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Файл:Optical methods of analysis. Educational aid
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The Ministry of Science and Higher Education of the Russian Federation
Kazan National Research Technological University
R. Bakeeva, S. Garmonov
OPTICAL METHODS OF ANALYSIS
Educational aid
Kazan
KNRTU Press
2023
UDC 543.4(075)
Published by the decision of the Editorial Review Board
Bakeeva R.
Оptical methods of analysis : Educational aid / R. Bakeeva, S. Garmonov;
The Ministry of Education and Science of the Russian Federation, Kazan
National Research Technological University. – Kazan : KNRTU Press,
2023. – 84 p.
ISBN 978-5-7882-3398-7
ISBN 978-5-7882-3398-7
©
R. Bakeeva, S. Garmonov, 2023
©
Kazan National Research Technological
University, 2023
UDC 543.4(075)
of the Kazan National Research Technological University
Reviewers:
PhD., Professor G. Budnikov
PhD., Professor S. Egorova
The basic principles of optical methods of analysis based on the use of electromagnetic radiation in the ultraviolet, visible and infrared regions of the spectrum are described. Laboratory works on spectrophotometry and photometry, IR-spectroscopy,
flame emission spectroscopy and refractometry are given. Examples of control questions are given. Methods for conducting laboratory work are given. A glossary of optical
terms is provided.
Designed for students majoring in Chemical Technology programmes and studying Analytical Chemistry in English.
Prepared at the Department of Analytical Chemistry, Certification and Quality
Management.
2
C O N T E N T S
Introduction ..................................................................................................................... 5
1. CLASSIFICATION OF OPTICAL METHODS OF ANALYSIS ............................. 7
2. ABSORPTION OF LIGHT BY A SUBSTANCE...................................................... 9
2.1. Classification of absorption methods of analysis ................................................. 9
2.2. The device and principle of operation of the spectrophotometer ....................... 14
3. PHOTOMETRY AND SPECTROPHOTOMETRY ................................................ 17
3.1. Basic law of light absorption .............................................................................. 17
3.2. Conditions for conducting quantitative determinations in photometry
and spectrophotometry ............................................................................................... 19
4. METHODS FOR DETERMINING CONCENTRATION IN OPTICAL
METHODS OF ANALYSIS ......................................................................................... 23
4.1. Calibration curve method .................................................................................... 23
4.2. Comparison method ............................................................................................ 24
4.3. Standard addition method ................................................................................... 25
4.4. Differential photometry method ......................................................................... 26
5. LABORATORY WORK ON SPECTROPHOTOMETRY
AND PHOTOMETRY .................................................................................................. 28
5.1. Laboratory work on photometry ......................................................................... 28
5.1.1. Choice of optimal conditions for photometric determinations ..................... 28
5.1.2. Photometric determination of copper ........................................................... 29
5.1.3. Photocolorimetric determination of titanium (IV) ....................................... 31
5.2. Laboratory work on spectrophotometry ............................................................. 33
5.2.1. Obtaining absorption spectra of a methyl orange solution at different
pH of the solution .................................................................................................... 33
5.2.2. Determination of the dissociation constant of methyl orange ...................... 35
5.2.3. Obtaining an absorption spectrum and quantifying acetone ........................ 36
5.2.4. Spectrophotometric determination of phosphates ........................................ 37
Control questions ....................................................................................................... 38
6. INFRARED SPECTROSCOPY ............................................................................... 40
6.1. Origin of absorption bands in the IR-spectrum ................................................. 41
6.2. The main types of oscillations and bands in the IR-spectra ............................... 42
3
6.3. Qualitative IR-analysis ........................................................................................ 43
6.4. Device of IR-spectrometers ................................................................................ 49
6.5. Methods for preparing samples for analysis ....................................................... 50
6.6 Quantitative analysis by IR-spectra ..................................................................... 52
6.7. Fields of application, advantages and disadvantages of the IR-spectroscopy
method ........................................................................................................................ 54
6.8. Questions and tasks ............................................................................................. 55
6.8.1. Questions to prepare for the colloquium ...................................................... 55
6.8.2. Control questions .......................................................................................... 56
6.8.3. Device diagram ............................................................................................. 57
6.8.4. Laboratory work 1. Obtaining an IR-spectrum and identification of
polymeric materials ................................................................................................. 58
6.8.5. Laboratory work 2. Quantitative analysis of a mixture of xylene
isomers .................................................................................................................... 59
7. ATOMIC EMISSION SPECTROSCOPY ............................................................... 61
7.1. Theoretical foundations....................................................................................... 61
7.2. Flame emission spectroscopy ............................................................................. 65
7.2.1. Equipment and measurement technique ....................................................... 65
7.2.2. Flame photometric determination of sodium ions ........................................ 68
7.2.3. Flame photometric determination of potassium ions ................................... 69
7.2.4. Flame photometric determination of calcium ions ....................................... 70
7.2.5. Flame photometric determination of sodium and potassium ions
in drinking water ..................................................................................................... 71
Control questions ....................................................................................................... 72
8. REFRACTOMETRIC ANALYSIS .......................................................................... 73
8.1. Fundamentals of method ..................................................................................... 73
8.2. Determination of carbohydrate content in aqueous solutions
by refractometric method ........................................................................................... 75
Control questions ....................................................................................................... 77
Conclusion ..................................................................................................................... 78
Glossary ......................................................................................................................... 80
Bibliography .................................................................................................................. 83
4
I N T R O D U C T I O N
Light and substances are what constantly surrounds a person and is used
by him in everyday activities. Progress in science and technology of the last
century has led to the development of a large number of various methods and
techniques that make it possible to study both new substances and obtain information about the structure, quality and composition of various materials
using the effect of electromagnetic radiation on them.
Spectroscopic methods represent one of the most important branches of
modern analytical chemistry. For more than two hundred years, optical spectroscopy has been used in various fields of science and production.
The high specificity of optical spectroscopy is explained by the fact that
each substance has its own spectral properties that differ from the spectral
characteristics of other substances. With its help, it is possible to establish
the qualitative and quantitative composition of various samples. Unlike other
spectroscopy methods, such as nuclear magnetic resonance, electron paramagnetic resonance, mass spectrometry, there are practically no restrictions
for samples analyzed using optical spectroscopy. Measurements of various
optical parameters depending on the wavelength/energy of the radiation
(spectrum) or time parameters (kinetics) provide valuable information that is
not always available with other analytical methods. Optical methods are
widely used in scientific and certification practice. They are quite informative and allow obtaining a significant amount of information about the structure, composition and quality of various substances. Spectral analysis methods are widely used in research and production chemical-analytical laboratories because they are distinguished by the speed of analysis, high sensitivity, accuracy and the possibility of automating the analytical control of production. Compared to other analytical methods, optical spectroscopy has
a number of advantages: the method is not destructive or aggressive; in terms
of the width of the covered range of concentrations, spectroscopic methods
cover all other analytical methods; liquid, solid or gaseous samples are easily
examined, regardless of their optical properties. There is a wide variety of
spectroscopic methods. Therefore, mastering this field of analytical chemistry is far from an easy task. When choosing a specific method, the researcher
must answer many questions regarding the fundamental methodological
foundations. For example: in which wavelength range one have to measure,
which method to choose for excitation of radiation, what kind of device is
available, what is its design and capabilities, whether device suitable for
5
solving the task. Such a systematic approach also facilitates the study of
the latest achievements in the field of spectroscopy, which, as a rule, are carried out through the gradual improvement of both measurement methods and
measuring equipment.
Laboratory workshop is of great importance when studying chemical
disciplines. In addition, practical works contributes to the development of
a methodology for analytical and philosophical thinking. In addition, practical works contributes to the development of a methodology for analytical and
philosophical thinking. As practice shows, during laboratory classes, certain
skills of perception of certain life phenomena, organization of workplaces,
compliance with safety regulations, etc. are formed. The manual includes
methods for performing laboratory work on optical methods of analysis.
All sections provide for the development of theoretical material on the topics
under study before the start of work and the subsequent defense of each work,
taking into account the existing control questions on the topic.
6
1. C L A S S I F I C A T I O N O F O P T I C A L M E T H O D S
OF A N A L Y S I S
The course “Analytical Chemistry and Physicochemical Methods of
Analysis” deals with spectroscopy methods based on the use of radiation in
the optical range of the electromagnetic wave scale. They are called “optical
methods of analysis”.
Optical methods of analysis are the most important branch of spectroscopy, which is widely used in scientific research, in various branches of industry and technology. Spectroscopic methods of analysis are based on
the ability of atoms and molecules of matter to emit, absorb or scatter electromagnetic radiation. The change in the intensity of electromagnetic radiation after interaction with a substance is associated with the qualitative and
quantitative composition of the substance, which leads to the widespread and
intensive development of spectroscopy methods in analysis.
Optical methods include:
– molecular absorption spectral analysis, based on the measurement of
the attenuation of the light flux occurring due to the selective absorption of
light by the analyte – photocolorimetry, spectrophotometry, infrared (IR) and
ultraviolet (UV) spectroscopy;
– atomic spectroscopy, which uses the emission or absorption spectra
of a substance after it has been transferred to the atomic state by an external
high-energy effect – emission and absorption atomic spectroscopy, flame
photometry;
– refractometric analysis based on the dependence of the refractive in-
dex of light on the nature and concentration of the substance;
– luminescent analysis, which is based on the dependence of the intensity
of the luminescence of a substance upon absorption of external energy on its
composition – fluorescence, cathodoluminescence, chemiluminescence.
Since light has a dual nature – wave and corpuscular, wave and quantum characteristics are used to describe it. Wave characteristics include oscillation frequency ν, wavelength λ and wave number υ, quantum character-
istics include photon energy (quantum of light) E.
The wavelength and frequency are related by the relationship:
ν = с/λ,
where c is the speed of light.
7
The reciprocal of the wavelength is called the wave number – υ and is
usually expressed in reciprocal centimeters (cm-1).
The energy of electromagnetic radiation is determined by the relation:
Е = h・ν,
where h is Planck's constant equal to 6.62・10
-34
J・s.
Optical methods of analysis are based on the study of the interaction of
electromagnetic radiation with atoms or molecules of a substance, accompanied by radiation, absorption or reflection of radiant energy.
Methods accompanied by the emission of energy are called emission
methods. These methods are based on the study of the radiation spectrum that
occurs when an electron passes from an excited state to the ground state.
Emission spectroscopy requires the transfer of the analyte to the atomic
state. This can be done either with a gas burner flame (2000–5000 °C), or an
electric arc (5000–7000 °C) or a high voltage spark (7000–15000 °C).
At such temperatures, the valence electrons of atoms pass to higher energy
levels and after a short period of time return to the ground state. In this case,
radiant energy quanta are released. Light emitted by atoms, passing through
the spectrograph prism, is refracted and decomposed into components.
As a result, the so-called “linear” spectrum is observed. The line spectrum of
each element is the passport of this element. Each element is unique in terms
of a set of quantum numbers – n, l, m and s. Each element has its own set of
“characteristic” lines (lines of the most probable transitions), which is a qualitative characteristic.
Emission flame photometry uses the property of elements (alkali, alkaline earth, and some transition metals) that are easily excited in the flame of
a gas burner to give a colored flame, i.e. obtain characteristic lines in the visible part of the world (400–760 nm). The radiation intensity, as a rule, is
directly proportional to the concentration of the element being determined.
Methods based on the absorption of radiant energy are called absorp-
tion methods. There are molecular absorption analysis and atomic absorption
analysis. Qualitative and quantitative analysis is carried out according
to the absorption spectra – the dependence of the light absorption on the char-
acteristics of the incident light (λ, ν, υ).
8
2 . A B S O R P T I O N O F L I G H T B Y A S U B S T A N C E
2 . 1 . C l a s s i f i c a t i o n o f a b s o r p t i o n m e t h o d s
of a n a l y s i s
Absorption spectral analysis is based on the measurement of the attenuation of the light flux that occurs due to the selective absorption of light by
the analyzed substance (system).
Absorption spectral analysis is carried out in various spectral regions
(Fig. 2.1):
– in the visible region of the spectrum (λ = 400–760 nm,
ν = 4・10-5 – 7.6・10-5 s-1, υ = 2.5・104 – 1.3・104 cm-1) – photometry and
spectrophotometry;
– in the infrared region (λ = 760–1100 nm, ν = 7.6・10-5 – 1.1・10
-4 s-1
,
υ = 2.5・103 – 9.0・104 cm-1) – IR-spectroscopy;
– in the ultraviolet region of the spectrum (λ = 200–400 nm,
ν = 2.0・10-5 – 4・10-5 s-1, υ = 5・104 – 1.3・104 cm-1) – UV-spectroscopy.
When white light (all light rays in the range of 400–760 nm) interacts
with various bodies, the following is observed:
1) all rays pass through a transparent body – it seems colorless;
2) all rays are reflected from the opaque – the body appears white;
3) all rays are completely absorbed – the body appears black;
4) all rays are partially absorbed – the body is gray;
5) the body selectively absorbs some rays of the visible part of the spectrum, the rest pass through it or are reflected by the body. The body seems to
be colored, and the color is determined by the action of all light rays minus
the absorbed ones, the so-called additional color.
For example, if the body absorbs light with a wavelength of 500–560 nm
(green light), its color appears purple and is made up of the exposure
of the eye to rays with a wavelength of 400–500 and 560–760 nm.
The reason for the absorption of light by a substance is an increase in
the internal energy of the substance due to the energy of the light passing
through the substance:
Е = h ν = hс/λ.
9
Fig. 2.1. The electromagnetic spectrum showing the colors of the visible
spectrum
When radiation passes through a transparent layer of a solid, liquid or
gas, selective (selective) absorption (absorption) of radiation occurs with certain frequencies. The electromagnetic energy of the radiation is transferred to
the atoms and molecules of the substance and transfers them from the ground
state to the excited energy state, which can be expressed as follows:
Е
+ h ν = Е1.
0
The internal energy of a substance is the sum of the rotational motion
of molecules around the center of gravity, the vibrational motion of atoms in
molecules, and the energy of motion of electrons in an atom:
Е
int
= Е
rot
+ Е
+ Еel.
osc
The ratio of energy values: 1:100:1000.
Each type of internal energy of a molecule has a quantum character and
can be characterized by a certain set of energy levels inherent in a given substance. This is due to the selective absorption of light by a substance, when
light quanta are intensely absorbed, the energy of which is equal to the excitation energy of a molecule or atom. Thus, each substance is characterized
by its own absorption spectrum. The spectrum is a graph of the dependence
of light absorption on the characteristics of the incident radiation (λ, ν, υ).
The absorption spectra have the form of bands with a maximum (peak).
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