1、Designation: D6656 01 (Reapproved 2010)1D6656 14Standard Test Method forDetermination of Chromic Oxide in Wet Blue (PerchloricAcid Oxidation)1This standard is issued under the fixed designation D6656; the number immediately following the designation indicates the year oforiginal adoption or, in the
2、case of 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.1 NOTEThe SI statement was added and the Subcommittee name corrected in footnote 1 in November 2010.
3、1. Scope1.1 This test method covers the determination of chromic oxide in wet blue that has been partly or completely tanned withchromium compounds. In general, the samples will contain chromium content between 1 % and 5 % when calculated as chromicoxide expressed upon a dry basis otherwise referred
4、 to as moisture-free basis (mfb).1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibilityof the use
5、r of this standard to establish appropriate safety and health practices and determine the applicability of regulatorylimitations prior to use. See Section 9 for specific safety hazards.2. Referenced Documents2.1 ASTM Standards:2D6658 Test Method for Volatile Matter (Moisture) of Wet Blue by Oven Dry
6、ingD6659 Practice for Sampling and Preparation of Wet Blue for Physical and Chemical TestsE180 Practice for Determining the Precision of ASTM Methods for Analysis and Testing of Industrial and Specialty Chemicals(Withdrawn 2009)3E691 Practice for Conducting an Interlaboratory Study to Determine the
7、Precision of a Test MethodE177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods3. Terminology3.1 DefinitionsThe terms and definitions employed within this method are commonly used in normal laboratory practice andrequire no special comment.4. Summary of Test Method4.1 The perchl
8、oric acid method may be applied to wet blue. Accurately weighed wet blue samples are digested in a blend ofconcentrated nitric acid and a prepared “oxidation mixture” consisting of sulfuric and perchloric acids. Once completed, oxidationof all tri-valent to haxavalent chrome is executed by controlle
9、d heating. Upon dilution, the chromium is indirectly (back) titratedvolumetrically with standardized thiosulfate using released iodine as the titrate. The perchloric acid method requires lessmanipulation than procedures based upon fusion of the ash. However, care must be taken because of potential h
10、azards in the useof this reagent.5. Significance and Use5.1 The procedure described is specific for chromium in wet blue. Vanadium is the only common interfering element and israrely present in quantity. The precision and accuracy of the methods are usually, at least, as good as the sampling of wet
11、blue itself.1 This test method is under the jurisdiction of ASTM Committee D31 on Leather and is the direct responsibility of Subcommittee D31.02 on Wet Blue.Current edition approved Oct. 1, 2010April 1, 2014. Published November 2010June 2014. Originally approved in 1996. Last previous edition appro
12、ved in 20062010 asD6656 - 02 (2006).D6656 - 01 (2010)1. DOI: 10.1520/D6656-01R10E01.10.1520/D6656-14.2 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Do
13、cument Summary page on the ASTM website.3 The last approved version of this historical standard is referenced on www.astm.org.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Becau
14、seit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official document.Copyright ASTM International, 100 Ba
15、rr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States15.2 The chromium content of wet blue is related to the degree of tannage obtained, and hence may be a matter for specificationin the purchase of wet blue. The procedure described provides adequate accuracy for this purpose
16、.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 of 0.1 mL.6.4 Glass Anti-Bumping Beadsor equivalent.6.5 Measuring Cylindersof 50 mL capacity or equiva
17、lent.6.6 Small Glass Filter Funnel.6.7 Dessicatorof suitable size and design and charged with fresh dessicant.6.8 Weighing Vesselsof suitable size and design.6.9 Drying Ovenwith accurate variable temperature controls.7. Reagents and Materials7.1 Purity of ReagentsAnalytical Reagent (AR) grade shall
18、be used in all tests. Unless otherwise indicated, it is intended thatall reagents shall conform to specifications of the Committee on Analytical Reagents of the American Chemical Society, wheresuch specifications are available.4 Other grades may be used, provided it is first ascertained that the rea
19、gent is of sufficiently highpurity to permit its use without lessening the accuracy of the determination.7.2 Purity of WaterUnless otherwise indicated, reference to water shall be understood to mean distilled water or water of equalpurity.7.3 Commercial ReagentsThe use of commercially available pre-
20、standardized analytical reagents and solutions is appropriate,providing those reagents and solutions have been prepared according to and conform to the previously mentioned 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 ac
21、id may be substituted; however, storage issomewhat more hazardous.7.6 Sulfuric Acid(H2SO4), 96-98 % w/w.7.7 Potassium Iodide(KI), 99-100 % purity.7.8 Potassium Iodide Solution(KI), 10 % w/w. Dissolve 10 g (6 0.1 g) of potassium iodide into 100 ml of water.7.9 Starch Indicator Solution2 % or equivale
22、nt. Prepared according to accepted procedures available in analytical handbooks.7.10 Oxidizing MixtureMix 1666 mL of concentrated sulfuric acid into an appropriate glass container that contains 2500 mLof perchloric acid using extreme caution while adding. Cool the mixture to room temperature before
23、use.7.11 Phosphoric Acid(H3PO4), 40 % w/w. Dilute 45 mL of 85 % phosphoric acid with water to 100 ml.7.12 Hydrochloric Acid(HCl, 1:1 dilution of 37 %, w/w). Used in standardization of thiosulfate solution.7.13 Potassium Dichromate(K2Cr2O7). Used in standardization of thiosulfate solution.7.14 Sodium
24、 Carbonate(Na2CO3). Used in preparation of thiosulfate solution.7.15 Sodium Thiosulfate(Na2S2O3 5H2O). Used to prepare thiosulfate solution.7.16 Sodium Thiosulfate Standard Solution(Na2S2O3), 0.1N (6 0.0002). Dissolve 24.85 g of sodium thiosulfate (Na2S2O3 5H2O) in previously boiled water, add 1.00
25、g of sodium carbonate (Na2CO3) and dilute to 1000 mL.8. Standardization8.1 Dry potassium dichromate (K2Cr2O3) in an oven at 130 C for 2 h and cool in a dessicator. Once cool, weigh about 0.2 g(6 0.0001 g) of this dry potassium dichromate into a clean 250 ml, glass-stoppered Erlenmeyer flask. Dissolv
26、e in 50 mL water;add 4 ml of hydrochloric acid (HCl, 1:1) and 20 ml of KI solution. Stopper the flask and allow to stand for 5 min. in the dark.Titrate with the thiosulfate solution that is to be standardized. When the solution color has faded to brownish-green, add 2 ml of4 Reagent Chemicals, Ameri
27、can Chemical Society Specifications , American Chemical Society, Washington, DC. For suggestions on the testing of reagents not listed bythe American Chemical Society, see Analar Standards for Laboratory Chemicals, BDH Ltd. Poole, Dorset, UK and the United States Pharmacopoeia and NationalFormulary,
28、 U.S. Pharmacopoeial Convention, Inc. (USPC), Rockville, MD.D6656 1422 % starch solution and continue titrating until the deep blue color changes to a clear green. Record the volume of titrant used.Calculate the Normality of the thiosulfate solution as follows:Normality5A0.049033B!where:A = grams of
29、 K2Cr2O7 used, andB = ml required for titration.8.2 Shelf LifeThe thiosulfate solution is relatively stable. However, it should be re-standardized at least once every month.Alternatively, a commercially available pre-standardized analytical solution may be substituted.9. Hazards9.1 Chemicals used ca
30、n be harmful or explosive, or both.9.2 The improper use of perchloric acid can lead to violent and serious explosions. In general, these can be traced to situationswhere concentrated perchloric acid has come in contact with organic or easily oxidized materials.9.3 The exact procedures given must be
31、followed and the digestion, once started, should be kept from possible contact withother organic matter. The digestion should never be allowed to boil dry. The perchloric acid should never be used without theaccompanying use of nitric and sulfuric acids.9.4 Any spills involving perchloric acid shoul
32、d be flushed with water and a liquid acid neutralizer.9.5 The use of a perchloric acid hood, reserved for perchloric acid digestion, equipped with wash-down facilities andconstructed entirely of non-porous inorganic material is required.9.6 Perchloric acid bottles should be stored on a ceramic or no
33、n-porous tray or shelf and never on a wooden or pervious shelf.9.7 Perchloric acid must not be permitted to go dry in the presence of organics, metals or metal salts.9.8 It is advisable to keep only a one pound (0.45 kg) bottle of acid in a working area.10. Test Specimens10.1 The test specimen shall
34、 be 3-5 g of wet blue from a composite sample prepared according to Practice D6659 and weighedto an accuracy of 0.001 g.11. Procedure11.1 Accurately weigh between 3 and 5 g (6 0.001 g) of the prepared wet blue sample into a 250 mL Erlenmeyer flask. Recordthe weight to the nearest 0.001 g.11.2 Using
35、extreme care, add 20 mL of nitric acid (HNO3) to the flask.11.3 Next, carefully and slowly add 25 mL of the Oxidizing Mixture to the flask.11.4 Add a few glass anti-bumping beads (these will allow the solution to boil evenly), then place the filter funnel into the neckof the flask.11.5 Under the per
36、chloric hood, heat the solution gently under reflux conditions (using the funnel as a condenser in the neckof the flask) until all organic matter is destroyed and the solution color changes to a clear red-orange, indicating oxidation of thechromium. Do not allow the sample to boil to dryness.11.6 He
37、at the solution for an additional 2 min to ensure complete oxidation; then rapidly cool the solution, rinse and removethe funnel, then dilute to 125 mL with water. Rapid cooling can be achieved with the use of a cold water bath.11.7 Re-heat the solution to boiling and continue for 7 to 10 min in ord
38、er to expel and any chlorine or oxides of nitrogen.11.8 Remove the flask from the heat and allow the solution to cool to room temperature.11.9 Once cool, add 30 mL of phosphoric acid and 25 mL of the 10 % potassium iodide solution, then stopper the flask.11.10 Place the flask in the dark and allow t
39、o stand for 5 min to allow complete release of iodine (dark brown color).11.11 Titrate volumetrically with 0.1 N, standardized sodium thiosulfate.11.12 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 d
40、eep blue color changes to a clear blue-green.11.13 Record the titration volume.11.14 If the titration volume is less than 5 mLor more than 50 ml, repeat the entire procedure, after adjusting the sample weightso that the titration volume is toward the middle of the burette.11.15 Calculate the results
41、 in accordance with Section 12.D6656 14312. Calculation12.1 Calculate the chromium content as the percentage of chromic oxide (Cr2O3) in the leather as follows:Chromic Oxide Cr2O3!,%5T 3N 3E 3100W!where:T = titration volume of sodium thiosulfate solution used in milliliters (ml),N = Normality of the
42、 sodium thiosulfate solution,E = 0.025332 = chromic oxide (Cr2O3) equivalence weight per milliliter (g/mL),W = weight of original wet blue specimen (as received) in grams (g), and100 = conversion to percent.12.2 The above calculation provides the Cr2O3 content of the wet blue on an “as received” bas
43、is. Since the chromium contentof wet blue is expressed upon a dry (moisture-free) basis, a moisture determination must be run in accordance with Test MethodD6658. If D is found to be the moisture content in the wet blue sample, then calculate the Cr2O3 upon a dry basis (mfb) as follows:Chromic Oxide
44、 Cr2O3!,%5T 3N 3E 3100W! 3112D100!#!where:T, N, E and W = the same significance as in the previous calculation.13. Report13.1 Report the percentage of Cr2O3 to the nearest 0.01 % for each and all values.13.2 Duplicate runs that agree within 0.09 % absolute are acceptable for averaging (95 % confiden
45、ce level).14. Precision and Bias14.1 ReproducibilityThe average difference between two results (each the average of duplicate determinations) obtained byanalysis in different laboratories will approximate 0.06 % on an absolute basis. Two such values should be considered suspect(95 % confidence level
46、) if they differ by more than 0.2 % absolute.14.2 BiasThe test method yields results that average 1.75 % lower in relation to the standard sample of NBS K2Cr2O7. The99 % confidence limits on this value are 1.50 to 2.00 %, as determined by triplicate analyses in five laboratories.NOTE 1The estimates
47、of checks for duplicates and reproducibility in 14.1 and 14.2 are based on an interlaboratory study of four leather samples runin triplicate in each of laboratories. The precision statements were developed5 using Practice E180.14.3 The precision of this test method is based on an intralaboratoy stud
48、y of ASTM WK40322, New Standard Test Method forthe Determination of Chromic Oxide inWet Blue andWetWhite, conducted between 2011 and 2013. Nine laboratories participatedin this study, testing 16 Wet Blue samples. Every “test result” represents an individual determination. The laboratories were asked
49、to report a single test result for 13 materials and duplicate test results for 3 materials. Except for the absence of replicate test resultsfrom all of the study materials, Practice E691 was followed for the design and analysis of the data; the details are give in anASTMresearch report.614.3.1 Repeatability (r)The difference between repetitive results obtained by the same operator in a given laboratory applyingthe same test method with the same apparatus under constant operating conditions on identical test material within short i
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