1、Designation: D 5015 02Standard Test Method forpH of Atmospheric Wet Deposition Samples byElectrometric Determination1This standard is issued under the fixed designation D 5015; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the ye
2、ar of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method is applicable to the determination ofpH in atmospheric wet deposition samples by electrometric
3、measurement using either a pH half cell with a reference probeor a combination electrode as the sensor.1.2 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 h
4、ealth practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D 1129 Terminology Relating to WaterD 1193 Specification for Reagent WaterD 1356 Terminology Relating to Sampling and Analysis ofAtmospheresD 2777 Practice for Determina
5、tion of Precision and Bias ofApplicable Methods of Committee D19 on WaterD 5012 Guide for Preparation of Materials Used for theCollection and Preservation of Atmospheric Wet Deposi-tionD 5111 Guide for Choosing Locations and Sampling Meth-ods to Monitor Atmospheric Deposition at Non-UrbanLocationsE1
6、 Specification for ASTM ThermometersE 380 Practice for Use of the International System of Units(SI) (the Modernized Metric System)3. Terminology3.1 Definitions:3.1.1 pHthe negative logarithm to the base ten of theconventional hydrogen ion activity.3.1.2 For definitions of other terms used in this te
7、st method,refer to Terminology D 1129 and D 1356. For an explanationof the metric system including units, symbols, and conversionfactors, see Practice E 380.4. Summary of Test Method4.1 The pH meter and the associated electrodes are cali-brated with two reference buffer solutions that bracket theant
8、icipated sample pH. The pH of the wet deposition sample isdetermined from this calibration and a quality control standard.The quality control standard is necessary in this application toevaluate the bias due to residual liquid junction potentials andto correct for this bias.4.2 The pH of a solution
9、is related to the EMF (millivolts)of a pH electrode system according to the operational definitionfor a two-point calibration:pH X! 5 pH S1! 1EX2 ES1ES22 ES1pH S2! 2 pH S1!# (1)where:pH(X) = pH of an unknown sample,pH (S1) = pH of a Standard Solution 1,pH (S2) = pH of a Standard Solution 2,EX= EMF (
10、mV) measured in an unknown sample,ES1= EMF (mV) measured in Standard Solution 1,andES2= EMF (mV) measured in Standard Solution 2.5. Significance and Use5.1 The accurate measurement of pH in atmospheric wetdeposition is an essential and critically important component inthe monitoring of atmospheric w
11、et deposition for trends in theacidity and overall air quality. Atmospheric wet deposition is,in general, a low ionic strength, unbuffered solution. Specialprecautions, as detailed in this test method, are necessary toensure accurate pH measurements (1).3Special emphasis mustbe placed on minimizing
12、the effect of the residual liquidjunction potential bias.1This test method is under the jurisdiction of ASTM Committee D22 on AirQuality and is the direct responsibility of Subcommittee D22.03 on AmbientAtmospheres and Source Emissions.Current edition approved October 10, 2002. Published December 20
13、02. Origi-nally published as D 5015 89. Last previous edition D 5015 95.2For 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 AS
14、TM website.3The boldface numbers in parentheses refer to the list of references at the end ofthis standard.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.5.2 This test method is applicable only to the measurementof pH in atmospheric
15、 wet deposition. Its use in other applica-tions may result in inaccuracies.5.3 Fig. 1 provides a frequency distribution of precipitationpH values measured in conjunction with a national monitoringprogram within the United States. These data are an indicationof the range of pH values common to atmosp
16、heric wetdeposition.6. Interferences6.1 The pH meter and the associated electrodes reliablymeasure pH in nearly all aqueous solutions and in general arenot subject to solution interferences from color, turbidity,oxidants, or reductants.6.2 The pH of an aqueous solution is affected by thetemperature.
17、 The electromotive force (EMF) between the glassand the reference electrode is a function of temperature as wellas pH. Temperature effects can be approximately compensatedfor automatically or manually depending on the pH meterselected.6.3 Organic materials dispersed in water appear to poisonthe glas
18、s electrode, particularly when analyzing low ionicstrength solutions. Difficulty encountered when standardizingthe electrode(s), erratic readings, or slow response times maybe an indication of contamination of the glass bulb or the liquidjunction of the reference electrode. To remove these coatings,
19、refer to the manual accompanying the probe for the manufac-turers recommendations.6.4 When analyzing samples that have low ionic strengths,such as wet deposition, an effect known as “residual junctionpotential” can lead to errors as large as 0.3 pH units. This erroroccurs when the junction potential
20、 of the sample differs greatlyfrom that of the standard. These conditions are frequently metin wet deposition analyses when the electrodes are calibratedwith high ionic strength standard reference buffers. In manycases, this error has been reduced by using a referenceelectrode with a ceramic junctio
21、n (2, 3).6.5 To speed electrode equilibration, the sample should beagitated prior to measurement. Care must be taken, however, toavoid introducing a source of error known as “residual stream-ing potential that can result in a significant differencebetween the stirred and unstirred pH of the sample (
22、4). Themagnitude of the streaming potential is dependent on theelectrodes and on the stirring rate. Differences in pH for stirredand unstirred wet deposition samples when the electrodeassembly has been calibrated only with quiescent referencestandards average 0.05 pH units at a stirring rate of four
23、revolutions per second.6.5.1 Eliminate the errors associated with residual stream-ing potentials by agitating all calibration standards and wetdeposition samples thoroughly to speed electrode equilibrationand then allowing each aliquot to become quiescent beforetaking a pH reading.6.5.2 If magnetic
24、stirring is used, take care not to contami-nate the sample when inserting the stirring bar. Maintain an airspace between the surface of the stirring motor and the samplecontainer to prevent heating the wet deposition sample.6.6 Laboratories used for the measurements of pH should befree from gaseous
25、and particulate contaminants that may affectthe true solution pH. Fumes from mineral acids such ashydrochloric acid, sulfuric acid, and nitric acid should be keptisolated from areas where pH measurements are made as wellas alkaline fumes from solutions such as ammonia.7. Apparatus and Equipment7.1 L
26、aboratory pH MeterThe meter may have either ananalog or digital display with a readability of at least 0.01 pHunits. A meter that has separate calibration and slope adjust-ment features and is electrically shielded to avoid interferencesfrom stray currents or static charge is necessary. It may bepow
27、ered by battery or 110 VAC; if battery powered, the metermust have a battery check feature. A temperature compensatorcontrol for measurements at temperatures other than 25C isdesirable.7.2 Sensing ElectrodeSelect a general purpose glass elec-trode that meets the performance criteria described in 12.
28、2.This electrode type is characterized by a quick response, andhas a useful range from 2 to 11 pH units. This electrode shouldbe used exclusively for atmospheric wet deposition measure-ments.7.3 Reference ElectrodeThe reference electrode recom-mended for wet deposition analysis is one equipped with
29、aceramic junction with controlled leakage of the internal elec-trolyte fill solution. The ceramic construction minimizes dif-ferences in potential between high ionic strength buffers andlow ionic strength samples thus reducing errors from residualjunction potential (1). This electrode should be used
30、 exclu-sively for atmospheric wet deposition measurements.7.4 Combination ElectrodeThe combination electrodecombines the indicating and reference elements in a single unit.A ceramic reference junction is recommended (see 7.3). Sincesample volume requirements are a consideration when analyz-ing wet d
31、eposition samples, combination electrodes are moreconvenient than separate glass and reference electrodes. Thiselectrode should be used exclusively for atmospheric wetdeposition measurements and must meet the criteria stated in12.2.7.5 Temperature ControlUse either a constant tempera-ture water bath
32、, a temperature compensator, or a thermometer(see Specification E1) to verify that all standards and samplesFIG. 1 Frequency Distribution of Measured Laboratory pH ofAtmospheric Wet Deposition From the 1984 NationalAtmospheric Deposition Program (NADP)/National TrendsNetwork (NTN)D5015022are maintai
33、ned at temperatures within 61C of one another. Ifa thermometer is used, select one capable of being read to thenearest 1C and covering the range from 0 to 40C.7.6 Stirring Device (Optional)Electric or water-driven. Ifan electric stirrer is selected, leave an air gap or place aninsulating pad between
34、 the stirrer surface and the solutioncontainer to minimize heating of the sample. Use afluorocarbon-coated stirring bar.7.7 Storage of ElectrodesWhen not in use, soak theelectrodes in a solution that is 0.1 mol/L of potassium chlorideand 0.1 mmol/L of hydrochloric acid. Do not store theelectrodes in
35、 buffers, concentrated acids, concentrated potas-sium chloride, basic solutions, or distilled water. Some manu-facturers recommend dry storage for specific types of elec-trodes. If the electrode is of this specific type, store dry. Usethese electrodes exclusively for atmospheric wet depositionmeasur
36、ements.8. Reagents and Materials8.1 Purity of ReagentsUse reagent or higher grade chemi-cals for all solutions. All reagents shall conform minimally tothe specifications of the Committee on Analytical Reagents ofthe American Chemical Society (ACS)4where such specifica-tions are available.8.2 Purity
37、of WaterUse water conforming to SpecificationD 1193, Type I. Point of use 0.2 m filters are recommendedfor all faucets supplying ASTM Type I water to prevent theintroduction of bacteria or ion exchange resins into reagents,standard solutions, and internally formulated quality controlcheck solutions.
38、8.3 Buffer SolutionsEither NIST buffers or commerciallyavailable buffer solutions traceable to NIST buffers must beused for standardization. These buffer solutions usually havepH values near 3, 4, 6, and 7, the exact pH and use temperaturebeing provided by the supplier of the specific buffer. Table
39、1identifies each buffer salt by its National Institute of Standardsand Technology (NIST) number. Store the reference buffersolutions in polyethylene or chemical-resistant glass bottlesand replace after one year or sooner if a visible change such asthe development of colloidal or particulate material
40、s is ob-served. Follow the directions on the Certificate of Analysis forpreparing solutions of known pH (5).8.4 Quality Control Sample (QCS)Quality controlsamples of verified pH in an atmospheric wet deposition matrixare to be used. Internally formulated quality control samples(see 8.4.1) may be pre
41、pared by dilutions of strong acids withwater. The pH of such samples must be verified by comparisonwith a NIST traceable low-ionic strength solution of knownpH.8.4.1 Dilute Nitric Acid (5.0 3 105mol/L HNO3)Add1.0 mL of concentrated nitric acid (HNO3, sp gr 1.42) to 0.5 Lwater, dilute to 1 L and mix
42、well. Dilute 3.2 mL of this stocksolution to 1 L with water. The resulting solution has a pH of4.30 6 0.10 at 25C. Store at room temperature in a high-density polyethylene or polypropylene container. Various fac-tors may affect the stability of this solution. Verify the pH ofthis solution with a NIS
43、T traceable standard at monthlyintervals.9. Safety Hazards9.1 The reference buffer solutions, sample types, and mostreagents used in this test method pose no hazard to the analystas used in this test method. Use a fume hood, protectiveclothing, and safety glasses when handling concentrated nitricaci
44、d.9.2 Follow American Chemical Society guidelines regard-ing the safe handling of chemicals used in this test method (6).10. Sample Collection, Preservation, and Storage10.1 Collect samples in high-density polyethylene (HDPE)containers that have been thoroughly rinsed with water. Do notuse strong mi
45、neral acids or alkaline detergent solutions forcleaning collection vessels. Residual acids may remain in thepolyethylene matrix and slowly leach back into the sample.Alkaline detergents may also leave residues that may affect thesample chemistry. Cap collection containers after cleaning toprevent co
46、ntamination from airborne contaminants; air drycollection containers in a laminar flow clean air work stationand wrap in polyethylene bags prior to use. If a laminar flowwork station is not available, pour out any residual rinse waterand bag or cap the containers immediately. Do not dry thecontainer
47、s interior by any method other than air drying in alaminar flow clean air work station.10.2 The frequency of sample collection and the choice ofsampler design are dependent on the monitoring objectives.Guide 5111 further discusses some of these issues. In general,the use of wet-only samplers is reco
48、mmended to exclude drydeposition contributions, minimize sample contamination, re-tard evaporation, and enhance sample stability. Sample collec-tion frequency may vary from subevent to monthly samplingperiods. Collection periods of more than one week are notrecommended since sample integrity may be
49、compromised bylonger exposure periods.10.3 The dissolution of particulate materials and the pres-ence of microbial activity will affect the stability of hydrogenions (pH) in wet deposition samples (7,8). This instabilitygenerally results in a decrease in hydrogen ions (higher pH).Refrigeration of samples at 4C will minimize but will notprevent a change in the hydrogen ion content.10.3.1 A biocide such as chloroform (CHCl3) may be usedto stabilize the organic acid component of the measured pH and4Reagent Chemicals, American Chemical Society Specification
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