1、Designation: D2503 92 (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 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 Department of D
3、efense.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 establ
4、ished 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
5、 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.
6、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 pressur
7、e 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 previou
8、sly 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 (m
9、olecularweight) 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 gra
10、de 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 asc
11、ertained 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 u
12、se polarsolvents for polar samples and nonpolar solvents for nonpolar1This 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, 201
13、2. 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 footno
14、te 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 test
15、ing 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.1Copyright ASTM International, 100 Barr Harbor D
16、rive, PO Box C700, West Conshohocken, PA 19428-2959, United States.samples. The solvents listed have been found suitable forhydrocarbons and petroleum fractions.5.2.1 Benzene (WarningPoison. Carcinogen. Harmful ifswallowed. Extremely flammable. Vapors may cause flash fire.Vapor harmful, may be absor
17、bed 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.)NOTE 1The preci
18、sion 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 (relative molecular mass).5.3
19、 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 squalane(422.8).6. Sampling
20、6.1 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, 0.02, 0.04, 0.06, 0.08, and 0.1 Msolutions of the cal
21、ibrating compound in the solvent 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 recess, align thevapor wick
22、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 wicks as they may bebent out
23、 of alignment. Turn on the thermostat and allow thetemperature of the sample chamber to reach equilibrium at37C.NOTE 2If the block is at room temperature, 2 to 3 h will be required.To avoid such delay, it is desirable to always leave the thermostat switchin the “on” position. If the chamber is at eq
24、uilibrium and is opened briefly,30 to 45 min will generally be required before temperature stabilization isregained. The exchange or refilling of syringes does not necessitate anywaiting period.7.3 Thoroughly rinse all syringes with the solvent beingused and allow to dry.7.4 Fill the syringes from g
25、uide 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 “NullDetector” switch (Note 3). Set
26、 the sensitivity control tosufficient gain so that a 1.0-V shift in the “Dekastat” producesone major division shift of the meter needle.NOTE 3No measurements should be attempted until the “Null Detec-tor” switch has been on for at least 30 min.7.6 Turn on the “Bridge” switch and turn the “T-DT” swit
27、chto “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 the chamber is still warming up
28、; if “T” isstable, switch the selector to the “DT” 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 thermistor, turn the feed screw
29、 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 approximately the size of the
30、 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 assure a good reference zero.
31、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 more than 0.01 ohm. Record t
32、he DRvalue, estimating to the nearest 0.01 V 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, deposit a drop, and recheckt
33、he 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 DR values for each concentr
34、ation 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. Procedure8.1 Select the solv
35、ent 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 0.9 to 1.3Record the we
36、ight to the nearest 0.1 mg and dilute to volumewith solvent.NOTE 5If the amount of sample is limited, weigh the sample into a 5or 1-mL volumetric flask, using one-fifth or one twenty-fifth respectivelyD2503 92 (2012)2of the amount indicated in the table. Weigh to the nearest 0.001 mg usinga microbal
37、ance.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 or 4 drops of thesample
38、solution and deposit 1 drop on the thermistor. Start thestop watch. Center the meter with the decade dials and recordDR at the time interval determined during the standardizationfor the solvent being employed (7.8). When running a series ofsamples, check the zero point frequently as described in 7.9
39、.8.4 Using the appropriate calibration curve, obtain themolarity corresponding to the observed DR value.9. Calculation9.1 Calculate the relative molecular mass (molecularweight) of the sample as follows:Relative Molecular Mass molecular weight!5c/m (1)where:c = concentration of sample solution, g/L
40、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 PrecisionThe precision of this test method as ob-tained by statistical examination of interlaboratory test resultsis as follows:11.1.1 RepeatabilityThe difference
41、 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 in twenty.11.1.2 Reprod
42、ucibilityThe 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 only in one casein twenty
43、.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 hydrocarbons; molecular weight;
44、 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 determination of the valid
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47、ot 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 (single or multiple co
48、pies) 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 ASTM website (www.astm.org/CO
49、PYRIGHT/).4Filed 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 (2012)3
