ASTM D6656-16(2021)
(Test Method)Standard Test Method for Determination of Chromic Oxide in Wet Blue (Perchloric Acid Oxidation)
Standard Test Method for Determination of Chromic Oxide in Wet Blue (Perchloric Acid Oxidation)
SIGNIFICANCE AND USE
5.1 The procedure described is specific for chromium in Wet Blue. Vanadium is the only common interfering element and is rarely 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 matter for specification in the purchase of Wet Blue. The procedure described provides adequate accuracy for this purpose.
SCOPE
1.1 This test method covers the determination of chromic oxide in Wet Blue that has been partly or completely tanned with chromium compounds. In general, the samples will contain chromium content between 1 % and 5 % when calculated as chromic oxide 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 measurement are included in this standard.
1.3 This test method does not apply to Wet White.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. See Section 9 for specific safety hazards.
1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
General Information
Relations
Standards Content (Sample)
This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D6656 − 16 (Reapproved 2021)
Standard Test Method for
Determination of Chromic Oxide in Wet Blue (Perchloric
Acid Oxidation)
This standard is issued under the fixed designation D6656; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope cialty Chemicals (Withdrawn 2009)
E177 Practice for Use of the Terms Precision and Bias in
1.1 This test method covers the determination of chromic
ASTM Test Methods
oxide in Wet Blue that has been partly or completely tanned
E691 Practice for Conducting an Interlaboratory Study to
with chromium compounds. In general, the samples will
Determine the Precision of a Test Method
contain chromium content between 1 % and 5 % when calcu-
lated as chromic oxide expressed upon a dry basis otherwise
3. Terminology
referred to as moisture-free basis (mfb).
3.1 Definitions—The terms and definitions employed within
1.2 The values stated in SI units are to be regarded as
this method are commonly used in normal laboratory practice
standard. No other units of measurement are included in this
and require no special comment.
standard.
4. Summary of Test Method
1.3 This test method does not apply to Wet White.
1.4 This standard does not purport to address all of the
4.1 The perchloric acid method may be applied toWet Blue.
safety concerns, if any, associated with its use. It is the Accurately weighed Wet Blue samples are digested in a blend
responsibility of the user of this standard to establish appro-
of concentrated nitric acid and a prepared “oxidation mixture”
priate safety, health, and environmental practices and deter- consisting of sulfuric and perchloric acids. Once completed,
mine the applicability of regulatory limitations prior to use.
oxidation of all tri-valent to haxavalent chrome is executed by
See Section 9 for specific safety hazards. controlled heating. Upon dilution, the chromium is indirectly
1.5 This international standard was developed in accor-
(back) titrated volumetrically with standardized thiosulfate
dance with internationally recognized principles on standard- using released iodine as the titrate. The perchloric acid method
ization established in the Decision on Principles for the requires less manipulation than procedures based upon fusion
Development of International Standards, Guides and Recom-
of the ash. However, care must be taken because of potential
mendations issued by the World Trade Organization Technical hazards in the use of this reagent.
Barriers to Trade (TBT) Committee.
5. Significance and Use
2. Referenced Documents
5.1 TheproceduredescribedisspecificforchromiuminWet
Blue. Vanadium is the only common interfering element and is
2.1 ASTM Standards:
rarely present in quantity. The precision and accuracy of the
D6658 Test Method for Volatile Matter (Moisture) of Wet
methods are usually, at least, as good as the sampling of Wet
Blue by Oven Drying
Blue itself.
D6659 Practice for Sampling and Preparation of Wet Blue
and Wet White for Physical and Chemical Tests
5.2 The chromium content of Wet Blue is related to the
E180 Practice for Determining the Precision of ASTM
degree of tannage obtained, and hence may be a matter for
Methods for Analysis and Testing of Industrial and Spe-
specification in the purchase of Wet Blue. The procedure
described provides adequate accuracy for this purpose.
This test method is under the jurisdiction ofASTM Committee D31 on Leather
6. Apparatus
and is the direct responsibility of Subcommittee D31.02 on Wet Blue.
6.1 Analytical Balance—accurate and calibrated to 0.001 g.
Current edition approved Sept. 1, 2021. Published October 2021. Originally
approved in 1996. Last previous edition approved in 2016 as D6656 - 16. DOI:
6.2 Erlenmeyer Flasks—250 mL capacity or equivalent.
10.1520/D6656-16R21.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on The last approved version of this historical standard is referenced on
the ASTM website. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6656 − 16 (2021)
6.3 Burette—50 mL capacity of suitable calibration grade, 7.12 Hydrochloric Acid—(HCl, 1:1 dilution of 37 %, w/w).
minimum calibration of 0.1 mL. Used in standardization of thiosulfate solution.
6.4 Glass Anti-Bumping Beads—or equivalent. 7.13 Potassium Dichromate—(K Cr O ). Used in standard-
2 2 7
ization of thiosulfate solution.
6.5 Measuring Cylinders—of 50 mLcapacity or equivalent.
7.14 Sodium Carbonate—(Na CO ). Used in preparation of
2 3
6.6 Small Glass Filter Funnel.
thiosulfate solution.
6.7 Dessicator—of suitable size and design and charged
7.15 Sodium Thiosulfate—(Na S O ·5H O). Used to pre-
2 2 3 2
with fresh dessicant.
pare thiosulfate solution.
6.8 Weighing Vessels—of suitable size and design.
7.16 Sodium Thiosulfate Standard Solution—(Na S O ),
2 2 3
6.9 Drying Oven—with accurate variable temperature con-
0.1N (60.0002). Dissolve 24.85 g of sodium thiosulfate
trols.
(Na S O ·5H O) in previously boiled water, add 1.00 g of
2 2 3 2
sodium carbonate (Na CO ) and dilute to 1000 mL.
2 3
7. Reagents and Materials
8. Standardization
7.1 Purity of Reagents—Analytical Reagent (AR) grade
8.1 Dry potassium dichromate (K Cr O ) in an oven at 130
2 2 3
shall be used in all tests. Unless otherwise indicated, it is
°C for 2 h and cool in a dessicator. Once cool, weigh about 0.2
intended that all reagents shall conform to specifications of the
g(6 0.0001 g) of this dry potassium dichromate into a clean
Committee on Analytical Reagents of the American Chemical
250 ml, glass-stoppered Erlenmeyer flask. Dissolve in 50 mL
Society, where such specifications are available. Other grades
water; add 4 ml of hydrochloric acid (HCl, 1:1) and 20 ml of
may be used, provided it is first ascertained that the reagent is
KI solution. Stopper the flask and allow to stand for 5 min. in
of sufficiently high purity to permit its use without lessening
the dark. Titrate with the thiosulfate solution that is to be
the accuracy of the determination.
standardized. When the solution color has faded to brownish-
7.2 Purity of Water—Unless otherwise indicated, reference
green, add 2 ml of 2 % starch solution and continue titrating
to water shall be understood to mean distilled water or water of
until the deep blue color changes to a clear green. Record the
equal purity.
volume of titrant used. Calculate the Normality of the thiosul-
7.3 Commercial Reagents—The use of commercially avail- fate solution as follows:
able pre-standardized analytical reagents and solutions is
Normality 5 A÷ 0.04903 3B
~ !
appropriate, providing those reagents and solutions have been
where:
prepared according to and conform to the previously men-
tioned specifications (see 7.1). A = grams of K Cr O used, and
2 2 7
B = ml required for titration.
7.4 Nitric Acid—(HNO ), 70 % w/w.
8.2 Shelf Life—The thiosulfate solution is relatively stable.
7.5 Perchloric Acid—(HClO ), 60-62 % w/w. 70 % or 72 %
However, it should be re-standardized at least once every
w/w perchloric acid may be substituted; however, storage is
month. Alternatively, a commercially available pre-
somewhat more hazardous.
standardized analytical solution may be substituted.
7.6 Sulfuric Acid—(H SO ), 96-98 % w/w.
2 4
9. Hazards
7.7 Potassium Iodide—(KI), 99-100 % purity.
9.1 Chemicals used can be harmful or explosive, or both.
7.8 Potassium Iodide Solution—(KI), 10 % w/w. Dissolve
9.2 The improper use of perchloric acid can lead to violent
10g(6 0.1 g) of potassium iodide into 100 ml of water.
and serious explosions. In general, these can be traced to
7.9 Starch Indicator Solution—2 % or equivalent. Prepared
situations where concentrated perchloric acid has come in
according to accepted procedures available in analytical hand-
contact with organic or easily oxidized materials.
books.
9.3 The exact procedures given must be followed and the
7.10 Oxidizing Mixture—Mix 1666 mL of concentrated
digestion, once started, should be kept from possible contact
sulfuric acid into an appropriate glass container that contains
with other organic matter. The digestion should never be
2500 mL of perchloric acid using extreme caution while
allowed to boil dry. The perchloric acid should never be used
adding. Cool the mixture to room temperature before use.
without the accompanying use of nitric and sulfuric acids.
NOTE 1—The reagents may be added individually according to 11.3.1.
9.4 Any spills involving perchloric acid should be flushed
7.11 Phosphoric Acid—(H PO ), 40 % w/w. Dilute 45 mL
3 4
with water and a liquid acid neutralizer.
of 85 % phosphoric acid with water to 100 ml.
9.5 The use of a perchloric acid hood, reserved for perchlo-
ric acid digestion, equipped with wash-down facilities and
constructed entirely of non-porous inorganic material is re-
Reagent Chemicals, American Chemical Society Specifications , American
quired.
Chemical Society, Washington, DC. For suggestions on the testing of reagents not
listed by the American Chemical Society, see Analar Standards for Laboratory
9.6 Perchloric acid bottles should be stored on a ceramic or
Chemicals, BDH Ltd. Poole, Dorset, UK and the United States Pharmacopoeia and
non-porous tray or shelf and never on a wooden or pervious
National Formulary, U.S. Pharmacopoeial Convention, Inc. (USPC), Rockville,
MD. shelf.
D6656 − 16 (2021)
9.7 Perchloric acid must not be permitted to go dry in the 12. Calculation
presence of organics, metals or metal salts.
12.1 Calculate the chromium content as the percentage of
9.8 It is advisable to keep only a one pound (0.45 kg) bottle
chromic oxide (Cr O ) in the leather as follows:
2 3
of acid in a working area.
Chromic Oxide Cr O ,% 5 T 3N 3E 3 100÷W
~ ! ~ !
2 3
10. Test Specimens
where:
10.1 The test specimen shall be 3-5 g of Wet Blue from a T = titration volume of sodium thiosulfate solution used
composite sample prepared according to Practice D6659 and in milliliters (ml),
N = Normality of the sodium thiosulfate solution,
weighed to an accuracy of 0.001 g.
E = 0.025332 = chromic oxide (Cr O ) equivalence
2 3
weight per milliliter (g/mL),
11. Procedure
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