1、Designation: D 6656 01 (Reapproved 2006)Standard Test Method forDetermination of Chromic Oxide in Wet Blue (PerchloricAcid Oxidation)1This standard is issued under the fixed designation D 6656; 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 (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the determination of chromicoxide in wet blue that has been partly or
3、completely tannedwith chromium compounds. In general, the samples willcontain chromium content between 1 % and 5 % when calcu-lated as chromic oxide expressed upon a dry basis otherwisereferred to as moisture-free basis (mfb).1.2 This standard does not purport to address all of thesafety concerns, i
4、f 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. See Section 9 forspecific safety hazards.2. Referenced Documents2.1 ASTM Standards:2D 66
5、58 Test Method for Volatile Matter (Moisture) of WetBlue by Oven DryingD 6659 Practice for Sampling and Preparation of Wet Bluefor Physical and Chemical TestsE 180 Practice for Determining the Precision of ASTMMethods for Analysis and Testing of Industrial and Spe-cialty Chemicals3. Terminology3.1 D
6、efinitionsThe terms and definitions employed withinthis method are commonly used in normal laboratory practiceand require no special comment.4. Summary of Test Method4.1 The perchloric acid method may be applied to wet blue.Accurately weighed wet blue samples are digested in a blend ofconcentrated n
7、itric acid and a prepared “oxidation mixture”consisting of sulfuric and perchloric acids. Once completed,oxidation of all tri-valent to haxavalent chrome is executed bycontrolled heating. Upon dilution, the chromium is indirectly(back) titrated volumetrically with standardized thiosulfateusing relea
8、sed iodine as the titrate. The perchloric acid methodrequires less manipulation than procedures based upon fusionof the ash. However, care must be taken because of potentialhazards in the use of this reagent.5. Significance and Use5.1 The procedure described is specific for chromium in wetblue. Vana
9、dium is the only common interfering element and israrely present in quantity. The precision and accuracy of themethods are usually, at least, as good as the sampling of wetblue itself.5.2 The chromium content of wet blue is related to thedegree of tannage obtained, and hence may be a matter forspeci
10、fication in the purchase of wet blue. The proceduredescribed provides adequate accuracy for this purpose.6. Apparatus6.1 Analytical Balanceaccurate and calibrated to 0.001 g.6.2 Erlenmeyer Flasks250 ml capacity or equivalent.6.3 Burette50 ml capacity of suitable calibration grade,minimum calibration
11、 of 0.1 ml.6.4 Glass Anti-Bumping Beadsor equivalent.6.5 Measuring Cylindersof 50 ml capacity or equivalent.6.6 Small Glass Filter Funnel.6.7 Dessicatorof suitable size and design and chargedwith fresh dessicant.6.8 Weighing Vesselsof suitable size and design.6.9 Drying Ovenwith accurate variable te
12、mperature con-trols.1This test method is under the jurisdiction of ASTM Committee D31 on Leatherand is the direct responsibility of Subcommittee D31.02 on Blue Stock.Current edition approved Oct. 1, 2006. Published November 2006. Originallyapproved in 1996. Last previous edition approved in 2001 as
13、D 6656 - 01.2Annual Book of ASTM Standards, Vol 15.04.For referenced ASTM standards, visit the ASTM 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.1Copyrig
14、ht ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.7. Reagents and Materials7.1 Purity of ReagentsAnalytical Reagent (AR) gradeshall be used in all tests. Unless otherwise indicated, it isintended that all reagents shall conform to specificatio
15、ns of theCommittee on Analytical Reagents of the American ChemicalSociety, where such specifications are available.3Other gradesmay be used, provided it is first ascertained that the reagent isof sufficiently high purity to permit its use without lesseningthe accuracy of the determination.7.2 Purity
16、 of WaterUnless otherwise indicated, referenceto water shall be understood to mean distilled water or water ofequal purity.7.3 Commercial ReagentsThe use of commercially avail-able pre-standardized analytical reagents and solutions isappropriate, providing those reagents and solutions have beenprepa
17、red according to and conform to the previously men-tioned specifications (see 7.1).7.4 Nitric Acid(HNO3), 70 % w/w.7.5 Perchloric Acid(HClO4), 60-62 % w/w. 70 % or 72 %w/w perchloric acid may be substituted; however, storage issomewhat more hazardous.7.6 Sulfuric Acid(H2SO4), 96-98 % w/w.7.7 Potassi
18、um Iodide(KI), 99-100 % purity.7.8 Potassium Iodide Solution(KI), 10 % w/w. Dissolve10g(6 0.1 g) of potassium iodide into 100 ml of water.7.9 Starch Indicator Solution2 % or equivalent. Preparedaccording to accepted procedures available in analytical hand-books.7.10 Oxidizing MixtureMix 1666 ml of c
19、oncentratedsulfuric acid into an appropriate glass container that contains2500 ml of perchloric acid using extreme caution while adding.Cool the mixture to room temperature before use.7.11 Phosphoric Acid(H3PO4), 40 % w/w. Dilute 45 ml of85 % phosphoric acid with water to 100 ml.7.12 Hydrochloric Ac
20、id(HCl, 1:1 dilution of 37 %, w/w).Used in standardization of thiosulfate solution.7.13 Potassium Dichromate(K2Cr2O7). Used in standard-ization of thiosulfate solution.7.14 Sodium Carbonate(Na2CO3). Used in preparation ofthiosulfate solution.7.15 Sodium Thiosulfate(Na2S2O35H2O). Used to pre-pare thi
21、osulfate solution.7.16 Sodium Thiosulfate Standard Solution(Na2S2O3),0.1N (6 0.0002). Dissolve 24.85 g of sodium thiosulfate(Na2S2O35H2O) in previously boiled water, add 1.00 g ofsodium carbonate (Na2CO3) and dilute to 1000 ml.8. Standardization8.1 Dry potassium dichromate (K2Cr2O3) in an oven at 13
22、0C for 2 h and cool in a dessicator. Once cool, weigh about 0.2g(6 0.0001 g) of this dry potassium dichromate into a clean250 ml, glass-stoppered Erlenmeyer flask. Dissolve in 50 mlwater; add 4 ml of hydrochloric acid (HCl, 1:1) and 20 ml ofKI solution. Stopper the flask and allow to stand for 5 min
23、. inthe dark. Titrate with the thiosulfate solution that is to bestandardized. When the solution color has faded to brownish-green, add 2 ml of 2 % starch solution and continue titratinguntil the deep blue color changes to a clear green. Record thevolume of titrant used. Calculate the Normality of t
24、he thiosul-fate solution as follows:Normality 5 A 4 0.04903 3 B!where:A = grams of K2Cr2O7used, andB = ml required for titration.8.2 Shelf LifeThe thiosulfate solution is relatively stable.However, it should be re-standardized at least once everymonth. Alternatively, a commercially available pre-sta
25、ndardized analytical solution may be substituted.9. Hazards9.1 Chemicals used can be harmful and/or explosive.9.2 The improper use of perchloric acid can lead to violentand serious explosions. In general, these can be traced tosituations where concentrated perchloric acid has come incontact with org
26、anic or easily oxidized materials.9.3 The exact procedures given must be followed and thedigestion, once started, should be kept from possible contactwith other organic matter. The digestion should never beallowed to boil dry. The perchloric acid should never be usedwithout the accompanying use of n
27、itric and sulfuric acids.9.4 Any spills involving perchloric acid should be flushedwith water and a liquid acid neutralizer.9.5 The use of a perchloric acid hood, reserved for perchlo-ric acid digestion, equipped with wash-down facilities andconstructed entirely of non-porous inorganic material is r
28、e-quired.9.6 Perchloric acid bottles should be stored on a ceramic ornon-porous tray or shelf and never on a wooden or perviousshelf.9.7 Perchloric acid must not be permitted to go dry in thepresence of organics, metals or metal salts.9.8 It is advisable to keep only a one pound (0.45 kg) bottleof a
29、cid in a working area.10. Test Specimens10.1 The test specimen shall be 3-5 g of wet blue from acomposite sample prepared according to Practice D 6659 andweighed to an accuracy of 0.001 g.11. Procedure11.1 Accurately weigh between 3 and5g(6 0.001 g) of theprepared wet blue sample into a 250 ml Erlen
30、meyer flask.Record the weight to the nearest 0.001 g.11.2 Using extreme care, add 20 ml of nitric acid (HNO3)tothe flask.11.3 Next, carefully and slowly add 25 ml of the OxidizingMixture to the flask.11.4 Add a few glass anti-bumping beads (these will allowthe solution to boil evenly), then place th
31、e filter funnel into theneck of the flask.3Reagent Chemicals, American Chemical Society Specifications , AmericanChemical Society, Washington, DC. For suggestions on the testing of reagents notlisted by the American Chemical Society, see Analar Standards for LaboratoryChemicals, BDH Ltd. Poole, Dors
32、et, UK and the United States Pharmacopoeia andNational Formulary, U.S. Pharmacopoeial Convention, Inc. (USPC), Rockville,MD.D 6656 01 (2006)211.5 Under the perchloric hood, heat the solution gentlyunder reflux conditions (using the funnel as a condenser in theneck of the flask) until all organic mat
33、ter is destroyed and thesolution color changes to a clear red-orange, indicating oxida-tion of the chromium. Do not allow the sample to boil todryness.11.6 Heat the solution for an additional 2 min to ensurecomplete oxidation; then rapidly cool the solution, rinse andremove the funnel, then dilute t
34、o 125 ml with water. Rapidcooling can be achieved with the use of a cold water bath.11.7 Re-heat the solution to boiling and continue for 7 to 10min in order to expel and any chlorine or oxides of nitrogen.11.8 Remove the flask from the heat and allow the solutionto cool to room temperature.11.9 Onc
35、e cool, add 30 ml of phosphoric acid and 25 ml ofthe 10 % potassium iodide solution, then stopper the flask.11.10 Place the flask in the dark and allow to stand for 5 minto allow complete release of iodine (dark brown color).11.11 Titrate volumetrically with 0.1 N, standardized so-dium thiosulfate.1
36、1.12 When the solution color has faded to a pale yellow-brown, add approximately 2 ml of the starch indicator solution.Continue the titration until the deep blue color changes to aclear blue-green.11.13 Record the titration volume.11.14 If the titration volume is less than 5 ml or more than50 ml, re
37、peat the entire procedure, after adjusting the sampleweight so that the titration volume is toward the middle of theburette.11.15 Calculate the results in accordance with Section 12.12. Calculation12.1 Calculate the chromium content as the percentage ofchromic oxide (Cr2O3) in the leather as follows
38、:Chromic Oxide Cr2O3!,% 5 T 3 N 3 E 3 100 4 W!where:T = titration volume of sodium thiosulfate solution usedin milliliters (ml),N = Normality of the sodium thiosulfate solution,E = 0.025332 = chromic oxide (Cr2O3) equivalenceweight per milliliter (g/ml),W = weight of original wet blue specimen (as r
39、eceived) ingrams (g), and100 = conversion to percent.12.2 The above calculation provides the Cr2O3content ofthe wet blue on an “as received” basis. Since the chromiumcontent of wet blue is expressed upon a dry (moisture-free)basis, a moisture determination must be run in accordance withTest Method D
40、 6658.IfD is found to be the moisture contentin the wet blue sample, then calculate the Cr2O3upon a drybasis (mfb) as follows:Chromic Oxide Cr2O3!,%5 T 3 N 3 E 3 100 4 W! 3 1 4 1 D4 100!#!where:T, N, E and W = the same significance as in the previouscalculation.13. Report13.1 Report the percentage o
41、f Cr2O3to the nearest 0.01 %for each and all values.13.2 Duplicate runs that agree within 0.09 % absolute areacceptable for averaging (95 % confidence level).14. Precision and Bias14.1 ReproducibilityThe average difference between tworesults (each the average of duplicate determinations) obtainedby
42、analysis in different laboratories will approximate 0.06 %on an absolute basis. Two such values should be consideredsuspect (95 % confidence level) if they differ by more than0.2 % absolute.14.2 BiasThe test method yields results that average1.75 % lower in relation to the standard sample of NBSK2Cr
43、2O7. The 99 % confidence limits on this value are 1.50 to2.00 %, as determined by triplicate analyses in five laborato-ries.NOTE 1The estimates of checks for duplicates and reproducibility in14.1 and 14.2 are based on an interlaboratory study of four leather samplesrun in triplicate in each of labor
44、atories. The precision statements weredeveloped4using Practice E 180.15. Keywords15.1 blue stock; chrome content; chromic oxide; perchloricacid oxidation; wet blueASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentionedin this s
45、tandard. 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 responsibility.This standard is subject to revision at any time by the responsible technical committee and must be re
46、viewed every five years andif not revised, either reapproved or withdrawn. Your comments are invited either for revision of this standard or for additional standardsand should be addressed to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of therespons
47、ible technical committee, which you may attend. If you feel that your comments have not received a fair hearing you shouldmake your views known to the ASTM Committee on Standards, at the address shown below.This standard is copyrighted by ASTM International, 100 Barr Harbor Drive, PO Box C700, West
48、Conshohocken, PA 19428-2959,United States. Individual reprints (single or multiple copies) of this standard may be obtained by contacting ASTM at the aboveaddress or at 610-832-9585 (phone), 610-832-9555 (fax), or serviceastm.org (e-mail); or through the ASTM website(www.astm.org).4The actual data upon which the results are based are reported in Journal,American Leather Chemists Assn. JALCA, Vol. 54, 1959, p. 2.D 6656 01 (2006)3
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