ASTM_D_3913_-_03_2015.pdf

Designation:D391303(Reapproved 2015)Standard Test Method forAcidity in Basic Chromium Tanning Liquors1This standard is issued under the fixed designation D3913;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 titrat-able acidity of chromium tanning liquors.By titratable acidityis meant not only free acidity,which is rarely present,but alsoanions combined with weakly basic cations such as Chromium(III),which can be titrated with base at the phenolphthalein endpoint.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 and health practices and determine the applica-bility of regulatory limitations prior to use.2.Referenced Documents2.1 ASTM Standards:2D3898 Test Method for Chromic Oxide in Basic ChromiumTanning Liquors3.Summary of Test Method3.1 A sample of suitable size is taken by diluting andaliquoting,if necessary.The sample is then heated to boilingand titrated hot to the first appearance of a pink color,stable toboiling in the supernatant solution,as determined by viewingthe settled solution against the rim of a porcelain dish,oragainst the light in a tall beaker or Erlenmeyer flask.4.Significance and Use4.1 The acidity of a chrome tanning liquor,as determined bythis test method,is rarely employed as such.This result isnormally combined with the results of a chromium analysis(see Test Method D3898)to determine a property of the liquorcalled basicity.This property,equal to the percentage by whichanions in a normal chromic salt have been replaced byhydroxyl in the solution being analyzed,is closely related tothe tanning behavior of the solution.5.Interferences5.1 Although for most purposes,the results of this analysisare intended to give a measure of the acid anions associatedwith chromium,it should be mentioned that any weak basiccation will behave similarly.Hence,aluminum,zirconium,ferric iron,and weak organic bases will all yield salts withacidity titratable by this test method.A discussion of the effectof these interferences has been published.35.2 If there are anions which form very stable complexeswith chromium present in the solution,the titration:CrOHxXyH2O!62x2y!x1y23!213 2 x!OH2(1)CrOH!31yX216 2 x 2 y!H2Owhere:X=stable complexes,will not go to completion at the phenolphthalein endpointand the acidity reported will be low.Large excess of oxalate orcomplexing anions of lesser affinity used for masking effectwill introduce this error.4To obviate this,an entirely differentmethod has been developed in which the hydroxyl groups aretitrated and the chromium is completely complexed by oxalate.This is generally referred to as the Lehigh method and may beused if complex ions are present in interfering amount.5,66.Reagents and Materials6.1 Purity of ReagentsReagent grade chemicals shall beused in all tests.Unless otherwise specified,it is intended thatall reagents shall conform to the specifications of the Commit-tee on Analytical Reagents of the American Chemical Society,1This test method is under the jurisdiction of ASTM Committee D31 on Leatherand is the direct responsibility of Subcommittee D31.06 on Chemical Analysis Thistest method was developed in cooperation with the American Leather ChemistsAssoc.Current edition approved Sept.1,2015.Published October 2015.Originallyapproved in 1991.Last previous edition approved in 2009 as D3913 03(2009).DOI:10.1520/D3913-03R15.2For referenced ASTM standards,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.3Hartford,W.H.,JALCA,Vol 56,1961,p.568.4Thorstensen,T.C.,and Theis,E.R.,JALCA,Vol 47,1952,p.583.5Theis,E.R.,and Thorstensen,T.C.,Journal International Society LeatherTrade Chemists,Vol 31,1947,p.124.6Serfass,E.J.,Theis,E.R.,and Clark,A.,Jr.,The Journal of the LeatherChemists Association,Vol 42,1947,p.360.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 where such specifications are available.7Other grades may beused,provided it is first ascertained that the reagent is ofsufficient high purity to permit its use without lessening theaccuracy of the determination.6.2 Purity of WaterUnless otherwise specified,referenceto water shall be understood to mean distilled water or water ofequal purity.6.3 Sodium Hydroxide,0.1 NDissolve 42 g NaOH inabout 60 mL water.Cool and transfer to a 100-mL volumetricflask;d