1、Designation: D2503 92 (Reapproved 2016)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 ofori
2、ginal 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 U.S. Department
3、 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 e
4、stablished above800 molecular weight (relative molecular mass). This testmethod should not be applied to oils having initial boilingpoints lower than 220 C.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 do
5、es 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
6、.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 p
7、ressure 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 p
8、reviously 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 m
9、ass (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 ReagentsReage
10、nt 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 fir
11、st 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 desirabl
12、e 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, Liquid Fuels, and Lubricants and is the dir
13、ect responsibility ofSubcommittee D02.04.0D on Physical and Chemical Methods.Current edition approved Oct. 1, 2016. Published November 2016. Originallyapproved in 1966. Last previous edition approved in 2012 as D2503 92 (2012).DOI: 10.1520/D2503-92R16.2A vapor pressure osmometer is available from H.
14、 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 Chem
15、icals, 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 Nationa
16、l 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 States15.2.1 Benzene (WarningPoison. Carcinogen. Harmful ifswallowed. Extremely flammable. Vapors may cause flash fire.Vapo
17、r 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 derm
18、atitis.)NOTE 1The precision data given in 10.1 will apply only whenbenzene is used as the solvent. There is also some evidence thatdeterminations on the same oil sample carried out in different solvents willproduce results that differ somewhat in absolute magnitude of apparentmolecular weight (relat
19、ive molecular mass).5.3 Reference StandardsA calibration curve must be con-structed for each new lot of solvent using a pure compoundwhose relative molecular mass (molecular weight) is accu-rately known. Compounds that have been used successfullyinclude benzil (210.2), n-octadecane (254.5), and squa
20、lane(422.8).6. Sampling6.1 The sample must be completely soluble in the selectedsolvent at concentrations of at least 0.10 M, and it must nothave an appreciable vapor pressure at the test temperature.7. Preparation and Calibration of Apparatus7.1 Prepare standard 0.01 M,0.02M,0.04M,0.06M,0.08 M, and
21、 0.1 M solutions of the calibrating compound in thesolvent selected.7.2 Remove the upper sample chamber assembly. Rinse thesolvent cap with the solvent to be used. Install a vapor wick inthe cup and fill with solvent to the bottom of the notches in theinner wick. Place the cup in the chamber base re
22、cess, align thevapor wick openings with the viewing tubes, and replace theupper assembly. Take care that the guide pins properly engagematching holes in the thermal block and that the matchingsurfaces of the base and block are clean. Be careful not to allowthe thermistor beads to touch the cup or wi
23、cks as they may bebent out of alignment. Turn on the thermostat and allow thetemperature of the sample chamber to reach equilibrium at37 C.NOTE 2If the block is at room temperature, 2 h to 3 h will be required.To avoid such delay, it is desirable to always leave the thermostat switchin the “on” posi
24、tion. If the chamber is at equilibrium and is opened briefly,30 min to 45 min will generally be required before temperature stabiliza-tion is regained. The exchange or refilling of syringes does not necessitateany waiting period.7.3 Thoroughly rinse all syringes with the solvent beingused and allow
25、to dry.7.4 Fill the syringes from guide tubes “5” and “6” with thesolvent. Fill the syringes for guide tubes “1” through “4” withthe standard solutions in order of increasing concentration.7.5 Insert the syringes into the thermal block, keeping theguide pins pointed away from the probe. Turn on the
26、“NullDetector” switch (Note 3). Set the sensitivity control tosufficient gain so that a 1.0 shift in the “Dekastat” producesone major division shift of the meter needle.NOTE 3No measurements should be attempted until the “NullDetector” switch has been on for at least 30 min.7.6 Turn on the “Bridge”
27、switch and turn the “T-T” switchto “T”. Approximately zero the meter with the “T” potentiom-eter and observe the drift of the needle. If the solvent chamberis at equilibrium, the needle should not drift more than 1 to 2mm during one complete heating cycle; a steady drift to theright indicates that t
28、he chamber is still warming up; if “T” isstable, switch the selector to the “T” position.7.7 While observing the thermistors in the viewing mirror,lower the syringe in position “5”, by rotating the knurled collarof the holder fully clockwise. With the end of the needledirectly above the reference th
29、ermistor, turn the feed screw andrinse the thermistor with about 4 drops of solvent. Finally,deposit a drop of solvent on the thermistor bead and raise thesyringe by rotating the knurled collar in a counterclockwisedirection. Rinse the sample thermistor with solvent fromsyringe “6” and apply a drop
30、approximately the size of the dropon the reference thermistor. Depress the zero button, and zerothe meter with the “Zero” control. Set the decade resistance tozero, and balance the bridge using the “Balance” control.Repeat the balancing of the bridge with fresh drops of solventon each thermistor to
31、assure a good reference zero.7.8 Lower syringe “1” and rinse the sample thermistor with3 to 4 drops of solution, finally applying one drop to the bead.Start the stop watch. Center the meter by means of the decadedials and take readings at 1-min intervals until two successivereadings do not differ by
32、 more than 0.01 . Record the Rvalue, estimating to the nearest 0.01 from the meter. Recordthe time required to reach this steady state, and use this time forall subsequent readings for the solvent used.7.9 Upon completing each series of sample readings, rinsethe sample thermistor with solvent, depos
33、it a drop, and recheckthe zero point. The meter should reproduce the originalindication within 0.5 mm. If the needle shows a negativedeflection, the sample thermistor should be rinsed again. If itshows a positive deflection, the drop on the reference therm-istor should be replaced.7.10 Plot the R va
34、lues for each concentration of standardagainst the molarity of the standard for the solvent used.NOTE 4The calibration must be repeated for each of the solvents to beemployed and separate working curves constructed. Recalibration isnecessary each time a new batch of solvent is put into use.8. Proced
35、ure8.1 Select the solvent to be used and fill the solvent cup asdescribed in 7.2. Weigh into a 25 mL volumetric flask theamount of sample suggested in the following table (Note 2):Estimated Relative Molecular Mass Sample Size, gLess than 200 0.3200 to 500 0.3 to 0.6500 to 700 0.6 to 0.9700 to 1000 0
36、.9 to 1.3Record the mass to the nearest 0.1 mg and dilute to volumewith solvent.NOTE 5If the amount of sample is limited, weigh the sample into a5 mL or 1 mL volumetric flask, using one-fifth or one twenty-fifthD2503 92 (2016)2respectively of the amount indicated in the table. Weigh to the nearest0.
37、001 mg using a microbalance.8.2 Fill syringes “5” and “6” with solvent and fill one of theremaining syringes with the sample solution. With the samplechamber at thermal equilibrium, balance the bridge to establishthe reference zero as described in 7.6 and 7.7.8.3 Rinse the sample thermistor with 3 o
38、r 4 drops of thesample solution and deposit 1 drop on the thermistor. Start thestop watch. Center the meter with the decade dials and recordR at the time interval determined during the standardizationfor the solvent being employed (7.8). When running a series ofsamples, check the zero point frequent
39、ly as described in 7.9.8.4 Using the appropriate calibration curve, obtain themolarity corresponding to the observed R value.9. Calculation9.1 Calculate the relative molecular mass (molecularweight) of the sample as follows:Relative Molecular Mass molecular weight! 5 c/m (1)where:c = concentration o
40、f sample solution, g/L andm = molarity of solution, as determined in 8.4.10. Report10.1 Report the result to the nearest whole number.11. Precision and Bias11.1 Precision The precision of this test method asobtained by statistical examination of interlaboratory testresults is as follows:11.1.1 Repea
41、tability The difference between successivetest results obtained by the same operator with the sameapparatus under constant operating conditions on identical testmaterial, would in the long run, in the normal and correctoperation of the test method, exceed the values shown in Table1 only in one case
42、in twenty.11.1.2 ReproducibilityThe difference between two singleand independent results, obtained by different operators, work-ing in different laboratories on identical test material, would inthe long run, in the normal and correct operation of the testmethod, exceed the values shown in Table 1 on
43、ly in one casein twenty.11.1.3 The precision was not obtained in accordance withCommittee D02 Research Report RR:D02-1007, “Manual onDetermining Precision Data for ASTM Methods on PetroleumProducts and Lubricants.”411.2 BiasBias for this test method has not been deter-mined.12. Keywords12.1 hydrocar
44、bons; molecular weight; osmometer; relativemolecular mass; thermoelectric measurement; vapor pressureASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentionedin this standard. Users of this standard are expressly advised that det
45、ermination of the validity of any such patent rights, and the riskof infringement of such rights, are entirely their own responsibility.This standard is subject to revision at any time by the responsible technical committee and must be reviewed every five years andif not revised, either reapproved o
46、r withdrawn. Your comments are invited either for revision of this standard or for additional standardsand should be addressed to ASTM International Headquarters. Your comments will receive careful consideration at a meeting of theresponsible technical committee, which you may attend. If you feel th
47、at your comments have not received a fair hearing you shouldmake your views known to the ASTM Committee on Standards, at the address shown below.This standard is copyrighted by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959,United States. Individual reprints
48、 (single or multiple copies) of this standard may be obtained by contacting ASTM at the aboveaddress or at 610-832-9585 (phone), 610-832-9555 (fax), or serviceastm.org (e-mail); or through the ASTM website(www.astm.org). Permission rights to photocopy the standard may also be secured from the Copyri
49、ght Clearance Center, 222Rosewood Drive, Danvers, MA 01923, Tel: (978) 646-2600; http:/ at ASTM International Headquarters and may be obtained by requestingResearch Report RR:D02-1007.TABLE 1 Precision Data (Benzene Solvent)Relative Molecular Mass(Molecular Weight) RangeRepeatability,g/molReproducibility,g/mol245 to 399 5 14400 to 599 12 32600 to 800 30 94D2503 92 (2016)3
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