1、Designation: D 5462 08Standard Test Method forOn-Line Measurement of Low-Level Dissolved Oxygen inWater1This standard is issued under the fixed designation D 5462; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last re
2、vision. 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 covers the on-line determination ofdissolved oxygen (DO) in water samples primarily in rangesfrom 0 to 500
3、g/L (ppb), although higher ranges may be usedfor calibration. On-line instrumentation is used for continuousmeasurements of DO in samples that are brought throughsample lines and conditioned from high-temperature and high-pressure sources when necessary.1.2 The values stated in SI units are to be re
4、garded 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 and health practices and deter
5、mine the applica-bility of regulatory limitations prior to use. For specific hazardsstatements, see 6.5.2. Referenced Documents2.1 ASTM Standards:2D 1066 Practice for Sampling SteamD 1129 Terminology Relating to WaterD 1193 Specification for Reagent WaterD 2777 Practice for Determination of Precisio
6、n and Bias ofApplicable Test Methods of Committee D19 on WaterD 3370 Practices for Sampling Water from Closed ConduitsD 3864 Guide for Continual On-Line Monitoring Systemsfor Water Analysis3. Terminology3.1 DefinitionsFor definitions of terms used in this testmethod, refer to Terminology D 1129.3.2
7、Definitions of Terms Specific to This Standard:3.2.1 diffusion-type probesgalvanic or polarographic sen-sors that depend on the continuous influx of oxygen throughthe membrane to develop the electrical signal.3.2.2 equilibrium-type probesmodified polarographicsensing probes that have a negligible in
8、flux of oxygen throughthe membrane except during changes of sample DO concen-tration. Oxygen consumption and regeneration balance eachother within the probes under stable conditions, and the net fluxthrough the membrane is insignificant.3.2.3 galvanic systemssensing probes and measuring in-struments
9、 that develop an electrical current from two elec-trodes inside the probe from which the final measurement isderived.3.2.4 partial pressure (of oxygen)the volume fraction ofoxygen multiplied by the total pressure. The partial pressure ofoxygen is the actual parameter detected by DO probes, whetherin
10、 air or dissolved in water.3.2.5 polarographic systemssensing probes and measur-ing instruments that include circuitry to control the operatingvoltage of the system, usually using a third (reference) elec-trode in the probe.4. Summary of Test Method4.1 Dissolved oxygen is measured by means of an ele
11、ctro-chemical cell separated from the sample by a gas-permeablemembrane. Behind the membrane and inside the probe, elec-trodes immersed in an electrolyte develop an electrical currentproportional to the oxygen partial pressure of the sample.4.2 The partial pressure signal is temperature compensateda
12、utomatically to account for variations with temperature of thefollowing: oxygen solubility in water; electrochemical celloutput; and, when necessary, diffusion rate of oxygen throughthe membrane. This yields a direct readout in concentration ofg/L (ppb) or mg/L (ppm).1This test method is under the j
13、urisdiction of ASTM Committee D19 on Waterand is the direct responsibility of Subcommittee D19.03 on Sampling Water andWater-Formed Deposits, Analysis of Water for Power Generation and Process Use,On-Line Water Analysis, and Surveillance of Water.Current edition approved May 1, 2008. Published May 2
14、008. Originallyapproved in 1993. Last previous edition approved in 2002 as D 5462 02.2For 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
15、page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.4.3 Diffusion-type probes rely on a continuous diffusion ofoxygen through the membrane. Immediately inside the mem-brane, oxygen is reduced at the noble metal ca
16、thode, usuallyplatinum or gold. An electrical current is developed that isdirectly proportional to the arrival rate of oxygen molecules atthe cathode, which is in turn dependent on the diffusion ratethrough the membrane. The less noble anode, usually silver orlead, completes the circuit and is oxidi
17、zed in proportion to thecurrent flow.At steady state, the resulting current signal is thenproportional to the oxygen partial pressure of the sample.Thorough descriptions of diffusion-type probes are given byHitchman (1)3and Fatt (2).4.4 Equilibrium-type probes rely on oxygen diffusionthrough the mem
18、brane only until equilibrium between theinside and outside is achieved. Oxygen is reduced at the noblemetal cathode, as with diffusion-type probes. However, themeasuring circuit forces electrical current to flow through thenoble metal anode equal and opposite to that at the cathode,and the resulting
19、 oxidation reaction produces oxygen. This isthe exact reverse of the reaction at the cathode, so there is nonet consumption of oxygen by the probe. It reaches equilibriumin constant DO samples, and no net oxygen diffuses throughthe membrane. Accuracy is not dependent on membranesurface condition or
20、sample flowrate.5. Significance and Use5.1 DO may be either a corrosive or passivating agent inboiler/steam cycles and is therefore controlled to specificconcentrations that are low relative to environmental andwastewater treatment samples. Out-of-specification DO con-centrations may cause corrosion
21、 in boiler systems, which leadsto corrosion fatigue and corrosion productsall detrimental tothe life and efficient operation of a steam generator. Theefficiency of DO removal from boiler feedwater by mechanicalor chemical means, or both, may be monitored by continuouslymeasuring the DO concentration
22、 before and after the removalprocess with on-line instrumentation. DO measurement is alsoa check for air leakage into the boiler water cycle.5.2 Guidelines for feedwater to high-pressure boilers withall volatile treatment generally require a feedwater DO con-centration below 5 g/L (3).5.3 Boiler fee
23、dwater with oxygenated treatment is main-tained in a range of 50 to 300 g/L DO (4).5.4 In microelectronics production, DO can be detrimentalin some manufacturing processes, for example, causing unde-sirable oxidation on silicon wafers.6. Interferences6.1 The leakage of atmospheric air into samples i
24、s some-times difficult to avoid and detect.Although sample line fittingsand connections to flow chambers may be water tight, it is stillpossible for air to diffuse through the water film of a joint tocontaminate a low-g/L sample. Sample flow through fittings,valves and rotometers can create a ventur
25、i effect, which drawambient air into the sample. Section 9 provides further detailson this non-obvious interference.6.2 Diffusion-type probes consume oxygen and will depleteit from the sample in immediate contact with the membranesurface unless an adequate, turbulent sample flow is main-tained. The
26、manufacturers minimum flowrate recommenda-tions must be met or exceeded in order to prevent erroneouslylow readings.6.3 Diffusion-type probes are subject to negative errorsfrom the buildup of coatings such as iron oxides, which impedethe diffusion rate of oxygen. (Equilibrium-type probes are notsubj
27、ect to errors from flowrate or coating.)6.4 Calibration must be corrected for barometric pressureaccording to the manufacturers recommendations at atmo-spheric conditions that deviate from a nominal range of 745 to775 mmHg. See Table 1 for altitude corrections. Calibrationunder low-pressure conditio
28、ns without compensation wouldresult in positive measurement errors.6.5 The growth of bacteria in sample lines and flow cham-bers and on probe membranes can consume oxygen and causenegative errors. Chemical sterilization with hydrochloric acid(1 + 44) or sodium hypochlorite solution (10 mg/L) should
29、beperformed if errors from bacteria growth are suspected.(WarningDo not mix hydrochloric acid and sodium hy-pochlorite since hazardous chlorine gas would be releasedrapidly.)6.6 The passage of high-temperature samples containingboth DO and an oxygen scavenger through hot sample linescan allow contin
30、ued reaction of the two. With long samplelines, the DO measured at the probe may be significantly belowthat at the sample point. Short sample lines and cooling nearthe source are recommended.6.7 Volatile oxygen scavengers or suppressants, such ashydrazine, amines, and hydrogen, that pass through the
31、 probemembrane may cause unwanted reactions at the electrodes andnegative errors. The magnitude of errors depends on the3The boldface numbers in parentheses refer to the list of references at the end ofthis test method.TABLE 1 Solubility of Oxygen (mg/L) at Various Temperaturesand Elevations (Based
32、on Sea Level Barometric Pressure of760 mmHg) (5)Temperature,CElevation, ft above Sea Level0 1000 2000 3000 4000 5000 60000 14.6 14.1 13.6 13.2 12.7 12.3 11.82 13.8 13.3 12.9 12.4 12.0 11.6 11.24 13.1 12.7 12.2 11.9 11.4 11.0 10.66 12.4 12.0 11.6 11.2 10.8 10.4 10.18 11.8 11.4 11.0 10.6 10.3 9.9 9.61
33、0 11.3 10.9 10.5 10.2 9.8 9.5 9.212 10.8 10.4 10.1 9.7 9.4 9.1 8.814 10.3 9.9 9.6 9.3 9.0 8.7 8.316 9.9 9.7 9.2 8.9 8.6 8.3 8.018 9.5 9.2 8.7 8.6 8.3 8.0 7.720 9.1 8.8 8.5 8.2 7.9 7.7 7.422 8.7 8.4 8.1 7.8 7.7 7.3 7.124 8.4 8.1 7.8 7.6 7.3 7.1 6.826 8.1 7.8 7.6 7.3 7.0 6.8 6.628 7.8 7.5 7.3 7.0 6.8
34、6.6 6.330 7.5 7.2 7.0 6.8 6.5 6.3 6.132 7.3 7.1 6.8 6.6 6.4 6.1 5.934 7.1 6.9 6.6 6.4 6.2 6.0 5.836 6.8 6.6 6.3 6.1 5.9 5.7 5.538 6.6 6.4 6.2 5.9 5.7 5.6 5.440 6.4 6.2 6.0 5.8 5.6 5.4 5.2D5462082relative concentrations of DO and the oxygen scavenger orsuppressant as well as the type of electrochemic
35、al cell used.The probe manufacturers cautions and limitations should beconsidered.6.8 New sample lines require conditioning to achieve equi-librium conditions. See Practices D 3370 to avoid samplinginterferences.6.9 Iron oxides and other deposits accumulate in slow-flowing horizontal sample lines an
36、d can developchromatograph-like retention of dissolved species, resulting invery long delay times. Precautions are described in Section 9.6.10 The response time can be slow for large decreases inDO. This is especially true of measurements below 10 g/Lfollowing air calibration, which corresponds to a
37、 concentrationdecrease of 3 to 4 orders of magnitude. Hours may be requiredfor all traces of oxygen to diffuse out of the probe and toachieve accurate measurements at low g/L levels.7. Apparatus7.1 Measuring Instrument:7.1.1 The instrument should have both g/L (ppb) and mg/L(ppm) range capability. I
38、t must have a span calibration adjust-ment to match the readout to the sensitivity of a particularprobe.7.1.2 The direct readout of DO concentration requirestemperature compensation for effects of the following: (1)oxygen solubility in water; (2) electrochemical cell output; and(3) when necessary, d
39、iffusion rate of oxygen through themembrane. During air calibration, the instrument must disablethe oxygen solubility portion of the compensation to respondonly to partial pressure.7.1.3 If included, electrical output signal(s) from the instru-ment must be isolated from the probe measuring circuit a
40、ndfrom earth ground in order to prevent ground loop problemswhen the instrument is connected to grounded external devices.7.2 Probe:7.2.1 Diffusion-type probes use galvanic or polarographicsystems, with a noble metal cathode and oxidizable anodeimmersed in an electrolyte and separated from the sampl
41、e witha polyethylene or fluorocarbon gas-permeable membrane.7.2.2 Equilibrium-type probes are similar to polarographicprobes, except that both the anode and cathode are platinumand the anode is not oxidized.7.2.3 A sealed flow-through probe configuration must beused to prevent contamination from the
42、 atmosphere, as de-scribed in 6.1. The flowrate must be maintained within themanufacturers recommendations. The probe must be capableof withstanding the flowrate, temperature, and pressure condi-tions of the installation. The probe must incorporate an integralprecision temperature sensor to ensure t
43、hat it senses the sampletemperature at which the DO is being detected in order toensure accurate temperature compensation with fast response.7.2.4 Diffusion-type probes must have their electrodes,electrolyte, and membrane serviced or replaced according tothe manufacturers recommendations. Equilibriu
44、m-type probesdo not require internal maintenance.7.2.5 Probe membranes must be cleaned per the manufac-turers recommendations. The cleaning frequency is deter-mined by experience with the particular sample and must besufficient to maintain acceptable accuracy with diffusion-typeprobes (see 6.3). The
45、 cleaning of equilibrium-type probes is notnecessary unless a heavy coating increases response time orbecomes biologically active (see 6.5).8. Reagents8.1 Purity of ReagentsReagent grade chemicals shall beused in all tests. Unless otherwise indicated, it is intended thatall reagents shall conform to
46、 the specifications of the Commit-tee on Analytical Reagents of the American Chemical Society,where such specifications are available.4Other grades may beused, provided it is first ascertained that the reagent is ofsufficiently high purity to permit its use without lessening theaccuracy of the deter
47、mination.8.2 Purity of WaterReferences to water shall be under-stood to mean water that meets or exceeds the quantitativespecifications for type III reagent water conforming to Speci-fication D 1193, Section 1.1.8.3 Hydrochloric Acid (1 + 44)Add 1 volume of concen-trated HCl (sp gr 1.19) to 44 volum
48、es of water and mix.8.4 Sodium Hypochlorite (10 mg/L)Add approximately0.05 mL (1 drop) of 5 % NaOCl solution (commercial bleachis satisfactory for this purpose) to 250 mL of water.8.5 Cobalt Chloride Solution, SaturatedDissolve 4.5 g ofcobalt chloride (CoCl2) in 10 mL of water.8.6 Sodium Sulfite Zer
49、o Solution (10 g/200 mL)Dissolve10 g of sodium sulfite (Na2SO3) in 200 mL of water.NOTE 1To attain zero DO more rapidly, add two drops of saturatedcobalt chloride solution to the sodium sulfite zero solution.9. Sampling9.1 Design and operate the sample lines to maintain sampleintegrity and fast response. Follow the applicable samplingprecautions in Practices D 1066, D 3370, and D 3864.9.2 The preferred permanent material for sample lines is316SS. Higher alloys in sample coolers may be required forhigh chloride cooling waters. PVDF and Nylon can be used fo
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