ASTM E169-04(2014)
(Practice)Standard Practices for General Techniques of Ultraviolet-Visible Quantitative Analysis
Standard Practices for General Techniques of Ultraviolet-Visible Quantitative Analysis
SIGNIFICANCE AND USE
4.1 These practices are a source of general information on the techniques of ultraviolet and visible quantitative analyses. They provide the user with background information that should help ensure the reliability of spectrophotometric measurements.
4.2 These practices are not intended as a substitute for a thorough understanding of any particular analytical method. It is the responsibility of the users to familiarize themselves with the critical details of a method and the proper operation of the available instrumentation.
SCOPE
1.1 These practices are intended to provide general information on the techniques most often used in ultraviolet and visible quantitative analysis. The purpose is to render unnecessary the repetition of these descriptions of techniques in individual methods for quantitative analysis.
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 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E169 − 04(Reapproved 2014)
Standard Practices for
General Techniques of Ultraviolet-Visible Quantitative
Analysis
This standard is issued under the fixed designation E169; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope law are defined in Terminology E131. Beer’s law (Note 1)
holds at a single wavelength and when applied to a single
1.1 These practices are intended to provide general infor-
component sample it may be expressed in the following form
mation on the techniques most often used in ultraviolet and
(see Section 10):
visible quantitative analysis. The purpose is to render unnec-
essary the repetition of these descriptions of techniques in A 5 abc (1)
individual methods for quantitative analysis.
Whenappliedtoamixtureof nnon-interactingcomponents,
1.2 The values stated in SI units are to be regarded as
it may be expressed as follows:
standard. No other units of measurement are included in this
A 5 a bc 1a bc 1….1a bc (2)
1 1 2 2 n n
standard.
1.3 This standard does not purport to address all of the
NOTE 1—Detailed discussion of the origin and validity of Beer’s law
safety concerns, if any, associated with its use. It is the
maybefoundinthebooksandarticleslistedinthebibliographyattheend
responsibility of the user of this standard to establish appro-
of these practices.
priate safety and health practices and determine the applica-
3.2 This practice describes the application of Beer’s law in
bility of regulatory limitations prior to use.
typical spectrophotometric analytical applications. It also de-
2. Referenced Documents scribes operating parameters that must be considered when
2 using these techniques.
2.1 ASTM Standards:
E131Terminology Relating to Molecular Spectroscopy
4. Significance and Use
E168Practices for General Techniques of Infrared Quanti-
3 4.1 These practices are a source of general information on
tative Analysis (Withdrawn 2015)
the techniques of ultraviolet and visible quantitative analyses.
E275PracticeforDescribingandMeasuringPerformanceof
Theyprovidetheuserwithbackgroundinformationthatshould
Ultraviolet and Visible Spectrophotometers
help ensure the reliability of spectrophotometric measure-
E925Practice for Monitoring the Calibration of Ultraviolet-
ments.
Visible Spectrophotometers whose Spectral Bandwidth
does not Exceed 2 nm 4.2 These practices are not intended as a substitute for a
E958Practice for Estimation of the Spectral Bandwidth of thorough understanding of any particular analytical method. It
is the responsibility of the users to familiarize themselves with
Ultraviolet-Visible Spectrophotometers
the critical details of a method and the proper operation of the
3. Summary of Practice
available instrumentation.
3.1 Quantitative ultraviolet and visible analyses are based
5. Sample Preparation
upontheabsorptionlaw,knownasBeer’slaw.Theunitsofthis
5.1 Accurately weigh the specified amount of the sample
(solid or liquid). Dissolve in the appropriate solvent and dilute
These practices are under the jurisdiction of ASTM Committee E13 on
tothespecifiedvolumeinvolumetricglasswareoftherequired
Molecular Spectroscopy and Separation Science and are the direct responsibility of
Subcommittee E13.01 on Ultra-Violet, Visible, and Luminescence Spectroscopy.
accuracy, ensuring that all appropriate temperature range
Current edition approved Aug. 1, 2014. Published August 2014. Originally
tolerancesaremaintained.Ifneeded,adilutionshouldbemade
approvedin1960.Lastpreviouseditionapprovedin2009asE169–04(2009).DOI:
with a calibrated pipet and volumetric flask, using adequate
10.1520/E0169-04R14.
volumes for accuracy. With the availability of moderin wide
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
range electronic balances, (capable of reading kg quantities to
Standards volume information, refer to the standard’s Document Summary page on
four or five decimal places), gravimetric dilution should be
the ASTM website.
3 considered as a more accurate alternative to volumetric, if
The last approved version of this historical standard is referenced on
www.astm.org. available. Fill the absorption cell with the solution, and fill the
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E169 − 04 (2014)
comparisonorblankcellwiththepuresolvent,atleast2×to3× being used. Procedures for checking precision and accuracy of
(if sufficient sample or solvent is available), before measuring. these scales are presented in Practices E275 and E925.
6. Cell and Base-Line Checks 8. Resolution and Bandwidth
8.1 If the analytical method specifies a resolution or a
6.1 Clean and match the cells. Suggested cleaning proce-
dures are presented in Practice E275. spectral slit width, set the resolution of the instrument to the
specified value. If the instrument has only a mechanical
6.2 Establish the base line of a recording double-beam
bandwidth indicator, use the information provided in the
spectrophotometer by scanning over the appropriate wave-
manufacturer’s literature to calculate the bandwidth that cor-
length region with pure solvent in both cells. Determine
responds to the specified resolution.
apparent absorbance of the sample cell at each wavelength of
interest. These absorbances are cell corrections that are sub-
NOTE 2—The accuracy of resolution and mechanical bandwidth indi-
cators can be determined using the procedure given in Practice E958.
tracted from the absorbance of the sample solution at the
corresponding wavelengths.
8.2 If the analytical method does not state a required
resolution or a bandwidth value but includes an illustrative
6.3 Forsinglebeaminstruments,eitherusethesamecellfor
spectrum, set the resolution or bandwidth of the instrument to
pure solvent and sample measurements, use matched cells, or
obtain comparable data. As a rule of thumb, the resolution
apply appropriate cell corrections.
should be less than one-eighth of the bandwidth; thus for a
6.4 On most software-controlled instruments, the cell cor-
peak of bandwidth 40 nm, the resolution should not exceed 5
rections or the blank cell absorbance is stored in memory and
nm.
automatically incorporated into the sample absorbance mea-
8.3 If the method neither specifies resolution or bandwidth
surement.
nor provides an illustrative spectrum, use the smallest resolu-
6.5 An accurate determination of cell path length in the
tion or bandwidth that yields an acceptable signal-to-noise
1-cm range is not practical in most laboratories, and common
ratio. Record this value for future reference.
practiceistopurchasecellsofknownpathlength.Moderncell
NOTE 3—Changes in the day-to-day values of resolution or bandwidth
manufacturing techniques employed by a number of leading
obtained with a given gain, or changes in signal-to-noise ratio at a given
manufacturers can guarantee the path length of a 1-cm cell to
resolution, are indicative of present or potential problems. Increased
60.01 mm or better.
resolution or a lowering of the S/N ratio may result from a lower output
of the light source, deterioration of optical components, deposits on the
7. Analytical Wavelengths and Photometry
windows of the cell compartment or on the inside wall of the reference
cell, an absorbing impurity in the solvent, or a faulty electronic compo-
7.1 Analytical wavelengths are those wavelengths at which
nent.
absorbance readings are taken for use in calculations. These
may include readings taken for purposes of background cor-
9. Solvents and Solvent Effects
rections. To minimize the effect of wavelength error, the
9.1 The ultraviolet absorption spectrum of a compound will
analytical wavelengths are frequently chosen at absorption
varyindifferentsolventsdependingonthechemicalstructures
maxima, but this is not always necessary. If the wavelength
involved. Non-polar solvents have the least effect on the
accuracy of the spectrophotometer is such that the calculated
absorption spectrum. Non-polar molecules in most instances
uncertainty in the absorbance measurement is within accept-
are not affected in polar solvents. However, polar molecules in
ablelimitsattheextremesofthiswavelengthuncertainyrange,
polar solvents may show marked differences in their spectra.
then single point measurements on a slope can be used. For
Any interaction between solute and solvents leads to a broad-
example, the use of isoabsorptive or isosbestic points is
ening and change in structural resolution of the absorption
frequently useful.
bands. Ionic forms may be created in acidic or basic solutions.
7.2 Record the absorbance readings at the specified analyti-
In addition, there are possible chemical reactions between
cal wavelengths, operating the instrument in accordance with
solute and solvent, and also photochemical reactions arising
the recommendations of the manufacturer or Practice E275.
from either room illumination or the short wavelengths in the
beam of the spectrophotometer. It is important that the solvent
7.3 Absorbance values should be used only if they fall
used be specified in recording spectral data. (The change in
within the acceptably accurate range of the particular spectro-
spectrabetweenacidicandbasicconditionsmaysometimesbe
photometerandmethodemployed.Iftheabsorbanceistoolow,
employed in multicomponent analysis.)
either use a longer absorption cell or prepare a new solution of
higher concentration. If the absorbance is too high, use a
9.2 Reference solvent data is shown in Table 1.Availability
shorter cell or make a quantitative dilution. If different cells
of a particular solvent may be restricted by international
are used, a new base-line must be obtained.
agreement, and the users’ attention is directed to 1.3 of these
practices. The short wavelength limit is approximate, and
7.4 Theprecisionandbiasofthewavelengthandphotomet-
referstothewavelengthatwhicha1-cmlightpathlengthgives
ric scales of the instrument must be adequate for the method
an absorbance of unity.
9.3 Water,and0.1Msolutionsofhydrochloricacid,sulfuric
The errors associated with cell path lengths are significantly less than those
acid, and sodium hydroxide also are commonly used as
generatedbyvolumetricdilution,andthereforewherepossible,differentpathlength
cells should be used in preference to volumetric procedures. solvents. Buffered solutions, involving non-absorbing
E169 − 04 (2014)
A
TABLE 1 Solvents
Note that c and C , have the dimensions of grams per litre.
s
Solvent Cutoff, nm
Ifdilutionismade, C isnottheconcentrationinthecellatthe
s
Pyridine 305
timetheabsorbanceisdetermined;theconcentrationinthecell
Tetrachloroethylene 290
is C /f.
s
Benzene 280
N,N-Dimethylformamide 270
10.3 ChemicalCalibration—Theabsorptivityoftheabsorb-
Carbon tetrachloride 265
ing material, the concentration of which it is desired to
Methyl formate 260
Chloroform 245
determine, is obtained by examination of a series of quantita-
Dichloromethane 235
tive dilutions of a neat sample of this material. However, if no
Ethyl ether 220
such neat sample is available, the best available material is
Acetonitrile 215
Isopropyl alcohol 210
used, or a value of the absorptivity is taken from the literature.
Ethyl alcohol 210
Take care to specify this, by reporting values as “percentage
Methyl alcohol 210
against calibration material” or by noting that the accuracy of
Cyclohexane <210
Isooctane <210
the analysis is dependent upon a published value of the
A
Procedures for special purification of solvents for further improvement in the
absorptivity or molar absorptivity. (Areference must be cited.)
wavelength limit are given in Refs (1, 2). Solvents of high purity for use in
10.3.1 Some sample materials are highly fluorescent which
absorption spectroscopy also are available commercially.
cansignificantlyreducethemeasuredabsorbance.Theeffectof
samplefluorescencemayvarydependinguponthespectropho-
tometerandwavelengthchosen.Samplefluorescencemaybea
materials, are frequently used; both the composition of the
particular problem when using published absorptivity values.
buffer and the measured pH should be specified. Mixtures of
10.4 Types of Analyses (see Fig. 1):
0.1 M di-hydrogen sodium phosphate and 0.1 M hydrogen
di-sodium phosphate are useful in the 4.5 to 8.9 pH range. A 10.4.1 One Component, No Background Correction:
table of non-absorbing buffers has been presented by Abbott
C 5 Af/~abC ! (7)
s
(3).
10.4.2 One Component, Simple Background Correction:
10. Calculations
A 2 A 3f
~ !
1 2
C 5 (8)
10.1 Quantitative analysis by ultraviolet spectrophotometry
a bC
1 s
depends upon Beer’s law. The terms and symbols used are
where the subscripts refer to analytical wavelengths. The
those defined in Terminology E131. According to Beer’s law:
term A is the absorbance at the wavelength used for making a
A 5 abc 5 ~ε/M! 3bc (3)
simple subtractive correction. It is usually selected from
where: examination of the spectral curve of the reference material at a
wavelengthlongerthanthatofA ,preferablywherea ismuch
A = absorbance, 1 2
less than a .
a = absorptivity,
b = cell length, cm, 10.4.3 One Component, with Slope-Type Background Cor-
c = concentration, g/L,
rection:
ε = molar absorptivity, and
A 2 A 1S λ 2 λ f
@ ~ !#
1 2 2 1
M = molecular weight.
C 5 (9)
a bC
1 s
10.1.1 Inpractice,adistinctionmustbemadebetweenc,the
where:
concentration of the absorbing material in the cell at the time
S = slopebetweenwavelengths1and2forthebackground.
ofobservation,andtheconcentrationoftheabsorbingmaterial
in the sample as received. This is here designated as a mass
10.4.3.1 The background absorption is usually not linear
fraction C (g/g). The solution to be examined has a concentra-
between the analytical wavelength and the wavelength at
tion of sample in solution, C , which is in units of grams per
s
which a simple subtractive background correction may be
litre.
obtained. When it is possible to determine the slope between
c 5 A/ab (4) wavelengths 1 and 2 by observation of the samples that do not
contain the absorbing material that is to be determined, this
C 5 c/C 5 A/~abC ! (5)
s s
may be used as a correction for the background absorption.
10.2 If one or more dilutions are then made, the quantity
10.4.4 One Component, With Linear Background Correc-
calledthedilutionfactormustbeincluded.Dilutionfactor,f,is
tion:
the ratio of the final volume to the initial volume. If more than
10.4.4.1 The
...
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. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E169 − 04 (Reapproved 2009) E169 − 04 (Reapproved 2014)
Standard Practices for
General Techniques of Ultraviolet-Visible
Quantitative Analysis
This standard is issued under the fixed designation E169; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 These practices are intended to provide general information on the techniques most often used in ultraviolet and visible
quantitative analysis. The purpose is to render unnecessary the repetition of these descriptions of techniques in individual methods
for quantitative analysis.
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 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
E131 Terminology Relating to Molecular Spectroscopy
E168 Practices for General Techniques of Infrared Quantitative Analysis
E275 Practice for Describing and Measuring Performance of Ultraviolet and Visible Spectrophotometers
E925 Practice for Monitoring the Calibration of Ultraviolet-Visible Spectrophotometers whose Spectral Bandwidth does not
Exceed 2 nm
E958 Practice for Estimation of the Spectral Bandwidth of Ultraviolet-Visible Spectrophotometers
3. Summary of Practice
3.1 Quantitative ultraviolet and visible analyses are based upon the absorption law, known as Beer’s law. The units of this law
are defined in Terminology E131. Beer’s law (Note 1) holds at a single wavelength and when applied to a single component sample
it may be expressed in the following form (see Section 10):
A 5 abc (1)
When applied to a mixture of n non-interacting components, it may be expressed as follows:
A 5 a bc 1a bc 1….1a bc (2)
1 1 2 2 n n
NOTE 1—Detailed discussion of the origin and validity of Beer’s law may be found in the books and articles listed in the bibliography at the end of
these practices.
3.2 This practice describes the application of Beer’s law in typical spectrophotometric analytical applications. It also describes
operating parameters that must be considered when using these techniques.
4. Significance and Use
4.1 These practices are a source of general information on the techniques of ultraviolet and visible quantitative analyses. They
provide the user with background information that should help ensure the reliability of spectrophotometric measurements.
These practices are under the jurisdiction of ASTM Committee E13 on Molecular Spectroscopy and Separation Science and are the direct responsibility of Subcommittee
E13.01 on Ultra-Violet, Visible, and Luminescence Spectroscopy.
Current edition approved Oct. 1, 2009Aug. 1, 2014. Published December 2009August 2014. Originally approved in 1960. Last previous edition approved in 20042009
as E169 – 04.E169 – 04(2009). DOI: 10.1520/E0169-04R09.10.1520/E0169-04R14.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E169 − 04 (2014)
4.2 These practices are not intended as a substitute for a thorough understanding of any particular analytical method. It is the
responsibility of the users to familiarize themselves with the critical details of a method and the proper operation of the available
instrumentation.
5. Sample Preparation
5.1 Accurately weigh the specified amount of the sample (solid or liquid). Dissolve in the appropriate solvent and dilute to the
specified volume in volumetric glassware of the required accuracy, ensuring that all appropriate temperature range tolerances are
maintained. If needed, a dilution should be made with a calibrated pipet and volumetric flask, using adequate volumes for accuracy.
With the availability of moderin wide range electronic balances, (capable of reading kg quantities to four or five decimal places),
gravimetric dilution should be considered as a more accurate alternative to volumetric, if available. Fill the absorption cell with
the solution, and fill the comparison or blank cell with the pure solvent, at least 2× to 3× (if sufficient sample or solvent is
available), before measuring.
6. Cell and Base-Line Checks
6.1 Clean and match the cells. Suggested cleaning procedures are presented in Practice E275.
6.2 Establish the base line of a recording double-beam spectrophotometer by scanning over the appropriate wavelength region
with pure solvent in both cells. Determine apparent absorbance of the sample cell at each wavelength of interest. These absorbances
are cell corrections that are subtracted from the absorbance of the sample solution at the corresponding wavelengths.
6.3 For single beam instruments, either use the same cell for pure solvent and sample measurements, use matched cells, or apply
appropriate cell corrections.
6.4 On most software-controlled instruments, the cell corrections or the blank cell absorbance is stored in memory and
automatically incorporated into the sample absorbance measurement.
6.5 An accurate determination of cell path length in the 1-cm range is not practical in most laboratories, and common practice
is to purchase cells of known path length. Modern cell manufacturing techniques employed by a number of leading manufacturers
can guarantee the path length of a 1-cm cell to 60.01 mm or better.
7. Analytical Wavelengths and Photometry
7.1 Analytical wavelengths are those wavelengths at which absorbance readings are taken for use in calculations. These may
include readings taken for purposes of background corrections. To minimize the effect of wavelength error, the analytical
wavelengths are frequently chosen at absorption maxima, but this is not always necessary. If the wavelength accuracy of the
spectrophotometer is such that the calculated uncertainty in the absorbance measurement is within acceptable limits at the extremes
of this wavelength uncertainy range, then single point measurements on a slope can be used. For example, the use of isoabsorptive
or isosbestic points is frequently useful.
7.2 Record the absorbance readings at the specified analytical wavelengths, operating the instrument in accordance with the
recommendations of the manufacturer or Practice E275.
7.3 Absorbance values should be used only if they fall within the acceptably accurate range of the particular spectrophotometer
and method employed. If the absorbance is too low, either use a longer absorption cell or prepare a new solution of higher
concentration. If the absorbance is too high, use a shorter cell or make a quantitative dilutiondilution. . If different cells are used,
a new base-line must be obtained.
7.4 The precision and bias of the wavelength and photometric scales of the instrument must be adequate for the method being
used. Procedures for checking precision and accuracy of these scales are presented in Practices E275 and E925.
8. Resolution and Bandwidth
8.1 If the analytical method specifies a resolution or a spectral slit width, set the resolution of the instrument to the specified
value. If the instrument has only a mechanical bandwidth indicator, use the information provided in the manufacturer’s literature
to calculate the bandwidth that corresponds to the specified resolution.
NOTE 2—The accuracy of resolution and mechanical bandwidth indicators can be determined using the procedure given in Practice E958.
8.2 If the analytical method does not state a required resolution or a bandwidth value but includes an illustrative spectrum, set
the resolution or bandwidth of the instrument to obtain comparable data. As a rule of thumb, the resolution should be less than
one-eighth of the bandwidth; thus for a peak of bandwidth 40 nm, the resolution should not exceed 5 nm.
8.3 If the method neither specifies resolution or bandwidth nor provides an illustrative spectrum, use the smallest resolution or
bandwidth that yields an acceptable signal-to-noise ratio. Record this value for future reference.
The errors associated with cell path lengths are significantly less than those generated by volumetric dilution, and therefore where possible, different path length cells
should be used in preference to volumetric procedures.
E169 − 04 (2014)
NOTE 3—Changes in the day-to-day values of resolution or bandwidth obtained with a given gain, or changes in signal-to-noise ratio at a given
resolution, are indicative of present or potential problems. Increased resolution or a lowering of the S/N ratio may result from a lower output of the light
source, deterioration of optical components, deposits on the windows of the cell compartment or on the inside wall of the reference cell, an absorbing
impurity in the solvent, or a faulty electronic component.
9. Solvents and Solvent Effects
9.1 The ultraviolet absorption spectrum of a compound will vary in different solvents depending on the chemical structures
involved. Non-polar solvents have the least effect on the absorption spectrum. Non-polar molecules in most instances are not
affected in polar solvents. However, polar molecules in polar solvents may show marked differences in their spectra. Any
interaction between solute and solvents leads to a broadening and change in structural resolution of the absorption bands. Ionic
forms may be created in acidic or basic solutions. In addition, there are possible chemical reactions between solute and solvent,
and also photochemical reactions arising from either room illumination or the short wavelengths in the beam of the
spectrophotometer. It is important that the solvent used be specified in recording spectral data. (The change in spectra between
acidic and basic conditions may sometimes be employed in multicomponent analysis.)
9.2 Reference solvent data is shown in Table 1. Availability of a particular solvent may be restricted by international agreement,
and the users’ attention is directed to 1.3 of this Practices.these practices. The short wavelength limit is approximate, and refers
to the wavelength at which a 1-cm light path length gives an absorbance of unity.
9.3 Water, and 0.1 M solutions of hydrochloric acid, sulfuric acid, and sodium hydroxide also are commonly used as solvents.
Buffered solutions, involving non-absorbing materials, are frequently used; both the composition of the buffer and the measured
pH should be specified. Mixtures of 0.1 M di-hydrogen sodium phosphate and 0.1 M hydrogen di-sodium phosphate are useful in
the 4.5 to 8.9 pH range. A table of non-absorbing buffers has been presented by Abbott (3).
10. Calculations
10.1 Quantitative analysis by ultraviolet spectrophotometry depends upon Beer’s law. The terms and symbols used are those
defined in Terminology E131. According to Beer’s law:
The boldface numbers in parentheses refer to a list of references at the end of this standard.
A
TABLE 1 Solvents
Solvent Cutoff, nm
Pyridine 305
Tetrachloroethylene 290
Benzene 280
N,N-Dimethylformamide 270
Carbon tetrachloride 265
Methyl formate 260
Chloroform 245
Dichloromethane 235
Ethyl ether 220
Acetonitrile 215
Isopropyl alcohol 210
Ethyl alcohol 210
Methyl alcohol 210
Cyclohexane <210
Isooctane <210
A
TABLE 1 Solvents
Solvent Cutoff, nm
Pyridine 305
Tetrachloroethylene 290
Benzene 280
N,N-Dimethylformamide 270
Carbon tetrachloride 265
Methyl formate 260
Chloroform 245
Dichloromethane 235
Ethyl ether 220
Acetonitrile 215
Isopropyl alcohol 210
Ethyl alcohol 210
Methyl alcohol 210
Cyclohexane <210
Isooctane <210
A
Procedures for special purification of solvents for further improvement in the
wavelength limit are given in Refs (1, 2). Solvents of high purity for use in
absorption spectroscopy also are available commercially.
E169 − 04 (2014)
A 5 abc 5 ε/M 3bc (3)
~ !
where:
A = absorbance,
a = absorptivity,
b = cell length, cm,
c = concentration, g/L,
ε = molar absorptivity, and
M = molecular weight.
10.1.1 In practice, a distinction must be made between c, the concentration of the absorbing material in the cell at the time of
observation, and the concentration of the absorbing material in the sample as received. This is here designated as a mass fraction
C (g/g). The solution to be examined has a concentration of sample in solution, C , which is in units of grams per litre.
s
c 5 A/ab (4)
C 5 c/C 5 A/~abC ! (5)
s s
10.2 If one or more dilutions are then made, the quantity called the dilution factor must be included. Dilution factor, f, is the
ratio of the final volume to the initial volume. If more than one dilution is performed, the dilution factor is the product of the factors
from each dilution. If dilutions are made, the equation becomes the following:
C 5 cf/C 5 Af/ abC (6)
~ !
s s
Note that c and C , have the dimensions of grams per litre. If dilution is made, C is not the concentration in the cell at the time
s s
the absorbance is determined; the concentration in the cell is C / f.
s
10.3 Chemical Calibration—The absorptivity of the absorbing material, the concentration of which it is desired to determine,
is obtained by examination of a series of quantitative dilutions of a neat sample of this material. However, if no such neat sample
is available, the best available material is used, or a value of the absorptivity is taken from the literature. Take care to specify this,
by reporting values as “percentage against calibration material” or by noting that the accuracy of the analysis is dependent upon
a published value of the absorptivity or molar absorptivity. (A reference must be cited.)
10.3.1 Some sample materials are highly fluorescent which can significantly reduce the measured absorbance. The effect of
sample fluorescence may vary depending upon the spectrophotometer and wavelength chosen. Sample fluorescence may be a
particular problem when using published absorpt
...
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