1、Designation: D7591 12Standard Test Method forDetermination of Free and Total Glycerin in BiodieselBlends by Anion Exchange Chromatography1This standard is issued under the fixed designation D7591; the number immediately following the designation indicates the year oforiginal adoption or, in the case
2、 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.1. Scope1.1 This test method covers and describes an anion ex-change chromatography procedure for determ
3、ining free andtotal glycerin content of biodiesel (B100) and blends (B0 toB20) with diesel fuel oils defined by Specification D975Grades 1-D, 2-D, and low sulfur 1-D and 2-D and SpecificationD6751 (for B100 feedstocks). It is intended for the analysis ofbiodiesel and blend samples containing between
4、 0.5 to 50mg/kg glycerin.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 s
5、tandard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D975 Specification for Diesel Fuel OilsD1193 Specification for Reagent WaterD4057 Practice for Manual Sampling of Petroleum
6、 andPetroleum ProductsD4177 Practice for Automatic Sampling of Petroleum andPetroleum ProductsD6299 Practice for Applying Statistical Quality Assuranceand Control Charting Techniques to Evaluate AnalyticalMeasurement System PerformanceD6751 Specification for Biodiesel Fuel Blend Stock (B100)for Midd
7、le Distillate FuelsD6792 Practice for Quality System in Petroleum Productsand Lubricants Testing LaboratoriesE177 Practice for Use of the Terms Precision and Bias inASTM Test MethodsE691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test Method3. Terminology3.1 Defi
8、nitions:3.1.1 free glycerin, nmeasure of the amount of glycerinremaining in the fuel.3.1.2 total glycerin, nsum of the free glycerin and theglycerin portion of any unreacted or partially reacted oil or fat.4. Summary of Test Method4.1 Free GlycerinA small volume of an extract of theblend sample is d
9、irectly injected into an ion chromatographconsisting of appropriate ion exchange columns and into anelectrochemical detector. Glycerin is separated based on itsaffinity for ion exchange sites of the resin with respect to theresins affinity for the eluent. An electrochemical detector isemployed for d
10、etection of glycerin. Glycerin is quantified bypeak area based on an external calibration curve, and isreported as g/g (mg/kg), or may be converted to wt%.Calibration standards are prepared from commercially avail-able glycerin (99+% purity) in an aqueous solution.4.2 Total GlycerinA small volume ex
11、tract of a saponifiedblend sample is directly injected into an ion chromatographconsisting of appropriate ion exchange columns and into anelectrochemical detector. Glycerin is separated based on itsaffinity for ion exchange sites of the resin with respect to theresins affinity for the eluent. An ele
12、ctrochemical detector isemployed for detection of glycerin. Glycerin is quantified bypeak area based on an external calibration curve, and isreported as g/g (mg/kg), or may be converted to wt%.Calibration standards are prepared from commercially avail-able glycerin (99+% purity) in an aqueous soluti
13、on.5. Significance and Use5.1 Petroleum-based diesel may be blended with biodiesel.High levels of free glycerin in biodiesel can cause injectordeposits (“gel effect”), as well as clogging fuel systems. High1This test method is under the jurisdiction of ASTM Committee D02 onPetroleum Products and Lub
14、ricants and is the direct responsibility of SubcommitteeD02.04.0C on Liquid Chromatography.Current edition approved Jan. 1, 2012. Published March 2012. DOI:10.1520/D7591122For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Ann
15、ual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.levels of unreacted glycerides can cause injector deposits andcan adversely
16、 affect cold weather operation and filter plugging.6. Interferences6.1 Interferences can be caused by substances with similarion chromatographic retention times, especially if they are inhigh concentration compared to the analyte of interest. Sampledilution can be used to minimize or resolve most in
17、terferenceproblems.Also, an excess of unreacted hydroxide (base) duringthe sample preparation step for total glycerin can cause a pHimbalance on the anion exchange column, resulting in anegative dip in front of the glycerin peak.6.2 A water dip (system void, negative peak as shown inFig. 1) can caus
18、e interference with some integrators. This dipcan be eliminated by dilution with the eluent. The water dipshould not be a problem since the glycerin peak is resolvedfrom the void peak.6.3 Interferences can be caused by contamination of glass-ware, eluent, reagents, etc. Take care to ensure that cont
19、ami-nation is kept at the lowest possible levels. The use of nitrilegloves is highly recommended to prevent contamination duringsample preparation.6.4 There are several known additives based on naturalproducts that might have similar retention times and detectorresponse similar to glycerin. In the c
20、ase of higher thanexpected values for biodiesel blends, it is highly recommendedthat the user needs to verify these higher than expected valuesfor glycerin using a different analytical technique.6.5 Pre-rinsing of the sample preparation containers withdeionized water is mandatory.7. Apparatus7.1 Ana
21、lytical Balancecapable of weighing up to 200 gaccurately to 60.0001 g.7.2 Desiccatorcontaining freshly activated silica gel (orequivalent desiccant) with moisture content indicator.7.3 Pipettes or Volumetric Transfer Devices 1and5mLclass A volumetric pipettes or calibrated variable volumeautomatic p
22、ipettes fitted with disposable polypropylene tips.7.4 Volumetric Flasks25, 50, 100 and 1000 mL class Avolumetric flasks.7.5 Containerstandard HDPE plastic 100 mL bottle withcap.7.6 Ion ChromatographAnalytical system with all re-quired accessories including syringes, columns, high-pressuredual piston
23、 pump, and detector.7.6.1 Injection Systemcapable of delivering 5 to 25 Lwith a precision better than 1%.7.6.2 Pumping Systemcapable of delivering mobile phaseflows between 0.1 and 5.0 mL/min with a precision better than2%. Due to the corrosive nature of the eluent, a PEEK pumphead is recommended.7.
24、6.3 Guard Columnfor protection of the analytical col-umn from strongly retained constituents.7.6.4 Anion Exchange Columncapable of producing sat-isfactory analyte separation.7.6.5 Electrochemical Detectorintegrated, temperaturecontrolled to 0.1C, capable of measuring at least 0 to 200 Aon a linear s
25、cale. Detector has a pulsed amperometric detectionmode for required sensitivity. Consult with the manufacturerfor optimal cell settings.7.6.6 Electrochemical Detector Cellminimum 3 mm goldworking electrode surface with wall jet design, solid statereference and counter electrodes. Ensure a minimal vo
26、lume inthe cell for enhanced sensitivity. A platinum working electrodemay also be used.7.6.7 Integrator or Chromatography Data SystemSoftwarecapable of measuring peak areas and retentiontimes, and performing a baseline correction.7.6.8 Sample Digestion Systemcapable of heating, andstirring with inte
27、grated reflux. Reflux is needed to minimizeloss of petroleum diesel in biodiesel blend samples. A chiller isrecommended for providing water to the reflux condenser forefficiency and to conserve water resources.7.7 Mechanical Wrist Shaker.7.8 Gloves, nitrile.8. Reagents and Materials8.1 Purity of Rea
28、gentsReagent grade or higher puritychemicals shall be used for the preparation of all samples,standards and eluent solutions. Unless otherwise indicated, it isintended that all reagents conform to the specifications of theFIG. 1 Typical Chromatogram of a Solution Containing 0.7 mg/kg of GlycerinD759
29、1 122Committee on Analytical Reagents of the American ChemicalSociety, where such specifications are available.3Other gradesmay be used, provided it is first ascertained that the reagent isof sufficiently high purity to permit its use without lesseningthe accuracy of the determination.8.2 Water Qual
30、ityUnless otherwise indicated, reference towater shall be understood to mean reagent water as defined byType II in Specification D1193 or better. For eluent preparationand handling, comply with all ion chromatograph instrumentand column vendor requirements (for example, filtering, de-gassing, etc.).
31、8.3 Eluent Stock Solution, sodium hydroxide (NaOH, 50%certified, ACS).8.3.1 Eluent Preparation, 0.10 M NaOH. Weigh 8.00 60.02 g of 50% NaOH in reagent water in a 1-L volumetric flaskand dilute to volume with degassed reagent water. The eluentsolution used may be different if other systems or analyti
32、calcolumns are used. Other volumes of stock solution may beprepared using appropriate ratios of reagents. Ready to usereagents may be used. Consult with the instrument manufac-turer for guidance and use. Do not store sodium hydroxidesolutions in glass.8.4 Potassium Hydroxide Solution for Total Glyce
33、rin, 1.0 MKOH. Weigh out 56.1 g of ACS grade potassium hydroxidepellets. Dissolve the pellets in approximately 250 mL DI waterina1Lvolumetric flask. Use caution when handling the flaskdue to the heat produced during the dissolution of the potas-sium hydroxide. Dilute to the mark with DI water. Prepa
34、redready to use 1.0 M potassium hydroxide solutions made withacceptable purity materials may also be used. Keep containerstightly closed when not in use to minimize carbonate formationfrom atmospheric carbon dioxide.9. Preparation of Standard Solutions9.1 Stock and working solutions.9.1.1 Glycerin S
35、tock Solution, 1000 mg/LAccuratelyweigh1gof99.5+% glycerin to the nearest tenth of amilligram (0.0001 g) and transfer toa1Lvolumetric flask.Dilute to the mark with water. Shake or swirl to mix thestandard for homogeneity. Other volumes of stock solutionmay be prepared using the appropriate ratio of
36、reagents.9.2 Working StandardsPrepare glycerin working stan-dards according to Table 1.9.2.1 Alternatively, commercial stock calibration solutionscan be used, provided that the solutions are traceable toprimary stock solutions or certified reference materials, and arefree from other analytes.10. Cal
37、ibration10.1 Set up the ion chromatograph according to the manu-facturers instructions. No specific parameters are given heresince different manufacturers equipment might requirechanges in eluent, flow conditions, and instrument settings toperform the separation and obtain the results. Calibrate the
38、 ionchromatograph with at least five concentration levels of glyc-erin, starting near but above the minimum detection limit, andcovering the expected working range of samples subsequentlyto be analyzed. Select concentrations of calibrant solutionsused that bracket the expected range for the samples
39、to beanalyzed. Use one or more mid-range standards to verify thelinearity of the calibration plot.10.1.1 Typical ion chromatographic conditions:Flow: 1.0 mL/minSample loop: 10 LOther analytical conditions may be used per the manufac-turers instructions.NOTE 1The sample loop volume will vary based on
40、 the columncapacity, sensitivity, and other factors. Refer to ion chromatographyequipment manuals and column information for instrument/column-specific details.10.1.2 Establish analytical curves with only one detectorscale setting. This will prevent a change of slope affecting theanalytical curve.10
41、.2 Verify the analytical calibration plot daily or wheneversamples are to be run, prior to the analysis of samples to verifythe system resolution, calibration, and sensitivity as part of thequality verification process (see Section 14).10.3 Repeat calibration after any change of the ion chroma-togra
42、phy eluent solution from 8.3, to reestablish ion retentiontimes and resolution. Use a check standard to verify calibra-tion, retention times, and resolution after any change in the ICeluent solution from 8.3. Recalibrate if needed.10.4 Measurement of the Calibration StandardsInject 10L of each calib
43、ration solution from 9.2 into the ion chromato-graph, and measure the areas of the peaks corresponding toglycerin. Generally, one injection per sample is sufficient.Refer to Section 14 for quality control discussion.10.5 Construct the glycerin calibration plots by plotting thepeak areas against the
44、glycerin concentrations. Use linearregression to determine the best straight-line calibration. Alinear least squares correlation coefficient of 0.99 or greater isrequired (see Fig. 2). The response factor for glycerin, Rf, isthe slope of the calibration plot straight line, in mg/kg/(areacount).10.5.
45、1 If the plot of the peak area values against the ionconcentrations is not linear (the correlation factor should be atleast 0.99), the procedure should be checked for errors, and ifnecessary, the calibration should be repeated starting fromSection 9.3Reagent Chemicals, American Chemical Society Spec
46、ifications, 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 Conve
47、ntion, Inc. (USPC), Rockville,MD.TABLE 1 Preparation of Glycerin Standards in WaterGlycerin Standard,mg/kgWater(final weight), gGlycerin StockSolution, g50 100 5.020 100 2.010 100 1.05 100 0.51 100 0.10.5 100 0.05D7591 12311. Procedure11.1 For Free Glycerin in Biodiesel and its Blends:11.1.1 Obtain
48、samples in accordance with Practice D4057or Practice D4177. Mix the samples thoroughly to ensurehomogeneity. A representative portion shall be taken foranalysis. Ensure that the sample containers do not contain anyresidual glycerin. Use clean containers that have been rinsedwith Type II reagent wate
49、r and dried prior to use.11.1.2 Thoroughly mix the samples in their containersimmediately prior to withdrawal of a test sample.11.1.3 Accurately weigh5gofsample to the nearest 0.001g into a 100 mL container with a cap, and record the weight.11.1.4 Add 50 g of deionized water to the container (withpreviously weighed sample), and record the weight to thenearest 0.001 g. Close the container, and shake for 5 min on amechanical wrist shaker.11.1.5 After shaking the sample, let it settle until the oil andaqueous phases are separated.11.1.6 Set up the i
