1、Designation: E 60 98 (Reapproved 2004)Standard Practice forAnalysis of Metals, Ores, and Related Materials byMolecular Absorption Spectrometry1This standard is issued under the fixed designation E 60; the number immediately following the designation indicates the year of originaladoption or, in the
2、case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscriptepsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers general recommendations for pho-toelectric photometers and spectro
3、photometers and for photo-metric practice prescribed in ASTM methods for chemicalanalysis of metals, sufficient to supplement adequately theASTM methods. A summary of the fundamental theory andpractice of photometry is given. No attempt has been made,however, to include in this practice a descriptio
4、n of everyapparatus or to present recommendations on every detail ofpractice in ASTM photometric or spectrophotometric methodsof chemical analysis of metals.21.2 These recommendations are intended to apply to theASTM photometric and spectrophotometric methods forchemical analysis of metals when such
5、 standards make definitereference to this practice, as covered in Section 4.1.3 In this practice, the terms “photometric” and “photom-etry” encompass both filter photometers and spectrophotom-eters, while “spectrophotometry” is reserved for spectropho-tometers alone.1.4 This standard does not purpor
6、t 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 determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3E 131
7、 Terminology Relating to Molecular SpectroscopyE 168 Practices for General Techniques of Infrared Quanti-tative AnalysisE 169 Practices for General Techniques of Ultraviolet-Visible Quantitative AnalysisE 275 Practice for Describing and Measuring Performanceof Ultraviolet, Visible, and Near Infrared
8、 Spectrophotom-eters3. Terminology Definitions and Symbols3.1 For definitions of terms relating to absorption spectros-copy, refer to Terminology E 131.3.2 background absorptionany absorption in the solutiondue to the presence of absorbing ions, molecules, or complexesof elements other than that bei
9、ng determined is called back-ground absorption.3.3 concentration rangethe recommended concentrationrange shall be designated on the basis of the optical path of thecell, in centimetres, and the final volume of solution asrecommended in a procedure. In general, the concentrationrange and path length
10、shall be specified as that which willproduce transmittance readings in the optimum range of theinstrument being used as covered in Section 14.3.4 initial setting the initial setting is the photometricreading (usually 100 on the percentage scale or zero on thelogarithmic scale) to which the instrumen
11、t is adjusted with thereference solution in the absorption cell. The scale will thenread directly in percentage transmittance or in absorbance.3.5 photometric readingthe term “photometric reading”refers to the scale reading of the instrument being used.Available instruments have scales calibrated in
12、 transmittance,T, (1)4or absorbance, A, (2) (see 5.2), or even arbitrary unitsproportional to transmittance or absorbance.3.6 reagent blank the reagent blank determination yieldsa value for the apparent concentration of the element sought,which is due only to the reagents used. It reflects both thea
13、mount of the element sought present as an impurity in thereagents, and the effect of interfering species.3.7 reference solutionphotometric readings consist of acomparison of the intensities of the radiant energy transmittedby the absorbing solution and the radiant energy transmitted by1This practice
14、 is under the jurisdiction of ASTM Committee E01 on AnalyticalChemistry for Metals, Ores and Related Materials and is the direct responsibility ofSubcommittee E01.20 on Fundamental Practices.Current edition approved May 1, 2004. Published June 2004. Originallyapproved in 1946. Last previous edition
15、approved in 1998 as E60 98.2For additional information on the theory and photoelectric photometry, see thelist of references at the end of this practice.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMSt
16、andards volume information, refer to the standards Document Summary page onthe ASTM website.4The boldface numbers in parentheses refer to the list of references appended tothis practice.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States
17、.the solvent. Any solution to which the transmittance of theabsorbing solution of the substance being measured is com-pared shall be known as the reference solution.4. Reference to This Practice in Standards4.1 The inclusion of the following paragraph, or a suitableequivalent, in any ASTM test metho
18、d (preferably after thesection on scope) shall constitute due notification that thephotometers, spectrophotometers, and photometric practiceprescribed in that test method are subject to the recommenda-tions set forth in this practice.“Photometers, Spectrophotometers, and PhotometricPracticePhotomete
19、rs, spectrophotometers, and photometricpractice prescribed in this test method shall conform to ASTMPractice E 60, Photometric and Spectrophotometric Methodsfor Chemical Analysis of Metals.”5. Theory5.1 Photoelectric photometry is based on Bouguers andBeers (or the Lambert-Beer) laws which are combi
20、ned in thefollowing expression:P 5 Po102abcwhere:P = transmitted radiant power,Po= incident radiant power, or a quantity proportional to it,as measured with pure solvent in the beam,a = absorptivity, a constant characteristic of the solutionand the frequency of the incident radiant energy,b = intern
21、al cell length (usually in centimetres) of thecolumn of absorbing material, andc = concentration of the absorbing substance, g/L.5.2 Transmittance, T, and absorbance, A, have the followingvalues:T 5 P/PoA 5 log101/T! 5 log10Po/P!where P and Pohave the values given in 5.1.5.3 From the transposed form
22、 of the Bouguer-Beer equa-tion, A = abc, it is evident that at constant b, a plot of A versusc gives a straight line if Beers law is followed. This line willpass through the origin if the usual practice of cancelling outsolvent reflections and absorption and other blanks is em-ployed.5.4 In photomet
23、ry it is customary to make indirect compari-son with solutions of known concentration by means ofcalibration curves or charts. When Beers law is obeyed andwhen a satisfactory instrument is employed, it is possible todispense with the curve or chart. Thus, from the transposedform of the Bouguer-Beer
24、law, c = A/ab, it is evident that oncea has been determined for any system, c can be obtained, sinceb is known and A can be measured.5.5 The value for a can be obtained from the equationa = A/cb by substituting the measured value of A for a given band c. Theoretically, in the determination of a for
25、an absorbingsystem, a single measurement at a given wavelength on asolution of known concentration will suffice. Actually, how-ever, it is safer to use the average value obtained with three ormore concentrations, covering the range over which the deter-minations are likely to be made and making seve
26、ral readings ateach concentration. The validity of the Bouguer-Beer law for aparticular system can be tested by showing that a remainsconstant when b and c are changed.APPARATUS6. General Requirements for Photometers andSpectrophotometers6.1 A photoelectric photometer consists essentially of thefoll
27、owing:NOTE 1The choice of an instrument may naturally be based on priceconsiderations, since there is no point in using a more elaborate (and,incidentally, more expensive) instrument than is necessary. In addition tosatisfactory performance from the purely physical standpoint, the instru-ment should
28、 be compact, rugged enough to stand routine use, and notrequire too much manipulation. The scales should be easily read, and theabsorption cells should be easily removed and replaced, as the clearing,refilling, and placing of the cells in the instrument consume a majorportion of the time required. I
29、t is advantageous to have an instrument thatpermits the use of cells of different depth (see Recommended PracticeE 275).6.1.1 An illuminant (radiant energy source),6.1.2 A device for selecting relatively monochromatic radi-ant energy (consisting of a diffraction grating or a prism withselection slit
30、, or a filter),6.1.3 One or more absorption cells to hold the sample,standards, reagent blank, or reference solution, and6.1.4 An arrangement for photometric measurement of theintensity of the transmitted radiant energy, consisting of one ormore photocells or photosensitive tubes, and suitable devic
31、esfor measuring current or potential.6.2 Precision instruments that employ monochromators ca-pable of supplying radiant energy of high purity at any chosenwavelength within their range are usually referred to asspectrophotometers. Instruments employing filters are knownas filter photometers or abrid
32、ged spectrophotometers, andusually isolate relatively broad bands of radiant energy. In mostcases the absorption peak of the compound being measured isrelatively broad, and sufficient accuracy can be obtained usinga fairly broad band (10 to 75 nm) of radiant energy for themeasurement (Note 2). In ot
33、her cases the absorption peaks arenarrow, and radiant energy of high purity (1 to 10 nm) isrequired. This applies particularly if accurate values are to beobtained in those systems of measurement based on theadditive nature of absorbance values.NOTE 2One nanometre (nm) equals one millimicron (m).7.
34、Types of Photometers and Spectrophotometers7.1 Single-Photocell InstrumentsIn most single-photocellinstruments, the radiant energy passes from the monochroma-tor or filter through the reference solution to a photocell. Thephotocurrent is measured by a galvanometer or a microamme-ter and its magnitud
35、e is a measure of the incident radiantpower, Po. An identical absorption cell containing the solutionof the absorbing component is now substituted for the cellcontaining the reference solution and the power of the trans-mitted radiant energy, P, is measured. The ratio of the currentcorresponding to
36、P to that of Pogives the transmittance, T,ofE 60 98 (2004)2the absorbing solution, provided the illuminant and photocellare constant during the interval in which the two photocurrentsare measured. It is customary to adjust the photocell output sothat the galvanometer or microammeter reads 100 on the
37、percentage scale or zero on the logarithmic scale when theincident radiant power is Po, in order that the scale will readdirectly in percentage transmittance or absorbance. This ad-justment is usually made in one of three ways. In the firstmethod, the position of the cross-hair or pointer is adjuste
38、delectrically by means of a resistance in the photocell-galvanometer circuit. In the second method, adjustment ismade with the aid of a rheostat in the source circuit (Note 3).The third method of adjustment is to control the quantity ofradiant energy striking the photocell with the aid of a dia-phra
39、gm somewhere in the path of radiant energy.NOTE 3Kortm (3) has pointed out on theoretical grounds thismethod of controls is faulty, since the change in voltage applied to thelamp not only changes the radiant energy emitted but also alters itschromaticity. Actually, however, instruments employing thi
40、s principle aregiving good service in industry, so the errors involved evidently are nottoo great.7.2 Two-Photocell InstrumentsIn order to eliminate theeffect of fluctuation of the source, a great many types oftwo-photocell instruments have been proposed. Most of theseare good, but some have poorly
41、designed circuits and do notaccomplish the purpose for which they are designed. Followingis a brief description of two types of two-photocell photometersand spectrophotometers that have been found satisfactory:7.2.1 In the first type of two-photocell instrument, beams ofradiant energy from the same
42、source are passed through thereference solution and the sample solution and are focused ontheir respective photocells. Prior to insertion of the sample, thereference solution is placed in both absorption cells, and thephotocells are balanced with the aid of a potentiometric bridgecircuit (Note 4). T
43、he reference solution and sample are theninserted and the balance reestablished by manipulation of thepotentiometer until the galvanometer again reads zero. Bychoosing suitable resistances and by using a graduated slidewire, the scale of the latter can be made to read directly intransmittance. It is
44、 important that both photocells show linearresponse, and that they have identical radiation sensitivity ifthe light is not monochromatic.NOTE 4Since b is defined as the internal cell length, the cancellationof radiant energy lost at the glass-liquid interfaces and within the glassmust be accomplishe
45、d by inserting the reference solution in the absorptioncells.7.2.2 The second type of two-photocell instrument is similarto the first, except that part of the radiant energy from thesource is passed through an absorption cell to the firstphotocell; the remainder is impinged on the second photocellwi
46、thout, however, passing through an absorption cell. Adjust-ment of the calibrated slide wire to read 100 on the percentagescale, with the reference solution in the cell, is accomplishedby rotating the second photocell. The reference solution is thenreplaced by the sample and the slide wire is turned
47、 until thegalvanometer again reads zero.8. Radiation Source8.1 In most of the commercially available instruments theilluminant is an incandescent lamp with a tungsten filament.This type of illuminant is not ideal for all work. For example,when an analysis calls for the use of radiant energy ofwavele
48、ngths below 400 nm, it is necessary to maintain thefilament at as high a temperature as possible in order to obtainsufficient radiant energy to ensure the necessary sensitivity forthe measurements. This is especially true when operating witha photovoltaic cell, for the response of the latter falls o
49、ffquickly in the near ultraviolet. The use of high-temperaturefilament sources may lead to serious errors in photometricwork if adequate ventilation is not provided in the instrumentin order to dissipate the heat.Another important source of errorresults from the change of the shape of the energy distributioncurve with age. As a lamp is used, tungsten will be vaporizedand deposited on the walls. As this condensation proceeds,there is a decrease in the radiation power emitted and, in someinstances, a change in the composition of the radiant energy.This change is es
