How to explain the origin of linear spectra. Origin of the emission spectrum

When at least one quantum number changes (main n, secondary - l; magnetic - m; spin - m s), the atom receives or gives off energy. This can happen when an atom interacts with an electromagnetic field, or when it directly exchanges energy with other atoms or molecules, such as during collisions or chemical reactions. In the absence of external influences, the atom is in the ground state, that is, it has the lowest energy. When receiving energy from the outside, the speed of the electrons increases and the atom becomes excited.

Fig.3. Energy transitions in the atom

One atom absorbs or emits only one photon with a certain energy (frequency) in one act. Matter consists of many identical atoms that are capable of moving to different energy levels, emitting or absorbing photons of different frequencies. The totality of all photons of the same frequency is spectral line, when absorbed it is called absorption, when emitted - emission. The set of all absorption or all emission lines is called absorption (absorption) or emission (emission) spectrum of a substance.

The absorption spectrum is obtained by placing the substance under study in the field of electromagnetic radiation (for example, in the path of a light flux), and to obtain the emission spectrum, the atoms of the substance are first transferred to an excited state, which is achieved by supplying some type of energy (thermal, chemical, electric discharge, electromagnetic radiation etc.); after excitation, the atoms return to the ground state after 10 – 9 – 10 – 7 s, emitting photons or heat. IN the latter case there will be a transition non-emitterbnom; in Fig. 3 it is depicted by a wavy arrow.

The frequency of emitted or absorbed radiation is determined by the energy difference between electron orbitals ∆E:

Where h is Planck's constant

Absolute energy of quantum states unknown, therefore hercount down from a certain level, conventionally accepted as zero, namely on ionization energy, i.e. complete removal of an electron from an atom.

The energy of atomic orbitals varies greatly. Thus, to excite an electron from the orbital closest to the nucleus (the main thing quantum number n=1) more than 6∙10 4 kJ mol - 1 is required (emitted photons have the frequency of X-ray radiation), and 150-600 kJ mol -1 is sufficient to excite external electrons (radiation from the ultraviolet and visible regions). As the principal quantum number increases, the excitation energy ∆E and radiation frequency decrease (Fig. 2.).

Most likely transitions from the first excited level to the main one E 0 ; the corresponding spectral lines are called resonant . An electron can also go to a higher energy state ( E 2 , E 3 etc.). Bringing it back to level E 0 may pass through a number of intermediate stages.

Rice. 4. Relative arrangement of energy levels of various quantum states and energy changes during electronic transitions

External easily excitable electrons are called optical , transitions with their participation give optical spectrum. Excitation energy outer electrons different elements is not the same. For example, to obtain a resonance line of alkali metals (transition E 1 → E 0) a relatively low energy is required (~ 2 eV, wavelengths are in the visible region), for non-metals this energy is significantly higher (~ 5 eV, wavelengths are in the UV region ). The more external electrons, the more opportunities an atom has for energy transitions, therefore the spectra of metals such as iron consist of thousands of lines , and the spectra alkaline elements poor in them.

Atomic spectroscopic methods of analysis.

All numerous energy transitions of electrons along the orbitals of an atom can be used for analytical purposes. Analysis methods based on changes in the energy state of atoms of substances are included in the group atomic spectroscopic methods, differing in the method of receiving and recording the signal.

Optical methods use energy transitions of outer (valence) electrons, common to them is the need for preliminary atomization (decomposition into atoms) of the substance.

Atomic emission spectrometry based on the emission of radiation by atoms excited by kinetic energy of plasma, arc or spark discharge, etc.

Atomic fluorescence spectroscopy uses the emission of radiation from atoms excited by electromagnetic radiation from an external source.

Atomic absorption spectroscopy founded on the absorption of radiation from an external source by atoms.

X-ray methods are based on energy transitions of the internal electrons of atoms. Depending on the method of receiving and recording the signal, there are X-ray emission, X-ray absorption And X-ray fluorescence spectroscopy. Varieties of these methods are - Auger spectroscopy, X-ray electron probe analysis, electron spectroscopy- used mainly to study the structure of substances. X-ray methods do not require atomization of the substance and allow the examination of solid samples without preliminary preparation.

Nuclear methods based on the excitation of atomic nuclei.

In Fig. 5. Various methods based on atomic emission or absorption are given. These methods are widely used and are characterized by high selectivity, exceptional sensitivity, speed and convenience; they are among the most selective analytical methods. These methods can determine approximately 70 elements. Sensitivity usually lies in the range 10 -4 -10 –10 %. Atomic spectral analysis can often be completed in a few minutes.

Rice. 5. Classification of atomic spectroscopy methods

In 1885 The Swiss physicist I. Balmer found a certain pattern in the arrangement of the spectral lines of hydrogen and showed that the wavelengths corresponding to the lines of the visible part of the spectrum can be calculated using the formula, which is now written in the form

Here to each integer n, more than two corresponds spectral line, A R called Rydberg constant

Later, other series of lines were found in the spectrum of hydrogen, obtained by replacing the two with the integer m, and the formula took the following form:

m And n– integers.

At m= 1, lines are obtained that lie in the ultraviolet part of the spectrum, constituting the Lyman series, and when m= 3 – in the infrared part – Paschen series.

From the point of view classical physics It was impossible to explain the presence of the lines themselves, much less the patterns in their arrangement, as well as the stability of the atom itself. Indeed, moving around the core with centripetal acceleration electron must radiate electromagnetic waves, losing energy and falling onto the core, which does not happen. The Danish physicist N. Bohr overcame the contradictions.

According to Bohr's idea, the energy of an atom cannot have an arbitrary value. For each atom there is a number of strictly defined discrete values the energies it can possess, which are called the energy levels of the atom.

1) The energy levels of an atom are determined by the formula:

The energy of an atom is determined only by the orbital number (principal quantum number) n, since all other quantities in this formula are fundamental constants.

At n = 1:

This energy is called the atomic binding energy or ionization energy - this amount of energy must be imparted to an electron in order to remove it from the atom.

At n = 2:

n = 3: etc.

Since the energy of an electron in an atom can only take on a discrete set of values, it is said to be quantized.

Bohr's theory is based on three postulates:

I (postulate of stationary states).

An electron in an atom can only be in special stationary (quantum) states, each of which corresponds to a certain energy. When the electron is in a stationary state, the atom does not radiate.

II postulate (frequency rule).

An electron in an atom can jump from one steady state to another. During this transition, a quantum is emitted or absorbed electromagnetic field with frequency u determined by the difference in electron energies in an atom in these states:

If , then energy is emitted, if - it is absorbed.

The state of the atom, which corresponds to the lowest energy, is called ground, the others are excited. The lifetime of an atom in an excited state is ~10 -8 s.

Bohr's III postulate (orbit quantization rule).

Stationary (allowed) electron orbits in an atom are found from the condition n= 1, 2, 3…– (orbit number is the main quantum number)

Where m– electron mass;

V- linear speed of its movement;

r n– radius n-th orbit;

h– Planck’s constant.

Louis de Broglie discovered the essence of the mysterious rule for quantizing electron orbits in an atom. According to de Broglie's hypothesis, each electron in an atom is stationary orbit corresponds standing wave with an integer number of wavelengths on the circle. If a whole number of wavelengths do not fit on the circle, then the wave “does not close” on itself and quickly decays. Thus, the allowed orbits of an electron in an atom correspond to the condition of maximum interference of the corresponding de Broglie wave.

This article introduces the basic concepts needed to understand how light is emitted and absorbed by atoms. The application of these phenomena is also described here.

Smartphone and physics

A person who was born after 1990 lived his life without a variety of electronic devices can't imagine. The smartphone not only replaces the telephone, but also makes it possible to monitor exchange rates, make transactions, call a taxi and even correspond with astronauts on board the ISS through its applications. Accordingly, all these digital assistants are taken for granted. The emission and absorption of light by atoms, which made possible the era of shrinkage of all kinds of devices, will seem to such readers only a boring topic in physics lessons. But there is a lot of interesting and exciting things in this section of physics.

Theoretical background for the discovery of spectra

There is a saying: “Curiosity will never lead you to any good.” But this expression rather refers to the fact that it is better not to interfere in other people’s relationships. If you show curiosity about the world around you, nothing bad will happen. At the end of the nineteenth century, people began to understand the nature of magnetism (it is well described in Maxwell's system of equations). Next question What scientists wanted to resolve was the structure of matter. We must immediately clarify: what is valuable for science is not the emission and absorption of light by atoms. Line spectra are a consequence this phenomenon and the basis for studying the structure of substances.

Atomic structure

Scientists are still in Ancient Greece suggested that marble consists of some indivisible pieces, “atoms”. And until the end of the nineteenth century, people thought that these were the smallest particles of matter. But Rutherford's experiment on the scattering of heavy particles on gold foil showed: the atom also has internal structure. The heavy nucleus is located in the center and is positively charged; light negative electrons revolve around it.

Paradoxes of the atom within the framework of Maxwell's theory

These data gave rise to several paradoxes: according to Maxwell's equations, any moving charged particle emits an electromagnetic field, and therefore loses energy. Why then do the electrons not fall onto the nucleus, but continue to rotate? It was also not clear why each atom absorbs or emits photons of only a certain wavelength. Bohr's theory made it possible to eliminate these inconsistencies by introducing orbitals. According to the postulates of this theory, electrons can only be around the nucleus in these orbitals. The transition between two neighboring states is accompanied by either the emission or absorption of a quantum with a certain energy. The emission and absorption of light by atoms occurs precisely because of this.

Wavelength, frequency, energy

For more full picture It is necessary to tell a little about photons. This elementary particles, which have no rest mass. They exist only while they move through the medium. But they still have mass: when they hit a surface, they transfer momentum to it, which would be impossible without mass. They simply convert their mass into energy, making the substance they hit and are absorbed by a little warmer. Bohr's theory does not explain this fact. The properties of the photon and the features of its behavior are described by quantum physics. So, a photon is both a wave and a particle with mass. The photon, and like a wave, has the following characteristics: length (λ), frequency (ν), energy (E). The longer the wavelength, the lower the frequency, and the lower the energy.

Spectrum of an atom

The atomic spectrum is formed in several stages.

1. An electron in an atom moves from orbital 2 (from more high energy) to orbital 1 (lower energy).
2. A certain amount of energy is released, which is formed as a quantum of light (hν).
3. This quantum is emitted into the surrounding space.

This is how it turns out line spectrum atom. Why it is called that way is explained by its shape: when special devices “catch” outgoing photons of light, a series of lines are recorded on the recording device. To separate photons of different wavelengths, the phenomenon of diffraction is used: waves with different frequencies have different refractive indexes, therefore some are deflected more than others.

Properties of substances and spectra

The line spectrum of a substance is unique for each type of atom. That is, hydrogen, when emitted, will give one set of lines, and gold - another. This fact is the basis for the use of spectrometry. Having obtained the spectrum of anything, you can understand what the substance consists of, how the atoms in it are located relative to each other. This method allows you to determine and various properties materials that chemistry and physics often use. The absorption and emission of light by atoms is one of the most common tools for studying the world around us.

Disadvantages of the emission spectra method

To at this moment it was rather about how atoms radiate. But usually all electrons are in their orbitals in a state of equilibrium; they have no reason to move to other states. For a substance to emit something, it must first absorb energy. This is a disadvantage of the method, which exploits the absorption and emission of light by an atom. Briefly, a substance must first be heated or illuminated before we obtain a spectrum. No questions will arise if a scientist studies stars; they already glow thanks to their own internal processes. But if you need to study a piece of ore or food product, then to obtain a spectrum it actually needs to be burned. This method is not always suitable.

Absorption spectra

Emission and absorption of light by atoms as a method “works” in two directions. You can shine broadband light on a substance (that is, one in which photons of different wavelengths are present), and then see which wavelengths are absorbed. But this method is not always suitable: it is necessary that the substance be transparent for the desired part electromagnetic scale.

Qualitative and quantitative analysis

It became clear: the spectra are unique for each substance. The reader could conclude that such analysis is used only to determine what the material is made of. However, the possibilities of the spectra are much wider. Using special techniques for examining and recognizing the width and intensity of the resulting lines, it is possible to determine the number of atoms included in the compound. Moreover, this indicator can be expressed in different units:

• as a percentage (for example, this alloy contains 1% aluminum);
• in moles (3 moles of table salt are dissolved in this liquid);
• in grams (this sample contains 0.2 g of uranium and 0.4 grams of thorium).

Sometimes the analysis is mixed: qualitative and quantitative at the same time. But if earlier physicists memorized the position of lines and assessed their shade using special tables, now all this is done by programs.

Application of spectra

We have already discussed in some detail what the emission and absorption of light by atoms is. Spectral analysis is used very widely. There is no area human activity, wherever the phenomenon we are considering is used. Here are some of them:

1. At the very beginning of the article we talked about smartphones. Silicon semiconductor elements have become so small thanks in part to studies of crystals using spectral analysis.
2. In any incident, it is the uniqueness electron shell each atom makes it possible to determine which bullet was fired first, why the frame of a car broke or a tower crane fell, as well as what poison a person was poisoned with and how long he spent in the water.
3. Medicine uses spectral analysis for its own purposes most often in relation to body fluids, but it happens that this method is also applied to tissues.
4. Distant galaxies, clouds of cosmic gas, planets near alien stars - all this is studied with the help of light and its decomposition into spectra. Scientists learn the composition of these objects, their speed and the processes that occur in them by being able to record and analyze the photons they emit or absorb.

Electromagnetic scale

What we pay most attention to is visible light. But on the electromagnetic scale this segment is very small. What the human eye cannot detect is much wider than the seven colors of the rainbow. Not only visible photons (λ = 380-780 nanometers), but also other quanta can be emitted and absorbed. Electromagnetic scale includes:

1. Radio waves(λ = 100 kilometers) transmit information to long distances. Because of the very long length The waves have very low energy. They are very easily absorbed.
2. Terahertz waves(λ = 1-0.1 millimeters) were difficult to access until recently. Previously, their range was included in radio waves, but now this segment of the electromagnetic scale is allocated to a separate class.
3. Infrared waves (λ = 0.74-2000 micrometers) transfer heat. A fire, a lamp, the Sun emit them in abundance.

We have considered visible light, so we will not write about it in more detail.

Ultraviolet waves(λ = 10-400 nanometers) are lethal to humans in excess, but their deficiency also causes irreversible processes. Our central star produces a lot of ultraviolet light, but the Earth's atmosphere blocks most of it.

X-ray and gamma quanta (λ < 10 нанометров) имеют общий диапазон, но различаются по происхождению. Чтобы получить их, нужно разогнать электроны или атомы до очень high speeds. Human laboratories are capable of this, but in nature such energies are found only inside stars or during collisions of massive objects. Example last process could be supernova explosions, the absorption of a star by a black hole, the meeting of two galaxies or a galaxy and a massive cloud of gas.

Electromagnetic waves of all ranges, namely their ability to be emitted and absorbed by atoms, are used in human activity. Regardless of what the reader has chosen (or is just about to choose) as his life path, he will definitely encounter the results of spectral research. The seller uses a modern payment terminal only because a scientist once studied the properties of substances and created a microchip. The farmer fertilizes the fields and now reaps large harvests only because a geologist once discovered phosphorus in a piece of ore. The girl wears bright clothes only thanks to the invention of permanent chemical dyes.

But if the reader wants to connect his life with the world of science, then he will have to study much more than the basic concepts of the process of emission and absorption of light quanta in atoms.

Molecular absorption spectroscopy. Basic law of light absorption. Practical application of the method.

Absorption spectroscopy. The basic law of light absorption (law B-L-B).

An atom, ion or molecule, absorbing a quantum of light, goes into a higher energy state. Usually this is a transition from a basic, unexcited level to one of the higher ones, most often to the first excited level. Due to the absorption of radiation as it passes through a layer of substance, the intensity of the radiation decreases and the greater, the higher the concentration of the light-absorbing substance.

Bouguer-Lambert-Beer law connects the decrease in the intensity of light passing through a layer of light-absorbing substance with the concentration of the substance and the thickness of the layer. To take into account light losses due to reflection and scattering, compare the intensities of light transmitted through the test solution and the solvent. With the same layer thickness in cuvettes made of the same material containing the same solvent, losses due to reflection and light scattering will depend on the concentration of the substance.

Reducing the intensity of light passing through the solution, characterized by transmittance(or just by passing) T: T= I / I 0, where I and I 0 are the intensity of light transmitted through the solution and solvent, respectively.

The logarithm T taken with the opposite sign is called optical density A:

Lg T= -lg (I / I 0)=lg (I 0 / I)=A.

The decrease in light intensity as it passes through a solution is subject to Bouguer-Lambert-Beer law: I=I 0 10 - elc, or I / I 0 =10 -elc,or -lg T=A=el c(1)

where e – molar absorption coefficient; l – thickness of the light-absorbing layer;c – solution concentration.

Physical meaning e becomes clear if we take I=1 cm and c=1 mol/l, then A= e . Hence, molar absorption coefficient equals optical density one-molar solution with a layer thickness of 1 cm.

The optical density of a solution containing several colored substances has the property of additivity, which is sometimes called by law additivity of light absorption. According to this law, the absorption of light by any substance does not depend on the presence of other substances in the solution. If there are several colored substances in a solution, each of them will make its own additive contribution to

Limitations and conditions of applicability of the Bouguer-Lambert-Beer law:

1. The law is fair for monochromatic light. To note this limitation, indices are introduced into equation (1) and written in the form: A l = e l l c . (2)

Index l indicates that the quantities A and e refer to monochromatic radiation with wavelength l .

2. Coefficient e in equation (1) depends on the refractive index of the medium. If the concentration of the solution is relatively low, its refractive index remains the same as that of a pure solvent, and no deviations from the law are observed for this reason.

3. Temperature should remain during measurements constant at least within a few degrees.

4. beam of light must be parallel.

5. Equation (1) is observed only for systems in which light-absorbing centers are particles of only one type. If, when the concentration changes, the nature of these particles changes due to, for example, acid-base interaction, polymerization, dissociation, etc., then the dependence of A on c will not remain linear, since the molar absorption coefficient of the newly formed and initial particles will not be generally the same.

For example, when a solution of potassium dichromate is diluted, not only does the concentration of the dichromate ion decrease, but chemical interaction processes occur:

Cr 2 O 2- 7 +H 2 O= 2HCrO - 4 = 2CrO 2- 4 +2H +

Instead of dichromate ions, hydrochromate and chromate ions appear in the solution. The dependence of optical density on the total concentration of chromium in the solution will not be linear.

Absorption spectra.

Light is absorbed by the solution selectively: at some wavelengths, light absorption occurs intensely, and at others, light is not absorbed. Quanta of light are intensively absorbed, the energy of which hv equal to the particle excitation energy and the probability of their absorption greater than zero. The molar absorption coefficient at these frequencies (or wavelengths) reaches large values. The frequency (or wavelength) distribution of molar absorption coefficient values ​​is called absorption spectrum.

Typically, the absorption spectrum is expressed as a graphical dependence of the optical density A or molar absorption coefficient e on frequency n or wavelength l incident light. Instead of A or e their logarithms are often plotted.

Curves in Ig A coordinates - l , as shown in Fig. 1, when the concentration or thickness of the layer changes, they move up or down along the ordinate parallel to themselves, while the curves in coordinates A- l (Fig. 2) do not have this property. This feature is essential for qualitative analysis. When studying infrared spectra, the percentage of light transmittance is usually plotted as a function n " or n.

Fig.1. Dependence of Ig A on l .

1 - concentration solution With in a cuvette of thickness l, cm; 2 - solution of concentration 1/4 s or in a cuvette with a thickness of l, cm

Origin of absorption spectra.

The appearance of absorption bands is due to the discreteness of the energy states of the absorbing particles and quantum nature electromagnetic radiation. When light quanta are absorbed, the internal energy of the particle increases, which consists of the rotational energy of the particle as a whole, the energy of vibration of atoms and the movement of electrons:

E= E vr + E count + E el (5)

where E is the rotational energy, E is the vibrational energy, and E is the electronic energy.

Equation (5) should also include the fine and hyperfine structure energy terms associated with the electron and nuclear spin, a correction for the approximation of the additive scheme, and some other terms that can be neglected as a first approximation.

In terms of energy, rotational, vibrational and electronic motion differ quite significantly, and E vr<

1. Rotational spectra.

The rotational energy of molecules is usually considered using rigid rotator models, which represents two masses located at a fixed distance from one another.

Excitation of rotational energy levels occurs already upon absorption of far infrared (IR) and microwave radiation having a wavelength l >=10 2 µm or wave number n " >= 10 2 cm -1 . The energy of quanta in this region of the spectrum is equal to: E bp = 2.8 10 -3 n "= 1.2 kJ/mol or less. This value is comparable to the energy of thermal motion kT, therefore, already at room temperature, part of the rotational levels is populated.

Purely rotational spectra are not used for analytical purposes. They are used to study the structure of molecules, determination of internuclear distances, etc.

2. Vibrational spectra.

The bands associated with the excitation of vibrational energy levels are located in the spectral region from approximately 200...300 to 4000...5000 cm -1, which corresponds to a quantum energy of 3 to 60 kJ/mol. Therefore, at ordinary temperatures, the energy state of molecules is, as a rule, characterized by the ground vibrational level. The simplest model that is used when considering the vibrations of a diatomic molecule is harmonic oscillator model. This is a system of two masses connected by elastic force. The potential energy curve of a harmonic oscillator is usually approximated by a parabola (Fig. 3, curve 1).

It should only be noted that not all molecules have vibrational infrared spectra, but only those whose electric dipole moment changes during vibration.

For example, molecules HC1, HBr, etc., have IR spectra, but not H2, O2, etc.

The vibrational spectra of polyatomic molecules are interpreted on the basis of the doctrine of molecular symmetry and group theory. The mathematical apparatus of group theory allows one to calculate the number of frequencies and selection rules for molecules of different symmetries (to determine molecular constants and study the structure of molecules). To solve chemical analytical problems, so-called characteristic frequencies(usually for qualitative analysis). Analysis of the IR spectra showed that some of the observed frequencies can be matched to vibrations of individual atoms or groups of atoms. For example, it was found that in the spectra of all molecules containing C-H bonds, there are frequencies in the region of 2800...3000 cm -1, triple bond C- C is characterized by a frequency of 1650 cm -1, and the C-C triple bond has a frequency of 2100 cm -1.

3. Electronic spectra.

The upper energy limit of the vibrational spectrum is usually considered to be photon energy of approximately 5000 cm -1, or about 60 kJ/mol. A further increase in the energy of irradiating quanta will most often lead to the excitation of electrons and the appearance in the spectrum of bands characterizing electronic transitions. Interpretation of electronic spectra can be made on the basis of quantum mechanical concepts, such as the molecular orbital (MO) method.

Electronic transitions are the most complex due to the superposition of vibrational and, under certain conditions, rotational transitions. The superposition of a large number of vibrational transitions often leads to a significant broadening of the bands of electronic spectra, since the vibrational structure is not always resolved.

Absorption intensity.

For the analytical characterization of compounds, it is not so much the integral absorption that matters, but light absorption at a certain wavelength. Important analytical characteristics are molar absorption coefficient at the maximum point emax And half-width of absorption band(Fig. 6).

The bands caused by the transfer of an electron from one atom to another (charge transfer bands) have the greatest intensity in the absorption spectra. Often these bands are associated with the transfer of an electron from the p-orbital of the ligand to the d-orbital of the central ion and vice versa (molar absorption coefficient of the order of 10 4). Charge transfer explains, for example, the intense color of the MnO - 4, CrO 2- 4 ions , coloring of thiocyanate complexes of iron, cobalt, molybdenum, iron sulfosalicylate complexes and others.

Rice. 6. Absorption band.

Significantly less intense are the bands associated with intra-atomic d-d- or f - f -transitions. The spectra of colored compounds in solution are usually characterized by fairly wide absorption bands. The broadening of the bands is associated with the strong influence of solvent molecules on the energy levels of electrons responsible for light absorption and the superposition of vibrational transitions on the electronic transition.

Obviously, the higher the molar absorption coefficient and the smaller the band width, the more valuable the chemical-analytical properties of the compound, since these band characteristics also determine important indicators such as the detection limit and selectivity.

Main components of adsorption spectroscopy devices.

The main components are: a light source, a light monochromatizer, a cuvette with the test substance, a receptor (light receiver), an optical system (consisting of lenses, prisms and mirrors, which serves to create a parallel beam of light, change the direction and focus of light), as well as a system to equalize the intensity of light fluxes (diaphragms, optical wedges, etc.).

In absorption spectroscopy devices, light from an illumination source passes through a monochromatizer and falls on a cuvette with the substance being studied. The intensity of monochromatic light passing through the cuvette is measured by a light receiver (receptor). In practice, the ratio of the intensities of monochromatic light passing through the test solution and through the solvent or a specially selected reference solution is usually determined.

Light sources.

1. Tungsten incandescent lamps. A tungsten filament produces light in a wide spectral range. However, glass transmits light only in the wavelength range 350...1000 nm, i.e. in the visible part of the spectrum and the closest ultraviolet and infrared regions.

2. Gas-filled lamps (hydrogen, mercury). In a hydrogen lamp, hydrogen glows during discharge. The excitation conditions are selected so that almost continuous radiation occurs in the region of 200...400 nm. In a mercury lamp, the discharge occurs in mercury vapor. Excited mercury atoms emit a line spectrum, in which radiation with wavelengths of 254, 302, 334 nm predominates.

3. A Nernst pin is a column pressed from oxides of rare earth elements. When heated by passing an electric current, it produces IR radiation in the region of 1.6...2.0 or 5.6...6.0 microns.

4. A globar pin made of SiC carborundum produces radiation in the range of 2...16 µm also when passing an electric current.

5.The simplest devices use daylight as a lighting source.

Monochromators (monuromators).

Monochromatizers or monochromators are devices for producing light with a given wavelength. When designing monochromatizers, various optical phenomena are used: light absorption, interference, dispersion, etc. The most widely used devices in the practice of absorption spectroscopy are those that use light filters (absorption, interference, or interference-polarization) and prisms as monochromatizers.

The action of absorption filters is based on the fact that when light passes through a thin layer, due to absorption, a change occurs in the magnitude and spectral composition of the passing light flux. Absorption filters have low transparency (T = 0.1) and a fairly wide bandwidth ( D l = 30 nm or more). Characteristics interference filters much better. The filter consists of two thinnest translucent layers of silver, between which there is a dielectric layer. As a result of the interference of light, rays with a wavelength equal to twice the thickness of the dielectric layer remain in the passing beam. The transparency of interference filters is T = 0.3...0.8. The effective transmission width usually does not exceed 5...10 nm. To further narrow the passbands, a system of two sequential interference filters is sometimes used.

The most universal monochromatizers are prisms made of quartz, glass and some other materials. For infrared spectroscopy, prisms made of LiF, NaCl, KBr and other alkali and alkaline earth metal halides are used. The same materials are used to make cuvettes. Prisms make it possible to obtain highly monochromatic light over a wide range of wavelengths.

Light receivers (receptors).

Photocells and photomultipliers are mainly used as receptors in absorption spectroscopy devices, and sometimes the light intensity is assessed by eye. Photocells, thermoelements and bolometers are used to measure the intensity of infrared radiation. Light receivers are characterized spectral sensitivity– the ability to perceive radiation of different wavelengths – and integral sensitivity, which is measured by the effect on the receptor of radiation not decomposed into a spectrum.

Thermoelements use thermal emf, which occurs when the temperature of the junction between metals or alloys changes under the influence of infrared radiation. A heat-sensitive element, which is a blackened platinum, antimony or other thin metal plate, is included in the bridge circuit. The principle of operation of a bolometer is based on a change in the electrical resistance of a material when heated.

The industry produces various absorption spectroscopy instruments: colorimeters, photometers, photoelectro-colorimeters, spectrophotometers, etc., which use various combinations of illuminators, monochromatizers and light receivers.

Qualitative analysis.

From the point of view of qualitative analysis, vibrational (or rather vibrational-rotational) spectra are of greatest interest. Experimental studies of vibrational-rotational spectra have shown that bands at certain frequencies can be brought into correspondence with vibrations of certain groups of atoms or individual atoms in a molecule. Such frequencies called characteristic. Different molecules containing the same bond or the same atomic group will produce absorption bands in the region of the same characteristic frequency in the IR spectrum. This is the basis of qualitative analysis of infrared spectra. For example, bands in the region of 3000...3600 cm -1 can only be attributed to O-H or N-H bonds.

To date, the infrared spectra of more than 20,000 compounds have been studied and compiled into appropriate atlases and tables, which greatly facilitates practical analysis. To obtain the first approximate data, the so-called Koltup map is often used, which indicates the spectral regions of occurrence of many characteristic frequencies and their possible assignment. Infrared spectroscopy is also successfully used in the analysis of inorganic substances. For example, the characteristic frequency of CO 2- 3 is 1450 cm -1, SO 2- 4 - 1130, NO - 3 - 1380, NH + 4 - 3300 cm -1, etc. Electronic absorption spectra for the purposes of qualitative analysis are used much less frequently than vibrational ones, since they are usually represented by a small number of broad absorption bands, which often overlap one on top of the other and completely or partially overlap.

Quantitative analysis.

Quantitative analysis methods are based on the Bouguer-Lambert-Beer law. Main parameters photometric determination are the wavelength at which the measurement is made, optical density, cuvette thickness and concentration of the colored solution. A significant influence is exerted by various chemical factors associated with the completeness and conditions of the photometric reaction, the concentration of colored and other reagents, their stability, etc. Depending on the properties of the analyzed system and the characteristics of the photometric device used, certain analysis conditions are selected.

Optimal conditions for photometric determination.

Wavelength.When determining a single light-absorbing substance in a solution, the analytical wavelength is usually chosen at the maximum of the absorption band. If there are several bands in the spectrum, the choice is usually made on the most intense one, since work in the region of maximum light absorption provides the highest detection sensitivity. Flat maxima are more preferable, since in this case the error in establishing the wavelength is less affected than in the case of sharp maxima or steep sections of the curve. It is also desirable that the sensitivity of the radiation receiver in the region of the analytical wavelength be maximum.

Light transmittance (optical density). The measuring device of a photometric instrument usually has a constant error D T in transmittance value T throughout the entire range of its values. Error in optical density units D In this regard, A will not be the same throughout the entire interval. Therefore, when solving some problems, it is more convenient to operate with transmittance rather than with optical density. The relative error increases sharply at very small and very large values T. In the area of ​​average values T the curve passes through the minimum (Fig. 7). Highest measurement accuracy. will be achieved. at lnT+1=0, i.e. at the optical density value A =0,435.

Fig.7. Dependence of relative error on solution transmission

The calculation did not take into account the error due to other sources, such as, for example, the error when setting the device to zero and full transmission. A more rigorous theoretical consideration and experience have shown that the optimal optical density is at 0.6...0.7.

Thickness of the light-absorbing layer. The Bouguer-Lambert-Beer law equation shows that the greater the thickness of the layer, the greater the optical density and, therefore, the more sensitive the determination will be, all other things being equal. However, with increasing layer thickness (optical path length), light scattering losses increase, especially when working with solutions. Cuvettes with a layer thickness greater than 5 cm are usually not used for photometry of solutions.

Concentration conditions for carrying out the photometric reaction. The equation of the basic law of light absorption includes the concentration of a colored (light-absorbing) compound, therefore the transformation of the component being determined into such a compound is one of the most important operations, which largely determines the accuracy of the analysis. Colored compounds in solution are obtained mainly as a result of oxidation-reduction and complexation reactions. Redox reactions used in photometry, for example, the oxidation of manganese to MnO - 4, usually proceed almost completely to completion.

Much more complex is the question of the concentration conditions for the occurrence of complexation reactions in solution. A complicating influence here can be exerted by the processes of stepwise complex formation, protolytic equilibria, insufficient stability of the resulting complex, the intrinsic color of the reagent, etc. The effect of most of these factors can be foreseen if the equilibria in the system of interest have been studied in sufficient detail and the constants corresponding to the equilibrium are known (stability constants of coordination compounds , dissociation of reagents, etc.). Using these data, it is possible to calculate, for example, at what pH values ​​and reagent concentrations the required completeness of the reaction will be achieved, how the accompanying elements will influence, etc.

Sensitivity and accuracy of the method. The minimum concentration that can be determined photometrically is usually calculated from the relation

c min =A min /(e l).

If for approximate calculations we assume that A min = 0.01, l=1 cm and e =10 3, then

c min =0.01/10 3 =1*10 -5 mol/l.

This is not the minimum concentration of the photometric method, since e may be several orders of magnitude greater. The accuracy of photometric methods depends on the individual characteristics of the photometric reaction, the characteristics of the device used and other factors and varies over a fairly wide range. The usual error of photometric methods is approximately 1...2% (relative).

Basic techniques for photometric measurements

Calibration graph method. In accordance with the Bouguer-Lambert-Beer law, the graph in coordinates optical density - concentration must be linear and the straight line must pass through the origin of coordinates. The calibration graph is usually constructed using at least three points, which increases the accuracy and reliability of the determinations. In case of deviations from the Bouguer-Lambert-Beer law, i.e. when linear dependence is violated A from c, the number of points on the graph should be increased. The main limitations of the method are associated with the difficulties of preparing standard solutions and taking into account the influence of the so-called third components, i.e. components that are in the sample are not themselves determined, but influence the result.

Molar absorption coefficient method. When working using this method, the optical density of several standard solutions A st is determined, for each solution it is calculated e = A st /(1/ c st) and the resulting value e average. Then measure the optical density of the analyzed solution A x and calculate the concentration c x using the formula: c x = A x /( e l).

A limitation of the method is the obligatory subordination of the analyzed system to the Bouguer-Lambert-Beer law, at least in the region of the studied concentrations.

Additive method.This method is used when analyzing solutions of complex composition, since it allows one to automatically take into account the influence of “third” components. Its essence is as follows. First, determine the optical density A x of the analyzed solution containing the analyte component of unknown concentration c x , and then a known amount of the analyte component (c st) is added to the analyzed solution and the optical density A x + st is measured again.

The optical density A x of the analyzed solution is equal to: A x = e l c x

and the optical density of the analyzed solution with the addition of a standard one:

A x+st = e l(c x +c st)

From here we find the concentration of the analyzed solution:

The concentration of the analyte in the additive method can also be found from the graph in coordinates A x+st =f(c st .).E If we plot A x+st as a function of c st, we get a straight line, the extrapolation of which to the intersection with the x-axis will give a segment equal to – c x

Method of differential photometry. Photometry of intensely colored solutions is successfully carried out using the method of differential photometry. Conventional photometry compares the light intensity I x , passed through the analyzed solution of unknown concentration, with the intensity of light I 0 passed through the solvent. The transmittance of such a solution will be equal to the intensity ratio:

Quantitative analysis using infrared spectra. Analysis by IR spectra is also based on the application of the Bouguer-Lambert-Beer law. The most commonly used method here is the calibration graph method. The use of the molar absorption coefficient method in IR spectroscopy is significantly complicated by the fact that due to scattering, continuous absorption and other effects, it is often impossible to determine the position of the 100% transmission line, i.e. determine the intensity of light passing through the sample without the analyzed component (I 0).

Many difficulties of quantitative IR spectroscopy are successfully overcome using baseline method, which has become widespread in practice. Its essence is easy to understand from Fig. 11, which shows a section of the IR spectrum with two absorption bands (their wave numbers n A and n B). The baseline is drawn at the base of the absorption band (shown as a dotted line). Transmittance is determined in this method as the ratio T A = I A /I 0(A) or T B = I B /I 0(B). Based on the data obtained, a calibration graph is drawn and determinations are made.

Determination of a mixture of light-absorbing substances. The spectrophotometric method, in principle, makes it possible to determine several light-absorbing substances in one solution without prior separation. Of great practical importance is the special case of such a system—the analysis of a mixture of two colored substances. In accordance with the law of additivity of light absorption for such a mixture of substances, for example A and

A fairly common example of such an analysis is also determination using a reagent that has its own color. This method can be extended to more complex multicomponent mixtures. When the light absorption of individual components is subject to the Bouguer-Lambert-Beer law and the law of light absorption additivity is observed, the number of terms in equations of type (3.21) increases in proportion to the number of components being determined and the number of equations increases accordingly. Computers are successfully used to solve systems of such equations.

Determination of the composition and stability of complex compounds in solution. The simplicity and sufficient accuracy of photometric measurements have led to the widespread use of photometric methods for studying reactions in solution and especially color reactions that have chemical and analytical significance. The isomolar series method is often used to determine the composition of compounds. Using this method, a series of solutions is prepared in which the ratio of the central ion concentration to the ligand concentration (c M:c L) varies from 9:1 to 1:9, and the total concentration (c M +c L) remains the same in all solutions ( isomolar series). Then measure the optical density of the solutions and plot the dependence of optical density on the concentration ratio c M:c L . The maximum on this graph indicates the composition of the complex. The isomolar series method has limitations and disadvantages, however, it is one of the most widely used in practice.

Practical application.

Photometric and spectrophotometric methods of analysis are used to determine many (more than 50) elements of the periodic table, mainly metals. Absorption spectroscopy methods are used to analyze ores, minerals and other natural objects, processing products from processing and hydrometallurgical enterprises. These methods are effectively used in metallurgical, electronic, chemical and other industries, in medicine, biology, etc. They are of great importance in the analytical monitoring of environmental pollution and solving environmental problems. The areas of practical application of absorption spectroscopy methods have expanded significantly due to the wider use of the infrared region of the spectrum and devices with a built-in computer. This made it possible to develop methods for analyzing complex multicomponent systems without their chemical separation. Absorption spectroscopy methods continue to be successfully developed and improved.

General characteristics of the method.

Advantages:

1. High sensitivity (low detection limit).

2. Accuracy. The error of photometric methods is usually 3...5%, decreasing in favorable cases to 1...2% and often to 0.5. ..1.0%.

3. The methods can be applied to analyze large and low grades.

4. Possibility of determining impurities (up to 10 -5 ...10 -6%).

5. High selectivity of many photometric methods, allowing determination of elements in complex samples without chemical separation of components.

6. Simplicity.

7. Expressness.

Flame photometry

Determination of small amounts of fluorescein

Laboratory work

The method is based on the ability of fluorescein to glow green. The fluorescence intensity is proportional to the concentration of the substance when its content in solution is low.

The goal of the work is to master the techniques of fluorimetric determination and the calibration graph method.

1 Task: determine, using the calibration graph method, the fluorescein content in the control solution.

2 Equipment, chemical glassware, reagents:

1) fluorimeter;

2) volumetric flasks with a capacity of 50 ml – 7 pcs;

3) graduated pipette with a capacity of 10.0 ml;

4) fluorescein, alkaline solution with a titer of 7 ∙ 10 -10 g/ml;

5) distilled water.

3 Determination process:

1) add sequentially 1, 2, 3, 4, 5 and 6 ml of standard fluorescein solution into 50 ml volumetric flasks, adjust the volumes in the flasks to the mark with distilled water, mix;

2) prepare the device for operation in accordance with the instructions for the device or the recommendations of the teacher. Measure the fluorescence intensity of each solution of the prepared series, and then the control solution;

3) based on the measurement data, construct a calibration graph in the coordinates “fluorescence intensity – volume of the standard solution.”

Find the volume of control solution given by the teacher from the graph;

4) calculate, using the concentration of the standard solution of fluorescein, its content in the control solution in mg and the error.

Flame emission photometry (or simply flame photometry) is based on the use of the emission of light energy by the atoms of elements in a flame.

The structure of the outer electron shells of atoms determines the features of atomic spectra. Atoms with similar outer electron shells have optical spectra that are similar in structure. The optical spectra of electrons can be observed only when the atoms are isolated from each other. This is achieved by spraying a solution of the metal compound into the flame. In this case, the solvent evaporates, the molecules of the substance become atomized, and then the atoms are excited. The spectrum of such a flame can contain both lines and bands of emission. A line spectrum is characteristic of metal atoms, striped spectra are characteristic of molecules of oxides (MeO) and hydroxides (Me(OH) n) formed in some cases. The latter are often formed during the study of alkaline earth and rare earth elements. For alkali metals, atomization occurs more easily and almost completely. At a flame temperature of the gas-air mixture of about 1800-1900 degrees Celsius, only alkali and alkaline earth metals are excited. To obtain the spectra of most other elements, it is necessary to use oxygen as an oxidizer and other gases (acetylene, hydrogen), which make the flame temperature higher.

The appearance of spectra is associated with the transition valence electrons atoms from normal to higher energy levels. The energy expended on this movement is called excitation energy and is expressed in kJ (eV).

Alkali metals have the lowest excitation energy, and inert gases have the highest. After some time (about 10 -8 s), the excited atoms return to their normal state. The energy released in this case (∆E) is emitted in the form of a quantum of a certain wavelength λ.

∆E = E 1 – E 0 = hν = (hc)/λ,

where E 1 is the energy of the excited state, V;

E 0 – energy of the initial state, V;

h – Planck’s constant;

ν – radiation frequency;

λ – radiation wavelength, nm;

с – speed of light, s.

Since many atoms with different initial energies take part in the radiation, lines caused by all possible transitions inherent in the atoms of a given element are observed in the radiation spectrum. The intensity of the spectral lines depends on the number of atoms involved in the absorption and subsequent emission of energy; on the temperature of the source; from the energy of the upper level of the atom. An increase in temperature leads to an increase in intensity, but at the same time, ionization of atoms is possible with such an increase. For example, at temperatures above 2000 degrees, potassium atoms become ionized, and the ions emit radiation at a different wavelength than the atoms, which can lead to errors in measuring radiation intensity.

To obtain reliable results of quantitative flame photometric measurements, a number of requirements must be strictly observed. Standard solutions used to obtain the calibration curve should, if possible, have the same general composition as the solution being analyzed. Calibration solutions must be photometered simultaneously with the analyzed ones. The composition of the sample for analysis should be relatively simple, and the component being determined should be the main one and be contained in larger quantities.

The method of constructing a calibration graph and the method of additions are equally applicable.