1、Designation: D 5462 02Standard 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 This standard does not purport to address
4、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 limitations prior to use. For specific hazardsstatements, see 6.5.2. Referenced Docume
5、nts2.1 ASTM Standards:D 1066 Practice for Sampling Steam2D 1129 Terminology Relating to Water2D 1192 Specification for Equipment for Sampling Waterand Steam in Closed Conduits2D 1193 Specification for Reagent Water2D 2777 Practice for Determination of Precision and Bias ofApplicable Methods of Commi
6、ttee D-19 on Water2D 3370 Practices for Sampling Water from Closed Con-duits2D 3864 Practice for Continual On-Line Monitoring Systemsfor Water Analysis23. Terminology3.1 DefinitionsFor definitions of terms used in this testmethod, refer to Terminology D 1129.3.2 Definitions of Terms Specific to This
7、 Standard:3.2.1 diffusion-type probesgalvanic or polarographic sen-sors that depend on the continuous influx of oxygen throughthe membrane to develop the measurement signal.3.2.2 equilibrium-type probesmodified polarographicsensing probes that have a negligible influx of oxygen throughthe membrane e
8、xcept 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 that develop an electrical current
9、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 air or dissolved in water.3.2.5 pol
10、arographic 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 electro-chemical cell separated from th
11、e 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 compensatedautomatically to account for variatio
12、ns 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).4.3 Diffusion-type probes rely on a continuous diffusion ofoxygen th
13、rough the membrane. Immediately inside the mem-brane, oxygen is reduced at the noble metal cathode, 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
14、the membrane. The less noble anode, usually silver orlead, completes the circuit and is oxidized in proportion to thecurrent flow. At steady state, the resulting current signal is thenproportional to the oxygen partial pressure of the sample.1This test method is under the jurisdiction of ASTM Commit
15、tee D19 on Waterand is the direct responsibility of Subcommittee D19.03 on Sampling of Water andWater-Formed Deposits, Surveillance of Water, and Flow Measurement of Water.Current edition approved June 10, 2002. Published August 2002. Originallypublished as D 5462 93. Last previous edition D 5462 93
16、 (01).2Annual Book of ASTM Standards, Vol 11.01.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.Thorough descriptions of diffusion-type probes are given byHitchman (1)3and Fatt (2).4.4 Equilibrium-type probes rely on oxygen diffusion
17、through the membrane 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,an
18、d the resulting oxidation reaction producesoxygen. This is the exact reverse of the reaction at the cathode,so there is no net consumption of oxygen by the probe. Itreaches equilibrium in constant DO samples, and no netoxygen diffuses through the membrane. Accuracy is notdependent on membrane surfac
19、e condition or sample flow-rate.45. 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 ma
20、y cause corrosion in boiler systems, which leadsto corrosion fatigue and corrosion productsall detrimental tothe life and efficient operation of a power unit. The efficiencyof DO removal from boiler feedwater by mechanical orchemical means, or both, may be monitored by continuouslymeasuring the DO c
21、oncentration 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.
22、3 Boiler feedwater 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 in
23、to samples is 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. Section 9 provides furtherdetails on this non-obvious inte
24、rference.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 manufacturers minimum flowrate recommenda-tions must be met or exceeded in order to prevent erroneouslylow
25、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 notsubject to errors from flowrate or coating.)6.4 Calibration must be corrected for barometric pressureaccording
26、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 conditions without compensation wouldresult in positive measurement errors.6.5 The growth of bacteria in sample lin
27、es 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 beperformed if errors from bacteria growth are suspected.(WarningDo not mix hydrochloric acid and sodium hy
28、-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 continued reaction of the two. With long samplelines, the DO measured at the probe may be significantly belowthat
29、 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 probemembrane may cause unwanted reactions at the electrodes andnegative errors. The magnitude of errors d
30、epends on therelative concentrations of DO and the oxygen scavenger orsuppressant as well as the type of electrochemical 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 t
31、o avoid samplinginterferences.6.9 Iron oxides and other deposits accumulate in slow-flowing horizontal sample lines and can develop3The boldface numbers in parentheses refer to the list of references at the end ofthis test method.4Leeds (2) electrochemical cell output; and(3) when necessary, diffusi
32、on 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 andfrom
33、 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 sample with
34、a 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 atmos
35、phere, 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 that it
36、 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. Equilibrium-type
37、 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 clean
38、ing 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 the s
39、pecifications of the Commit-tee on Analytical Reagents of the American Chemical Society,where such specifications are available.5Other grades may beused, provided it is first ascertained that the reagent is ofsufficiently high purity to permit its use without lessening theaccuracy of the determinati
40、on.8.2 Purity of Water Unless otherwise indicated, refer-ences to water shall be understood to mean reagent water asdefined by Type I of Specification D 1193.8.3 Hydrochloric Acid (1 + 44)Add 1 volume of concen-trated HCl (sp gr 1.19) to 44 volumes of water and mix.8.4 Sodium Hypochlorite (10 mg/L)A
41、dd 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 Zero Solution (10 g/200 mL)Dissolve10 g of sodium sulfite
42、 (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 Pra
43、ctices D 1066, D 3370, and D 3864 andSpecification D 1192.9.2 Use sample lines of compatible materials. Do not usecopper because it can oxidize and consume oxygen. Thepreferred materials are 316SS followed by PVDF and Nylon.Most other plastic and rubber tubing materials are gas perme-able and allow
44、significant oxygen diffusion into the sample. Adiscussion of suitable sampling materials for this purpose isgiven by Carr (12).9.3 Maintain a continuous, stable flowrate to enable thesample line to reach equilibrium with the sample conditions.Measurements following changes to the sample flowrate ort
45、emperature may not represent actual process conditions duringthe period of time required to recover from transient effects.9.4 Seal the sample from the atmosphere to prevent oxygenabsorption. Leakage of the equivalent of only one 2-mm-diameter air bubble/min into a sample flowing at 100 mL/minadds a
46、pproximately 11 g/L oxygen to the sample. Test thesample line integrity by observing the measurement understeady state conditions of DO and increasing the sampleflowrate approximately 50 %, but not exceeding the manufac-turers recommendations. A significant decrease in DO readingis usually an indica
47、tion of air leakage since the higher flowratedilutes the leak. (An increase in DO reading could be observedwith a diffusion-type probe and indicates that the originalflowrate was too low, as described in 6.2. The flowrate mustthen be increased until a stable plateau of response is reached.)9.5 In po
48、wer plant installations in which iron oxide andother solids occur in pure water samples, control flowrates tominimize the accumulation of deposits, which could delay the5“Reagent Chemicals, American Chemical Society Specifications,” Am. Chemi-cal Soc., Washington, DC. For suggestions on the testing
49、of reagents not listed bythe American Chemical Society, see “Analar Standards for Laboratory Chemicals,”BDH Ltd., Poole, Dorset, UK, and the “United States Pharmacopeia.”D5462023transport of dissolved materials greatly. A water sample flowvelocity of 1.8 m/s (6 ft/s) in sample lines has been foundoptimum. Additional sample line design criteria have beendocumented in Refs (7-9).10. Calibration10.1 map edit amMeasuring Instrument Evaluate themeasuring instrument calibration and performance using sub-stitute resistors in place of the probe, or by other means
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