g citric acid (C H O ·H O) into a 1l beaker. Add 100 ml water and stir to
6 8 7 2
dissolve, then add 85 ml nitric acid (HNO , 65 % to 68 %).
To make solution c, add solution a into solution b, and mix well.
To make solution d, mix 85 ml nitric acid (HNO , 65 % to 68 %) with 100 ml water in a 400 ml beaker,
then add 5 ml quinoline (2-azabicyclo[4.4.0]deca-1(6),2,4,7,9-pentaene, C H N,CAS: 91-22-5). Mix well.
9 7
Add solution d into solution c, mix well and stand overnight. Filter the mixed solution with filter paper,
add 280 ml of acetone into the filtrate, then dilute the solution by adding water to 1 l. The as-prepared
quimociac reagent should be stored in a polyethylene bottle and preserved in a dark place to avoid light
and heat.
NOTE If the colour of quimociac reagent turns to light blue (caused by light), an appropriate amount of
potassium bromate solution (KBrO3, 10 g/l) can be added into the quimociac reagent until the colour disappears.
5.3 Nitric acid solution (HNO , 1 + 1).
Dilute a certain volume of 65 % to 68 % nitric acid (ρ = 1,39 g/ml to 1,40 g/ml) with equal volume of
water.
6 Apparatus and materials
6.1 Ordinary laboratory apparatus.
6.2 Electric thermostatic drying oven, temperature can be maintained at 180 °C ± 2 °C.
2 © ISO 2021 – All rights reserved

6.3 Glass filtering crucible, type IV (pore size: 5 μm to 15 μm), volume of 30 ml.
6.4 Thermostatic water bath oscillator, equipped with reciprocating oscillator or rotating oscillator
which temperature can be maintained at 60 °C ± 2 °C.
7 Test procedure
7.1 General
Replicate experiments shall be done for the determination.
7.2 Preparation of test sample
Prepare the test samples for analysis in accordance with ISO 8358 and ISO 14820-2.
7.3 Weigh of test portion
Weigh the appropriate amount of the test portion (accurately to 0,000 2 g) to obtain between 100 mg to
200 mg P O .
2 5
7.4 Extraction of EDTA soluble phosphorus
According to 7.3, weigh the appropriate amount of the test portion on the filter paper and wrap it,
put it into a 250 ml volumetric flask. Add 150 ml EDTA solution (5.1), plug the flask tightly, shake the
volumetric flask to break apart the filter paper and disperse the sample into the solution, then put it in the
thermostatic water bath oscillator (6.4) pre-set at 60 °C ± 2 °C, oscillate for 1 h at constant temperature
(set the oscillation frequency to make the sample in volumetric flask can flip over freely, and to keep the
sample suspended in the extracting solution while avoiding collection of excessive amounts of sample
on the upper portions of the interior of the volumetric flask). Then take the volumetric flask out, cool
down to ambient temperature, dilute the solution by water to scale, mix well and dry filter. Discard
the first part of the filtrate. Label the remaining filtrate as solution I for determining EDTA soluble
phosphorus content.
7.5 Determination of the EDTA soluble phosphorus content
Draw the appropriate amount of solution I (volume of v ) by single-line pipette, place into a 500 ml
beaker, add 10 ml nitric acid solution (5.3), dilute by water to 100 ml, heat on the hot plate till boiling,
and hold for 2 min to 3 min, remove and add 35 ml quimociac reagent (5.2), then cover with a watch
glass. Continue heating on the hot plate for 1 min or place covered-beaker into water bath pre-set at
close to boil until the precipitation is complete and the precipitate has settled to the bottom of the
beaker. Remove the beaker from the hot plate or water bath and allow to cool to ambient temperature.
Filter it by glass filter crucible (6.3), which was pre-dried to constant weight in drying oven at
180 °C ± 2 °C beforehand. Filter the supernatant first, and then wash the precipitate by decantation 1
to 2 times, use 25 ml of water each time, then transfer the precipitate to the glass filter crucible with
water and wash with an additional water, the total amount of water was 125 ml to 150 ml. Place the
glass filter crucible containing the precipitate in the drying oven pre-set at 180 °C ± 2 °C, drying for
45 min after the temperature reach 180 °C, take it out and transfer it into a desiccator and cool it down
to ambient temperature, weigh the precipitation together with the filter.
7.6 Blank test
Except for not adding samples, the blank test should be carried out exactly the same as the sample test
using the same amount of reagent by identical analysis procedures.
8 Calculation and expression of results
8.1 General
The EDTA soluble phosphorus content (w ) is expressed by a mass fraction percentage of P O as given
1 2 5
in Formula (1):
()mm−×0,03207
w = ×100 (1)
v
m ×
A
where
m is the weight of quinolinium molybdophosphate precipitation during the sample test, in g;
m is the weight of quinolinium molybdophosphate precipitation during the blank test, in g;
m is the weight of test portion used during the sample test for determination of EDTA soluble
A
phosphorus content, in g;
v is the volume of test solution I used during the sample test for determination of EDTA soluble
phosphorus content, in ml;
250 is the total volume of test solution I, in ml.
The reported value is the
...