ASTM D2503-92(2007)
(Test Method)Standard Test Method for Relative Molecular Mass (Molecular Weight) of Hydrocarbons by Thermoelectric Measurement of Vapor Pressure
Standard Test Method for Relative Molecular Mass (Molecular Weight) of Hydrocarbons by Thermoelectric Measurement of Vapor Pressure
SIGNIFICANCE AND USE
Relative molecular mass (molecular weight) is a fundamental physical constant that can be used in conjunction with other physical properties to characterize pure hydrocarbons and their mixtures.
A knowledge of the relative molecular mass (molecular weight) is required for the application of a number of correlative methods that are useful in determining the gross composition of the heavier fractions of petroleum.
SCOPE
1.1 This test method covers the determination of the average relative molecular mass (molecular weight) of hydrocarbon oils. It can be applied to petroleum fractions with molecular weights (relative molecular mass) up to 3000; however, the precision of this test method has not been established above 800 molecular weight (relative molecular mass). This test method should not be applied to oils having initial boiling points lower than 220°C.
1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.3 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 and health practices and determine the applicability of regulatory limitations prior to use. For specific hazard statements, 5.2.1, 5.2.3, and 5.2.3.
General Information
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation:D2503–92 (Reapproved 2007)
Standard Test Method for
Relative Molecular Mass (Molecular Weight) of
Hydrocarbons by Thermoelectric Measurement of Vapor
Pressure
This standard is issued under the fixed designation D2503; 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.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope 3. Significance and Use
1.1 Thistestmethodcoversthedeterminationoftheaverage 3.1 Relative molecular mass (molecular weight) is a funda-
relative molecular mass (molecular weight) of hydrocarbon mental physical constant that can be used in conjunction with
oils. It can be applied to petroleum fractions with molecular otherphysicalpropertiestocharacterizepurehydrocarbonsand
weights (relative molecular mass) up to 3000; however, the their mixtures.
precision of this test method has not been established above 3.2 A knowledge of the relative molecular mass (molecular
800 molecular weight (relative molecular mass). This test weight) is required for the application of a number of correla-
method should not be applied to oils having initial boiling tive methods that are useful in determining the gross compo-
points lower than 220°C. sition of the heavier fractions of petroleum.
1.2 The values stated in SI units are to be regarded as
4. Apparatus
standard. No other units of measurement are included in this
standard. 4.1 Vapor Pressure Osmometer, with operating diagram.
1.3 This standard does not purport to address all of the
5. Reagents and Materials
safety concerns, if any, associated with its use. It is the
5.1 Purity of Reagents—Reagent grade chemicals shall be
responsibility of the user of this standard to establish appro-
used in all tests. Unless otherwise indicated, it is intended that
priate safety and health practices and determine the applica-
all reagents shall conform to the specifications of the Commit-
bility of regulatory limitations prior to use. For specific hazard
tee onAnalytical Reagents of theAmerican Chemical Society,
statements, 5.2.1, 5.2.3, and 5.2.3.
where such specifications are available. Other grades may be
2. Summary of Test Method
used, provided it is first ascertained that the reagent is of
sufficiently high purity to permit its use without lessening the
2.1 Aweighedportionofthesampleisdissolvedinaknown
quantity of appropriate solvent. A drop of this solution and a accuracy of the determination.
5.2 Solvents—Solvents that do not react with the sample
drop of solvent are suspended, side by side, on separate
thermistors in a closed chamber saturated with solvent vapor. must be used. Since many organic materials exhibit a tendency
to associate or dissociate in solution, it is desirable to use polar
Sincethevaporpressureofthesolutionislowerthanthatofthe
solvent, solvent condenses on the sample drop and causes a solvents for polar samples and nonpolar solvents for nonpolar
temperature difference between the two drops. The resultant
change in temperature is measured and used to determine the
Avapor pressure osmometer is available from H. Knauer and Co., Berlin, West
relative molecular mass (molecular weight) of the sample by
Germany. The manufacture of the Mechrolab instrument previously referred to in
reference to a previously prepared calibration curve.
this footnote has been discontinued. However, some models may be available from
stocks on hand at laboratory supply houses, or as used equipment from laboratory
instrument exchanges.
1 3
This test method is under the jurisdiction of ASTM Committee D02 on Reagent Chemicals, American Chemical Society Specifications, American
Petroleum Products and Lubricants and is the direct responsibility of Subcommittee Chemical Society, Washington, DC. For Suggestions on the testing of reagents not
D02.04.0D on Physical and Chemical Methods. listed by the American Chemical Society, see Annual Standards for Laboratory
Current edition approved Nov. 1, 2007. Published January 2008. Originally Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia
´1
approved in 1966. Last previous edition approved in 2002 as D2503–92(2002) . and National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville,
DOI: 10.1520/D2503-92R07. MD.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D2503–92 (2007)
NOTE 3—No measurements should be attempted until the “Null Detec-
samples. The solvents listed have been found suitable for
tor” switch has been on for at least 30 min.
hydrocarbons and petroleum fractions.
5.2.1 Benzene (Warning—Poison. Carcinogen. Harmful if
7.6 Turn on the “Bridge” switch and turn the “T-DT” switch
swallowed. Extremely flammable. Vapors may cause flash fire.
to “T”.Approximately zero the meter with the “T” potentiom-
Vapor harmful, may be absorbed through skin.)
eter and observe the drift of the needle. If the solvent chamber
5.2.2 Chloroform (Warning—May be fatal if swallowed.
is at equilibrium, the needle should not drift more than 1 to 2
Harmful if inhaled. May produce toxic vapors if burned.)
mm during one complete heating cycle; a steady drift to the
5.2.3 1,1,1-Trichloroethane(Warning—Harmfulifinhaled.
right indicates that the chamber is still warming up; if “T” is
High concentrations may cause unconsciousness or death.
stable, switch the selector to the “DT” position.
Contact may cause skin irritation and dermatitis.)
7.7 While observing the thermistors in the viewing mirror,
lower the syringe in position “5”, by rotating the knurled collar
NOTE 1—The precision data given in 10.1 will apply only when
benzene is used as the solvent. There is also some evidence that
of the holder fully clockwise. With the end of the needle
determinationsonthesameoilsamplecarriedoutindifferentsolventswill
directly above the reference thermistor, turn the feed screw and
produce results that differ somewhat in absolute magnitude of apparent
rinse the thermistor with about 4 drops of solvent. Finally,
molecular weight (relative molecular mass).
deposit a drop of solvent on the thermistor bead and raise the
5.3 Reference Standards—Acalibration curve must be con-
syringe by rotating the knurled collar in a counterclockwise
structed for each new lot of solvent using a pure compound
direction. Rinse the sample thermistor with solvent from
whose relative molecular mass (molecular weight) is accu-
syringe“6”andapplyadropapproximatelythesizeofthedrop
rately known. Compounds that have been used successfully
on the reference thermistor. Depress the zero button, and zero
include benzil (210.2), n-octadecane (254.5), and squalane
the meter with the “Zero” control. Set the decade resistance to
(422.8).
zero, and balance the bridge using the “Balance” control.
Repeat the balancing of the bridge with fresh drops of solvent
6. Sampling
on each thermistor to assure a good reference zero.
6.1 The sample must be completely soluble in the selected 7.8 Lower syringe “1” and rinse the sample thermistor with
solvent at concentrations of at least 0.10 M
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