1、Designation: D7946 14Standard Test Method forInitial pH (i-pH)-Value of Petroleum Products1This standard is issued under the fixed designation D7946; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A numb
2、er 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 procedures for the determina-tion of initial pH (i-pH) in new and in-service lubricants.NOTE 1In new and used oils,
3、 the constituents that may be consideredto have characteristics influencing the i-pH value include organic andinorganic acids, esters, phenolic compounds, lactones, resins, salts ofheavy metals, salts of ammonia and other weak bases, acid salts ofpolybasic acids, and addition agents such as inhibito
4、rs and detergents.“Initial” is used to differentiate from aqueous systems. The analysis isterminated after a defined time interval whenever equilibrium conditions,as known for pH measurements in aqueous systems, are not reached (see3.1.1.2)1.2 This test method is used to indicate relative changes th
5、atoccur in oil during use under oxidizing conditions or due tocontamination by blow-by gases of combustion processes ofbiogases regardless of the color or other properties of thein-service lubricants. Although the initial pH is made underdefinite equilibrium conditions, the test method is not intend
6、edto measure an absolute acidic property that can be used topredict performance of oil under service conditions. No generalcorrelation between corrosion of non-ferrous bearing metalsand initial pH value is known.1.3 The values stated in SI units are to be regarded asstandard. No other units of measu
7、rement are included in thisstandard.1.4 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 applica-bility of regulatory limi
8、tations prior to use.2. Referenced Documents2.1 ASTM Standards:2D1193 Specification for Reagent WaterD4057 Practice for Manual Sampling of Petroleum andPetroleum ProductsD4177 Practice for Automatic Sampling of Petroleum andPetroleum Products3. Terminology3.1 Definitions:3.1.1 initial pH (i-pH), nth
9、e measure of the hydrogen ion(H+) concentration when a fixed sample amount of lubricationoil is dissolved into a fixed volume of solvent and using aspecified detection system.3.1.1.1 DiscussionThis test method expresses the amountof dissociated acidic components as i-pH of a sample in amixture of to
10、luene and propan-2-ol to which a small amount ofwater has been added.3.1.1.2 DiscussionThe measured i-pH is called initialsince the original definition of the pH-scale depends onaqueous solutions. Its application to organic solutions will notgive stable electrode potential for all types of samples.
11、Apotential reading after a defined time is used in those casesinstead.4. Summary of Test Method4.1 The oil sample is dissolved in solvent mixture and thei-pH value is measured using a glass indicating electrode anda reference electrode or a combination electrode. The meterreadings are measured manua
12、lly or automatically.5. Significance and Use5.1 New and used petroleum products may contain acidicconstituents that are present as additives, degradation productsformed during service, such as oxidation products or compo-nents formed from combustion gases. The i-pH-value is ameasure of the amount of
13、 acidic substances and their aciditydefined under the conditions of test. The i-pH-value is used asa measure of lubricant degradation in service.5.2 The corrosiveness of acidic components depends ontheir concentration and acidity. The i-pH-value is a measure ofthe amount of dissociated acidic compon
14、ents with the potentialof corrosiveness towards metals.5.3 Since a variety of oxidation and blow-by productscontribute to the i-pH-value, this test method cannot be used topredict corrosiveness of oil or blends under service conditionsagainst metallic components. No general correlation is knownbetwe
15、en i-pH-value and the corrosive tendency of blends oroils toward metals.1This test method is under the jurisdiction of ASTM Committee D02 onPetroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility ofSubcommittee D02.06 on Analysis of Lubricants.Current edition approved July
16、1, 2014. Published August 2014. DOI: 10.1520/D7946-14.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.Copyrig
17、ht ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States16. Apparatus6.1 pH-MeterMeter, a voltmeter or a potentiometer, thatwill operate with an accuracy of 60.005 V and a sensitivity of60.002 V over a range of at least 6 0.5 V when the meter isused
18、with the electrodes specified in 6.2 to 6.4 and when theresistance between the electrodes falls within the range from0.2 M to 20 M. The meter shall be protected from strayelectrostatic fields so that no permanent change in the meterreadings over the entire operating range is produced bytouching, wit
19、h a grounded lead, any part of the exposed surfaceof the glass electrode, the glass electrode lead, the stirrer, or themeter.NOTE 2A suitable apparatus could consist of a continuous-readingelectronic voltmeter designed to operate on an input of less than510-12A, when an electrode system having a res
20、istance of 1000-Mis connectedacross the meter terminals and provided with a metal shield connected tothe ground, as well as a satisfactory terminal to connect the shieldedconnection wire from the glass electrode to the meter without interferencefrom any external electrostatic field.6.2 Sensing Elect
21、rode, Standard pH, suitable for nonaque-ous media.6.3 Reference Electrode, Silver/Silver Chloride (Ag/AgCl)Reference Electrode, filled with 1 mol/L to 3 molL LiCl inethanol.6.4 Combination ElectrodesSensing electrodes may havethe Ag/AgCl reference electrode built into the same electrodebody, which o
22、ffers the convenience of working with andmaintaining only one electrode. The combination electrodeshall have a sleeve junction on the reference compartment andshall use an inert ethanol electrolyte, for example, 1 mol/Lto3 molL LiCl in ethanol. These combination electrodes shallhave the same respons
23、e or better response than a dual electrodesystem. They shall have movable sleeves for easy rinsing andaddition of electrolyte.6.5 Variable-Speed Mechanical Stirrer, a suitable type,equipped with a propeller-type stirring paddle. The rate ofstirring shall be sufficient to produce vigorous agitation w
24、ithoutspattering and without stirring air into the solution.Apropellerwith blades 6 mm in radius and set at a pitch of 30 to 45 issatisfactory. A magnetic stirrer is also satisfactory. If anelectrical stirring apparatus is used, it shall be electricallycorrect and grounded so that connecting or disc
25、onnecting thepower to the motor will not produce a permanent change in themeter reading during the course of the determination.6.6 Beaker, 250 mL capacity, made of borosilicate glass orother suitable material.6.7 Beaker, 100 mL, or suitable capacity, made of borosili-cate glass or other suitable mat
26、erial.6.8 Stand, suitable for supporting the electrodes, and stirrer.NOTE 3An arrangement that allows the removal of the beaker withoutdisturbing the electrodes and stirrer is desirable.6.9 Graduated Cylinder, 100 mL, or dispensing devicecapable of delivering 70 mL 6 0.5 mL.6.10 Pipette, 2.0 mL, Cla
27、ss A.7. Reagents7.1 Purity of ReagentsReagent 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.3
28、Other grades may beused, provided it is first ascertained that the reagent is ofsufficiently high purity to permit its use without lessening theaccuracy of the determination.7.2 Commercially available solutions may be used in placeof laboratory preparations provided the solutions have beencertified
29、as being equivalent.7.3 Alternative volumes of the solutions may be prepared,provided the final solution concentration is equivalent.7.4 Purity of WaterUnless otherwise indicated, referenceto water shall be understood to mean reagent water that meetsthe requirements of either Type I, II, or III of S
30、pecificationD1193.7.5 Ethanol, (WarningFlammable and toxic, especiallywhen denatured.)7.6 Lithium Chloride, LiCl.7.7 Lithium Chloride ElectrolytePrepare a 1 mol/L to3 molL solution of lithium chloride (LiCl) in ethanol.7.8 Propan-2-ol, Anhydrous, (less than 0.1 % H2O).(WarningFlammable.) If adequate
31、ly dry reagent cannot beprocured, it can be dried by distillation through a multiple platecolumn, discarding the first5%ofmaterial distilling overheadand using the 95 % remaining. Drying can also be accom-plished using molecular sieves such as Linde Type 4A, bypassing the solvent upward through a mo
32、lecular sieve columnusing one part of molecular sieve per ten parts of solvent.NOTE 4It has been reported that, if not originally inhibited against it,propan-2-ol can contain peroxides. When this occurs, an explosion ispossible when the storage of the vessel or other equipment such as adispensing bo
33、ttle, is near empty and approaching dryness.7.9 Toluene, (WarningFlammable.)7.10 Commercial Aqueous pH 4 and, pH 7 BufferSolutionsThese solutions shall be replaced at regular inter-vals consistent with their stability or when contamination issuspected. Information relating to their stability should
34、beobtained from the manufacturer.8. Electrode System8.1 Preparation of ElectrodesWhen a Ag/AgCl referenceelectrode is used for the determination and it contains anelectrolyte which is not 1 molL to 3 molL LiCl in ethanol,replace the electrolyte. Drain the electrolyte from the electrode,wash away all
35、 the salt (if present) with water and then rinsewith ethanol. Rinse several times with the LiCl electrolytesolution. Finally, replace the sleeve and fill the electrode withthe LiCl electrolyte to the filling hole. When refitting thesleeve, ensure that there will be a free flow of electrolyte intothe
36、 system. A combination electrode shall be prepared in the3Reagent Chemicals, American Chemical Society Specifications, AmericanChemical Society, Washington, DC. For suggestions on the testing of reagents notlisted by the American Chemical Society, see Analar Standards for LaboratoryChemicals, BDH Lt
37、d., Poole, Dorset, U.K., and the United States Pharmacopeiaand National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville,MD.D7946 142same manner. The electrolyte in a combination electrode canbe removed with the aid of a vacuum suction.8.2 Testing of ElectrodesTest the meter-electrod
38、e combi-nation when first put into use, or when new electrodes areinstalled, and retest at intervals thereafter. Rinse the electrodeswith solvent then with water, and dip them into a pH 4 aqueousbuffer solution. Read the mV value after stirring 1 min.Remove the electrodes and rinse with water. Dip t
39、he electrodesinto a pH 7 aqueous buffer. Read the mV value after stirring1 min. Calculate the mV difference. A good electrode systemwill have a difference of at least 162 mV (20 C to 25 C). Ifthe difference is less than 162 mV, lift the sleeve of theelectrode and insure electrolyte flow. Repeat the
40、measure-ments. If the difference is still less than 162 mV, clean orreplace the electrode(s).8.3 When the sensing electrode and the reference electrodeare separate, one pair of electrodes shall be considered as oneunit. If one or the other is changed, it shall be considered asdifferent pair and shal
41、l be retested.8.4 Maintenance and Storage of ElectrodesCleaning theelectrodes thoroughly, keeping the ground-glass joint free offoreign materials, and regular testing of the electrodes are veryimportant in obtaining repeatable potentials, since contamina-tion may introduce uncertain erratic and unno
42、ticeable liquidcontact potentials.NOTE 5See Appendix X1 for a possible procedure to check theelectrode performance.8.5 Clean the glass electrode at frequent intervals based onuse and type of samples being analyzed (not less than onceevery week during continual use) by immersing in non-chromium conta
43、ining, strongly oxidizing cleaning solution.The reference electrode shall be cleaned periodically when inuse or when a new electrode is installed. Drain the referenceelectrode at least once each week and refill with the fresh LiClelectrolyte as far as the filling hole. Ensure that there are no airbu
44、bbles in the electrode liquid. If air bubbles are observed,hold the electrode in a vertical position and gently tap it torelease the bubbles. Maintain the electrolyte level in thereference electrode above that of the liquid in the samplebeaker or vessel at all times.8.6 Prior to each i-pH measuremen
45、t soak the preparedelectrodes in deionized water for at least 2 min. Rinse theelectrodes with propan-2-ol immediately before use, and thenwith the dilution solvent.8.7 When not in use, the glass indicator electrode membraneshould be kept hydrated and stored according to the manufac-turers recommenda
46、tion.8.8 Electrode LifeTypically, electrode usage is limited to3 months to 6 months depending, upon usage. Electrodes havea limited shelf life and shall be tested before use (see 8.2).9. Standardization of Apparatus9.1 Calibration of the pH-meter with the Aqueous BufferSolutionsThe electrode should
47、be calibrated daily usingaqueous pH buffer solution standards 4 and 7. When usingdirect electrode potential reading, establish a linear regressionfor conversion of sample potential readings to i-pH.10. Preparation of Sample of Used Oil10.1 Strict observance of the sampling procedure is neces-sary si
48、nce the sediment, eventually present in the sample canbe acidic or basic or has absorbed acidic or basic material fromthe sample. Failure to obtain a representative sample causesserious errors.10.2 When applicable, refer to Practice D4057 (ManualSampling) or Practice D4177 (Automatic Sampling) for p
49、ropersampling techniques.10.3 When sampling used lubricants, the specimen shall berepresentative of the system sampled and shall be free ofcontamination from external sources.NOTE 6As used oil can change appreciably in storage, test samples assoon as possible after removal from the lubricating system and note thedates of sampling and testing.10.4 Heat the sample (see Note 7) of used oil to 60 C 65 C in the original container and agitate until all of thesediment is homogeneously suspended in the oil. If the originalcontainer is a can or if it is g
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