ISO/DIS 25749

Fertilizers -- Determination of sulfates content using three different methods

Engrais -- Dosage des sulfates selon trois méthodes différentes

Standards Content (sample)

DRAFT INTERNATIONAL STANDARD
ISO/DIS 25749
ISO/TC 134 Secretariat: ISIRI
Voting begins on: Voting terminates on:
2014-12-04 2015-03-04
Fertilizers — Determination of sulfates content using three
different methods
Engrais — Dosage des sulfates selon trois méthodes différentes
ICS: 65.080
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NATIONAL REGULATIONS.
ISO/DIS 25749:2014(E)
RECIPIENTS OF THIS DRAFT ARE INVITED
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
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PROVIDE SUPPORTING DOCUMENTATION. ISO 2014
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ISO/DIS 25749:2014(E)
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EN 15749:2009 (E)
Contents Page

Foreword ..............................................................................................................................................................3

Introduction .........................................................................................................................................................4

1 Scope ......................................................................................................................................................5

2 Normative references ............................................................................................................................5

3 Terms and definitions ...........................................................................................................................5

4 Principle ..................................................................................................................................................5

5 Sampling and sample preparation .......................................................................................................6

6 Method A – Gravimetric method ..........................................................................................................6

7 Method B – ICP-OES method ................................................................................................................7

8 Method C – IC method ........................................................................................................................ 11

9 Precision of methods A, B and C ...................................................................................................... 17

10 Test report ........................................................................................................................................... 18

Annex A (informative) Statistical results of the inter-laboratory tests ....................................................... 19

Bibliography ..................................................................................................................................................... 22

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EN 15749:2009 (E)
Foreword

This document (EN 15749:2009) has been prepared by Technical Committee CEN/TC 260 “Fertilizers and

liming materials”, the secretariat of which is held by DIN.

This European Standard shall be given the status of a national standard, either by publication of an identical

text or by endorsement, at the latest by April 2010, and conflicting national standards shall be withdrawn at the

latest by April 2010.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent

rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.

This document has been prepared under a mandate given to CEN by the European Commission and the

European Free Trade Association.
This document supersedes CEN/TS 15749:2008.

According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following

countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Cyprus, Czech

Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,

Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,

Sweden, Switzerland and the United Kingdom.
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EN 15749:2009 (E)
Introduction

This document specifies three different methods for the determination of sulfur. Based on the statistical results

of the inter-laboratory tests, obtained with the same samples, the three methods produce equivalent results,

and hence can be used all three methods on decision of the user and availability of equipment.

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EN 15749:2009 (E)
1 Scope

This European Standard specifies three different methods (Methods A, B and C) for the determination of sulfur

present in fertilizers extracts in the form of sulfates. Method A specifies the gravimetric method. Method B

specifies the method using inductively coupled plasma optical spectrometry (ICP-OES). Method C specifies

the method using ion chromatography (IC).
2 Normative references

The following referenced documents are indispensable for the application 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.

EN 1482-2, Fertilizers and liming materials - Sampling and sample preparation - Part 2: Sample preparation

EN 12944-1:1999, Fertilizers and liming materials and soil improvers - Vocabulary - Part 1: General terms

EN 12944-2:1999, Fertilizers and liming materials and soil improvers - Vocabulary - Part 2: Terms relating to

fertilizers
CEN/TS 15925, Fertilizers — Extraction of total sulfur present in various forms

CEN/TS 15926, Fertilizers — Extraction of water soluble sulfur where the sulfur is in various forms

CEN/TS 15960, Fertilizers — Extraction of total calcium, total magnesium, total sodium and total sulfur in the

forms of sulfates

CEN/TS 15961, Fertilizers — Extraction of water soluble calcium, magnesium, sodium and sulfur (in the forms

of sulfates)

EN ISO 3696, Water for analytical laboratory use - Specification and test methods (ISO 3696:1987)

3 Terms and definitions

For the purposes of this document, the terms and definitions given in EN 12944-1:1999 and EN 12944-2:1999

apply.
4 Principle
4.1 Method A: Gravimetric method

Sulfur is extracted from the sample according to the methods described in CEN/TS 15925, CEN/TS 15926,

CEN/TS 15960 or CEN/TS 15961 and determined based on the gravimetric determination as barium sulfate.

4.2 Method B: ICP-OES

Sulfur is extracted from the sample according to the methods described in CEN/TS 15925, CEN/TS 15926,

CEN/TS 15960 or CEN/TS 15961 and its concentration in the extract is measured by inductively coupled

plasma-optical emission spectrometry (ICP-OES).
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EN 15749:2009 (E)
4.3 Method C: IC

Sulfur is extracted from the fertilizer according to the methods described in CEN/TS 15925, CEN/TS 15926,

CEN/TS 15960 or CEN/TS 15961 in the form of sulfate. The sulfate concentration of the extract is measured

by ion chromatography (IC) equipped with a suppressor device and a conductivity detector.

5 Sampling and sample preparation

Sampling is not part of the methods specified in this European Standard. A recommended sampling method is

given in EN 1482-1.
Sample preparation shall be carried out in accordance with EN 1482-2.
6 Method A – Gravimetric method
6.1 Reagents

Use only reagents of recognized analytical grade and distilled or demineralized water (grade 3 according to

EN ISO 3696).
6.1.1 Diluted hydrochloric acid
Mix one volume of ρ(HCl) = 1,18 g/ml with one volume of water.
6.1.2 Barium chloride solution, ρ(BaCl 2 H O) = 122 g/l.
2 2
6.1.3 Silver nitrate solution, ρ = 5 g/l.
6.2 Apparatus
6.2.1 Porcelain crucibles
6.2.2 Hot water bath
6.2.3 Drying oven, set at 105 °C ± 1 °C.
6.2.4 Electric oven, set at 800 °C ± 50 °C.
6.3 Procedure
6.3.1 Sampling of the solution

Pipette an aliquot part of one of the extraction solutions containing between 20 mg and 100 mg of sulfur or

50 mg and 250 mg of SO .

Place this aliquot in a beaker of suitable capacity. Add 20 ml of diluted hydrochloric acid (6.1.1). Make up to

about 300 ml with water.
6.3.2 Preparation of the precipitate

Bring the solution to the boil. Add, drop by drop, about 20 ml of the barium chloride solution (6.1.2) while

stirring the solution vigorously. Boil for a few minutes.
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EN 15749:2009 (E)

Place the beaker, covered with a watch glass, in a boiling hot water bath (6.2.2) for 1 h. Then leave standing

hot (± 60 °C) until the supernatant liquor is clear. Decant the clear solution through a slow filtration ash-free

filter. Wash the precipitate several times with hot water. Continue to wash the precipitate on the filter until the

filtrate is chloride free. This can be checked by using a silver nitrate solution (6.1.3).

6.3.3 Incineration and weighing of the precipitate

Place the filter paper and precipitate in a porcelain crucible (6.2.1) previously weighed to the nearest 0,1 mg.

Dry in the oven (6.2.3) and ash at approximately 800 °C for half an hour (6.2.4). Allow to cool in a desiccator

and weigh to 0,1 mg.
6.4 Calculation and expression of the result
1 mg of barium sulfate corresponds to 0,137 mg of sulfur or to 0,343 mg of SO .

Calculate the sulfates content, w , as mass fraction in percent of the fertilizer according to Equation (1):

w =m × 0,0137 × (1)
S 1
v ×m
2 2

Calculate the SO content, wSO , as mass fraction in percent of the fertilizer according to Equation (2):

3 3
w = w × 2,5 (2)
SO S
where
m is the mass of the barium sulfate precipitate, in mg;
m is the mass of the test portion, in g;
v is the volume of the extraction solution, in ml;
v is the aliquot volume, in ml.
7 Method B – ICP-OES method
7.1 Reagents

Use only reagents of recognized analytical grade, and water conforming to grade 2 of EN ISO 3696.

Stock solutions shall be replaced after a maximum of one year, but the standard solution shall be freshly

prepared monthly as a minimum.
7.1.1 Hydrochloric acid, ρ approximately 1,18 g/ml;

7.1.2 Diluted hydrochloric acid, mix 40 ml of hydrochloric acid (7.1.1) in 1 l of water;

7.1.3 Sulfur stock solution, corresponding to 1 000 mg/l sulfur.

Dry in a pre-treatment step some grams of sodium sulfate (Na SO ) at 105 °C for 1 h. Let cool in a desiccator.

2 4

Weigh to the nearest 0,1 mg, approximately 4,437 5 g of sodium sulfate (Na SO ). Dissolve the weighted

2 4
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EN 15749:2009 (E)

mass in a small quantity of water in volumetric flasks of nominal capacity of 1 000 ml, fill to the mark with

water.
The solution is stable for several months if stored at 4 °C to 6 °C.

NOTE Sulfur stock solution of 1 000 mg/l is also readily available commercially, and may be used instead.

7.2 Apparatus

Common laboratory equipment and glassware, in particular equipment according to 7.2.1 to 7.2.2.

7.2.1 Analytical balance, capable of weighing to an accuracy of 0,1 mg.
7.2.2 Inductively coupled plasma – Optical emission spectrometer

Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) with radial viewing of the plasma and

simultaneous measurement of emission signals.

For measuring sulfur at wavelengths below 190 nm the optical system shall be evacuated or be filled or

continuously flushed with an inert gas as recommended by the instrument manufacturer to obtain high and

stable signal intensities.

The instrument shall be equipped with radial plasma as a minimum requirement; axial plasma is equally

acceptable, as long as it can be shown that the results are statistically equal to the results obtained with radial

plasma. Background correction shall also be performed. Settings of the working conditions (e.g. viewing

height, gas flows, RF or plasma power, sample uptake rate, integration time, number of replicates) shall be

optimized according the manufacturer’s instructions.
7.3 Preparation of the extract

The sulfur is extracted from the sample according to one of the methods described in CEN/TS 15925,

CEN/TS 15926, CEN/TS 15960 or CEN/TS 15961.
7.4 Procedure
7.4.1 General

Calibration shall be performed by means of the standard addition technique. This method allows the analysis

of fertilizers with unknown matrix composition or with a matrix that cannot be synthetically imitated easily.

7.4.2 Preparation of the test solution

Dilute the extraction solution with the diluted hydrochloric acid (7.1.1) to obtain a concentration between

10 mg/l and 150 mg/l of sulfur.
7.4.3 Preparation of the blank test solution

Carry out a blank test at the same time as the extraction, with only the reagents.

7.4.4 Preparation of the calibration solutions

The additions to the test solution, prior to the dilution step, should be about 20 %, 50 % and 100 % of the

expected sulfur content. After the additions, dilute each of the samples with the diluted hydrochloric acid

(7.1.1).
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EN 15749:2009 (E)

A (external) calibration curve method can also be used instead of the standard addition method where the

analytical results are demonstrated to be statistically equal. Appropriate matrix matching of the calibration

solutions shall be performed if an (external) calibration method is used.
7.5 Determination
7.5.1 General

Set up the instrument according to the manufacturer’s instructions using appropriate conditions, and with the

suitable background correction system in operation.

For each instrument used, selectivity, limits of detection and quantification, precision, linear working area, and

interference shall be established separately.
7.5.2 Determination by ICP-OES

Aspirate the blank test solution (7.4.3), the test solution (7.4.2) and the various additions (7.4.4) in ascending

order separately into the plasma, and measure the emission of sulfur. Perform at least two replicates and

average the values if the values fall within an accepted range. After each measurement, aspirate water or

diluted hydrochloric acid (7.1.2).

The following special procedures can be used to obtain high precision and accuracy necessary to analyse

high sulfur contents in fertilizers:
a) optimised instrument settings;

b) simultaneous measurement of the signals of analyte and an appropriate internal standard element;

c) adjustment of the matrix of calibration solutions to the sample matrix;

d) consistent identical treatment of samples and calibration solutions during preparation and measurement,

or use gravimetric dilution instead of volumetric dilution;
e) correction of instrumental drift and background (if required).
Table 1 — General analytical conditions for sulfur determination by ICP-OES
181,978
alternatives:
Wavelength, nm
180,676
182,568
Background correction Yes
Working range mg/l 1 to 150
202,030 nm
Possible internal standard:
Molybdenum
281,615 nm
7.6 Calculation and expression of the results

NOTE Net signal is defined as the number of counts per time unit at the selected wavelength, corrected for

background contributions.
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EN 15749:2009 (E)
7.6.1 Calculation

In case of several additions, regression techniques on the linear model of variable y as a function of variable x,

have to be used to determine the sulfur concentration of the test solution. Generally, this model can be written

as:
y = a +b ⋅x (3)
i i
In this particular case of three standard additions:
y = S (i = 0, 1, 2, 3) (4)
i i
x = ρ ⋅V (i = 0, 1, 2, 3) (5)
i s i
where
ρ is the concentration, in mg/l, of the standard solution;
V are the various volumes, in l, of the standard solution added;
S are the net signals after the various additions.
Calculate the values of a and b as follows:
n ⋅ x y − x y
¦ i i¦¦i i
b = (6)
n ⋅ x −()x
¦¦i i
y −b ⋅ x
¦¦i i
a = (7)
where
n is the number of solutions measured (n = 4 in case of three additions).

Calculate the sulfur concentration ρ , in mg/l, of the filtrate of the test portion using the following equation:

ρ = (8)
where

V is the volume, in l, of the filtrate of the test portion used to prepare the test solution.

7.6.2 Expression of the sulfur content in the sample

The sulfur content in the sample or mass fraction of sulfur, w , expressed in mg of sulfur per kg of fertilizer, is

determined using the following equation:
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EN 15749:2009 (E)
(ρ − ρ )
f bl
w = ⋅V (9)
S t
where
ρ is the concentrat
...