1、Designation: D7318 11Standard 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 1.020 mg/kg existent inorganic sulfate.1.2 The values stated in SI units are to be regarded asstanda
4、rd. 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 and health practices and determine the applic
5、a-bility of regulatory limitations prior to use. Material SafetyData Sheets are available for reagents and materials. Reviewthem for hazards prior to usage.2. Referenced Documents2.1 ASTM Standards:2D1193 Specification for Reagent WaterD4052 Test Method for Density, Relative Density, and APIGravity
6、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 Charting Techniques to Evaluate AnalyticalMeasurement S
7、ystem Performance3. Terminology3.1 Definitions:3.1.1 existent inorganic sulfate, nsulfate species presentas sulfuric acid, ionic salts of this acid, or mixtures of these.3.1.1.1 DiscussionSpecifically in this test method, inor-ganic sulfate is present as sulfate in ethanol.4. Summary of Test Method4
8、.1 An ethanol sample containing inorganic sulfate istitrated in ethanolic medium with a standard lead nitratesolution. 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 ionac
9、tivity, as measured by a leadselective electrode.5. Significance and Use5.1 Ethanol is used as a blending agent 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 acce
10、ptability foruse depends on the sulfate content. Sulfate content, as mea-sured by this test method, can be used as one measure ofdetermination of the acceptability of ethanol for automotivespark-ignition engine fuel use.6. Apparatus6.1 Potentiometric Titration AssemblyA titration assem-bly consistin
11、g of an automatic titrator fitted with a leadion-selective electrode, a double-junction reference electrode,buret, and stirring is used. Stirring may be accomplished bymeans of magnetic or propeller type stirrer mechanisms. Theburet size should ideally be 10 or 20 mL.6.2 Reference ElectrodeA double
12、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 inethanol. This configuration is used to prevent silver ion, a leadelectrode poison, from le
13、aching 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. Othertypes of reference electrodes may be considered with somecaveats (for example, singl
14、e junction, combination, or glassycarbon), but the data presented in this test method were1This test method is under the jurisdiction of ASTM Committee D02 onPetroleum Products and Lubricants and is the direct responsibility of SubcommitteeD02.03 on Elemental Analysis.Current edition approved July 1
15、, 2011. Published August 2011. Originallyapproved in 2007. Last previous edition approved in 2007 as D731807. DOI:10.1520/D7318-07.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume informa
16、tion, refer to the standards Document Summary page onthe ASTM website.1*A Summary of Changes section appears at the end of this standard.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.generated using exclusively a double junction ele
17、ctrode, whichis the best choice for this determination.6.3 Lead ElectrodeAlead sulfidebased crystalline sensortype lead ion selective electrode (ISE) is used.6.4 Drying OvenA drying oven for drying sodium sulfateat 110C is required.6.5 Pipets or Volumetric Transferring DevicesClass Aglass pipets or
18、their equivalent.6.6 Polishing MaterialLead sulfide based crystalline sen-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
19、beakers or titrationvessels supplied with titration equipment.7. Reagents 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 Sodi
20、um SulfateAnhydrous, reagent grade, 99% mini-mum purity. (WarningDo not 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 mea
21、surable sulfate. (WarningCorrosive;keep away from skin and eyes. Perchloric acid is a strongoxidizer.)7.4 EthanolDenatured with methanol, formula 3A orhistological grade ethanol, anhydrous, denatured with ethylacetate, methyl isobutyl ketone and hydrocarbon naphtha. Itmust be free of any measurable
22、sulfate. (WarningFlammable, toxic, may be harmful or fatal if ingested 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 Specifi-cation D1193.7.8 Anhydrous Calcium Sul
23、fate Desiccant.8. Preparation of Standard Solutions8.1 Lead Nitrate 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 acommerci
24、al vendor, and its exact molarity shall be determinedin accordance with 10.1.8.2 Aqueous Sulfate Standard, 0.01 MDry 5 g anhydroussodium sulfate at 110C 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
25、 balance to thenearest tenth of a milligram, and place it in a 500-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)Molarity = molarity of sulfate standard solution, mol
26、/L,G = weight in grams of Na2SO4, dissolved in 500mL, and142.02 = gram molecular weight of Na2SO4.8.3 Aqueous Sulfate Stock Solution for Standards in Etha-nol, 2000 mg/LAccurately weigh 2.95 g anhydrous sodiumsulfate to the nearest tenth of a milligram and transfer it to a1-L volumetric flask. (Drie
27、d anhydrous sodium sulfate shouldbe stored in a desiccator.) Add Type III water to dissolve thesodium sulfate, and make to volume. Calculate the concentra-tion of sulfate in the solution in accordance with Eq 2.Aqueous Stock Sulfate mg/L!5gNa2SO4! 0.6764! 1000 mg/g!1L(2)gNa2SO4= weight in grams of N
28、a2SO4dissolved in 1 L,and0.6764 = fraction of sulfate 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 sulfate
29、stock solution from 8.3 is added to the solution in accordancewith 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 su
30、lfatestock solution and dividing by the final solution weight inaccordance with Eq 3.EtOH Sulfate Standard mg/kg!5V 3 CW(3)V = volume of aqueous sulfate 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 Aqueo
31、us 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 measurable 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
32、well and dilute to the mark with Type III water.TABLE 1 Preparation of Sulfate Standards in EthanolEthanolic Sulfate Standard,mg sulfate/kg ethanolEthanol,gAqueous Sulfate StockSolution, mL50 975 2520 990 1010 995 55 997.5 2.51 999.5 0.5D7318 1128.7 Lithium Chloride in Ethanol, 1 MAdd lithium chlo-r
33、ide (8.49 g) to absolute ethanol (200 mL) while stirring untilit is 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 monotonictitra
34、tion mode, using either 25-L or 50-L titrant additionincrements. Use a 10 mV/min drift condition or 20-s wait timebetween additions (whichever is achieved first).9.2 Electrode PreparationProper care of the leadselec-tive electrode is essential for obtaining highquality titrationcurves. Preparation o
35、f the lead electrode should be performedas specified by the manufacturer. A lead electrode utilizing asolid crystal sensor requires polishing when performancedeteriorates. The voltage range for a 10 ppm sulfate in ethanoltitration should span at least 50 mV and produce an acceptablyshaped titration
36、curve (see Fig. 1). If this level of electrodeperformance is not met, this indicates a need for lead electrodepolishing. The lead electrode normally comes supplied with apolishing grit and a cloth rectangle. The polishing grit mediumis placed on the cloth, wet with ethanol or water, moved overthe su
37、rface of the cloth for a minute, rinsed with water toremove the polishing medium, and wiped dry with a tissue. Theelectrode is then soaked in lead titrant (8.1) for 2 min tore-equilibrate the electrode surface. The double junction refer-ence electrode is filled with 1 M lithium chloride in absolutee
38、thanol (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. Record thisvolume as V.10.1.2 Add approxi
39、mately 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. Titration shouldrequire approximately 4.0-mL
40、 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 Eq 4. The standardization should be re
41、peated untilprecision comparable to that shown in Section 15 and Table 2has been achieved.MPb5V 3 ST Tb(4)FIG. 1 Potentiometric Titration Curves for Sulfate Standards in EthanolD7318 113where:MPb= molarity of lead in the titrant, mol/L,V = volume of sulfate standard solution added, mL(10.1.1),S = mo
42、larity of the sulfate solution from 8.2, 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 samplesho
43、uld be well mixed to ensure homogeneity. 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
44、with water,they must be rinsed with Type 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. An aliquot of aqueous sulfateblank solution (8.5) is adde
45、d to all samples as part of the blank.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 1
46、00 g) into a 150-mL beaker, and record 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
47、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 acid and recheck the pH.11.
48、3.4 To prepare the autotitrator for operation, immerse theelectrodes in the titration solution, initiate stirring and titrate tothe potentiometric endpoint with lead titrant. Record thevolume of titrant used as V.11.3.5 Duplicate determinations should agree to within therepeatability limits in Secti
49、on 15 and Table 2. If not, additionaltitrations should be performed until agreement is achieved.11.3.6 If satisfactory titration curves are not obtained, thelead electrode should be polished (9.2). Typical titration curvesfor sulfate standards in denatured ethanol are shown in Fig. 1,and examples from commercial denatured ethanol samples areshown in Fig. 2.TABLE 2 Test Method PrecisionAnalyte mg/kg Repeatability (r) Reproducibility (R)Existent Inorganic Sulfate 1.0 0.18 0.604.0 0.25 0.8520.0 0.38 1.30FIG. 2 Potentiometric Titration
