1、Designation: G51 95 (Reapproved 2012)G51 18Standard Test Method forMeasuring pH of Soil for Use in Corrosion Testing1This standard is issued under the fixed designation G51; the number immediately following the designation indicates the year of originaladoption or, in the case of revision, the year
2、of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers a procedure for determining the pH of a soil in corrosion testing. The principle use of the tes
3、t isto supplement soil resistivity measurements and thereby identify conditions under which the corrosion of metals in soil may beaccentuated (see G5778(1984). 78 (2012).1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.1.3
4、This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine theapplicability of regulatory limitations prior
5、 to use.1.4 This international standard was developed in accordance with internationally recognized principles on standardizationestablished in the Decision on Principles for the Development of International Standards, Guides and Recommendations issuedby the World Trade Organization Technical Barrie
6、rs to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:2D1193 Specification for Reagent WaterE177 Practice for Use of the Terms Precision and Bias in ASTM Test MethodsE691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test MethodG57 Test Method for F
7、ield Measurement of Soil Resistivity Using the Wenner Four-Electrode MethodNACE/ASTM G193 Terminology and Acronyms Relating to CorrosionG215 Guide for Electrode Potential Measurement3. Terminology3.1 Definitions:3.1.1 calibration solution, na commercially available solution with a stable pH used for
8、 calibrating a pH measuring system(meter and probe).3.1.2 calomel electrode, nan electrode that develops a standard electric potential and is used to provide a reference voltagein the circuit for sensing electrodes. It is composed of an amalgam of mercury and mercury (I) chloride.3.1.2.1 DiscussionD
9、ue to shipping restrictions on mercury containing materials, calomel electrodes have been replaced by silver/silver chlorideelectrodes in newer pH meters.3.1.3 combination electrode, nan electrochemical apparatus that incorporates an ion-selective electrode and a referenceelectrode in a single assem
10、bly thereby avoiding the need for a separate reference electrode.3.1.4 pH, nthe negative logarithm of the hydrogen ion activity written as pH = log10 (aH+) where aH+ = Hydrogen ion activity= the molar concentration of hydrogen ions multiplied by the mean ion-activity coefficient.1 This test method i
11、s under the jurisdiction of ASTM Committee G01 on Corrosion of Metals and is the direct responsibility of Subcommittee G01.10 on Corrosion inSoils.Current edition approved May 1, 2012Nov. 1, 2018. Published June 2012December 2018. Originally approved in 1977. Last previous edition approved in 200520
12、12 asG5195(2005).G51 95(2012). DOI: 10.1520/G0051-95R12.10.1520/G0051-18.2 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of ASTM Standardsvolume information, refer to the standards Document Summary page on the A
13、STM website.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consu
14、lt prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.1.5 pH meter, nan instrument with
15、a display of pH units. For field use, it is portable and battery-powered.3.1.6 purified water, nwater that meets Specification D1193, Type IV requirements.3.1.7 silver/silver chloride electrode, nan electrode that develops a standard electric potential and is used to provide areference voltage. The
16、electrode functions as a redox electrode and the equilibrium is between the silver metal (Ag) and it salt Silver Chloride (AgCl).3.1.7.1 DiscussionRefer to detailed description and discussion in Guide G215.3.2 The terminology used herein, if not specifically defined otherwise, shall be in accordance
17、 with NACE/ASTM G193.Definitions provided herein and not given in NACE/ASTM G193 are limited only to this standard.4. Significance and Use4.1 Information on pH of soil is used as an aid in evaluating the corrosivity of a soil environment. Some metals are moresensitive to the pH of their environment
18、than others, and information on the stability of a metal as a function of pH and potentialis available in the literature.35. Apparatus5.1 pH MetersMeterA portable, battery-powered pH meter that reads or attaches the electrodes is necessary for fieldmeasurements. Most instruments can also function as
19、 a high-impedance voltmeter. An LCD display An LCD (liquid-crystaldisplay) is preferred for its readability in a bright, outdoor environment.5.2 Calomel and Glass Electrodes:5.2.1 Silver/Silver Chloride ElectrodeUse a saturated calomel A saturated silver/silver chloride reference electrode or itsequ
20、ivalent is used in the pH determination.Afew crystals of solid potassium chloride should always be present within the chambersurrounding the calomel to assure that the solution is saturated under the conditions of use. The design of the electrode must permitthe formation of a fresh liquid junction b
21、etween the solution of potassium chloride and the buffer or test soil for each test and allowtraces of soil to be readily removed by washing.NOTE 1A few crystals of solid potassium chloride should always be present within the chamber surrounding the silver to assure that the solution issaturated und
22、er the conditions of use. The design of the electrode must permit the formation of a fresh liquid junction between the solution of potassiumchloride and the buffer or test soil for each test and allow traces of soil to be readily removed by washing.5.2.2 Glass ElectrodeA glass electrode of rugged co
23、nstruction is required. The performance of the glass electrode issatisfactory if it furnishes the correct pH value (60.1 pH unit) for standard buffered solutions.5.2.3 Combination ElectrodeA combination electrode consisting of a saturated calomel reference electrode and a glasselectrode (4.2.15.2.1
24、and 4.2.25.2.2) combined as a single electrode is acceptable. However, the requirements outlined above areequally applicable to the electrodes used in this combination unit.5.3 Subsurface ProbeWhen pH measurements below the surface of the soil are required, it is necessary to use a probe ofsuitable
25、length which will allow measurements to be made at the depth of interest. This probe consists of a glass electrode or acombination electrode in a rubber housing at the end of a plastic tube. One type of probe is illustrated in Fig. 1.3 Pourbaix, M., Atlas of Electrochemical Equilibria in Aqueous Sol
26、utions, Pergamon Press, 1966.FIG. 1 Subsurface ProbeG51 1825.4 Soil ThermometerSome pH electrodes have temperature compensation built in as part of the pH electrode, but most donot (see manufacturers specifications).Athermometer of rugged construction is required for soil use, and a stainless steel
27、sheathedthermometer is preferred. Metal sheathed thermometers come in different lengths, and a length appropriate for the depth of interestshould be chosen.5.5 MiscellaneousDepending on how and where the soil will be analyzed, one or more of the following may be needed: handauger, post-hole diggers,
28、 sample bags, glass container, etc.6. Reagents and Materials6.1 During the calibration procedure for the pH meter, standard buffered solutions of known pH are necessary. These solutions,or tablets to make up these solutions, can be purchased from chemical supply companies or pH equipment manufacture
29、rs. Frompractical experience, standard solutions of pH 4, 7, and 10 are recommended. Reagent grade water such as those described inSpecification D1193 may be required to make solutions.7. SamplingSampling, Test Specimens, and Test Units7.1 By the nature of the measurement, pH is determined for a sma
30、ll volume of soil at each reading, and it is important that atleast three measurements at different locations be made and a simple average calculated. The regions of interest, surface,subsurface, or both, where applicable, must be sampled.7.2 In-situ Soil SamplingIdeally the pH measurement should be
31、 made in the field with the glass electrode contacting the soilat the specific depth of interest. If the surface soil pH is desired, then the soil can be broken up if compacted so as to accept theelectrodes. If a subsurface pH is desired, then a boring or an excavation must be done so that the elect
32、rode can be placed in thesoil at the desired depth. The subsurface probe (Fig. 1) is ideal for use with a boring tool such as hand auger.After boring throughthe soil to the depth of interest, carefully lower the probe into the cavity without further preparation of the soil.7.3 Disturbed Soil Samplin
33、gSoil samples can be brought to the surface with a boring tool or a post-hole digger, and themeasurement made in the field on the soil specimen thus obtained. This technique is less desirable than the probe method describedin 7.2.7.4 Laboratory TestingThe least desirable pH measurement of soil is th
34、at which is based on a soil sample transported to alaboratory for evaluation. However, if this must be done, then make the pH measurement as quickly as possible after the soil istaken from the field. Place the sample in a clean, airtight, glass container or plastic bag so that the soil is not in con
35、tact with anymetal. If the pH measurement is not made within 24 h from the time the soil sample is taken, then it is recommended that the soilsample be packed in dry ice to retard any change in pH due to chemical or biological reactions. Make the pH measurement on thesoil at room temperature and as
36、received.8. Calibration and Standardization8.1 Test for LinearityPrior to field use, or periodically when used extensively in the field, test the apparatus for linearity ofresponse. This procedure is as follows:7.1.1 Turn on the instrument, allow to warm up thoroughly, and bring to electrical balanc
37、e in accordance with themanufacturers instructions. Before use, clean and rinse the glass and calomel electrodes in distilled water.48.1.1 At least two standard buffered pH solutions that span the soil pH to be measured are required. From practical experiencestandard solutions of pH 4, 7, and 8 are
38、recommended. For the test, the temperature of these solutions shall not differ from eachother by more than 5C. A laboratory thermometer can be used for these measurements.8.1.2 Adjust the temperature compensating dial on the pH meter to the standard solution temperature.8.1.3 Immerse the electrodes
39、in a small volume of the first known standard solution. Now adjust the pH meter to read this knownpH.8.1.4 Remove the electrodes from the first standard solution, and rinse in distilledpurified water. Immerse the electrodes in thesecond known standard solution and read the pH value. Judge the system
40、 to be operating satisfactorily if the reading obtained forthe second standard agrees within 60.1 unit of the assigned pH.8.2 Calibration of pH MeterCalibrate the pH meter in accordance with manufacturers recommendations immediately beforeuse. If a series of measurements are to be made, repeat the c
41、alibration procedure at intervals of about 30 min. Perform the pH metercalibration as follows:8.2.1 Use After calibration, use a standard pH solution in the range of the pH of the soil to be tested, if such information isknown beforehand. Otherwise, begin with a standard solution having a pH of 7. S
42、tabilize the temperature of the solution so thatit matches the temperature of the soil to within 10C.7.2.2 Immerse the electrodes in the known standard solution and calibrate the meter in accordance with the manufacturersinstructions.9. Procedure9.1 Soil Preparation for pH Determination:G51 1838.1.1
43、 Ideally the pH measurement should be made in the field with the glass electrode contacting the soil at the specific depthof interest. If the surface soil pH is desired, then the soil can be broken up if compacted so as to accept the electrodes. If asubsurface pH is desired, then a boring or an exca
44、vation must be done so that the electrode can be placed in the soil at the desireddepth. The subsurface probe (Fig. 1) is ideal for use with a boring tool such as hand auger. After boring through the soil to thedepth of interest, carefully lower the probe into the cavity without further preparation
45、of the soil.8.1.2 Soil samples can be brought to the surface with a boring tool or a post-hole digger, and the measurement made in the fieldon the soil thus obtained. This technique is less desirable than the probe method described in this section.9.1.1 The least desirable pH measurement of soil is
46、that which is based on a soil sample transported to a laboratory forevaluation. However, if this must be done, then make the pH measurement as quickly as possible after the soil is taken from thefield. Place the sample in a clean, airtight, glass container or plastic bag so that addition of water to
47、 the soil is not recommendedbecause some soils are so poorly buffered that added moisture may change their pH. When the soil is not in contact with any metal.If the pH measurement is not made within 24 h from the time the soil sample is taken, then it is recommended that the soil samplebe packed in
48、dry ice to retard any change in pH due to chemical or biological reactions. Make the pH measurement on the soilat room temperature and as received.so dry that a pH reading cannot be obtained without the addition of water, only a minimalamount of purified water necessary to obtain a steady reading sh
49、ould be added.8.1.3.1 The addition of water to the soil is not recommended for any case because some soils are so poorly buffered that addedmoisture will change their pH. However, if a soil is extremely dry with a resistivity exceeding 106 -cm, a condition not normallyencountered, then the pH measurement cannot and should not be made. This situation will manifest itself with a random driftingof the pH meter reading.9.1.1.1 If the soil is frozen, it must be thawed prior to making the measurement. Frozen soil manifests itself by random driftin
