SIST EN ISO 8968-3:2007

SLOVENSKI STANDARD
SIST EN ISO 8968-3:2007
01-oktober-2007
0OHNR'RORþHYDQMHGXãLNDGHO0HWRGDUD]NURMDYHOHNWULþQHPEORNXSROPLNUR
KLWUDUXWLQVNDPHWRGD,62
Milk - Determination of nitrogen content - Part 3: Block-digestion method (Semi-micro
rapid routine method) (ISO 8968-3:2004)
Milch - Bestimmung des Stickstoffgehaltes - Teil 3: Blockaufschluss-Verfahren
(Halbmikro-Schnellverfahren) (ISO 8968-3:2004)
Lait - Détermination de la teneur en azote - Partie 3: Méthode de minéralisation en bloc
(Méthode de routine semi-micro rapide) (ISO 8968-3:2004)
Ta slovenski standard je istoveten z: EN ISO 8968-3:2007
ICS:
67.100.10 0OHNRLQSUHGHODQLPOHþQL Milk and processed milk
SURL]YRGL products
SIST EN ISO 8968-3:2007 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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EUROPEAN STANDARD
EN ISO 8968-3
NORME EUROPÉENNE
EUROPÄISCHE NORM
June 2007
ICS 67.100.10

English Version
Milk - Determination of nitrogen content - Part 3: Block-digestion
method (Semi-micro rapid routine method) (ISO 8968-3:2004)
Lait - Détermination de la teneur en azote - Partie 3: Milch - Bestimmung des Stickstoffgehaltes - Teil 3:
Méthode de minéralisation en bloc (Méthode de routine Blockaufschluss-Verfahren (Halbmikro-Schnellverfahren)
semi-micro rapide) (ISO 8968-3:2004) (ISO 8968-3:2004)
This European Standard was approved by CEN on 19 May 2007.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the CEN Management Centre or to any CEN member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CEN member into its own language and notified to the CEN Management Centre has the same status as the
official versions.
CEN members are the national standards bodies of 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 United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2007 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 8968-3:2007: E
worldwide for CEN national Members.

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EN ISO 8968-3:2007 (E)






Foreword



The text of ISO 8968-3:2004 has been prepared by Technical Committee ISO/TC 34
"Agricultural food products” of the International Organization for Standardization (ISO) and has
been taken over as EN ISO 8968-3:2007 by Technical Committee CEN/TC 302 "Milk and milk
products - Methods of sampling and analysis", the secretariat of which is held by NEN.

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 December 2007, and conflicting national
standards shall be withdrawn at the latest by December 2007.

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 United
Kingdom.


Endorsement notice

The text of ISO 8968-3:2004 has been approved by CEN as EN ISO 8968-3:2007 without any
modifications.

2

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INTERNATIONAL ISO
STANDARD 8968-3
IDF
20-3
First edition
2004-09-01

Milk — Determination of nitrogen
content —
Part 3:
Block-digestion method (Semi-micro
rapid routine method)
Lait — Détermination de la teneur en azote —
Partie 3: Méthode de minéralisation en bloc (Méthode de routine semi-
micro rapide)




Reference numbers
ISO 8968-3:2004(E)
IDF 20-3:2004(E)
©
ISO and IDF 2004

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ISO 8968-3:2004(E)
IDF 20-3:2004(E)
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©  ISO and IDF 2004
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO or IDF at the respective
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Published in Switzerland

ii © ISO and IDF 2004 – All rights reserved

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ISO 8968-3:2004(E)
IDF 20-3:2004(E)
Contents Page
Foreword. iv
Foreword. v
1 Scope. 1
2 Normative references . 1
3 Terms and definitions. 1
4 Principle . 2
5 Reagents . 2
6 Apparatus . 3
7 Sampling . 4
8 Preparation of test sample. 4
9 Procedure . 4
9.1 Test portion and pretreatment. 4
9.2 Determination . 5
9.3 Blank test . 6
9.4 Recovery tests. 6
10 Calculation and expression of results. 7
10.1 Calculation of nitrogen content. 7
11 Precision . 8
12 Test report . 8
Annex A (informative) Results of interlaboratory test . 9
Annex B (informative) Modified procedure for analysis of other milk products. 10
Bibliography . 11

© ISO and IDF 2004 – All rights reserved iii

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ISO 8968-3:2004(E)
IDF 20-3:2004(E)
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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 8968-3IDF 20-3 was prepared by Technical Committee ISO/TC 34, Food products, Subcommittee SC 5,
Milk and milk products, and the International Dairy Federation (IDF), in collaboration with AOAC International.
It is being published jointly by ISO and IDF and separately by AOAC International.
ISO 8968IDF 20 consists of the following parts, under the general title Milk — Determination of nitrogen
content:
 Part 1: Kjeldahl method
 Part 2: Block-digestion method (Macro method)
 Part 3: Block-digestion method (Semi-micro rapid routine method)
 Part 4: Determination of non-protein-nitrogen content
 Part 5: Determination of protein-nitrogen content

iv © ISO and IDF 2004 – All rights reserved

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ISO 8968-3:2004(E)
IDF 20-3:2004(E)
Foreword
IDF (the International Dairy Federation) is a worldwide federation of the dairy sector with a National
Committee in every member country. Every National Committee has the right to be represented on the IDF
Standing Committees carrying out the technical work. IDF collaborates with ISO and AOAC International in
the development of standard methods of analysis and sampling for milk and milk products.
Draft International Standards adopted by the Action Teams and Standing Committees are circulated to the
National Committees for voting. Publication as an International Standard requires approval by at least 50 % of
IDF National Committees casting a vote.
ISO 8968-3IDF 20-3 was prepared by Technical Committee ISO/TC 34, Food products, Subcommittee SC 5,
Milk and milk products, and the International Dairy Federation (IDF), in collaboration with AOAC International.
It is being published jointly by ISO and IDF and separately by AOAC International.
All work was carried out by the Joint ISO/IDF/AOAC Action Team, Nitrogen compounds, of the Standing
Committee on Main components of milk, under the aegis of its project leader Mrs S. Berman (IL).
This edition of ISO 8968-3IDF 20-3 cancels and replaces IDF 20-3:1993, which has been technically revised.
ISO 8968IDF 20 consists of the following parts, under the general title Milk — Determination of nitrogen
content:
 Part 1: Kjeldahl method
 Part 2: Block-digestion method (Macro method)
 Part 3: Block-digestion method (Semi-micro rapid routine method)
 Part 4: Determination of non-protein-nitrogen content
 Part 5: Determination of protein-nitrogen content

© ISO and IDF 2004 – All rights reserved v

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ISO 8968-3:2004(E)
INTERNATIONAL STANDARD
IDF 20-3:2004(E)

Milk — Determination of nitrogen content —
Part 3:
Block-digestion method (Semi-micro rapid routine method)
WARNING — The use of this part of ISO 8968IDF 20 may involve the use of hazardous materials,
operations and equipment. This standard does not purport to address all the safety risks associated
with its use. It is the responsibility of the user of this standard to establish appropriate safety and
health practices and determine the applicability of local regulatory limitations prior to use.
1 Scope
This part of ISO 8968IDF 20 specifies a method for the determination of the nitrogen content of liquid, whole
or skimmed milk.
It concerns a semi-micro rapid routine method following the block-digestion principle.
NOTE The method is a more rapid method than that described in ISO 8968-1IDF 20-1 and ISO 8968-2IDF 20-2
since the digestion time is reduced by taking a lower mass of test portion and using hydrogen peroxide together with
sulfuric acid and a catalyst in the digestion.
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.
1)
ISO 385:— , Laboratory glassware — Burettes
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
nitrogen content
mass fraction of nitrogen determined by the procedure specified in this part of ISO 8968IDF 20.
NOTE The nitrogen content is expressed as a percentage by mass.

1) To be published. (Revision of ISO 385-1:1984, ISO 385-2:1984 and ISO 385-3:1984)
© ISO and IDF 2004 – All rights reserved 1

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ISO 8968-3:2004(E)
IDF 20-3:2004(E)
4 Principle
A test portion is digested by using a block-digestion apparatus with a mixture of concentrated sulfuric acid,
hydrogen peroxide and potassium sulfate, together with a catalyst to convert the organic nitrogen present to
ammonium sulfate. Excess sodium hydroxide is added to the cooled digest to liberate ammonia.
The liberated ammonia is distilled using a manual, semi-automatic or fully automatic steam distillation unit. In
the case of manual or semi-automatic steam distillation, the ammonia is distilled into excess boric acid
solution then titrated with hydrochloric acid solution. Where a fully automatic distillation unit is employed,
titration of the ammonia is carried out automatically with endpoint detection using a photometric or pH system.
The nitrogen content is calculated from the amount of ammonia produced.
5 Reagents
Use only reagents of recognized analytical grade, unless otherwise specified, and distilled or demineralized
water.
5.1 Kjeldahl catalyst tablets
These tablets may be purchased commercially. Tablets comprising 3,5 g of potassium sulfate, 0,105 g of
copper(II) sulfate pentahydrate and 0,105 g of titanium dioxide per tablet are suitable.
Other types of tablet may be used provided that
a) they contain a quantity of potassium sulfate such that 7 g of potassium sulfate can be dispensed using an
integer number of whole tablets, and
b) they contain no salts of toxic metals such as selenium or mercury.
5.2 Sulfuric acid (H SO), with mass fraction of at least 95 % to 98 %, nitrogen free
2 4
[ρ (H SO ) = 1,84 g/ml approximately].
20 2 4
5.3 Hydrogen peroxide solution (H O ), approximately 30 g of H O per 100 ml.
2 2 2 2
5.4 Antifoaming agent
A silicone preparation is recommended, for example with a mass fraction of 30 % aqueous emulsion.
5.5 Sodium hydroxide solution (NaOH), nitrogen free, containing approximately 40 g of NaOH per
100 ml.
5.6 Boric acid solutions
5.6.1 Boric acid solution, c(H BO ) = 40,0 g/l.
3 3
Dissolve 40,0 g of boric acid in 1 litre of hot water in a 1 000 ml one-mark volumetric flask. Allow the flask and
its contents to cool to 20 °C. Add 3 ml of indicator solution (5.7.1). Adjust to the mark with water and mix well.
Store the solution, which will be light orange in colour, in a borosilicate glass bottle. Protect the solution from
light and sources of ammonia fumes during storage.
5.6.2 Boric acid solution, c(H BO ) = 10,0 g/l, to be used in the photometric endpoint titration.
3 3
Dissolve 10,0 g of boric acid in 1 litre of hot water in a 1 000 ml one-mark volumetric flask. Allow the flask and
its contents to cool to 20 °C. Add 7 ml of methyl red solution (5.7.2) and 10 ml of bromocresol green solution
(5.7.2) and mix.
2 © ISO and IDF 2004 – All rights reserved

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ISO 8968-3:2004(E)
IDF 20-3:2004(E)
Dilute to the 1 000 ml mark with water and mix well. Neutralize the boric acid solution with 0,1 mol/l sodium
hydroxide until its colour changes to green.
NOTE The addition of 3 ml of 0,1 mol/l NaOH into 1 litre of 1 % boric acid usually gives good adjustments.
5.7 Indicator solutions
5.7.1 Indicator solution to be used in the pH endpoint titration
Dissolve 0,1 g of methyl red in 95 % (volume fraction) ethanol. Dilute to 50 ml with the ethanol. Dissolve 0,5 g
of bromocresol green in a small quantity of 95 % (volume fraction) ethanol. Dilute to 250 ml with the ethanol.
Mix amounts of one part of methyl red solution with five parts of bromocresol green solution, or combine and
mix both solutions.
5.7.2 Indicator solution to be used in the boric acid solution (5.6.2) for the photometric endpoint
titration
a) Dissolve 0,1 g of bromocresol green in 100 ml of 95 % (volume
...