ASTM_D_2503_-_92_2012.pdf

Designation:D250392(Reapproved 2012)Standard Test Method forRelative Molecular Mass(Molecular Weight)ofHydrocarbons by Thermoelectric Measurement of VaporPressure1This standard is issued under the fixed designation D2503;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.This standard has been approved for use by agencies of the Department of Defense.1.Scope1.1 This test method covers the determination of the averagerelative molecular mass(molecular weight)of hydrocarbonoils.It can be applied to petroleum fractions with molecularweights(relative molecular mass)up to 3000;however,theprecision of this test method has not been established above800 molecular weight(relative molecular mass).This testmethod should not be applied to oils having initial boilingpoints lower than 220C.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.For specific hazardstatements,5.2.1,5.2.3,and 5.2.3.2.Summary of Test Method2.1 Aweighed portion of the sample is dissolved in a knownquantity of appropriate solvent.A drop of this solution and adrop of solvent are suspended,side by side,on separatethermistors in a closed chamber saturated with solvent vapor.Since the vapor pressure of the solution is lower than that of thesolvent,solvent condenses on the sample drop and causes atemperature difference between the two drops.The resultantchange in temperature is measured and used to determine therelative molecular mass(molecular weight)of the sample byreference to a previously prepared calibration curve.3.Significance and Use3.1 Relative molecular mass(molecular weight)is a funda-mental physical constant that can be used in conjunction withother physical properties to characterize pure hydrocarbons andtheir mixtures.3.2 A knowledge of the relative molecular mass(molecularweight)is required for the application of a number of correla-tive methods that are useful in determining the gross compo-sition of the heavier fractions of petroleum.4.Apparatus4.1 Vapor Pressure Osmometer,with operating diagram.25.Reagents and Materials5.1 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.3Other grades may beused,provided it is first ascertained that the reagent is ofsufficiently high purity to permit its use without lessening theaccuracy of the determination.5.2 SolventsSolvents that do not react with the samplemust be used.Since many organic materials exhibit a tendencyto associate or dissociate in solution,it is desirable to use polarsolvents for polar samples and nonpolar solvents for nonpolarsamples.The solvents listed have been found suitable forhydrocarbons and petroleum fractions.1This test method is under the jurisdiction of ASTM Committee D02 onPetroleum Products and Lubricants and is the direct responsibility of SubcommitteeD02.04.0D on Physical and Chemical Methods.Current edition approved April 15,2012.Published April 2012.Originallyapproved in 1966.Last previous edition approved in 2007 as D250392(2007).DOI:10.1520/D2503-92R12.2A vapor pressure osmometer is available from H.Knauer and Co.,Berlin,WestGermany.The manufacture of the Mechrolab instrument previously referred to inthis footnote has been discontinued.However,some models may be available fromstocks on hand at laboratory supply houses,or as used equipment from laboratoryinstrument exchanges.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 5.2.1 Benzene(WarningPoison.Carcinogen.Harmful ifswallowed.Extremely flammable.Vapors may cause flash fire.Vapor harmful,may be absorbed through skin.)5.2.2 Chloroform(WarningMay be fatal if swallowed.Harmful if inhaled.May produce toxic vapors if burned.)5.2.3 1,1,1-Trichloroethane(WarningHarmful if inhaled.High concentrations may cause unconsciousness or death.Contact may cause skin irritation and dermatitis.)NOTE1The