1、Designation: D7318 18Standard Test Method forExistent Inorganic Sulfate in Ethanol by PotentiometricTitration1This standard is issued under the fixed designation D7318; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of la
2、st revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope*1.1 This test method covers a potentiometric titration pro-cedure for determining the existent inorganic sulfate content of
3、hydrous, anhydrous ethanol, and anhydrous denatured ethanol,which is added as a blending agent with spark ignition fuels. Itis intended for the analysis of denatured ethanol samplescontaining between 1.0 mg kg to 20 mg kg existent inorganicsulfate.1.2 The values stated in SI units are to be regarded
4、 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 standard to establish appro-priate safety, health, and environmental practices
5、 and deter-mine the applicability of regulatory limitations prior to use.Material Safety Data Sheets are available for reagents andmaterials. Review them for hazards prior to usage.1.4 This international standard was developed in accor-dance with internationally recognized principles on standard-iza
6、tion established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:2D1193 Specification for Reagent WaterD4052 Test Metho
7、d for Density, Relative Density, and APIGravity of Liquids by Digital Density MeterD4057 Practice for Manual Sampling of Petroleum andPetroleum ProductsD4177 Practice for Automatic Sampling of Petroleum andPetroleum ProductsD6299 Practice for Applying Statistical Quality Assuranceand Control Chartin
8、g Techniques to Evaluate AnalyticalMeasurement System Performance3. Terminology3.1 Definitions:3.1.1 existent inorganic sulfate, ninorganic sulfate speciesactually present in the sample at the time of analysis with nooxidation treatment.3.1.1.1 DiscussionSpecifically in this test method, inor-ganic
9、sulfate is present as sulfate in ethanol.3.1.2 inorganic sulfate, nsulfate (SO42) species presentas sulfuric acid, ionic salts of this acid, or mixtures of these.4. Summary of Test Method4.1 An ethanol sample containing inorganic sulfate istitrated in ethanolic medium with a standard lead nitratesol
10、ution. Lead sulfate precipitate is formed during the titration.Perchloric acid is added to remove possible interference fromcarbonate. The endpoint is signaled by an increase in lead ionactivity, as measured by a lead-selective electrode.5. Significance and Use5.1 Ethanol is used as a blending agent
11、 added to gasoline.Sulfates are indicated in filter plugging deposits and fuelinjector deposits. When fuel ethanol is burned, sulfates maycontribute to sulfuric acid emissions. Ethanol acceptability foruse depends on the sulfate content. Sulfate content, as mea-sured by this test method, can be used
12、 as one measure ofdetermination of the acceptability of ethanol for automotivespark-ignition engine fuel use.6. Apparatus6.1 Potentiometric Titration AssemblyA titration assem-bly consisting of an automatic titrator fitted with a leadion-selective electrode, a double-junction reference electrode,bur
13、et, and stirring is used. Stirring may be accomplished bymeans of magnetic or propeller type stirrer mechanisms. Theburet size should ideally be 10 mL or 20 mL.1This test method is under the jurisdiction of ASTM Committee D02 onPetroleum Products, Liquid Fuels, and Lubricants and is the direct respo
14、nsibility ofSubcommittee D02.03 on Elemental Analysis.Current edition approved Oct. 1, 2018. Published October 2018. Originallyapproved in 2007. Last previous edition approved in 2013 as D7318 13. DOI:10.1520/D7318-18.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact AS
15、TM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.*A Summary of Changes section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshoh
16、ocken, PA 19428-2959. United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organ
17、ization Technical Barriers to Trade (TBT) Committee.16.2 Reference ElectrodeA double junction reference elec-trode with the inner electrode composed of silver/silver chlo-ride with a potassium chloride solution as internal electrolyte.The external solution is composed of 1 M lithium chloride inethan
18、ol. This configuration is used to prevent silver ion, a leadelectrode poison, from leaching into the analyte solution duringtitration. Preferred electrolytes for use in double junctionelectrodes may vary with the manufacturer; use the manufac-turers recommended electrolytes for the application. Othe
19、rtypes of reference electrodes may be considered with somecaveats (for example, single junction, combination, or glassycarbon), but the data presented in this test method weregenerated using exclusively a double junction electrode, whichis the best choice for this determination.6.3 Lead ElectrodeA l
20、ead sulfide-based crystalline sensortype lead ion selective electrode (ISE) is used.6.4 Drying OvenA drying oven for drying sodium sulfateat 110 C is required.6.5 Pipets or Volumetric Transferring DevicesClass Aglass pipets or their equivalent.6.6 Polishing MaterialLead sulfide based crystalline sen
21、-sor electrodes require polishing to remove oxidation products.These materials are supplied with the electrode from themanufacturer.6.7 pH Test StripsTest strips in the range of pH 1 to pH 7.6.8 Titration VesselsStandard glass beakers or titrationvessels supplied with titration equipment.7. Reagents
22、 and Materials7.1 Lead NitrateReagent grade, 99 % minimum purity.(WarningPoison, harmful by inhalation and if ingested.Avoid contact with the skin.) Dispose of this material inaccordance with accepted local requirements.7.2 Sodium SulfateAnhydrous, reagent grade, 99 % mini-mum purity. (WarningDo not
23、 ingest. Avoid unnecessaryexposure.)7.3 Perchloric Acid 70 %A.C.S. reagent grade, minimumpurity with sulfate concentration 0.001 % (m/M). Dispose ofthis material in accordance with accepted local requirements. Itmust contain no measurable sulfate. (WarningCorrosive;keep away from skin and eyes. Perc
24、hloric acid is a strongoxidizer.)7.4 EthanolDenatured with methanol, formula 3A or his-tological grade ethanol, anhydrous, denatured with ethylacetate, methyl isobutyl ketone and hydrocarbon naphtha. Itmust be free of any measurable sulfate. (WarningFlammable, toxic, may be harmful or fatal if inges
25、ted orinhaled. Avoid skin contact.)7.5 EthanolAbsolute, 200 proof, 99.5 %, A.C.S. reagentgrade.7.6 Lithium Chloride99+ %, A.C.S. reagent grade.7.7 WaterType III reagent water conforming to Specifica-tion D1193.7.8 Anhydrous Calcium Sulfate Desiccant.8. Preparation of Standard Solutions8.1 Lead Nitra
26、te Titrant, 0.0025 MDissolve 0.833 g leadnitrate in 300 mL water. Pour into a 1 L bottle and fill withdenatured ethanol and mix well. Standardize in accordancewith 10.1.8.1.1 Alternatively, this solution may be purchased from acommercial vendor, and its exact molarity shall be determinedin accordanc
27、e with 10.1.8.2 Aqueous Sulfate Standard, 0.01 MDry 5 g anhydroussodium sulfate at 110 C for 1 h. Remove it from the oven, andallow it to cool in a desiccator over anhydrous calcium sulfate.Accurately weigh about 0.70 g on an analytical balance to thenearest tenth of a milligram, and place it in a 5
28、00 mLvolumetric flask. Add Type III water to dissolve the sodiumsulfate, then dilute to volume. Calculate the exact concentra-tion in accordance with Eq 1.G142.02!0.500!5 Molarity (1)where:Molarity = molarity of sulfate standard solution, mol/L,G = weight in grams of Na2SO4, dissolved in500 mL, and1
29、42.02 = gram molecular weight of Na2SO4.8.3 Aqueous Sulfate Stock Solution for Standards inEthanol, 2000 mg LAccurately weigh 2.95 g anhydrous so-dium sulfate to the nearest tenth of a milligram and transfer itto a 1 L volumetric flask. (Dried anhydrous sodium sulfateshould be stored in a desiccator
30、.)Add Type III water to dissolvethe sodium sulfate, and make to volume. Calculate the concen-tration of sulfate in the solution in accordance with Eq 2.Aqueous Stock Sulfate mg/L! 5gNa2SO4!0.6764!1000 mg/g!1L(2)where:gNa2SO4= weight in grams of Na2SO4dissolved in 1 L,and0.6764 = fraction of sulfate
31、in Na2SO4.8.4 Sulfate Standards in EthanolEthanol (denatured con-taining no measurable sulfate) is weighed into a container(equipped with a closure to prevent evaporation) in accordancewith Table 1 to achieve the desired standard. Aqueous sulfatestock solution from 8.3 is added to the solution in ac
32、cordancewith Table 1, and the final weight of the solution recorded.Standards should be remade weekly or if recovery of less than90 % is noted. The concentration of the standard is calculatedby dividing the number of milligrams sulfate from the sulfateTABLE 1 Preparation of Sulfate Standards in Etha
33、nolEthanolic Sulfate Standard,mg sulfate/kg ethanolEthanol,gAqueous Sulfate StockSolution, mL50 975 2520 990 1010 995 55 997.5 2.51 999.5 0.5D7318 182stock solution and dividing by the final solution weight inaccordance with Eq 3.EtOH Sulfate Standard mg/kg! 5V 3CW(3)where:V = volume of aqueous sulf
34、ate stock (8.3), mL,C = concentration of aqueous sulfate stock (8.3), mg/L, andW = final weight of ethanol and aqueous sulfate stockaliquot, g.8.5 Aqueous Sulfate Blank Solution, 0.01 MThis is thesame solution as in 8.2. This solution contains sulfate and willbe added to all samples to allow a measu
35、rable sulfate blank tobe measured.8.6 Dilute Perchloric Acid, 0.1 MDissolve 8.8 mL per-chloric acid (7.3) in 250 mL water in a 1 L volumetric flask.Mix well and dilute to the mark with Type III water.8.7 Lithium Chloride in Ethanol, 1 MAdd lithium chlo-ride (8.49 g) to absolute ethanol (200 mL) whil
36、e stirring until itis dissolved.9. Titration Equipment Preparation9.1 TitratorPrepare the titrator by filling the titrator res-ervoir with lead titrant (8.1). Follow the manufacturers pro-cedure for filling the buret. Perform titrations in monotonictitration mode, using either 25 L or 50 L titrant a
37、dditionincrements. Use a 10 mV min drift condition or 20 s wait timebetween additions (whichever is achieved first).9.2 Electrode PreparationProper care of the lead-selective electrode is essential for obtaining high-quality titra-tion curves. Preparation of the lead electrode should beperformed as
38、specified by the manufacturer. A lead electrodeutilizing a solid crystal sensor requires polishing when perfor-mance deteriorates. The voltage range for a 10 ppm sulfate inethanol titration should span at least 50 mV and produce anacceptably shaped titration curve (see Fig. 1). If this level ofelect
39、rode performance is not met, this indicates a need for leadelectrode polishing. The lead electrode normally comes sup-plied with a polishing grit and a cloth rectangle. The polishinggrit medium is placed on the cloth, wet with ethanol or water,moved over the surface of the cloth for a minute, rinsed
40、 withwater to remove the polishing medium, and wiped dry with aFIG. 1 Potentiometric Titration Curves for Sulfate Standards in EthanolD7318 183tissue. The electrode is then soaked in lead titrant (8.1) for2 min to re-equilibrate the electrode surface. The doublejunction reference electrode is filled
41、 with 1 M lithium chloridein absolute ethanol (8.7) in the outer chamber.10. Standardization10.1 Measure the exact concentration of the lead nitratetitrant by titration of the sulfate standard (8.2).10.1.1 Transfer 1.00 mL of the 0.01 M aqueous sulfatestandard solution (8.2) into a 150 mL beaker. Re
42、cord thisvolume as V.10.1.2 Add approximately 100 mL of denatured ethanol(7.4) and 1 mL of 0.1 M perchloric acid (8.6).10.1.3 Prepare the autotitrator for operation, immerse theelectrodes in the titration solution, initiate stirring, and titrate tothe potentiometric endpoint with lead titrant. Titra
43、tion shouldrequire approximately 4.0 mL of lead solution to reach theendpoint. Record this volume as T.10.2 Titrate a blank denatured ethanol sample in the samemanner without addition of sulfate standard. Record thisvolume as Tb.10.3 Calculate the concentration of the lead nitrate titrant asshown in
44、 Eq 4. The standardization should be repeated untilprecision comparable to that shown in Section 15 and Table 2has been achieved.MPb5V 3ST 2 Tb(4)where:MPb= molarity of lead in the titrant, mol/L,V = volume of sulfate standard solution added, mL(10.1.1),S = molarity of the sulfate solution from 8.2,
45、 mol/L,T = volume of lead titrant used to titrate the sulfatesolution, mL, andTb= volume of lead titrant used for the blank, mL (10.2).11. Procedure11.1 Sample AnalysisObtain an ethanol sample in accor-dance with Practice D4057 or Practice D4177. The sampleshould be well mixed to ensure homogeneity.
46、 A representativeportion shall be taken for analysis. Samples should be collectedin containers with closures that seal well to prevent evaporationof ethanol. Samples containers should not contain any residualsulfate. If containers have been cleaned and rinsed with water,they must be rinsed with Type
47、 III water and dried prior to use.11.2 Blank Measurement:11.2.1 Determine the blank value by analysis of a denaturedethanol sample used for making standards (100 g) using theprocedure described in 11.3. A 20 mL aliquot of aqueoussulfate blank solution (8.5) is added to all samples as part of theblan
48、k. This is necessary to provide a stable blank value.11.2.2 The average of three blank measurements is recordedas Vbfor use in the calculation of sulfate content of samples.11.3 Sample Measurement:11.3.1 Accurately weigh an ethanol sample or blank (ap-proximately 100 g) into a 150 mL beaker, and rec
49、ord theweight as G.11.3.2 Carefully pipet 0.200 mL of aqueous sulfate blanksolution (8.5) into the beaker containing the ethanol sample.11.3.3 Add 1 mL of 0.1 M perchloric acid (8.6). The sampleshould be slightly acidic to pH paper before titrating, pHshould be in the range of 3 to 5. The pH can be checked bydipping a glass stirring rod into the sample and testing a dropleton the rod with a moistened pH test strip. If 1 mL of 0.1 Mperchloric acid is insufficient to achieve a pH in the range of 3to 5, add an additional millilitre of
