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Designation: D6657 − 16 (Reapproved 2026)Standard Test Method forpH of Wet Blue and Wet White 1This standard is issued under the f i xed designation D6657; the number immediately following the designation indicates the year oforiginal adoption or, in the 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. Scope1.1 This test method covers the determination of the pH ofall types of Wet Blue and Wet White.1.2 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.3 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, health, and environmental practices and deter-mine the applicability of regulatory limitations prior to use.1.4 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards: 2D6659 Practice for Sampling and Preparation of Wet Blueand Wet White for Physical and Chemical TestsE177 Practice for Use of the Terms Precision and Bias inASTM Test MethodsE691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test Method3. Terminology3.1 Def i nitions of Term Specif i c to this Standard:3.1.1 The pH of a solution has been def i ned as the negativelogarithm of the hydrogen ion activity. A solution of pH 7 isneutral at 24 °C. Lower numbers indicate increasing acidity;higher numbers, increasing alkalinity.4. Signif i cance and Use4.1 This test method is designed to measure the pH of adistilled-water extract of Wet Blue and Wet White. This isconsidered to be a measure of the acidity or alkalinity of theWet Blue or Wet White. Excessive acidity or alkalinity mayhave a deleterious effect on the aging characteristics of WetBlue and Wet White and the leather made from it.4.2 This test method is suitable for development, control,and service evaluation of wet blue.5. Apparatus5.1 Electronic pH Meter, with a suitable electrode. Themeter shall have an accuracy of at least 0.01 pH unit andreproducibility of 0.05 pH unit.5.2 Analytical Balance, sensitive to 0.01 g or greater.6. Reagents6.1 Purity of Water—Unless otherwise indicated, referencesto water shall be understood to mean distilled water or water ofequal purity. Distilled water shall have a pH value of not lessthan 5.5 nor more than 7.0 and shall give a residue of not morethan 0.5 mg, when 100 mL is evaporated and dried in aplatinum dish.6.2 Commercially Standardized pH Solutions 3 :6.2.1 Alkaline Phosphate Buffer Solution (0.01 M trisodiumphosphate, pH = 11.72 at 25 °C)—Dissolve 1.42 g of anhy-drous disodium hydrogen phosphate (Na 2 HPO 4 ) in 100 mL ofa 0.1 N carbonate-free solution of sodium hydroxide (NaOH)and dilute to 1 L with water.6.2.2 Borax Buffer Solution (0.01 M, pH = 9.18 at 25 °C)—Dissolve 3.81 g of sodium tetraborate decahydrate (Na 2 B 4 O 7 ·10H 2 O) in water and dilute to 1 L.6.2.3 Hydrochloric Acid (pH = 1.10 at 25 °C)—Add 2 g ofconcentrated hydrochloric acid (HCl, sp gr 1.19) to 450 g ofwater. Standardize and dilute to 0.1 N.6.2.4 Phosphate Buffer Solution (0.025 M with respect toeach phosphate, salt pH = 6.86 at 2.5 °C)—Dissolve 3.40 g of1 This test method is under the jurisdiction of ASTM Committee D31 on Leatherand is the direct responsibility of Subcommittee D31.02 on Wet Blue.Current edition approved July 1, 2026. Published July 2026. Originally approvedin 2001. Last previous edition approved in 2021 as D6657 – 16 (2021). DOI:10.1520/D6657-16R26.2 For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at www.astm.org/contact. For Annual Book ofASTM Standards volume information, refer to the standard’s Document Summarypage on the ASTM website.3 Commercial buffer salts and solutions prepared in accordance with NationalBureau of Standards recommendations are sold by reputable laboratory supplyhouses and may be used.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.1