1、Designation: D 2868 07Standard Test Method forNitrogen Content (Kjeldahl) and Hide Substance Content ofLeather1This standard is issued under the fixed designation D 2868; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of
2、last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1. Scope1.1 This test method covers the determin
3、ation of the nitro-gen content of all types of leather. The nitrogen content is usedto calculate the hide substance (protein fiber) content of leather.NOTE 1This test method is essentially a composite of Method 6441 ofFederal Test Method Standard No. 311 and Method B 5 of the AmericanLeather Chemist
4、s Association.1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2
5、. Referenced Documents2.1 ASTM Standards:2D 2813 Practice for Sampling Leather for Physical andChemical TestsE 180 Practice for Determining the Precision of ASTMMethods for Analysis and Testing of Industrial and Spe-cialty Chemicals3. Summary of Test Method3.1 The ground leather specimen prepared ac
6、cording to anaccepted procedure3is digested with acid in the presence of acatalyst to convert the nitrogen to ammonium ion. The ammo-nium ion formed is nonvolatile under these highly acidconditions.3.2 The acid mixture is then made alkaline and the ammonialiberated is distilled into a boric acid sol
7、ution which absorbsthe ammonia.3.3 The amount of ammonia in the boric acid is thendetermined by back titration with standardized acid using asharp color change indicator (green to purple) to determine theend point.4. Significance and Use4.1 The nitrogen content as determined by this test methodis no
8、rmally considered to be related to the amount of hidesubstance (protein fiber) present in the leather sample. A factorof 5.62 is normally used to calculate the hide substance fromthe nitrogen content.4.1.1 The 5.62 factor represents the average result of manyanalyses of animal hides, but it cannot b
9、e considered to beaccurate since it varies somewhat from hide to hide of the sametype, from type of hide to type of hide, and also with thethickness of hide retained in the final leather (split thickness ascompared to original hide thickness). As a result of thesevariations, the true factor for any
10、given leather may beexpected to vary from 5.44 to 5.80 or about 63%.44.2 A given leather sample may contain nitrogenous sub-stances other than hide substance (protein fiber) which will beanalyzed for by this test method, such as resins, dyestuffs, etc.,that contain nitrogen. Therefore, although this
11、 test method isfairly accurate for determining the nitrogen content of leather,its use for determining hide substance may result in largeerrors.4.3 The hide substance value derived from this determina-tion has a large bearing on other chemical determinations of agiven leather. Any errors, such as th
12、ose described in 4.1.1 and4.2, will be carried over into these other analytical calculations.5. Apparatus5.1 Kjeldahl Apparatus consisting of:1This test method is under the jurisdiction of ASTM Committee D31 on Leatherand is the direct responsibility of Subcommittee D31.06 on ChemicalAnalysis. Thist
13、est method was developed in cooperation with the American Leather ChemistsAssn. (Standard Method B 5 1954).Current edition approved April 1, 2007. Published April 2007. Originallyapproved in 1970. Last previous edition approved in 2001 as D 2868 96 (2001).2For referenced ASTM standards, visit the AS
14、TM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Acceptable procedures are published in Journal of the American LeatherChemists Association, Vol 51, 1956
15、 p. 497; or Offcial Methods of Analysis, Am.Leather Chemists Assn., available through the Office of the Secretary-Treasurer,Campus Station, Cincinnati, Ohio 45221; see Practice D 2813.4Dahl, S., “Determination of Hide Substance in the Kjeldahl Method,” inChemistry and Technology of Leather, Vol 4, R
16、einhold Publishing Co., New York,NY, 1965.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.5.1.1 Kjeldahl Flask, of 500 or 800-mL capacity for diges-tion of the sample.5.1.2 Heater, (gas or electric) for the Kjeldahl flask withfume ho
17、od or other exhaust system.5.1.3 Distillation Apparatus, consisting of an efficient vaportrap that can be sealed tightly in the top of the Kjeldahl flaskand a condenser connected to the top of the trap. All elementsof the distillation system shall be constructed of block tin,borosilicate glass, or o
18、ther materials known not to react withhot ammonia vapor.5.2 Comparable semi-automated equipment may be avail-able. The sample sizes may need to be adjusted and resultsshould be compared to the above equipment to determinesuitability.6. Reagents6.1 Purity of ReagentsReagent grade chemicals shall beus
19、ed in all tests. Unless otherwise indicated, it is intended thatall reagents shall conform to the specifications of the Commit-tee on Analytical Reagents of the American Chemical Society,where such specifications are available.5Other grades may beused, provided it is first ascertained that the reage
20、nt is ofsufficiently high purity to permit its use without lessening theaccuracy of the determination.6.2 Purity of WaterUnless otherwise indicated, referencesto water shall be understood to mean distilled water or water ofequal purity.6.3 Boric Acid Indicator SolutionDissolve 40 g of boricacid (H3B
21、O3) (borax-free) in water, add 10 mL of mixedindicator solution (5.5) and dilute to 1 L.6.4 Catalyst Digestion Mixture6,7 20.0 g K2SO4+ 0.6 gCuSO4+ 0.2 g pumice.6.5 Mixed Indicator SolutionDissolve 0.060 g of methylred indicator and 0.040 g of methylene blue indicator in 100mL of 95 % ethyl alcohol.
22、6.6 Sodium Hydroxide, Concentrated Solution (450 g/L)Dissolve 450 g of sodium hydroxide (NaOH) pellets (98 %) inwater and dilute to 1 L.6.7 Sodium Hydroxide, Standard Solution (0.1 N)Dissolve 10 mL of the concentrated NaOH solution (6.6)in1L of boiled and cooled water. Determine the exact normality
23、bytitration against the standard sulfuric acid (6.11) using themixed indicator (6.5) for the end point.6.8 Sodium Thiosulfate Solution (80 g/L)Dissolve 80 g ofsodium thiosulfate (Na2S2O35H2O) in water and dilute to 1 L.6.9 Sucrose (C11H22O11).6.10 Sulfuric Acid (sp gr 1.84)Concentrated sulfuric acid
24、(H2SO4), free from nitrogen.6.11 Sulfuric Acid, Standard (0.3 N)Dissolve 9 mL ofconcentrated H2SO4(5.10) in water and dilute to 1 L. Deter-mine the exact normality by titration against an equivalentsolution of a primary standard such as anhydrous sodiumcarbonate or tris (hydroxymethyl) amino methane
25、.7. Standardization7.1 BlanksRun a blank determination substituting 1.0 g ofsucrose in place of the leather specimen by the procedureshown in Section 8. Calculate the blank results, as shown inSection 9.7.2 StandardTris (hydroxymethyl) amino methane can beused as an internal nitrogen standard for th
26、e method. Weigh outto 0.001 g approximately1goftris (hydroxymethyl) aminomethane and transfer to the Kjeldahl flask. Run this standard bythe same procedure shown in Section 8. One gram of thisreagent is equal to 0.1156 g of N2or 8.255 meq of N2.8. Procedure8.1 Sample and SpecimenWeigh out two specim
27、ens fromthe ground sample2of 0.5 6 0.05 g accurately to 0.001 g andrecord the weight of each specimen.NOTE 2The specimens for all chemical tests to be performed on theleather should be weighed at the same time to keep the moisture contentconstant among the specimens. If only the nitrogen content is
28、beinganalyzed for, then specimens for moisture analysis should be weighed outat the same time as those for the nitrogen analyses.8.2 DigestionTransfer the specimen to a Kjeldahl flask,being careful that all the powder is shaken down into the mainbulb of the flask. Add 10 6 0.5 g of the catalyst dige
29、stionmixture, a few glass beads or other anti-bumping agents, and25 mL of H2SO4. Mix the contents by gently swirling until allof the powder is wet by the acid. Place the flask over the heaterin the fume hood with the flask inclined at about 45. Adjustthe heat so that the contents boil gently (the co
30、ndensate lineshould be within the neck of the flask) and maintain this boilingfor 1.5 h. Cool and dilute with 250 to 300 mL of water. Add 25mL of Na2S2O3solution and swirl the flask gently to mix thecontents thoroughly. Set aside for 5 to 10 min with occasionalshaking.8.3 Distillation:8.3.1 Measure
31、out 125 mL of the H3BO3solution with agraduate and transfer to a 500-mL Erlenmeyer receiving flask.Place the receiving flask under the outlet tube from thecondenser so that the end of the tube dips below the surface ofthe H3BO3solution.8.3.2 Carefully pour an amount of the concentrated NaOHsolution
32、sufficient to make the contents of the flask stronglyalkaline slowly down the side of the digestion flask so that thecaustic settles to the bottom and does not mix with the acidlayer. The amount of concentrated NaOH solution required isabout 95 mL. Connect the Kjeldahl flask to the trap immedi-ately
33、 and be sure that the rubber stopper is tightly in place.Swirl the contents gently to mix the two layers and then heatsufficiently to boil the solution in the flask. Continue heatinguntil 150 to 200 mL has distilled over and been collected in theH3BO3solution in the receiver. Disconnect the flask an
34、d trapbefore turning off the heat to prevent sucking the solution fromthe receiver back into the flask. Disconnect the condenser5Reagent Chemicals, American Chemical Society Specifications , AmericanChemical Society, Washington, DC. For suggestions on the testing of reagents notlisted by the America
35、n Chemical Society, see Analar Standards for LaboratoryChemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeiaand National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville,MD.6Dahl, S., and Oehler, R., “The Determination of Nitrogen in Leather by theKjeldahl Meth
36、od,” JALCA, Vol 46, 1951, pp. 317355.7Available as a prepared catalyst mixture from some laboratory supply compa-nies, for example, EM Science Kelmate.D2868072outlet tube and rinse it off into the receiver. Dilute the contentsof the receiver to approximately 350 mL.8.3.3 TitrationTitrate the receive
37、r contents immediatelywith the 0.3 N H2SO4(5.11) to a purple end point (pH about4.9). Blank determinations run in accordance with 7.1 mayrequire titration with alkali (if they are purple at the end of thedistillation). In this case, titrate with the 0.1 N NaOH solution.9. Calculations9.1 BlankIf the
38、 blank was acid (purple) and requiredtitration by the 0.1 N NaOH solution, then convert this value toequivalent millilitres of 0.3 N H2SO4as follows:B 5 blank, ml of standard 5 VbNb/Na(1)where:Vb= millilitres of NaOH solution required for titration ofthe blank,Nb= normality of the NaOH solution, and
39、Na= normality of the H2SO4.9.2 Nitrogen in SampleCalculate as follows for the asre-ceived basis:Nitrogen, % 5 A 6 B! 3 N 3 0.014!/W 3 100 (2)where:A = millilitres of H2SO4required for titration of thesample,B = millilitres of H2SO4required for titration of the blank(or equivalent millilitres of H2SO
40、4in terms of 0.1 NNaOH solution calculated in 9.1) (use plus B in theformula above if the blank was acidic and requiredtitration by alkali. Use minus B if the blank wasalkaline and required titration by acid),N = normality of the H2SO4, andW = grams of sample.9.2.1 Calculate the percent nitrogen on
41、the moisture-freebasis as follows:Nitrogen, % moisture2free basis!5C/100 2 M!# 3 100 (3)where:C = percent nitrogen (as-received basis), andM = percent volatile matter or moisture determined accord-ing to an accepted procedure.29.3 Calculate the percent of hide substance (protein fiber) onthe moistur
42、e free basis as follows:Hide Substance, % Moisture Free Basis!5 5.62 3 nitrogen Moisture Free Basis! (4)10. Report10.1 Report the following information:10.1.1 Nitrogen ContentReport the percent nitrogen onthe moisture-free basis.10.1.2 Hide SubstanceReport the hide substance on themoisture-free basi
43、s.11. Precision and Bias811.1 The following criteria should be used to judge theacceptability of results:11.2 RepeatabilityThe average difference between tworesults obtained by the same analyst on different days, willapproximate 0.9 % relative. Two such values should be con-sidered suspect (95 % con
44、fidence level) if they differ by morethan 1.8 % relative.11.2.1 ReproducibilityThe average difference betweentwo results (each the average of duplicate determinations),obtained by analysts in different laboratories, will approximate1.1 % relative. Two such values should be considered suspect(95 % co
45、nfidence level) if they differ by more than 4.2 %relative.NOTE 3The above precision estimates are based on an interlaboratorystudy of four leathers run on four different days by three modifications ofthis method in four different laboratories for a total of 192 samples.Practice E 180, was used in de
46、veloping these estimates. Data supportingthe precision statements may be found in Footnote 10. A new estimate isexpected to be developed based on a larger range of specimen sizes toaccommodate semiautomatic equipment.912. Keywords12.1 hide substance; Kjeldahl; nitrogenASTM International takes no pos
47、ition respecting the validity of any patent rights asserted in connection with any item mentionedin this standard. Users of this standard are expressly advised that determination of the validity of any such patent rights, and the riskof infringement of such rights, are entirely their own responsibil
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