1、Designation: D6656 16Standard 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 case of revision, the year
2、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. Scope1.1 This test method covers the determination of chromicoxide in Wet Blue that has been partly or completely tannedwith
3、 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 The values stated in SI units are to be regarded asstandard. No other units of measurement
4、are included in thisstandard.1.3 This test method does not apply to Wet White.1.4 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 deter
5、mine the applica-bility of regulatory limitations prior to use. See Section 9 forspecific safety hazards.2. Referenced Documents2.1 ASTM Standards:2D6658 Test Method for Volatile Matter (Moisture) of WetBlue by Oven DryingD6659 Practice for Sampling and Preparation of Wet Blueand Wet White for Physi
6、cal and Chemical TestsE180 Practice for Determining the Precision of ASTMMethods for Analysis and Testing of Industrial and Spe-cialty Chemicals (Withdrawn 2009)3E177 Practice for Use of the Terms Precision and Bias inASTM Test MethodsE691 Practice for Conducting an Interlaboratory Study toDetermine
7、 the Precision of a Test Method3. Terminology3.1 DefinitionsThe 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 Blu
8、e samples are digested in a blendof concentrated nitric 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 volum
9、etrically with standardized thiosulfateusing released 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 d
10、escribed is specific for chromium in WetBlue. Vanadium 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 ta
11、nnage obtained, and hence may be a matter forspecification 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
12、 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 equivalent.6.6 Small Glass Filter Funnel.6.7 Dessicatorof suitable size and design and chargedwith fresh dessicant.1This test method is under the jurisdi
13、ction of ASTM Committee D31 on Leatherand is the direct responsibility of Subcommittee D31.02 on Wet Blue.Current edition approved Sept. 1, 2016. Published October 2016. Originallyapproved in 1996. Last previous edition approved in 2014 as D6656 - 14b. DOI:10.1520/D6656-16.2For referenced ASTM stand
14、ards, 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.3The last approved version of this historical standard is referenced onwww.astm.org.Cop
15、yright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States16.8 Weighing Vesselsof suitable size and design.6.9 Drying Ovenwith accurate variable temperature con-trols.7. Reagents and Materials7.1 Purity of ReagentsAnalytical Reagent (AR) gradeshall
16、 be used in all tests. Unless otherwise indicated, it isintended that all reagents shall conform to specifications of theCommittee on Analytical Reagents of the American ChemicalSociety, where such specifications are available.4Other gradesmay be used, provided it is first ascertained that the reage
17、nt isof sufficiently high purity to permit its use without lesseningthe accuracy of the determination.7.2 Purity 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-sta
18、ndardized analytical reagents and solutions isappropriate, providing those reagents and solutions have beenprepared 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
19、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. Dissolve10g(6 0.1 g) of potassium iodide into 100 ml of water.7.9 Starch Indicator Solution2 % or equivalent. Pr
20、eparedaccording to accepted procedures available in analytical hand-books.7.10 Oxidizing MixtureMix 1666 mL of concentratedsulfuric acid into an appropriate glass container that contains2500 mL of perchloric acid using extreme caution whileadding. Cool the mixture to room temperature before use.NOTE
21、 1The reagents may be added individually according to 11.3.1.7.11 Phosphoric Acid(H3PO4), 40 % w/w. Dilute 45 mLof 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
22、 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 thiosulfate solution.7.16 Sodium Thiosulfate Standard Solution(Na2S2O3),0.1N (60.0002). Dissolve 24.85 g of sodium thiosulfate
23、(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 130C 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 clean25
24、0 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. inthe dark. Titrate with the thiosulfate solution that is to bestandardized. When the solution color has faded to brownish
25、-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 the thiosul-fate solution as follows:Normality 5 A0.04903 3B!where:A = grams of K2Cr2O7used, andB = ml required for titration
26、.8.2 Shelf LifeThe thiosulfate solution is relatively stable.However, it should be re-standardized at least once everymonth. Alternatively, a commercially available pre-standardized analytical solution may be substituted.9. Hazards9.1 Chemicals used can be harmful or explosive, or both.9.2 The impro
27、per 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 organic or easily oxidized materials.9.3 The exact procedures given must be followed and thedigestion, once started, should be
28、 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 nitric and sulfuric acids.9.4 Any spills involving perchloric acid should be flushedwith water and a liquid acid neutralizer
29、.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 re-quired.9.6 Perchloric acid bottles should be stored on a ceramic ornon-porous tray or shelf and never on a wooden or perv
30、iousshelf.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 acid in a working area.10. Test Specimens10.1 The test specimen shall be 3-5 g of Wet Blue from acomposite sample prepared a
31、ccording to Practice D6659 andweighed to an accuracy of 0.001 g.4Reagent 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
32、, BDH Ltd. Poole, Dorset, UK and the United States Pharmacopoeia andNational Formulary, U.S. Pharmacopoeial Convention, Inc. (USPC), Rockville,MD.D6656 16211. Procedure11.1 Accurately weigh between 3 and5g(6 0.001 g) of theprepared Wet Blue sample into a 250 mL Erlenmeyer flask.Record the weight to
33、the nearest 0.001 g.11.2 Using extreme care, add 20 mL of nitric acid (HNO3)to the flask.11.3 Next, carefully and slowly add 25 mL of the OxidizingMixture to the flask.11.3.1 Alternatively, add in this order: 20 mL HNO3,15mLHC1O4, and 10 mL H2SO4.11.4 Add a few glass anti-bumping beads (these will a
34、llowthe solution to boil evenly), then place the filter funnel into theneck of the flask.11.5 Under the perchloric hood, heat the solution gentlyunder reflux conditions (using the funnel as a condenser in theneck of the flask) until all organic matter is destroyed and thesolution color changes to a
35、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 to 125 mL with water. Rapidcooling can be achieved wi
36、th 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 Once cool, add 30 mL of phosphoric acid and 25 mL ofthe
37、 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.11.12 When the solution color has faded to a pale yel
38、low-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, repeat the entire procedure, after adjusting the sampl
39、eweight 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:Chromic Oxide Cr2O3!,%5 T 3N 3E 3 100W!where:T = ti
40、tration 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 received)in grams (g), and100 = conversion to percent.12.2
41、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 D6658.IfD is found to be the moisture content inthe Wet Blu
42、e sample, then calculate the Cr2O3upon a dry basis(mfb) as follows:Chromic Oxide Cr2O3!,%5 T 3N 3E 3 100W! 3 112 D100!#!where:T, N, E and W = the same significance as in the previouscalculation.13. Report13.1 Report the percentage of Cr2O3to the nearest 0.01 %for each and all values.13.2 Duplicate r
43、uns 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 analysis in different laboratories will approximate 0.06 %on an abso
44、lute 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 NBSK2Cr2O7. The 99 % confidence limits on this value are 1.50 to2.00 %, as
45、determined by triplicate analyses in five laborato-ries.NOTE 2The 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 laboratories. The precision statements weredeveloped5using Practice E180.
46、14.3 A real world precision statement was determinedthrough statistical examination6of 139 results from 9laboratories, on 16 materials over nearly 2 years. Practice E691was followed for the design and analysis of the data. The termsbelow (repeatability and reproducibility) are used as specifiedin Pr
47、actice E177.14.3.1 Repeatability (r)The difference between repetitiveresults obtained by the same operator in a given laboratoryapplying the same test method with the same apparatus underconstant operating conditions on identical test material within5The actual data upon which the results are based
48、are reported in Journal,American Leather Chemists Assn. JALCA, Vol. 54, 1959, p. 2.6Supporting data have been filed at ASTM International Headquarters and maybe obtained by requesting Research Report RR:D31-1020. ContactASTM CustomerService at serviceastm.org.D6656 163short intervals of time would i
49、n the long run, in the normal andcorrect operation of the test method, exceed the followingvalues only in one case in 20.14.3.2 Reproducibility (R)The difference between twosingle and independent results obtained by different operatorsapplying the same test method in different laboratories usingdifferent apparatus on identical test material would, in the longrun, in the normal and correct operation of the test method,exceed the following values only in one case in 20.Repeatability (r) Reproducibility (R)0.14 0.7014.4 The precision of
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