1、Designation: D 4517 04Standard Test Method forLow-Level Total Silica in High-Purity Water by FlamelessAtomic Absorption Spectroscopy1This standard is issued under the fixed designation D 4517; the number immediately following the designation indicates the year oforiginal adoption or, in the case of
2、revision, the year 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. Scope*1.1 This test method covers the determination of total silicain water.1.2 This test method is appl
3、icable in the range from 25 to250 g/L of silica as SiO2. Higher concentrations may bedetermined by decreasing the aliquot volume (see Note 6).Concentration range should not be extended by dilution.1.3 This test method determines total silica, and does notdistinguish between soluble and insoluble for
4、ms.1.4 This test method was tested on reagent water only. It isthe users responsibility to assure the validity of the testmethod for waters of other matrices.1.5 This standard does not purport to address the safetyconcerns, if any, associated with its use. It is the responsibilityof the user of this
5、 standard to establish appropriate safety andhealth practices and determine the applicability of regulatorylimitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D 859 Test Method for Silica in WaterD 1066 Practice for Sampling SteamD 1129 Terminology Relating to WaterD 1192 Guide for E
6、quipment for Sampling Water andSteam in Closed Conduits3D 1193 Specification for Reagent WaterD 2777 Practice for Determination of Precision and Bias ofApplicable Methods of Committee D19 on WaterD 3370 Practices for Sampling Water from Closed ConduitsD 3919 Practice for Measuring Trace Elements in
7、Water byGraphite Furnace Atomic Absorption SpectrophotometryD 4453 Practice for Handling of Ultra-Pure Water SamplesD 5810 Guide for Spiking into Aqueous SamplesD 5847 Practice for Writing Quality Control Specificationsfor Standard Test Methods for Water Analysis3. Terminology3.1 DefinitionsFor defi
8、nitions of terms used in this testmethod, refer to Terminology D 1129.4. Summary of Test Method4.1 Total silica is determined using an atomic absorptionspectrophotometer in conjunction with a graphite furnace. Asample is placed in a graphite tube, evaporated to dryness,charred, and atomized. Since t
9、he graphite furnace uses thesample much more efficiently than flame atomization, thedetection of low concentrations of elements in small samplevolumes is possible. Finally, the absorption signal duringatomization is recorded and compared to standards. A generalguide for the application of the graphi
10、te furnace is given inPractice D 3919. Pretreatment of the graphite tube may be usedto enhance the sensitivity and repeatability, or both, of the test.44.2 Total silica is determined using a freshly ultrasonicallytreated and shaken aliquot of sample.4.3 This test method determines low-level total si
11、lica inhigh purity water. Refer to Test Method D 859, Method B, fordetermination of molybdate-reactive silica.5. Significance and Use5.1 Control of silica in boiler feedwater and boiler water isnecessary to minimize the formation of scale-forming silicatesthat decrease heat transfer in the boiler. V
12、olatilization andcarryover of silica with the steam may cause hard, glassysiliceous deposits to form on turbine blades that reduce turbineefficiency.5.2 Colloidal silica that is not removed by boiler waterpretreatment processes may be solubilized in the boiler and1This test method is under the juris
13、diction of ASTM Committee 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 May 1, 2004. Published May 2004. Originallyapproved in 1985. Last previous editio
14、n approved in 1985 as D 4517 85 (1999).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.3Withdrawn.4Rawa, Judi
15、th A. and Earl L. Henn, “Determination of Trace Silica in IndustrialProcess Waters by Flameless Atomic Absorption Spectrometry,” Analytical Chem-istry, Vol 51, March 1979.1*A Summary of Changes section appears at the end of this standard.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C7
16、00, West Conshohocken, PA 19428-2959, United States.thus contribute to the dissolved silica concentration in theboiler. Both dissolved and total silica are of interest.6. Interferences6.1 For a complete discussion of general interferences withfurnace procedures, the analyst is referred to Practice D
17、 3919.6.2 Relatively pure waters such as demineralizer effluent,condensate, or high-pressure boiler feedwater are of sufficientpurity to minimize potential interferences such as sodium,orthophosphate, or sulfate.6.3 Graphite tube pretreatment with the calcium/lanthanumreagent has been found to impro
18、ve the silica response withsome graphite tube materials or designs. Since responseproblems may include memory effects as well as poor sensi-tivity, the following procedure must be used to determinewhether pretreatment is needed.6.3.1 Following instrument set-up and blank determinationas described in
19、 10.1-10.3, inject 10 to 12 replicates of astandard containing 100 to 150 g/L of SiO2. Treat the tube inaccordance with 6.4, and repeat. Note whether the response ofthe two sets is constant, indicating no memory effect, orwhether the first set shows increasing response. Note alsowhether the response
20、 of the second set is greater than that ofthe first. If either sensitivity of repeatability is improved, tubepretreatment is recommended.6.4 Pretreatment may be accomplished as follows:6.4.1 Inject into the furnace the diluted calcium/lanthanumreagent using the maximum aliquot recommended by themanu
21、facturer of the tube, and start the atomization program.Repeat three times, or more if necessary. Run blank atomiza-tion cycles until the furnace blank is constant.6.4.2 Proceed to calibrate the furnace and analyze samplesin accordance with Sections 10 and 11. The absorbanceenhancement may deteriora
22、te after numerous injections, inwhich case retreatment will be required. Refer to PracticeD 3919.7. Apparatus7.1 Atomic Absorption Spectrophotometer, for use at 251.6nm.NOTE 1A wavelength other than 251.6 nm may be used if it has beendetermined to be equally suitable.NOTE 2The manufacturers instruct
23、ions should be followed for allinstrument parameters.7.2 Silicon Light Source, silicon hollow cathode lamp.7.3 Graphite Furnace, capable of reaching temperaturessufficient to atomize the element of interest. Atomizationtemperature must be reached as rapidly as possible for maxi-mum sensitivity.NOTE
24、3Special furnace accessories are commercially available toprovide a means of accelerating atomization temperature.7.4 Graphite Tubes, compatible with furnace device. Stan-dard graphite tubes are preferred, particularly when tubepretreatment is practiced. Pyrolytic graphite tubes may be usedif it has
25、 been determined by the analyst that the precision, bias,and sensitivity of the determination are not compromised.7.5 Pipets:7.5.1 Microlitre, with disposable, polypropylene tips. Sizesmay range from 1 to 1000 L, as required.7.5.2 Millilitre, 50.0 mL.7.6 Data SystemFast transient signal data are col
26、lectedand processed using an internal microprocessor or externaldesktop computer systems. Data can be stored on disks,transmitted to central servers, or printed in hard copy. Datamay be evaluated and processed using the instruments dedi-cated systems to determine analyte concentrations. Users ofthis
27、 practice may use a strip chart recorder to obtain sample andcalibration data, if desired.7.7 Ultrasonic Cleaner, operable at 20 KHz.7.8 Flasks, volumetric, 100 and 1000-mL polyethylene.8. Reagents and Materials8.1 Purity of ReagentsReagent grade chemicals shall beused in all tests. Unless otherwise
28、 indicated, it is intended thatall reagents shall conform to the specifications of the Commit-tee on Analytical Reagents of the American Chemical Societywhere such specifications are available.5Other grades may beused, provided it is first ascertained that the reagent is ofsufficiently high purity t
29、o permit its use without lessening theaccuracy of the determination.8.2 Purity of Water All references to reagent water in thismethod shall be understood to conform to SpecificationD 1193, for reagent water Type II. In addition, the reagentwater shall be made silica-free and determined as such inacc
30、ordance with this method. The collecting apparatus andstorage containers for the reagent water must be made ofsuitable materials that will not contaminate the reagent waterwith silica (see Section 9).NOTE 4Silica-free water may be prepared by distillation, demineral-ization, and passage through a 0.
31、22 m filter. Total removal of colloidalsilica may be done by treating the above water with hydrofluoric acid,letting it stand for 24 h, and then processing it through strong baseion-exchange resin in the hydroxide form.8.3 Calcium/Lanthanum ReagentWet 2.90 g of lanthanumoxide (La2O399.99 % pure) wit
32、h about 50 mL of water in a100 mL volumetric flask. Carefully add 5.5 mL of concentratednitric acid (HNO3sp gr 1.42) and swirl to dissolve. Add 0.100g of calcium oxide (CaO) and swirl to dissolve. Dilute tovolume with water. This solution contains 100 mg/L CaO and2500 mg/L La. Dilute 1:100 for injec
33、tion. The diluted reagentis needed only for pretreatment of the graphite tube asdescribed in 6.4.8.4 Silica Stock Solution (1 mL = 1 mg SiO2) Dissolve 4.7g of sodium metasilicate (Na2SiO39H2O) and dilute to 1 Lwith water. Determine the silica concentration of this solutionin accordance with Test Met
34、hod D 859, Method A.8.5 Silica Working Solution (1 mL = 0.05 mg SiO2)Dilute50.0 mL of the silica stock solution (see 8.4) to 1 L with water5Reagent Chemicals, American Chemical Society Specifications, AmericanChemical Society, Washington, DC. For suggestions on the testing of reagents notlisted by t
35、he American Chemical Society, see Analar Standards for LaboratoryChemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeiaand National Formulary, U.S. Pharmaceutical Convention, Inc. (USPC), Rockville,MD.D 4517 042in a volumetric flask. The concentration of this diluted standardis
36、 calculated after confirming the concentration of the stocksolution.8.6 Argon, standard, welders grade, commercially avail-able.9. Sampling9.1 Collect the sample in accordance with the applicableASTM standards: Practice D 1066, Specification D 1192, andPractices D 3370.9.2 Since silica is such a ubi
37、quitous material, meticuloussample collection, handling, and injection into the furnace arenecessary to avoid contamination.9.2.1 Collect samples only in plastic or TFE-fluorocarboncontainers. Polystyrene, polypropylene, linear polyethylene,and TFE-fluorocarbon have been found to be generally suit-a
38、ble, but tests should be conducted on sample containers beforeuse to determine their suitability.9.2.2 Sample containers must be ultrasonically cleanedbefore use. Fill the sample containers with water and immersein an operating ultrasonic cleaner for at least 1 min. Remove,shake vigorously, and drai
39、n. Additional cycles of cleaning maybe found necessary.9.2.3 Before actual sample collection, rinse sample con-tainer with the sample it is to contain three times with vigorousshaking, then fill. Do not allow any object to touch the insideof the bottle or cap.10. Calibration10.1 Prepare standards fo
40、r test calibration containing about0, 50, 100, 150, and 250 g/L of SiO2by diluting 0, 100, 200,300, and 500 L of the silica working solution (see 8.4)to100mL with water in separate volumetric flasks.NOTE 5The exact concentration of the prepared silica standards isdetermined from the result obtained
41、by the gravimetric standardization ofthe stock solution in 8.4.10.2 Zero the instrument without making an injection orinitiating an atomization program.10.3 Set the atomization program (drying, charring, atom-izing) according to the manufacturers instructions.NOTE 6While sample size and furnace prog
42、ramming vary amonginstruments, the following analytical conditions have generally beenfound to be suitable. It is up to the analyst to optimize analyticalconditions for the instrument used.AliquotAn aliquot size of 10 to 100 L, depending on tube size andconcentration of sample. The same aliquot size
43、 must be used for allstandard, sample, and blank injections.Function Temperature, C Time, sDry 100 1 s/LChar 1000 30Atomize 2800 710.4 Determine the furnace blank by initiating the atomiza-tion program without making an injection. Repeat until thefurnace blank reproduces within 10 %.NOTE 7Determinat
44、ion of the furnace blank is a mechanism forevaluating the acceptability of the graphite tube for silica determination.10.5 Inject a measured aliquot of the 100 g/L of SiO2standard solution (see 10.1) into the furnace, and initiate thepresent atomization program. Repeat this analysis until therespons
45、e reproduces within 10 %.10.6 Determine the acceptability of the water by injecting ameasured aliquot of the “0” standard solution into the furnaceand initiating the preset atomization program. Repeat thisanalysis until the response reproduces within 10 %.10.7 Inject a measured aliquot of each of th
46、e remainingstandard solutions as described in 10.5.10.8 Prepare a calibration curve by plotting absorbance orpeak height versus concentration as micrograms per litre ofSiO2on linear paper if direct readout in terms of concentrationis not possible with the instrument.11. Procedure11.1 Rinse the micro
47、litre pipet with water and inject analiquot of water to test pipet and furnace cleanliness beforesample analysis. After samples containing a high concentrationof silica are analyzed, additional atomization cycles may beneeded to reduce the furnace blank to prior level.11.2 Ultrasonicate for at least
48、 1 min and then vigorouslyshake sample. Immediately withdraw an aliquot (refer to 10.3and Note 6) with the microlitre pipet. Do not touch pipet tip toany object.11.3 Analyze sample as described in 10.2-10.4.NOTE 8If the total silica concentration of the sample is below thedetection limit with the no
49、rmal maximum injection volume for theinstrument used, multiple injections may be used as follows. Inject analiquot, start program. Stop at completion of drying step. Repeat asrequired. Allow program to run to completion when enough sample hasbeen injected. Both precision and bias will decrease when this techniqueis used.12. Calculation12.1 Sample concentration is read directly from calibrationcurve prepared in 10.8. If multiple injections are used asdiscussed in Note 8, divide the result by the number ofinjections.13. Precision and Bias613.1 The single
