1、BSI Standards PublicationBS ISO 1656:2014Rubber, raw natural, andrubber latex, natural Determination of nitrogencontentBS ISO 1656:2014 BRITISH STANDARDNational forewordThis British Standard is the UK implementation of ISO 1656:2014. Itsupersedes BS 7164-21:1996 which is withdrawn.The UK participati
2、on in its preparation was entrusted to TechnicalCommittee PRI/23, Test methods for rubber and non-blackcompounding ingredients.A list of organizations represented on this committee can beobtained on request to its secretary.This publication does not purport to include all the necessaryprovisions of
3、a contract. Users are responsible for its correctapplication. The British Standards Institution 2014. Published by BSI StandardsLimited 2014ISBN 978 0 580 82378 7ICS 83.040.10Compliance with a British Standard cannot confer immunity fromlegal obligations.This British Standard was published under the
4、 authority of theStandards Policy and Strategy Committee on 31 October 2014.Amendments issued since publicationDate Text affectedBS ISO 1656:2014 ISO 2014Rubber, raw natural, and rubber latex, natural Determination of nitrogen contentCaoutchouc brut naturel et latex de caoutchouc naturel Dosage de l
5、azoteINTERNATIONAL STANDARDISO1656Fourth edition2014-10-15Reference numberISO 1656:2014(E)BS ISO 1656:2014ISO 1656:2014(E)ii ISO 2014 All rights reservedCOPYRIGHT PROTECTED DOCUMENT ISO 2014All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized oth
6、erwise 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 ISOs member body in the country of the requester.ISO copyright office
7、Case postale 56 CH-1211 Geneva 20Tel. + 41 22 749 01 11Fax + 41 22 749 09 47E-mail copyrightiso.orgWeb www.iso.orgPublished in SwitzerlandBS ISO 1656:2014ISO 1656:2014(E) ISO 2014 All rights reserved iiiContents PageForeword iv1 Scope . 12 Normative references 13 Principle 14 Macro-method . 14.1 Rea
8、gents 14.2 Apparatus 24.3 Sampling and preparation of test portion . 24.4 Procedure 34.5 Blank test . 34.6 Expression of results 35 Semi-micro method . 45.1 Reagents 45.2 Apparatus 55.3 Sampling and preparation of test portion . 55.4 Procedure 55.5 Blank test . 65.6 Expression of results 66 Precisio
9、n . 77 Test report . 7Annex A (informative) Guidance for using precision results .17Annex B (informative) Precision 18BS ISO 1656:2014ISO 1656:2014(E)ForewordISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work
10、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-government
11、al, 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 I
12、SO/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 possibil
13、ity 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 pa
14、tent 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 info
15、rmation about ISOs adherence to the WTO principles in the Technical Barriers to Trade (TBT) see the following URL: Foreword - Supplementary informationThe committee responsible for this document is ISO/TC 45, Rubber and rubber products, Subcommittee SC 2, Testing and analysis.This fourth edition can
16、cels and replaces the third edition (ISO 1656:1996), which has been technically revised.iv ISO 2014 All rights reservedBS ISO 1656:2014INTERNATIONAL STANDARD ISO 1656:2014(E)Rubber, raw natural, and rubber latex, natural Determination of nitrogen contentWARNING Persons using this International Stand
17、ard should be familiar with normal laboratory practice. This International Standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user to establish appropriate safety and health practices and to ensure compliance with any nat
18、ional regulatory conditions.1 ScopeThis International Standard specifies a macro-method and a semi-micro method for the determination of nitrogen in raw natural rubber and in natural rubber latex using variants of the Kjeldahl process.NOTE The determination of nitrogen in natural rubber is usually c
19、arried out in order to arrive at an estimate of the protein content. Minor amounts of non-proteinous nitrogen containing constituents are also present. However, in the dry solids prepared from natural rubber latex, these materials can make a substantial contribution to the total nitrogen content.2 N
20、ormative referencesThe 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 amendment
21、s) applies.ISO 123, Rubber latex SamplingISO 124, Latex, rubber Determination of total solids contentISO 1795, Rubber, raw natural and raw synthetic Sampling and further preparative proceduresISO/TR 9272, Rubber and rubber products Determination of precision for test method standards3 PrincipleA kno
22、wn mass of the sample is digested with a mixture of sulfuric acid, potassium sulfate, and catalytic amounts of copper sulfate and selenium or sodium selenate, thereby converting nitrogen compounds into ammonium hydrogen sulfate from which the ammonia is distilled after making the mixture alkaline.Th
23、e distilled ammonia is absorbed either in standard volumetric sulfuric acid solution followed by titration of the excess acid with a standard volumetric base solution or in boric acid solution followed by titration with standard volumetric acid solution (as boric acid is a weak acid, it does not aff
24、ect the indicator used for this titration).4 Macro-method4.1 ReagentsDuring the analysis, unless otherwise stated, use only reagents of recognized analytical grade and only distilled water or water of equivalent purity. ISO 2014 All rights reserved 1BS ISO 1656:2014ISO 1656:2014(E)4.1.1 Catalyst mix
25、ture or catalyst solutionCAUTION When working with selenium, avoid breathing vapours and/or contact with skin or clothing. Work only with adequate ventilation.4.1.1.1 Catalyst mixturePrepare a finely divided intimate mixture of the following: 30 parts by mass of anhydrous potassium sulfate (K2SO4);
26、four parts by mass of copper sulfate pentahydrate (CuSO45H2O); one part by mass of selenium powder or two parts by mass of sodium selenate decahydrate (Na2SeO410H2O).4.1.1.2 Catalyst solutionDissolve, with heating, the following: 110 g of anhydrous potassium sulfate; 14,7 g of copper sulfate pentahy
27、drate; 3,7 g of selenium or 7,49 g of sodium selenate, in 600 cm3of sulfuric acid (4.1.2).4.1.2 Sulfuric acid, = 1,84 g/cm3.4.1.3 Sulfuric acid standard volumetric solution, c(H2SO4) = 0,05 mol/dm3.4.1.4 Sodium hydroxide standard volumetric solution, c(NaOH) = 0,1 mol/dm3.4.1.5 Sodium hydroxide solu
28、tion, c(NaOH) approximately 10 mol/dm3.Dissolve 400 g of solid sodium hydroxide in 600 cm3of water and dilute to 1 000 cm3.4.1.6 Boric acid solution, c(H3BO3) approximately 0,17 mol/dm3.Dissolve 10,5 g of solid boric acid in water, warming if necessary, and dilute to 1 000 cm3then cool the solution
29、to room temperature.4.1.7 Mixed indicator solution.Dissolve 0,1 g of methyl red and 0,05 g of methylene blue in 100 cm3of at least 95 % ( V/ V ) ethanol.This indicator might deteriorate during storage and shall therefore be freshly prepared.4.2 ApparatusOrdinary laboratory apparatus and Kjeldahl app
30、aratus, with a digestion flask of capacity 800 cm3.4.3 Sampling and preparation of test portionFor the determination of nitrogen in raw solid rubber, a test portion shall be taken from the homogenized piece, sampled and prepared in accordance with ISO 1795.For the determination of nitrogen in latex,
31、 a representative portion (as specified in ISO 123) of thoroughly mixed latex containing about 2 g of total solids shall be dried to constant mass, as specified in ISO 124.2 ISO 2014 All rights reservedBS ISO 1656:2014ISO 1656:2014(E)4.4 Procedure4.4.1 Cut about 2 g of the rubber or dried latex, wei
32、ghed to the nearest 0,5 mg, into small pieces and place in the digestion flask (see 4.2). Add about 13 g of the catalyst mixture (4.1.1.1) and 60 cm3of the sulfuric acid (4.1.2) or, alternatively, 65 cm3of the catalyst solution (4.1.1.2). Mix the contents of the flask by swirling and then boil gentl
33、y until the solution is clear. Continue boiling for 1 h.NOTE Acidic fumes evolved during digestion will be trapped in an alkaline solution and will be neutralized before being discharged.Allow the digestion flask and its contents to cool to room temperature then cautiously add 200 cm3of water and mi
34、x by swirling.Place the receiving flask containing the absorbing solution in position, connect the distillation unit, and then slowly add 150 cm3of the sodium hydroxide solution (4.1.5) to the digestion flask from a dropping funnel.4.4.2 Carry out the absorption and titration of the liberated ammoni
35、a by the procedure described in 4.4.2.1 or 4.4.2.2. The temperature of the receiving flask shall be maintained below 30 C to prevent any loss of ammonia.NOTE Ensure proper disposal of the selenium-containing waste in the distillation flask.4.4.2.1 Place 75 cm3of water and, by means of a pipette, 25
36、cm3of the standard volumetric sulfuric acid solution (4.1.3) in the receiving flask of the distillation unit together with two drops of mixed indicator solution (4.1.7). Position the receiving flask so that the end of the delivery tube from the condenser dips below the surface of the absorbing solut
37、ion. While holding the stopper of the digestion flask in place, thoroughly mix the contents by swirling. Immediately commence distillation and continue at a steady rate until 200 cm3of distillate have been collected. If the colour of the indicator changes, it indicates alkalinity of the absorbing so
38、lution. Discontinue the determination and repeat the procedure using more sulfuric acid or a smaller test portion.When the distillation is complete (normally, when the volume in the flask reaches about 300 cm3), titrate the contents with the sodium hydroxide solution (4.1.4), reading the burette to
39、the nearest 0,02 cm3.4.4.2.2 Place 100 cm3of the boric acid solution (4.1.6) in the receiving flask of the distillation unit with two drops of the mixed indicator solution (4.1.7). Carry out the distillation as described in 4.4.2.1 and titrate the distillate with the sulfuric acid solution (4.1.3),
40、reading the burette to the nearest 0,02 cm3.NOTE If the concentrations of the standard volumetric solution used are not exactly as specified in the list of reagents, appropriate corrections are to be made.4.5 Blank testIn parallel with the determination, carry out a blank test using the same quantit
41、ies of reagents under the same operating conditions but omitting the test portion.4.6 Expression of results4.6.1 When sulfuric acid is used as the absorbing solution as specified in 4.4.2.1, the nitrogen content of the rubber, expressed as a percentage by mass, is given by Formula (1): ISO 2014 All
42、rights reserved 3BS ISO 1656:2014ISO 1656:2014(E)VV cm120 014100(),(1)whereV1is the volume, in cubic centimetres, of sodium hydroxide solution (4.1.4) required for the titra-tion;V2is the volume, in cubic centimetres, of sodium hydroxide solution (4.1.4) required for the titra-tion in the blank test
43、;c is the concentration of sodium hydroxide;m is the mass, in grams, of the test portion.Express the results to the nearest 0,01 %.4.6.2 When boric acid is used as the absorbing solution as specified in 4.4.2.2, the nitrogen content of the rubber, expressed as a percentage by mass, is given by Formu
44、la (2):VV cm340 028100(),(2)whereV3is the volume, in cubic centimetres, of sulfuric acid solution (4.1.3) required for the titration;V4is the volume, in cubic centimetres, of sulfuric acid solution (4.1.3) required for the titration in the blank test;c is the concentration of sulfuric acid;m is the
45、mass, in grams, of the test portion.Express the results to the nearest 0,01 %.5 Semi-micro method5.1 ReagentsDuring the analysis, unless otherwise stated, use only reagents of recognized analytical grade and only distilled water or water of equivalent purity.5.1.1 Catalyst mixtureCAUTION When workin
46、g with selenium, avoid breathing vapours and/or contact with skin or clothing. Work only with adequate ventilation.Prepare a finely divided intimate mixture of the following: 30 parts by mass of anhydrous potassium sulfate (K2SO4); four parts by mass of copper sulfate pentahydrate (CuSO45H2O); one p
47、art by mass of selenium powder or two parts by mass of sodium selenate decahydrate (Na2SeO410H2O).5.1.2 Sulfuric acid, = 1,84 g/cm3.4 ISO 2014 All rights reservedBS ISO 1656:2014ISO 1656:2014(E)5.1.3 Sulfuric acid standard volumetric solution, c(H2SO4) 0,01 mol/dm3.5.1.4 Sodium hydroxide solution, c
48、(NaOH) approximately 10 mol/dm3.Dissolve 400 g of solid sodium hydroxide in 600 cm3of water and dilute to 1 000 cm3.5.1.5 Sodium hydroxide, standard volumetric solution, c(NaOH) = 0,02 mol/dm3, carbonate-free.5.1.6 Boric acid solution, c(H3BO3) approximately 0,17 mol/dm3.Dissolve 10,5 g of solid bor
49、ic acid in 200 cm3of water, warming if necessary, and dilute to 1 000 cm3, then cool the solution to room temperature.5.1.7 Mixed indicator solution.Dissolve 0,1 g of methyl red and 0,05 g of methylene blue in 100 cm3of at least 95 % ( V/ V ) ethanol.This indicator might deteriorate during storage and shall therefore be freshly prepared.5.2 ApparatusOrdinary laboratory apparatus and the following are to be used.5.2.1 Semi-micro Kjeldahl digestion apparatus.5.2.1.1 Digestion flasks of capacity 30 cm3and 10 cm3(for
