oSIST prEN ISO 15604:2019

Fertilizers - Determination of different forms of nitrogen in the same sample, containing nitrogen as nitric, ammoniacal, urea and cyanamide nitrogen (ISO 15604:2016)

ISO 15604:2016 specifies a method for the determination of any one form of nitrogen in the presence of any other form.
The method is applicable to any fertilizer provided for in the Regulation (EC) No 2003/2003, Annex I[2] containing nitrogen in various forms.

Düngemittel - Bestimmung verschiedener, nebeneinander anwesender Stickstoff-Formen in derselben Probe mit Stickstoff in Form von Ammonium, Nitrat, Harnstoff und Cyanamid (ISO 15604:2016)

Engrais - Détermination des différentes formes d'azote dans un même échantillon d'engrais contenant l'azote sous forme nitrique ammoniacale, uréique et cyanamidique (ISO 15604:2016)

La présente Norme internationale spécifie une méthode de détermination d'une forme d'azote quelle qu'elle soit, en présence d'une autre forme, quelle qu'elle soit.
La méthode s'applique à tout engrais indiqué dans l'Annexe I du Règlement (CE) n° 2003/2003[2] contenant de l'azote sous différentes formes.

Gnojila - Določevanje različnih oblik dušika v istem vzorcu, ki vsebuje dušik v nitratni, amonijevi, sečninski in cianoamidni obliki (ISO 15604:2016)

General Information

• Status: Not Published
• Public Enquiry End Date: 03-Jul-2019
• ICS: 65.080 - Fertilizers
• Technical Committee: KAT - Soil quality
• Current Stage: 98 - Abandoned project (Adopted Project)
• Start Date: 20-Aug-2020
• Due Date: 25-Aug-2020
• Completion Date: 20-Aug-2020
• Ref Project: prEN ISO 15604 - Fertilizers - Determination of different forms of nitrogen in the same sample, containing nitrogen as nitric, ammoniacal, urea and cyanamide nitrogen (ISO 15604:2016)

Relations

• Revised: SIST EN 15604:2009 - Fertilizers - Determination of different forms of nitrogen in the same sample, containing nitrogen as nitric, ammoniacal, urea and cyanamide nitrogen
• Effective Date: 01-Jun-2019

Standards Content (Sample)

SLOVENSKI STANDARD
oSIST prEN ISO 15604:2019
01-junij-2019
Gnojila - Določevanje različnih oblik dušika v istem vzorcu, ki vsebuje dušik v
nitratni, amonijevi, sečninski in cianoamidni obliki (ISO 15604:2016)
Fertilizers - Determination of different forms of nitrogen in the same sample, containing
nitrogen as nitric, ammoniacal, urea and cyanamide nitrogen (ISO 15604:2016)
Düngemittel - Bestimmung verschiedener, nebeneinander anwesender Stickstoff-
Formen in derselben Probe mit Stickstoff in Form von Ammonium, Nitrat, Harnstoff und
Cyanamid; Deutsche und Englische Fassung prEN ISO 15604:2018
Engrais - Détermination des différentes formes d'azote dans un même échantillon
d'engrais contenant l'azote sous forme nitrique ammoniacale, uréique et cyanamidique
(ISO 15604:2016)
Ta slovenski standard je istoveten z: prEN ISO 15604
ICS:
65.080 Gnojila Fertilizers
oSIST prEN ISO 15604:2019 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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oSIST prEN ISO 15604:2019
INTERNATIONAL ISO
STANDARD 15604
First edition
2016-05-01
Fertilizers — Determination of
different forms of nitrogen in the same
sample, containing nitrogen as nitric,
ammoniacal, urea and cyanamide
nitrogen
Engrais — Détermination des différentes formes d’azote dans
un même échantillon contenant l’azote sous forme nitrique,
ammoniacale, uréique et cyanamidique
Reference number
ISO 15604:2016(E)
©
ISO 2016

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oSIST prEN ISO 15604:2019
ISO 15604:2016(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2016, Published in Switzerland
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
the requester.
ISO copyright office
Ch. de Blandonnet 8 • CP 401
CH-1214 Vernier, Geneva, Switzerland
Tel. [phone]
Fax [phone]
[e-mail]
www.iso.org
ii © ISO 2016 – All rights reserved

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Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
4.1 Total soluble and insoluble nitrogen . 1
4.2 Forms of soluble nitrogen. 2
4.2.1 General. 2
4.2.2 Total soluble nitrogen . 2
4.2.3 Total soluble nitrogen with the exception of nitrate nitrogen . 2
4.2.4 Nitrate nitrogen by difference . 2
4.2.5 Ammoniacal nitrogen . 2
4.2.6 Urea nitrogen . 2
4.2.7 Cyanamide nitrogen . 3
5 Reagents . 3
6 Apparatus . 5
7 Sampling and sample preparation .10
8 Procedure.10
8.1 Total soluble and insoluble nitrogen .10
8.1.1 In the absence of nitrates .10
8.1.2 In the presence of nitrate .11
8.2 Forms of soluble nitrogen.12
8.2.1 Preparation of the solution to be analysed .12
8.2.2 Total soluble nitrogen .13
8.2.3 Total soluble nitrogen with the exception of nitrate nitrogen .14
8.2.4 Nitrate nitrogen .14
8.2.5 Ammoniacal nitrogen .15
8.2.6 Urea nitrogen .16
8.2.7 Cyanamide nitrogen .18
9 Verification of the result .18
10 Test report .18
Bibliography .19
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Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity assessment,
as well as information about ISO’s adherence to the World Trade Organization (WTO) principles in the
Technical Barriers to Trade (TBT) see the following URL: www.iso.org/iso/foreword.html.
ISO 15604 was prepared by CEN/TC 260 as EN 15604:2009 and was adopted (without modification
other than those stipulated below) by Technical Committee ISO/TC 134, Fertilizers and soil conditioners.
Modifications were made as follows:
a) 5.2: p.a. = pro analysis = analytical grade;
b) 6.2: add “Refer to Figure 1”.
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oSIST prEN ISO 15604:2019
INTERNATIONAL STANDARD ISO 15604:2016(E)
Fertilizers — Determination of different forms of nitrogen
in the same sample, containing nitrogen as nitric,
ammoniacal, urea and cyanamide nitrogen
1 Scope
This International Standard specifies a method for the determination of any one form of nitrogen in the
presence of any other form.
[2]
The method is applicable to any fertilizer provided for in the Regulation (EC) No 2003/2003, Annex I
containing nitrogen in various forms.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 3696, Water for analytical laboratory use — Specification and test methods
ISO 14820-2, Fertilizers and liming materials — Sampling and sample preparation — Part 2: Sample
preparation
ISO 25475, Fertilizers — Determination of ammoniacal nitrogen
EN 12944-1, Fertilizers and liming materials — Vocabulary — Part 1: General terms
EN 12944-2, Fertilizers and liming materials — Vocabulary — Part 2: Terms relating to fertilizers
EN 15562, Fertilizers — Determination of cyanamide nitrogen
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 12944-1 and EN 12944-2 apply.
4 Principle
4.1 Total soluble and insoluble nitrogen
[2]
According to the list of standard fertilizers given in Regulation (EC) No 2003/2003, Annex I, this
determination is applicable to products containing calcium cyanamide.
In the absence of nitrates, the test sample is mineralized by direct Kjeldahl digestion.
In the presence of nitrates, the test sample is mineralized by Kjeldahl digestion after reduction with the
aid of metallic iron and stannous chloride.
In both cases, the ammonia is determined according to ISO 25475.
NOTE If analysis shows an insoluble nitrogen content of more than 0,5 %, one concludes that the fertilizer
[2]
contains other forms of insoluble nitrogen not included in the list in Regulation (EC) No 2003/2003, Annex I.
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4.2 Forms of soluble nitrogen
4.2.1 General
The forms of soluble nitrogen referred to in 4.2.2 to 4.2.7 are determined from different aliquots taken
from the same solution of the test sample.
4.2.2 Total soluble nitrogen
4.2.2.1 In the absence of nitrates, by direct Kjeldahl digestion. The ammonia is then determined (by
the same method as that described in ISO 25475).
4.2.2.2 In the presence of nitrates, by Kjeldahl digestion on an aliquot part taken from the solution after
reduction according to Ulsch. The ammonia is then determined (by the same method as that described in
ISO 25475).
4.2.3 Total soluble nitrogen with the exception of nitrate nitrogen
By Kjeldahl digestion after elimination in an acid medium of nitrate nitrogen with ferrous sulfate. The
ammonia is then determined (by the same method as that described in ISO 25475).
4.2.4 Nitrate nitrogen by difference
4.2.4.1 In the absence of calcium cyanamide, by determining the difference between the nitrogen
determined as summarized in 4.2.2.2 and that determined as summarized in 4.2.3 or between total soluble
nitrogen (see 4.2.2) and the sum of ammoniacal nitrogen and ureic organic nitrogen (4.2.5 + 4.2.6).
4.2.4.2 In the presence of calcium cyanamide, by determining the difference between the nitrogen
determined as summarized in 4.2.2.2 and that determined as summarized in 4.2.3 or between the
nitrogen determined as summarized in 4.2.2.2 and the sum of that determined as summarized in 4.2.5,
4.2.6 and 4.2.7.
4.2.5 Ammoniacal nitrogen
4.2.5.1 Solely in the presence of ammoniacal nitrogen and ammoniacal plus nitrate nitrogen, according
to ISO 25475.
4.2.5.2 In the presence of urea nitrogen and/or cyanamide nitrogen by cold distillation after making
slightly alkaline, the ammonia is absorbed in a standard solution of sulfuric acid and determined
according to ISO 25475.
4.2.6 Urea nitrogen
4.2.6.1 By conversion using urease into ammonia which is titrated with a standard solution of
hydrochloric acid.
or
4.2.6.2 By gravimetry with xanthydrol: the co-precipitated biuret can be counted with urea nitrogen
without great error, its content remaining generally low in absolute value in compound fertilizers.
or
4.2.6.3 By difference according to Table 1.
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Table 1 — Determination of urea nitrogen by difference
Nitrate Ammoniacal Cyanamidic
Case Difference
nitrogen nitrogen nitrogen
1 Absent Present Present (4.2.2.1) - (4.2.5.2 + 4.2.7)
2 Present Present Present (4.2.3) - (4.2.5.2 + 4.2.7)
3 Absent Present Absent (4.2.2.1) - (4.2.5.2)
4 Present Present Absent (4.2.3) - (4.2.5.2)
4.2.7 Cyanamide nitrogen
By precipitation as a silver compound, the nitrogen being determined in the precipitate by the
Kjeldahl method.
5 Reagents
5.1 General.
Use only reagents of recognized analytical grade and distilled or de-mineralized water of grade 3
according to ISO 3696.
5.2 Potassium sulfate, p.a. (p.a. = pro analysis = analytical grade).
5.3 Iron powder, reduced with hydrogen.
The prescribed quantity of iron shall be able to reduce at least 50 mg of nitrate nitrogen.
5.4 Potassium thiocyanate, p.a.
5.5 Potassium nitrate, p.a.
5.6 Ammonium sulfate, p.a.
5.7 Urea, p.a.
5.8 Sulfuric acid diluted.
Dilute one volume of sulfuric acid (ρ = 1,84 g/ml) in one volume of water.
20
5.9 Sulfuric acid, standard solution, c = 0,1 mol/l.
5.10 Sodium hydroxide solution, aqueous solution of about 30 % (mass concentration), free from
ammonia.
5.11 Sodium or potassium hydroxide, standard solution, c = 0,2 mol/l, free from carbonates.
5.12 Stannous chloride solution.
Dissolve 120 g of SnCl · 2H O in 400 ml of concentrated hydrochloric acid (ρ = 1,18 g/ml) and make
2 2 20
up to 1 l with water. The solution shall be perfectly clear and prepared immediately before use.
It is essential to check the reducing power of stannous chloride: dissolve 0,5 g of SnCl · 2H O in 2 ml
2 2
of concentrated hydrochloric acid (ρ = 1,18 g/ml) and make up to 50 ml with water. Then, add 5 g
20
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of Rochelle salt (potassium sodium tartrate), then a sufficient quantity of sodium bicarbonate for the
solution to be alkaline to litmus paper.
Titrate with an iodine solution (I ) of c = 0,05 mol/l in the presence of a starch solution as an indicator.
2
1 ml of iodine solution (I ) of c = 0,05 mol/l corresponds to 0,011 28 g of SnCl · 2H O.
2 2 2
At least 80 % of the total tin present in the solution thus prepared shall be in bivalent form. For the
titration, at least 35 ml of the c = 0,05 mol/l iodine solution (I ) should be used.
2
5.13 Sulfuric acid, ρ = 1,84 g/ml.
20
5.14 Hydrochloric acid diluted.
Mix one volume of hydrochloric acid (ρ = 1,18 g/ml) with one volume of water.
20
5.15 Acetic acid, 96 % to 100 %.
5.16 Sulfuric acid solution, containing about 30 % of H SO (mass concentration).
2 4
5.17 Ferrous sulfate, crystalline, FeSO · 7H O.
4 2
5.18 Sulfuric acid standard solution, c = 0,05 mol/l.
5.19 Octyl alcohol.
5.20 Potassium carbonate, saturated solution.
5.21 Sodium or potassium hydroxide standard solution, c = 0,1 mol/l (free from carbonates).
5.22 Barium hydroxide saturated solution.
5.23 Sodium carbonate solution, at 10 % (mass concentration).
5.24 Hydrochloric acid, c = 2 mol/l.
5.25 Hydrochloric acid standard solution, c = 0,1 mol/l.
5.26 Urease solution.
Suspend 0,5 g of active urease in 100 ml of water. Using hydrochloric acid 0,1 mol/l (5.25), adjust the pH
to 5,4, measured by a pH-meter.
5.27 Xanthydrol.
Use a solution at 5 % in ethanol or methanol (5.32) (do not use products giving a high proportion of
insoluble matter). The solution may be kept for three months in a well-stoppered bottle, away from
the light.
5.28 Catalyst.
Use 0,3 g to 0,4 g of copper oxide per determination or an equivalent quantity of copper sulfate
pentahydrate of 0,95 g to 1,25 g per determination.
5.29 Anti-bump granules, washed in hydrochloric acid and calcined.
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5.30 Indicator solutions.
5.30.1 Solution A.
Dissolve 1 g of methyl red in 37 ml of sodium hydroxide solution 0,1 mol/l and make up to 1 l with water.
5.30.2 Solution B.
Dissolve 1 g of methylene blue in water and make up to 1 l.
5.30.3 Combined indicator solution.
Mix one volume of solution A with two volumes of solution B.
This indicator is violet in acid solution, grey in neutral solution and green in alkaline solution. Use 0,5 ml
(10 drops) of this indicator solution.
5.30.4 Methyl red indicator solution.
Dissolve 0,1 g of methyl red in 50 ml of 95 % ethanol. Make up to 100 ml with water and filter if
necessary. This indicator (four or five drops) may be used instead of that described in 5.30.3.
5.31 Indicator papers, litmus bromothymol blue (or other papers sensitive to pH = 6 to pH = 8).
5.32 Ethanol or methanol, solution 95 %.
6 Apparatus
6.1 Distillation apparatus.
Consisting of a round-bottomed flask of suitable capacity connected to a condenser by means of a splash
head. The equipment is made of borosilicate glass.
NOTE The different types of equipment recommended for this determination are reproduced, showing all
the features of construction, in Figures 1, 2, 3 and 4.
An automatic distillation apparatus may also be used, provided that the results are statistically
equivalent.
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Dimensions in millimetres
3
6
2
4
a
1
5
Key
1 round-bottomed, long-necked flask of 1 000 ml capacity
2 distillation tube with a splash head, connected to the condenser by means of a spherical joint (No 18) (the
spherical joint for the connection to the condenser may be replaced by an appropriate rubber connection)
3 funnel with a polytetrafluoroethylene (PTFE) tap (6) for the addition of sodium hydroxide (the tap may likewise
be replaced by a rubber connection with a clip)
4 six-bulb condenser with spherical joint (No 18) at the entrance and joined at the issue to a glass extension tube
by means of a small rubber connection (when the connection to the distillation tube is effected by means of a
rubber tube, the spherical joint may be replaced by a suitable rubber bung)
5 500 ml flask in which the distillate is collected
6 PTFE tap
a
Hole.
Figure 1 — Distillation apparatus 1
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Dimensions in millimetres
3
5
2
a
1
4
Key
1 round-bottomed, short-necked flask of 1 000 ml capacity with a spherical joint (No 35)
2 distillation tube with a splash head, equipped with a spherical joint (No 35) at the entrance and a spherical
joint (No 18) at the issue, connected at the side to a funnel with a polytetrafluoroethylene (PTFE) tap (5) for the
addition of sodium hydroxide
3 six-bulb condenser with a spherical joint (No 18) at the entrance and joined at the issue to a glass extension
tube by means of a small rubber connection
4 500 ml flask in which the distillate is collected
5 PTFE tap
a
Enlarged cross-section.
Figure 2 — Distillation apparatus 2
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Dimensions in millimetres
3
2
4
1
5
Key
1 round-bottomed, long-necked flask of 750 ml or 1 000 ml capacity with a bell mouth
2 distillation tube with a splash head and a spherical joint (No 18) at the issue
3 elbow tube with a spherical joint (No 18) at the entrance, and a drip cone (the connection to the distillation
tube may be effected by means of a rubber tube instead of a spherical joint)
4 six-bulb condenser joined at the issue to a glass extension tube by means of a small rubber connection
5 500 ml flask in which the distillate is collected
Figure 3 — Distillation apparatus 3
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Dimensions in millimetres
2
3
5
4
1
Key
1 round-bottomed, long-necked flask of 1 000 ml capacity with a bell mouth
2 distillation tube with a splash head and a spherical joint (No 18), at the issue, connected at the side to a funnel
with a polytetrafluoroethylene (PTFE) tap (5) for the addition of sodium hydroxide (a suitable rubber bung may
be used instead of the spherical joint; the tap may be replaced by a rubber connection with an appropriate clip)
3 six-bulb condenser with a spherical joint (No 18) at the entrance, joined at the issue by a rubber connection, to
a glass extension tube (when the connection to the distillation tube is effected by means of a rubber tube, the
spherical joint may be replaced by a suitable rubber bung)
4 500 ml flask for the collection of the distillate
5 PTFE tap
Figure 4 — Distillation apparatus 4
6.2 Apparatus for the determination of ammoniacal nitrogen.
According to analytical technique (8.2.5.3). (Refer to Figure 1.)
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The apparatus is made up of a specially shaped receptacle with a ground glass neck, with a side neck,
a connecting tube with a splash head and a perpendicular tube for the introduction of air. The tubes
can be connected to the receptacle by means of a simple perforated rubber bung. It is important to
give a suitable shape to the end of the tubes introducing air, since the bubbles of gas should be
evenly distributed throughout the solutions contained in the receptacle and the absorber. The best
arrangement consists of small mushroom-shaped pieces with an external diameter of 20 mm and six
openings of 1 mm around the periphery.
6.3 Apparatus for the determination of urea nitrogen.
According to the urease technique (8.2.6.1).
It consists of a 300 ml Erlenmeyer flask, with a separating funnel and a small absorber.
6.4 Rotary shaker, 35 r/min to 40 r/min.
6.5 pH meter.
6.6 Oven, capable of being maintained at a temperature of 130 °C.
6.7 Glassware:
a) pipettes of 2 ml, 5 ml, 10 ml, 20 ml, 25 ml, 50 ml and 100 ml capacity;
b) long-necked Kjeldahl flasks of 300 ml and 500 ml capacity;
c) graduated flasks of 100 ml, 250 ml, 500 ml and 1 000 ml capacity;
d) crucibles of sintered glass, pore diameter, 5 µm to 15 µm;
e) mortars.
7 Sampling and sample preparation
Sampling is not part of the method specified in this International Standard. A recommended sampling
[1]
method is given in ISO 14820-1.
Sample preparation shall be carried out in accordance with ISO 14820-2.
8 Procedure
8.1 Total soluble and insoluble nitrogen
8.1.1 In the absence of nitrates
8.1.1.1 Digestion
Weigh, to an accuracy of 0,001 g, a quantity of the sample containing 100 mg of nitrogen at the most.
Place it in the flask of the distillation apparatus (see 6.1). Add 10 g to 15 g of potassium sulfate (see 5.2),
the catalyst (see 5.28), and a few anti-bump granules (see 5.29). Then add 50 ml of diluted sulfuric acid
(see 5.8), and mix thoroughly. First, heat gently, mixing from time to time, until foam no longer forms.
Then heat in such a way that the liquid boils regularly and keep it boiling for 1 h after the solution
has become clear, preventing any organic matter from sticking to the sides of the flask. Allow to cool.
Carefully add about 350 ml of water, with mixing. Ensure that the dissolution is as complete as possible.
Allow to cool and connect the flask to the distillation apparatus (see 6.1).
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8.1.1.2 Distillation of ammonia
Transfer with a precision pipette, into the receiver of the apparatus, 50 ml of a standard solution of
sulfuric acid 0,1 mol/l (see 5.9). Add the indicator (see 5.30.3 or 5.30.4). Ensure that the tip of the
condenser is at least 1 cm below the level of the solution.
Taking the necessary precautions to avoid any loss of ammonia, carefully add to the distillation flask
enough of the concentrated sodium hydroxide solution (see 5.10) to make the liquid strongly alkaline
(120 ml is generally sufficient). Check by adding a few drops of phenolphthalein. At the end of the
distillation, the solution in the flask shall still be clearly alkaline. Adjust the heating of the flask so as
to distil 150 ml in 0,5 h. Test with indicator paper (see 5.31) that the distillation has been completed. If
it has not, distil a further 50 ml and repeat the test until the supplementary distillate reacts neutrally
to the indicator paper (see 5.31). Then, lower the receiver, distil a few millilitres more and rinse the tip
of the condenser. Titrate the excess of acid with a standard solution of potassium or sodium hydroxide
0,2 mol/l (see 5.11) until the indicator changes colour.
8.1.1.3 Blank test
Carry out a blank test (omitting the sample) under the same conditions, and take account of it when
calculating the final result.
8.1.1.4 Expression of the result
Express the result as the mass fraction in percent of nitrogen contained in the fertilizer as received for
analysis.
()50−×V 02, 8
w = (1)
N
m
where
50 is the volume of the standard solution of sodium or potassium hydroxide 0,2 mol/l used for the
blank, carried out by pipetting into the receiver of the apparatus (see 6.1), in millilitre [50 ml
of standard solution of sulfuric acid 0,1 mol/l (see 5.9)];
V is the volume of the standard solution of sodium or potassium hydroxide 0,2 mol/l used for the
analysis, in millilitre;
m is the mass of the test sample, in grams.
8.1.2 In the presence of nitrate
8.1.2.1 Test sample
Weigh, to an accuracy of 0,001 g, a quantity of the sample containing not more than 40 mg of nitrate
nitrogen.
8.1.2.2 Reduction of the nitrate
Mix the test sample in a small mortar with 50 ml of water. Transfer with the minimum amount of water
into a 500 ml Kjeldahl flask. Add 5 g of reduced iron (see 5.3) and 50 ml of stannous chloride solution
(see 5.12). Shake and leave it to stand for 0,5 h. During the time it is standing, stir again after 10 min
and 20 min.
8.1.2.3 Kjeldahl digestion
Add 30 ml of sulfuric acid (see 5.13), 5 g of potassium sulfate (see 5.2), the prescribed quantity of
catalyst (see 5.28), and some anti-bump granules (see 5.29). Heat gently with
...