ASTM D2503-92(1997)
(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
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 the method has not been established above 800 molecular weight (relative molecular mass). The method should not be applied to oils having initial boiling points lower than 220°C.
1.2 Values stated in SI units are to be regarded as the standard. The values given in parentheses are provided for information purposes only.
1.3 This standard does not purport to address all of the safety problems, 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)
Designation: D 2503 – 92 (Reapproved 1997) An American National Standard
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
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 D 2503; 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 (e) 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 3.2 A knowledge of the relative molecular mass (molecular
weight ) is required for the application of a number of
1.1 This test method covers the determination of the average
correlative methods that are useful in determining the gross
relative molecular mass (molecular weight) of hydrocarbon
composition of the heavier fractions of petroleum.
oils. It can be applied to petroleum fractions with molecular
weights (relative molecular mass) up to 3000; however, the
4. Apparatus
precision of the method has not been established above 800
4.1 Vapor Pressure Osmometer, with operating diagram.
molecular weight (relative molecular mass). The method
should not be applied to oils having initial boiling points lower
5. Reagents and Materials
than 220°C.
1.2 Values stated in SI units are to be regarded as the 5.1 Purity of Reagents—Reagent grade chemicals shall be
standard. The values given in parentheses are provided for used in all tests. Unless otherwise indicated, it is intended that
information purposes only. all reagents shall conform to the specifications of the Commit-
1.3 This standard does not purport to address all of the tee on Analytical Reagents of the American Chemical Society,
safety concerns, if any, associated with its use. It is the where such specifications are available. Other grades may be
responsibility of the user of this standard to establish appro- used, provided it is first ascertained that the reagent is of
priate safety and health practices and determine the applica- sufficiently high purity to permit its use without lessening the
bility of regulatory limitations prior to use. For specific hazard accuracy of the determination.
statements, 5.2.1, 5.2.3, and 5.2.3. 5.2 Solvents—Solvents that do not react with the sample
must be used. Since many organic materials exhibit a tendency
2. Summary of Test Method
to associate or dissociate in solution, it is desirable to use polar
2.1 A weighed portion of the sample is dissolved in a known
solvents for polar samples and nonpolar solvents for nonpolar
quantity of appropriate solvent. A drop of this solution and a samples. The solvents listed have been found suitable for
drop of solvent are suspended, side by side, on separate
hydrocarbons and petroleum fractions.
thermistors in a closed chamber saturated with solvent vapor. 5.2.1 Benzene
Since the vapor pressure of the solution is lower than that of the
NOTE 1—Warning: Poison. Carcinogen. Harmful if swallowed. Ex-
solvent, solvent condenses on the sample drop and causes a
tremely flammable. Vapors may cause flash fire. Vapor harmful, may be
temperature difference between the two drops. The resultant
absorbed through skin.
change in temperature is measured and used to determine the
5.2.2 Chloroform
relative molecular mass (molecular weight) of the sample by
reference to a previously prepared calibration curve.
NOTE 2—Warning: May be fatal if swallowed. Harmful if inhaled.
May produce toxic vapors if burned.
3. Significance and Use
5.2.3 1,1,1-Trichloroethane
3.1 Relative molecular mass (molecular weight) is a funda-
mental physical constant that can be used in conjunction with
other physical properties to characterize pure hydrocarbons and
A vapor pressure osmometer is available from H. Knauer and Co., Berlin, West
their mixtures.
Germany. The manufacture of the Mechrolab instrument previously referred to in
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
This test method is under the jurisdiction of ASTM Committee D-2 on instrument exchanges.
Petroleum Products and Lubricantsand is the direct responsibility of Subcommittee “Reagent Chemicals, American Chemical Society Specifications,” Am. Chemi-
D02.04on Hydrocarbon Analysis. cal Soc., Washington, DC. For suggestions on the testing of reagents not listed by
Current edition approved Aug. 15, 1992. Published October 1992. Originally the American Chemical Society, see “Analar Standards for Laboratory U.K.
e1
published as D 2503 – 66T. Last previous edition D 2503 – 82 (1987) . Chemicals,” BDH Ltd., Poole, Dorset, and the “United States Pharmacopeia.”
D 2503
NOTE 3—Warning: Harmful if inhaled. High concentrations may cause
stable, switch the selector to the “DT” position.
unconsciousness or death. Contact may cause skin irritation and derma-
7.7 While observing the thermistors in the viewing mirror,
titis.
lower the syringe in position “5”, by rotating the knurled collar
NOTE 4—The precision data given in 10.1 and 10.2 will apply only
of the holder fully clockwise. With the end of the needle
when benzene is used as the solvent. There is also some evidence that
directly above the reference thermistor, turn the feed screw and
determinations on the same oil sample carried out in different solvents will
rinse the thermistor with about 4 drops of solvent. Finally,
produce results that differ somewhat in absolute magnitude of apparent
molecular weight (relative molecular mass). deposit a drop of solvent on the thermistor bead and raise the
syringe by rotating the knurled collar in a counterclockwise
5.3 Reference Standards—A calibration curve must be con-
direction. Rinse the sample thermistor with solvent from
structed for each new lot of solvent using a pure compound
syringe “6” and apply a drop approximately the size of the drop
whose relative molecular mass (molecular weight) is accu-
on the reference thermistor. Depress the zero button, and zero
rately known. Compounds that have been used successfully
the meter with the “Zero” control. Set the decade resistance to
include benzil (210.2), n-octadecane (254.5), and squalane
zero, and balance the bridge using the “Balance” control.
(422.8).
Repeat the balancing of the bridge with fresh drops of solvent
6. Sampling on each thermistor to assure a good reference zero.
7.8 Lower syringe “1” and rinse the sample thermistor with
6.1 The sample must be completely soluble in the selected
3 to 4 drops of solution, finally applying one drop to the bead.
solvent at concentrations of at least 0.10 M, and it must not
Start the stop watch. Center the meter by means of the decade
have an appreciable vapor pressure at the test temperature.
dials and take readings at 1-min intervals until two successive
7. Preparation and Calibration of Apparatus
readings do not differ by more than 0.01 ohm. Record the DR
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