ASTM_D_1991_-_15.pdf

Designation:D199115Standard Test Method forRubber Chemicals2-Mercaptobenzothiazole(MBT)Assay1This standard is issued under the fixed designation D1991;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 a procedure for the assay of2-mercaptobenzothiazole(MBT).It is based on a potentiomet-ric titration of MBT with sodium hydroxide.1.2 The assay is determined as percent by mass.1.3 The values stated in SI units are to be regarded asstandard.No other units of measurement are included in thisstandard.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 determine the applica-bility of regulatory limitations prior to use.2.Referenced Documents2.1 ASTM Standards:2D4483 Practice for Evaluating Precision for Test MethodStandards in the Rubber and Carbon Black ManufacturingIndustries3.Summary of Test Method3.1 A sample of MBT is dissolved in ethanol.The solutionis titrated potentiometrically using a solution of sodium hy-droxide as the titrant.4.Significance and Use4.1 MBT is commonly used as an accelerator for rubber andlatex vulcanization.The purity of MBT may be of importancein predicting performance in rubber compounds and this testmethod is designed to assess the purity of MBT.4.2 This test method may be used as a quality control tooland for research and development work.5.Apparatus5.1 Erlenmeyer Flask,250-cm3.5.2 Analytical Balance,having a sensitivity of 60.1 mg.5.3 Potentiograph.5.4 Glass pH Electrode,and reference electrode.5.5 Graduated Cylinder,200-cm3.6.Reagents6.1 Purity of ReagentsReagent grade chemicals shall beused in all tests.Unless otherwise indicated,it is intended thatall reagents shall conform to the specifications of the Commit-tee on Analytical Reagents of the American Chemical Society,where such specifications are available.36.2 Purity of WaterUnless otherwise indicated,referencesto water shall be understood to mean distilled water or water ofequal purity.6.3 Aqueous Sodium Hydroxide Solution(40 g NaOH in1 dm3of solution)Standardize by accepted analytical tech-niques to ensure that the maximum error of the normalizationfactor is not more than 0.001.6.4 Ethanol,denatured with toluene(in the ratio of 100volumes ethanol to 1 volume toluene).6.5 Toluene.7.Sampling7.1 Sampling shall be at the discretion of the analyst toobtain as representative a sample as possible,of the lot to betested.8.Procedure8.1 Weigh(to nearest 0.001 g)about 5 g of the specimen,and transfer into a 250-cm3Erlenmeyer flask.Using a gradu-ated cylinder,add 125 cm3denatured ethanol(6.4).Titrate the1This test method is under the jurisdiction of ASTM Committee D11 on Rubberand is the direct responsibility of Subcommittee D11.11 on Chemical Analysis.Current edition approved June 1,2015.Published July 2015.Originally approvedin 1991.Last previous edition approved in 2010 as D1991 05(2010).DOI:10.1520/D1991-15.2For 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.3Reagent Chemicals,American Chemical Society Specifications,AmericanChemical Society,Washington,DC.For Suggestions on the testing of reagents notlisted by the American Chemical Society,see Annual Standards for LaboratoryChemicals,BDH Ltd.,Poole,Dorset,U.K.,and the United States Pharmacopeiaand National Formulary,U.S.Pharmacopeial Convention,Inc.(USPC),Rockville,MD.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 solution potentiometrically,using a glass electrode system,with NaOH(6.3)as the titrant(A)(see Fig.1).9.Calculation9.1 From the plot of pH versus millilitres of NaOH,asillustrated in Fig.1,the equivalence point for the potentiomet-ric titration is defined as the inflection point of the curve.Thisinflection point may be estimated by determination of themidpoint of the tangent slope.9.1.1 This inflection point is identified as follows:9.1.1.1 Draw a tangent through the steepest part of thecurve.Determine the two points at which the curve departsfrom this tangent line.The midpoint of the line segmentbetween these two points represents the equivalence point.9.2 Calculate the MBT content by the following equation:MBT content,%5167.23A 3N 3100W 31000(1)where:A=volume of sodium hydroxide to the equivalencepoint,cm3,167.2=molecular mass of MBT,W=mass g of specimen,andN=normality of titrant.10.Report10.1 Report the following information:10.1.1 Proper identification of the sample,and10.1.2 Results obtained from