1、September 2007DEUTSCHE NORM English price group 11No part of this standard may be reproduced without prior permission ofDIN Deutsches Institut fr Normung e. V., Berlin. Beuth Verlag GmbH, 10772 Berlin, Germany,has the exclusive right of sale for German Standards (DIN-Normen).ICS 67.100.10!$I0p“13813
2、77www.din.deDDIN EN ISO 8968-3Milk Determination of nitrogen content Part 3: Block-digestion method (Semi-micro rapid routine method)(ISO 8968-3:2004)English version of DIN EN ISO 8968-3:2007-09Milch Bestimmung des Stickstoffgehaltes Teil 3: Blockaufschluss-Verfahren (Halbmikro-Schnellverfahren) (IS
3、O 8968-3:2004)Englische Fassung DIN EN ISO 8968-3:2007-09www.beuth.deDocument comprises 17 pages11.07DIN EN ISO 8968-3:2007-09 2 National foreword This standard falls in the domain of Technical Committee CEN/TC 302 “Milk and milk products Methods of sampling and analysis” (Secretariat: NEN, The Neth
4、erlands). This standard has been prepared by Subcommittee SC 5 “Milk and milk products” (Secretariat: NEN, The Netherlands) of Technical Committee ISO/TC 34 “Food products” (Secretariat: MSZT, Hungary) in collabora-tion with Technical Committee CEN/TC 302. The responsible German body involved in its
5、 preparation was the Normenausschuss Lebensmittel und land-wirtschaftliche Produkte (Food and Agricultural Products Standards Committee), Technical Committee Milch und Milchprodukte Probenahme- und Untersuchungsverfahren. Based on the results of the unique acceptance procedure, ISO 8968-3:2004 has b
6、een adopted as a European Standard. The DIN Standards corresponding to the International Standards referred to in the EN are as follows: ISO 385 DIN EN ISO 385 ISO 707 DIN EN ISO 707 ISO 5725-1 DIN ISO 5725-1 ISO 5725-2 DIN ISO 5725-2 ISO 8968-1 DIN EN ISO 8968-1 ISO 8968-2 DIN EN ISO 8968-2 Nationa
7、l Annex NA (informative) Bibliography DIN EN ISO 385, Laboratory glassware Burettes DIN EN ISO 707, Milk and milk products Guidance on sampling DIN EN ISO 8968-1, Milk Determination of nitrogen content Part 1: Kjeldahl method DIN EN ISO 8968-2, Milk Determination of nitrogen content Part 2: Block-di
8、gestion method (Macro method) DIN ISO 5725-1, Accuracy (trueness and precision) of measurement methods and results Part 1: General principles and definitions DIN ISO 5725-2, Accuracy (trueness and precision) of measurement methods and results Part 2: Basic method for the determination of repeatabili
9、ty and reproducibility of a standard measurement method EUROPEAN STANDARDNORME EUROPENNEEUROPISCHE NORMEN ISO 8968-3June 2007ICS 67.100.10English VersionMilk - Determination of nitrogen content - Part 3: Block-digestionmethod (Semi-micro rapid routine method) (ISO 8968-3:2004)Lait - Dtermination de
10、la teneur en azote - Partie 3:Mthode de minralisation en bloc (Mthode de routinesemi-micro rapide) (ISO 8968-3:2004)Milch - Bestimmung des Stickstoffgehaltes - Teil 3:Blockaufschluss-Verfahren (Halbmikro-Schnellverfahren)(ISO 8968-3:2004)This European Standard was approved by CEN on 19 May 2007.CEN
11、members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this EuropeanStandard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such nationalstandards may be obtained on applica
12、tion 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 translationunder the responsibility of a CEN member into its own language and notified to the CEN Management Centre has t
13、he same status as theofficial 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
14、,Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.EUROPEAN COMMITTEE FOR STANDARDIZATIONCOMIT EUROPEN DE NORMALISATIONEUROPISCHES KOMITEE FR NORMUNGManagement Centre: rue de Stassart, 36 B-1050 Brussels 2007 CEN All rights of exploitation in any form and by any means reserv
15、edworldwide for CEN national Members.Ref. No. EN ISO 8968-3:2007: EContents 1 2 3 4 5 6 7 8 9 9.1 9.2 9.3 9.4 10 10.1 11 12 2Page Blank test . 10 Recovery tests. 10 Determination 9 Foreword 3 Scope 5 Foreword. 4 Normative references . 5 Terms and definitions. 5 Reagents 6 Principle . 6 Apparatus 7 S
16、ampling 8 Test portion and pretreatment. 8 Preparation of test sample. 8 Procedure 8 Calculation and expression of results 11 Precision 12 Calculation of nitrogen content. 11 Test report . 12 Annex A (informative) Results of interlaboratory test 13 Annex B (informative) Modified procedure for analys
17、is of other milk products 14 Bibliography . 15 EN ISO 8968-3:2007 (E)EN ISO 8968-3:2007 (E) 3 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
18、 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
19、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
20、, 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 8968-3:2
21、004 has been approved by CEN as EN ISO 8968-3:2007 without any modifications. 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 Standin
22、g 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
23、Committees for voting. Publication as an International Standard requires approval by at least 50 % of IDF National Committees casting a vote. ISO 8968-3IDF 20-3 was prepared by Technical Committee ISO/TC 34, Food products, Subcommittee SC 5, Milk and milk products, and the International Dairy Federa
24、tion (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 pr
25、oject leader Mrs S. Berman (IL). This edition of ISO 8968-3IDF 20-3 cancels and replaces IDF 20-3:1993, which has been technically revised. ISO 8968IDF 20 consists of the following parts, under the general title Milk Determination of nitrogen content: Part 1: Kjeldahl method Part 2: Block-digestion
26、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 4EN ISO 8968-3:2007 (E)WARNING The use of this part of ISO 8968IDF 20 may involve the use of hazardous materi
27、als, 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 u
28、se. 1 Scope This part of ISO 8968IDF 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 8968-1ID
29、F 20-1 and ISO 8968-2IDF 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 do
30、cument. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. ISO 385:1), Laboratory glassware Burettes 3 Terms and definitions For the purposes of this document, the following terms and definit
31、ions apply. 3.1 nitrogen content mass fraction of nitrogen determined by the procedure specified in this part of ISO 8968IDF 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) 5EN ISO 8968-3:2007 (E)
32、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 dige
33、st 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. Wh
34、ere 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, un
35、less 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
36、 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 (H2SO4), with mass fraction of a
37、t least 95 % to 98 %, nitrogen free 20(H2SO4) = 1,84 g/ml approximately. 5.3 Hydrogen peroxide solution (H2O2), approximately 30 g of H2O2per 100 ml. 5.4 Antifoaming agent A silicone preparation is recommended, for example with a mass fraction of 30 % aqueous emulsion. 5.5 Sodium hydroxide solution
38、(NaOH), nitrogen free, containing approximately 40 g of NaOH per 100 ml. 5.6 Boric acid solutions 5.6.1 Boric acid solution, c(H3BO3) = 40,0 g/l. 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
39、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(H3BO3) = 10,0 g/l, to be used
40、 in the photometric endpoint titration. 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. 6EN ISO 8968-3:200
41、7 (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 Ind
42、icator 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 p
43、art 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 fraction) ethano
44、l. b) Dissolve 0,1 g of methyl red in 100 ml of 95 % (volume fraction) ethanol. 5.8 Hydrochloric acid standard volumetric solution, c(HCl) = (0,1 0,0005) mol/l. If the titration (9.2.3) is carried out manually, the use of a more diluted hydrochloric acid standard volumetric solution, c(HCl) = (0,05
45、0,0005) mol/l, is recommended. 5.9 Ammonium sulfate (NH4)2SO4, minimum assay 99,9 % (mass fraction) on dried material. Immediately before use, dry the ammonium sulfate at 102 C 2 C for not less than 2 h. Cool to room temperature in a desiccator. 5.10 Tryptophan (C11H12N2O2) or lysine hydrochloride (
46、C6H15CIN2O2), minimum assay 99 % (mass fraction). Do not dry the reagents in an oven before use. 5.11 Sucrose, with a nitrogen content of not more than 0,002 % (mass fraction). Do not dry the sucrose in an oven before use. 6 Apparatus Usual laboratory apparatus and, in particular, the following. 6.1
47、 Water bath, capable of being maintained at between 38 C and 40 C. 6.2 Analytical balance, capable of weighing to the nearest 1 mg, with readability to 0,1 mg. 6.3 Digestion block, aluminium alloy block or equivalent block, fitted with an adjustable temperature control and device for measuring block
48、 temperature. 6.4 Digestion tubes, of capacity 250 ml, suitable for use with the digestion block (6.3). 6.5 Exhaust manifold, suitable for use with the digestion tubes (6.4). 7EN ISO 8968-3:2007 (E)6.6 Centrifugal scrubber apparatus, filter pump or aspirator, constructed of acid-resistant material,
49、for use with mains water supply. 6.7 Automatic pipettes (dispensers), of capacity 5 ml and 10 ml. 6.8 Graduated measuring cylinders, of capacity 50 ml. 6.9 Distillation unit, capable of steam distilling, manual or semi-automatic, suited to accept the digestion tubes (6.4) and the conical flasks (6.10). 6.10 Conical flasks, of capacity 250 ml. 6.11 Burette, of capacity 25 ml, with a readability of at least 0,01 ml, complying with the requirements of ISO 385:, class A. Alternatively, an automatic b
