1、Designation: D2868 17Standard Test Method forNitrogen Content (Kjeldahl) and Hide Substance Content ofLeather, Wet Blue and Wet White1This standard is issued under the fixed designation D2868; the number immediately following the designation indicates the year oforiginal adoption or, in the case of
2、revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the U.S. Department of Defense.1. Scope1.1 This test
3、method covers the determination of the nitro-gen content of all types of leather, wet blue and wet white. Thenitrogen content is used to calculate the hide substance (proteinfiber) content of leather, wet blue and wet white.NOTE 1The original test method for leather was essentially acomposite of Met
4、hod 6441 of Federal Test Method Standard No. 311 andMethod B5 of the American Leather Chemists Association.NOTE 2Melamine, if present in bonded leather, could give anartificially high value for the calculation of protein fiber.1.2 The values stated in SI units are to be regarded asstandard. No other
5、 units of measurement are included in thisstandard.1.3 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
6、regulatory limitations prior to use.1.4 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Tra
7、de Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:2D2813 Practice for Sampling Leather for Physical andChemical TestsD6659 Practice for Sampling and Preparation of Wet Blueand Wet White for Physical and Chemical TestsE177 Practice for Use of the Ter
8、ms Precision and Bias inASTM Test MethodsE691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test Method3. Summary of Test Method3.1 The specimen prepared according to an accepted proce-dure (see Note 3) is digested with acid in the presence of acatalyst to convert t
9、he nitrogen to ammonium ion. The ammo-nium ion formed is nonvolatile under these highly acidconditions.NOTE 3For leather use specimen prepared per Practice D2813. Forwet blue and wet white, use specimen prepared per Practice D6659.3.2 The acid mixture is then made alkaline and the ammonialiberated i
10、s distilled into either a boric acid solution whichabsorbs the ammonia, or a sulfuric acid solution which absorbsthe ammonia.3.3 When the boric acid solution is used, the amount ofammonia in the boric acid is then determined by back titrationwith standardized acid using a sharp color change indicato
11、r(green to purple) to determine the end point. When the sulfuricacid solution is used, the amount of ammonia in the sulfuricacid solution is then determined by back titration with stan-dardized base using a sharp color change indicator (purple togreen-blue) to determine the end point.4. Significance
12、 and Use4.1 The nitrogen content as determined by this test methodis normally considered to be related to the amount of hidesubstance (protein fiber) present in the leather sample.Afactorof 5.62 is normally used to calculate the hide substance fromthe nitrogen content.4.1.1 The 5.62 factor represent
13、s the average result of manyanalyses of animal hides, but it cannot be 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
14、 thickness). As a result of these1This test method is under the jurisdiction ofASTM Committee D31 on Leatherand is the direct responsibility of Subcommittee D31.06 on ChemicalAnalysis. Thistest method was developed in cooperation with the American Leather ChemistsAssn. (Standard Method B5 1954).Curr
15、ent edition approved April 1, 2017. Published May 2017. Originallyapproved in 1970. Last previous edition approved in 2015 as D2868 10 (2015).DOI: 10.1520/D2868-17.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Boo
16、k of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance with internationally recogni
17、zed principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.1variations, the true factor for any given leather may beexpected t
18、o vary from 5.44 to 5.80 or about 63%.34.2 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 test method isfairly accurate
19、 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 those described in 4.1.1 and4.2,
20、 will be carried over into these other analytical calculations.5. Apparatus5.1 Kjeldahl Apparatus consisting of: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 hood or other exhaust system.5.1.3 Distillation
21、 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 other materials known not to react withhot amm
22、onia vapor.5.2 Semi-automated equipment (Kjeltec/micro-Kjeldahl)produce comparable results and may be substituted for Kjel-dahl apparatus. See Precision and Bias (12.1 12.4).6. Reagents6.1 Purity of ReagentsReagent grade chemicals shall beused in all tests. Unless otherwise indicated, it is intended
23、 thatall reagents shall conform to the specifications of the Commit-tee on Analytical Reagents of the American Chemical Society,where such specifications are available.4Other grades may beused, provided it is first ascertained that the reagent is ofsufficiently high purity to permit its use without
24、lessening theaccuracy of the determination.6.2 Purity of WaterUnless otherwise indicated, referencesto water shall be understood to mean distilled water, deionizedwater, or water of equal purity.6.3 Boric Acid Indicator SolutionDissolve 40 g of boricacid (H3BO3) (borax-free) in water, add 10 mL of m
25、ixedindicator solution (6.5) and dilute to 1 L.6.4 Catalyst Digestion Mixture5,620.0gK2SO4+ 0.6 gCuSO4+ 0.2 g pumice.6.5 Mixed Indicator Solution7Dissolve 0.060 g of methylred indicator and 0.040 g of methylene blue indicator in 100mL of 95 % ethyl alcohol.6.6 Sodium Hydroxide, Concentrated Solution
26、 (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)in1Lof boiled and cooled water. Determine the exact normality bytitration against the standard sulfuric ac
27、id (6.10) using themixed indicator (6.5) for the end point.6.8 Sucrose (C11H22O11).6.9 Sulfuric Acid (sp gr 1.84)Concentrated sulfuric acid(H2SO4), free from nitrogen.6.10 Sulfuric Acid, Standard (0.3 N)Dissolve 9 mL ofconcentrated H2SO4(6.9) in water and dilute to 1 L. Determinethe exact normality
28、by titration against an equivalent solutionof a primary standard such as anhydrous sodium carbonate ortris (hydroxymethyl) amino methane.6.11 Sulfuric Acid, Standard (0.5 N)Available commer-cially. Determine the exact normality by titration against anequivalent solution of a primary standard such as
29、 anhydroussodium carbonate or tris (hydroxymethyl) amino methane.6.12 Sodium Hydroxide (0.5 N)Available commercially.Determine the exact normality by titration against a knownsolution of a primary standard such as potassium hydrogenphthalate.6.13 When using semi-automatic equipment, follow theguidel
30、ines provided by the manufacturer.7. Hazards7.1 All reagents and chemicals should be handled with care.Before using any chemical, read and follow all safety precau-tions and instructions on the manufacturers label or SDS(Safety Data Sheet).8. Standardization8.1 BlanksRun a blank determination substi
31、tuting 1.0 g ofsucrose in place of the leather specimen by the procedureshown in Section 9. Calculate the blank results, as shown inSection 9.3.8.2 StandardTris (hydroxymethyl) amino methane can beused as an internal nitrogen standard for the method. Weigh outto 0.001 g approximately1goftris (hydrox
32、ymethyl) aminomethane and transfer to the Kjeldahl flask. Run this standard bythe same procedure shown in Section 9. One gram of thisreagent is equal to 0.1156 g of N2or 8.255 meq of N2.9. Procedure9.1 Procedure A Kjeldahl Apparatus9.1.1 Sample and Specimen:3Dahl, S., “Determination of Hide Substanc
33、e in the Kjeldahl Method,” inChemistry and Technology of Leather, Vol 4, Reinhold Publishing Co., New York,NY, 1965.4Reagent Chemicals, American Chemical Society Specifications , AmericanChemical Society, Washington, DC. For suggestions on the testing of reagents notlisted by the American Chemical S
34、ociety, 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.5Dahl, S., and Oehler, R., “The Determination of Nitrogen in Leather by theKjeldahl Method,” JALCA,
35、Vol 46, 1951, pp. 317355.6Available as a prepared catalyst mixture from some laboratory supplycompanies, for example, Alfie Packers, #20P.7Available as prepared solution from some laboratory supply companies. HachBromcresol Green Methyl Red Indicator.D2868 1729.1.1.1 LeatherWeigh out two specimens f
36、rom the pre-pared sample of 0.5 6 0.05 g accurately to 0.001 g and recordthe weight of each specimen.9.1.1.2 Wet Blue or Wet WhiteWeigh out two specimensfrom the prepared sample of 0.5 1.0 g accurately to 0.001 gand record the weight of each specimen.NOTE 4The specimens for all chemical tests to be
37、performed on theleather, wet blue or wet white, should be weighed at the same time to keepthe moisture content constant among the specimens. If only the nitrogencontent is being analyzed for, then specimens for moisture analysis shouldbe weighed out at the same time as those for the nitrogen analyse
38、s.9.1.2 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 digestionmixture, a few glass beads or other anti-bumping agents, and25 mL of H2SO4. Mix the contents by gently swirling until al
39、lof 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 condensate lineshould be within the neck of the flask) and maintain this boilingfor 1.5 h. Cool and dilute with 250 to 300 mL o
40、f water.NOTE 5For distillation, use either boric acid solution (9.1.3)orsulfuric acid (9.1.4) in the receiving flasks.9.1.3 Distillation (Boric acid solution):9.1.3.1 Measure out 125 mL of the H3BO3solution with agraduated cylinder and transfer to a 500-mL Erlenmeyerreceiving flask. Place the receiv
41、ing flask under the outlet tubefrom the condenser so that the end of the tube dips below thesurface of the H3BO3solution.9.1.3.2 Carefully pour an amount of the concentrated NaOHsolution, sufficient to make the contents of the flask stronglyalkaline, slowly down the side of the digestion flask so th
42、at 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 and be sure that the rubber stopper is tightly in place.Swirl the contents gently to mix the two layers and th
43、en 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 and trapbefore turning off the heat to prevent sucking the solution fromthe receiver back into the flask. Disconn
44、ect the condenseroutlet tube and rinse it off into the receiver. Dilute the contentsof the receiver to approximately 350 mL.9.1.4 Distillation (Sulfuric acid solution):9.1.4.1 Measure out 25.0 mL of the 0.5 N H2SO4solutionfrom a buret into a 500-mL Erlenmeyer receiving flask. Add0.5 mL of the mixed
45、indicator solution. Adjust the volume tothe 100 mLmark with DI water. Place the receiving flask underthe outlet tube from the condenser so that the end of the tubedips below the surface of the acidic receiving solution. Preparea receiving flask for each sample, plus a separate flask for theblank.9.1
46、.4.2 Carefully pour an amount of the concentrated NaOHsolution, 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 i
47、sabout 95 mL. Connect the Kjeldahl flask to the trap immedi-ately 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 colle
48、cted in thesolution in the receiver. Disconnect the flask and trap beforeturning off the heat to prevent sucking the solution from thereceiver back into the flask. Disconnect the condenser outlettube and rinse it off into the receiver. Dilute the contents of thereceiver to approximately 350 mL.9.1.5
49、 Titration (Boric acid solution)Titrate the receivercontents of the blank distillate immediately with the 0.3 NH2SO4(6.10) to a purple end point (pH about 4.9). Blankdeterminations run in accordance with 8.1 may require titrationwith alkali (if they are purple at the end of the distillation). Inthis case, titrate with the 0.1 N NaOH solution.9.1.6 Titration (Sulfuric acid solution)Titrate the receivercontents of the blank distillate immediately with the 0.5 NNaOH (6.12) to a blue-green end point. Record the volume oftitrant for the blank. Titrate th
