1、BS ISO 1871:2009ICS 67.050NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAWBRITISH STANDARDFood and feed products General guidelinesfor determination ofnitrogen by KjeldahlmethodThis British Standardwas published underthe authority of theStandards Policy andStrategy Committee on
2、 30September 2009 BSI 2009ISBN 978 0 580 56149 8Amendments/corrigenda issued since publicationDate CommentsBS ISO 1871:2009National forewordThis British Standard is the UK implementation of ISO 1871:2009.The UK participation in its preparation was entrusted to TechnicalCommittee AW/-/3, Food analysi
3、s - Horizontal methods.A list of organizations represented on this committee can be obtained onrequest to its secretary.This publication does not purport to include all the necessary provisionsof a contract. Users are responsible for its correct application.Compliance with a British Standard cannot
4、confer immunityfrom legal obligations.BS ISO 1871:2009Reference numberISO 1871:2009(E)ISO 2009INTERNATIONAL STANDARD ISO1871Second edition2009-09-01Food and feed products General guidelines for the determination of nitrogen by the Kjeldahl method Produits alimentaires et aliments des animaux Lignes
5、directrices gnrales pour le dosage de lazote selon la mthode de Kjeldahl BS ISO 1871:2009ISO 1871:2009(E) PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobes licensing policy, this file may be printed or viewed but shall not be edited unless the typefaces which are
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10、blished in Switzerland ii ISO 2009 All rights reservedBS ISO 1871:2009ISO 1871:2009(E) ISO 2009 All rights reserved iiiForeword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Sta
11、ndards 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
12、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
13、 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 possibi
14、lity 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 1871 was prepared by Technical Committee ISO/TC 34, Food products. This second edition cancels and replaces the first edition (ISO
15、 1871:1975), which has been technically revised. BS ISO 1871:2009ISO 1871:2009(E) iv ISO 2009 All rights reservedIntroduction The analysis of products of animal or plant origin, such as those used in food and feed products, often includes determining their nitrogen content according to the Kjeldahl
16、method. This method can be standardized in principle, as it is generally accepted that different apparatus or operating methods are equivalent if their results are similar. The purpose of this document is to describe the various stages of the method, the associated critical points and the minimum ob
17、jectives to be achieved to ensure that the method is applied correctly. This document provides general guidelines; it is not intended to replace existing International Standards which are in use. BS ISO 1871:2009INTERNATIONAL STANDARD ISO 1871:2009(E) ISO 2009 All rights reserved 1Food and feed prod
18、ucts General guidelines for the determination of nitrogen by the Kjeldahl method WARNING The use of this International Standard may involve hazardous materials, operations and equipment. This International Standard does not purport to address all the safety problems associated with its use. It is th
19、e responsibility of the user of this International Standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. 1 Scope This International Standard provides general guidelines for the determination of nitrogen by the Kjeldahl m
20、ethod. It applies to food and feed products containing nitrogenous compounds that can be directly determined by the Kjeldahl method. NOTE This measurement principle does not take into account the nitrogen from nitrates and nitrites. 2 Principle Digestion of a test portion with concentrated sulfuric
21、acid in the presence of catalysts to convert the organic nitrogen into ammonium sulfate. Excess sodium hydroxide is added to the cooled digest to release the ammonia. The released ammonia is distilled into an excess of boric acid solution and then titrated with a standard solution of sulfuric or hyd
22、rochloric acid. The nitrogen content is calculated from the quantity of ammonia produced. NOTE In the following text, the term “nitrogen” refers to organic nitrogen. 3 Reagents Use only reagents of recognized analytical grade, unless otherwise specified, and distilled or demineralized water or water
23、 of equivalent purity. 3.1 Sulfuric acid, virtually free from nitrogenous compounds and of mass density 20= 1,83 g/ml to 1,84 g/ml. 3.2 Catalysts (see 5.2.1). 3.3 Boric acid solution (10 g/l to 40 g/l depending on the apparatus used). If using the colorimetric end-point titration, boric acid solutio
24、n shall contain indicator (the pH or colour of this mixed solution shall be adjusted before use). 3.4 Standard hydrochloric acid (0,02 mol/l to 0,50 mol/l) or sulfuric acid solution (0,01 mol/l to 0,25 mol/l). The titre of the solution, ct, shall be known to at least within 0,001 mol/l. 3.5 Indicato
25、rs, which should change colour between pH 4 and pH 5. NOTE Various indicators are available. A methyl red and bromocresol green mixed indicator is most commonly used. Ready-to-use boric acid solutions containing mixed indicators are available. BS ISO 1871:2009ISO 1871:2009(E) 2 ISO 2009 All rights r
26、eserved3.6 Hydrogen peroxide (H2O2), min. 30 % mass fraction. 3.7 Sodium hydroxide solution, min. 30 % mass fraction. 3.8 Antifoaming agents. EXAMPLE Silicone, liquid paraffin. 3.9 Ammonium sulfate or ammonium chloride (minimum purity 99,9 %). Immediately before use, dry the ammonium sulfate or ammo
27、nium chloride at 104 C 4 C for at least 2 h. Allow it to cool at ambient temperature in a desiccator. NOTE Solutions of known concentration can be used. 3.10 Tryptophan or acetanilide or lysine hydrochloride (minimum purity 99 % mass fraction). These reagents should be kept away from humidity. WARNI
28、NG Do not dry these reagents in an oven before use. 3.11 Sucrose, with nitrogen content less than a mass fraction of 0,002 %. WARNING Do not dry sucrose in an oven before use. 4 Apparatus and materials Usual laboratory apparatus and, in particular, the following. 4.1 Analytical balance, capable of w
29、eighing to the nearest 0,001 g. 4.2 Digestion, steam distillation and titration systems. They are used to perform the operations described in Clause 5 and to ensure that the performance objectives described in 5.5.3 and 5.5.4 are met. 4.3 Boiling point regulators (if needed), for example pumice grai
30、ns, glass beads, aluminium oxide (corundum) or silicon carbide. 4.4 Weighing paper or medium, free from nitrogenous compounds and suitable for the test portion and type of product. 5 Operating method NOTE According to the nature of the sample, it may be necessary to prepare the test portion in advan
31、ce to obtain a homogeneous sample (grinding, homogenization, etc.). 5.1 Test portion The test portion, the quantity of which depends on the presumed nitrogen content determined by the Kjeldahl method, shall be representative of the sample and contain between 0,005 g and 0,2 g of nitrogen. The test p
32、ortion can be obtained by weighing with the analytical balance (4.1), to give mass, m, in grams or by using a pipette, to give volume, Vt, in millilitres. BS ISO 1871:2009ISO 1871:2009(E) ISO 2009 All rights reserved 3The test portion can be inserted into the tube directly or via a support (4.4). Th
33、e quantity of test portion can be adjusted according to the composition of the product under test and the quantity of sulfuric acid (see 5.2.2). 5.2 Digestion 5.2.1 Catalysts It is important to differentiate between the substances used to raise the boiling point of the liquid during digestion and th
34、e catalysts themselves that facilitate digestion. The former are usually potassium sulfate or, possibly, sodium sulfate. They are introduced in sufficient quantity to raise the boiling point of the acid to between 380 C and 430 C. The most commonly used catalyst is copper in the form of copper sulfa
35、te alone or mixed with titanium oxide. The optional addition of hydrogen peroxide (3.6) on the basis of 3 ml to 5 ml per tube prior to heating accelerates digestion, but should be used with the utmost care to ensure that no nitrogen is lost in the form of vapour. Moreover, great care should be taken
36、 when adding hydrogen peroxide to the tubes, as this causes a strong exothermic reaction. The quantity of potassium sulfate provided by the catalyst should not be less than 7 g. Depending on the sectors of activity, various compositions are used. They should meet the requirements of the blank test (
37、5.5.2) and the control tests (5.5.3 and 5.5.4). Operators should handle selenium-based catalysts and waste conditions with care. NOTE Ready-to-use composite catalysts are available on the market (for example in tablet or pellet form). 5.2.2 Addition of acid It is important to use a sufficient quanti
38、ty of sulfuric acid to ensure digestion after: acid consumption by the organic matter of the sample, bearing in mind the fact that 1 g of fat consumes 10 ml of sulfuric acid, 1 g of protein consumes 5 ml of sulfuric acid, 1 g of carbohydrate consumes 4 ml of sulfuric acid; acid consumption by the re
39、agents (salts); acid losses by evaporation. The addition of 20 ml to 25 ml of acid (3.1) is generally sufficient for good digestion and to maintain excess acid at the end of the reaction. 5.2.3 Heating WARNING The following operations should be performed under a very well ventilated fume hood. The m
40、anufacturers instructions relating to the use of the equipment should generally be followed. The digestion system should be made homogeneous, for example by creating a thermal or digestion efficiency process diagram (5.5.3). Foam-producing agents should be brought to boiling point by increasing the
41、temperature gradually or in steps. Three to four drops of antifoaming agent per tube (3.8) can also be used. For “dry” products (i.e. with no visible wetness), the tubes can be placed directly in a preheated unit. BS ISO 1871:2009ISO 1871:2009(E) 4 ISO 2009 All rights reservedAcid fumes shall be rem
42、oved with an extraction system suitable for the equipment used. Excessive extraction may cause crystallization and a loss of nitrogen (see Annex A). In all cases, the digestion temperature and time should be determined to meet the requirements of the digestion control test (5.5.3). NOTE 1 Heating at
43、 420 C for two hours is appropriate for numerous matrices. The digest obtained should be clear and free from black particles. At the end of the digestion process, allow the tubes to cool away from any possible contamination. At this stage, the test portions can be stored and distilled later. NOTE 2
44、Dilution step with water as described in 5.3.1 may be done at this stage in order to avoid crystallization. 5.3 Ammonia distillation 5.3.1 Alkalinization Dilute the digest with water, and then alkalise by adding at least 3,5 ml of sodium hydroxide solution (3.7) per millilitre of sulfuric acid (3.1)
45、 used for the digestion process. NOTE The volume of the added sodium hydroxide solution (3.7) may be lower if its mass fraction is higher than 30 %. WARNING Care should be taken when adding the solution, as the medium becomes very hot. 5.3.2 Distillation Perform the distillation with the apparatus u
46、nder consideration in its usual condition. Collect the distillate in the boric acid solution (3.3), which shall contain the indicator (3.5). Adjust the pH until there is a change of colour to grey (bromocresol green + methyl red indicator) before beginning the distillation. There are several criteri
47、a for determining when the distillation process is finished, for example, when a specific volume of distillate has been collected, after a fixed distillation time, and so on. Ensure that in compliance with the control tests (5.5), the ammonia distillation is complete and that there is no excess by e
48、ntrainment of the alkaline liquid. 5.4 Titration The distillate obtained is titrated with sulfuric (3.1) or hydrochloric acid (3.4); this can be done simultaneously or after distillation. Post-distillation titration should be performed as soon as possible after distillation. There are two methods of
49、 detecting the end point. By visual colorimetry or using an optical measurement system: The end point is reached when the indicator changes colour. In the case of visual colorimetry, it is important to titrate each test referring to the conditions obtained in the blank test. By potentiometric analysis with a pH measurement system: Depending on the equipment or operating methods, the end point may be a fixed pH (generally pH 4,6, which corresponds to the inflection point of the titration curve), the pH obtained in the
