1、Designation: D6656 14aD6656 14bStandard 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,
2、 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. Scope1.1 This test method covers the determination of chromic oxide in wet blue that has been partly or completely
3、 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 to as moisture-free basis (mfb).1.2 The values stated in SI units are to be regarded as standard. No other units of
4、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 user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
5、limitations 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 DryingD6659 Practice for Sampling and Preparation of Wet Blue for Physical and Chemical TestsE180 Practice for Determini
6、ng the Precision of ASTM Methods for Analysis and Testing of Industrial and Specialty Chemicals(Withdrawn 2009)3E177 Practice for Use of the Terms Precision and Bias in ASTM Test MethodsE691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method3. Terminology3.1
7、 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 perchloric acid method may be applied to wet blue. Accurately weighed wet blue samples are digested in a blend ofconcentrat
8、ed 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 controlled heating. Upon dilution, the chromium is indirectly (back) titratedvolumetrically with standardized thiosulfate usin
9、g 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 hazards in the useof this reagent.5. Significance and Use5.1 The procedure described is specific for chromium in wet b
10、lue. 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 blue itself.5.2 The chromium content of wet blue is related to the degree of tannage obtained, and hence may be a mat
11、ter for specificationin the purchase of wet blue. The procedure described provides adequate accuracy for this purpose.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 May 1, 2014
12、Nov. 1, 2014. Published June 2014December 2014. Originally approved in 1996. Last previous edition approved in 2014 asD6656 - 14.D6656 - 14a. DOI: 10.1520/D6656-14A.10.1520/D6656-14B.2 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.o
13、rg. For Annual Book of ASTM Standardsvolume information, refer to the standards Document 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 st
14、andard an indication of what changes have been made to the previous version. Becauseit 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 A
15、STM is to be considered the official document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States16. Apparatus6.1 Analytical Balanceaccurate and calibrated to 0.001 g.6.2 Erlenmeyer Flasks250 mL capacity or equivalent.6.3 Burette50 mL cap
16、acity 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 charged with fresh dessicant.6.8 Weighing Vesselsof suitable
17、 size and design.6.9 Drying Ovenwith accurate variable temperature controls.7. Reagents and Materials7.1 Purity of ReagentsAnalytical Reagent (AR) grade shall be used in all tests. Unless otherwise indicated, it is intended thatall reagents shall conform to specifications of the Committee on Analyti
18、cal Reagents of the American Chemical Society, wheresuch specifications are available.4 Other grades may be used, provided it is first ascertained that the reagent is of sufficiently highpurity to permit its use without lessening the accuracy of the determination.7.2 Purity of WaterUnless otherwise
19、indicated, reference to water shall be understood to mean distilled water or water of equalpurity.7.3 Commercial ReagentsThe use of commercially available pre-standardized analytical reagents and solutions is appropriate,providing those reagents and solutions have been prepared according to and conf
20、orm 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 acid may be substituted; however, storage issomewhat more hazardous.7.6 Sulfuric Acid(H2SO4), 96-98 % w/w.7.7 Potassium Iodide(KI), 99-100 % pu
21、rity.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 equivalent. Prepared according to accepted procedures available in analytical handbooks.7.10 Oxidizing MixtureMix 1666 mL of concentrated sulfuric ac
22、id into an appropriate glass container that contains 2500 mLof perchloric acid using extreme caution while adding. Cool the mixture to room temperature before use.NOTE 1The reagents may be added individually according to 11.3.1.7.11 Phosphoric Acid(H3PO4), 40 % w/w. Dilute 45 mL of 85 % phosphoric a
23、cid 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 Carbonate(Na2CO3). Used in preparation of thiosulfate solution.7.15 Sodium
24、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 g of sodium carbonate (Na2CO3) and dilute to 1000 mL.8. Standardization8.1 D
25、ry 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. Dissolve in 50 mL water;add 4 ml of hydrochloric acid (HCl, 1:1) and 20 ml of KI so
26、lution. 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 of2 % starch solution and continue titrating until the deep blue color changes to a clear green. Record
27、the volume of titrant used.Calculate the Normality of the thiosulfate solution as follows:4 Reagent Chemicals, American Chemical Society Specifications , American Chemical Society, Washington, DC. For suggestions on the testing of reagents not listed bythe American Chemical Society, see Analar Stand
28、ards for Laboratory Chemicals, BDH Ltd. Poole, Dorset, UK and the United States Pharmacopoeia and NationalFormulary, U.S. Pharmacopoeial Convention, Inc. (USPC), Rockville, MD.D6656 14b2Normality5A0.049033B!where:A = grams of K2Cr2O7 used, andB = ml required for titration.8.2 Shelf LifeThe thiosulfa
29、te 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 can be harmful or explosive, or both.9.2 The improper use of perchloric acid
30、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 followed and the digestion, once started, should be kept from possible cont
31、act 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 should be flushed with water and a liquid acid neutralizer.9.5 The use of a perc
32、hloric 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 non-porous tray or shelf and never on a wooden or pervious shelf.9.7 Perchlor
33、ic 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 be 3-5 g of wet blue from a composite sample prepared according to Practic
34、e 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 extreme care, add 20 mL of nitric acid (HNO3) to the flask.11.3 Next, caref
35、ully and slowly add 25 mL of the Oxidizing Mixture to the flask.11.3.1 Alternatively, add in this order: 20 mL HNO3, 15 mL HC1O4, and 10 mL H2SO4.11.4 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
36、the perchloric 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.
37、11.6 Heat 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
38、 in order 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
39、allow to 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 unti
40、l the deep 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
41、results in accordance with Section 12.D6656 14b312. 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 = Normalit
42、y of the 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 recei
43、ved” basis. 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:Chrom
44、ic Oxide 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 %
45、confidence 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 % confiden
46、ce level) 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 2The es
47、timates 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 Areal world precision statement was determined through stati
48、stical examination6 of 139 results from 9 laboratories, on 16materials over nearly 2 years. Practice E691 was followed for the design and analysis of the data. The terms below (repeatabilityand reproducibility) are used as specified in Practice E177.14.3.1 Repeatability (r)The difference between rep
49、etitive 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 intervalsof time would in the long run, in the normal and correct operation of the test method, exceed the following values only in one casein 20.14.3.2 Reproducibility (R)The difference between two single and independent results obtained by different operators applyingthe same test method in different laboratories using different apparatu
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