1. General characteristics of physico-chemical methods
2. Overview of spectroscopic analysis methods.
3. A photometric method of analysis: photocolorimetry, colorimetry, spectrophotometry.
4. Overview of nephelometric, fluorescent, polarimetric analysis methods.
5. Refractometric method of analysis.
6. Overview of mass spectral, radiometric analyses.
7. Electrochemical methods of analysis (potentiometry, conductometry, coulometry, amperometry, polarography).
8. Chromatographic method of analysis.
The essence physico-chemical methods of analysis. Their classification.
Physico-chemical methods of analysis and chemical methods, based on the conduct of a particular chemical reaction. In physical methods chemical reactions are either absent or of secondary importance, although in the spectral analysis of the line intensity is always significantly depends on chemical reactions in coal electrode or gas flame. So sometimes the physical methods include a group of physical and chemical methods, as sufficiently rigorous unambiguous distinction between physical and physical-chemical methods are not, and allocation of physical methods in a separate group does not matter.
Chemical methods of analysis were not able to meet the diverse demands of practice increased as a result of scientific-technical progress, development of the semiconductor industry, electronics and computers, wide applications pure and ultrapure substances in technology.
The application of physicochemical methods of analysis are reflected in the technical-chemical control of food production, research and production laboratories. These methods are characterized by high sensitivity and fast analysis. They are based on the use of physico-chemical properties of substances.
When performing analyses of physical-chemical methods of the equivalence point (end of response) is not determined visually and with instruments that record the change in the physical properties of the test substance at the equivalence point. For this purpose are usually used devices with relatively complex optical or electrical schemes, so these methods are called instrumental methods of analysis.
In many cases to perform the analysis these methods do not require a chemical reaction in contrast to chemical methods of analysis. It is only necessary to measure any physical properties of the analyte: conductivity, light absorption, the refraction of light and other Physical and chemical methods allow for industry continuous control of raw materials, semi-finished and finished products.
Physico-chemical methods of analysis began to be applied later than chemical methods of analysis, who established and studied the relationship between the physical properties of substances and their composition.
The precision of physico-chemical methods varies greatly depending on the method. The highest accuracy (up to 0.001%) has coulometry, based on the measurement of the amount of electricity required for the electrochemical oxidation or restore the designated ions or elements. Most physico-chemical methods have an error of 2-5 %, which exceeds the accuracy of chemical analysis methods. However, such a comparison of errors not quite correct, since it refers to different concentration areas. With a small content of the designated component (about 10-3 % or less) classical chemical methods of analysis generally unsuitable; if large concentrations of physico-chemical methods successfully compete with the chemical. Among the significant drawbacks of most physico-chemical methods applies mandatory standards and standard solutions.
Among physico-chemical methods the most practical use are:
1. spectral and other optical methods (refractometry, polarimetry);
2. electrochemical methods of analysis;
3. chromatographic methods of analysis.
In addition to this, there are another 2 groups of physico-chemical methods:
1. radiometric methods, based on measuring the radioactive radiation of the given element;
2. mass spectrometric analysis methods based on the determination of the masses of individual ionized atoms, molecules and radicals.
The most extensive number of methods and important practical value is a group of spectral and other optical methods. These methods are based on the interaction of substances with electromagnetic radiation. It is known many different types of electromagnetic radiation: x-rays, ultraviolet, visible, infrared, microwave and radio. Depending on the type of interaction of electromagnetic radiation with matter, optical methods are classified as follows.
On the measurement of the polarization effects of molecules based refractometry, polarimetry.
The analyte can absorb electromagnetic radiation and through the use of this phenomenon distinguish the group of optical absorption methods.
The absorption of light by atoms of the analyzed substances used in atomic absorption analysis. The ability to absorb light molecules and ions in the ultraviolet, visible and infrared regions of the spectrum allowed to create a molecular absorption analysis (colorimetry, photocolorimetry, the spectrophotometry).
Absorption and scattering of light by suspended particles in the solution (suspension) led to the emergence of methods of turbidimetry and nephelometry.
Methods based on measuring the intensity of radiation resulting from the energy excited molecules and atoms of the analyte are called emission methods. To molecular emission methods include luminescence (fluorescence), atomic-emission - emission spectral analysis and flame photometry.
Electrochemical methods of analysis based on measurement of electrical conductivity (conductometry); potential difference (potentiometry); quantity of electricity passed through the solution (coulometry); the dependence of current on applied potential (volts-amperometry).
The group of chromatographic methods of analysis include methods of gas and gas-liquid chromatography, chromatography, thin-layer, adsorption, ion exchange and other chromatography.
Spectroscopic methods of analysis: General information
The concept of spectroscopic analysis method, its varieties
Spectroscopic methods of analysis— physical methods based on the interaction of electromagnetic radiation with matter. The interaction leads to different energy transitions that register of tool in the form of radiation absorption, reflection and scattering of electromagnetic radiation.
Classification:
• Emission spectral analysis based on the study of the spectra of emission (emission) or the emission spectra of various substances. A variation of this analysis is a flame photometry based on the measurement of intensity of radiation of atoms excited by the heating substances in the flame.
• Absorption spectral analysis based on the study of the absorption spectra of analytes. If there is absorption of radiation by atoms, then absorption is called nuclear, if molecules, the molecular. There are several types of absorption spectral analysis:
1. Spectrophotometry — takes into account the absorption of the analyte with light of a specific wavelength, i.e. absorption of monochromatic radiation.
2. Photometry based on the measurement of the absorption of the analyte light is not strictly monochromatic radiation.
3. Colorimetry is based on measuring the absorption of light by coloured solutions in the visible spectrum.
4. Nephelometry based on measuring the intensity of light scattered by particles suspended in solution, i.e. light scattered by a suspension.
• Fluorescent spectroscopy uses the illumination of the investigated object and arising under the action of ultraviolet rays.
Depending on what part of the spectrum is the absorption or emission, distinguish spectroscopy in the ultraviolet, visible and infrared regions of the spectrum.
Spectroscopy is a sensitive method for the determination of over 60 elements. It is used for the analysis of many materials, including biological environment, substances of vegetable origin, cements, glass and natural water.
Photometric methods of analysis
Photometric analysis methods based on selective light absorption by the analyte or a compound with a suitable reagent. Absorption intensity can be measured by any method, regardless of the nature of the coloured compounds. The accuracy of the method depends on the method of measurement. Distinguish colorimetric, spectrophotometric and photocolorimetric methods.
Photocolorimetric method of analysis.
Photocolorimetric method of analysis allows to determine quantitatively the intensity of absorption of light by the analyzed solution by photoelectrocolorimeter (sometimes called photocolorimeters). To do this, prepare series of standard solutions and draw the dependence of the light absorption of the analyte to its concentration. This relationship is called a calibration graph. In photocolorimeters light flows passing through the solution, have a wide range of absorption - 30-50 nm, so the light here is polychromatic. This leads to loss of reproducibility, accuracy and selectivity of the analysis. Advantages of photocalorimetry is of simple construction and high sensitivity with a large aperture radiation source – incandescent bulbs.
Colorimetric method of analysis.
Colorimetric method of analysis based on measurement of light absorption by substance. While comparing the color intensity, i.e. the optical density of the examined solution with the coloration (optical density) of a standard solution, whose concentration is known. The method is highly sensitive and is used to determine micro - and polymicrobial.
For analysis a colorimetric method requires much less time than by chemical means.
Visual analysis achieve equality in the intensity of staining of the analyzed and the painted solution. This can be achieved in 2 ways:
1. call color, by changing the thickness of the layer;
2. select the standard solutions of different concentrations (method standard series).
However, visually it is impossible to establish quantitatively how many times one solution is colored more intensely than the other. In this case, you can only set the same color of the analyzed solution when compared with standard.
The basic law of light absorption.
If the light flux, whose intensity I0 to direct the solution in a flat glass vessel (cuvette), one part of his intensity Ir reflected from the surface of the cell, another part is absorbed intensity Ia solution and the third part of the intensity It passes through the solution. Between these values there is the dependence:
I0 = Ir + Ia + It (1)
Because the intensity Ir of the reflected part of the light flux when working with the same cuvettes constant and negligible, the calculations can be neglected. Then the equality (1) takes the form:
I0 = Ia + It (2)
This equality characterizes the optical properties of the solution, i.e. its ability to absorb or transmit light.
The intensity of the absorbed light depends on the number of painted particles in the solution that absorb more light than thinner.
The luminous flux passing through the solution loses some intensity – the greater, the greater the concentration and the thickness of the mortar layer. Colored solutions there is a relationship called the law of the Bouguer – Lambert – Bera (between the degree of absorption of light, intensity of incident light, the concentration of the colored substance and the thickness of the layer).
Under this law, the absorption monochromaticism light transmitted through the layer of dyed liquid is proportional to the concentration and the thickness of the layer:
I = I0·10-kCh,
where I is the intensity of light transmitted through the solution; I0 is the incident light intensity; C – concentration, mol/l; h – thickness of layer, cm, k is the molar absorption coefficient.
The molar absorption coefficient k – the optical density of a solution containing 1 mol/l of the absorbing material, with the layer thickness of 1 cm It depends on the chemical nature and physical condition of the light absorbing substance and the wavelength of monochromatic light.
Method standard series.
Standard series method is based on obtaining the same color intensity of the investigated and standard solutions with the same thickness of the layer. Coloration of the examined solution is compared with the color range of standard solutions. With the same intensity of staining concentrations of the studied and standard solutions are equal.
To prepare series of standard solutions take 11 vials of the same shape, size and from the same glass. Poured from a burette a standard solution in gradually increasing numbers, for example: 1 vial of 0.5 ml, in 2 1 ml in 1.5 ml 3rd, etc. up to 5 ml (in each of the following test tube 0.5 ml more than the previous). All the vials are poured equal volumes of solution, which yields by ion color reaction. The solutions were diluted so that the liquid levels in all tubes was the same. The vials are stoppered, the contents mixed thoroughly and placed in a tripod at increasing concentrations. Thus a color scale.
To the test solution into the same test tube add the same number of reagent, dilute with water to the same volume as other tubes. Close the tube, thoroughly mix the contents. Coloration of the examined solution is compared with color of standard solutions on a white background. The solutions should be well lit ambient light. If the intensity of staining of the investigated solution coincides with the intensity of the color of one of the solutions for color scales, the concentrations of this and researched solutions are equal. If the color intensity of the investigated solution of intermediate intensity between two neighboring solutions of the scale, its concentration is equal to the average concentration of these solutions.
Application of the method standard solutions useful only for determining the mass of any substance. Prepared a series of standard solutions is relatively short time.
The adjustment method color intensity of the solutions.
The adjustment method color intensity of the investigated and standard solutions made by changing the height of the layer of one of the solutions. To do this in 2 identical vessel put colored solutions: the test and standard. Change the layer height of the solution in one vessel until then, until the intensity of color in both the solutions will not be the same. In this case, determine the concentration of test solution Sissl., comparing it with the concentration of a standard solution:
Sissl. = SST·SST / ISL,
where St and ISL – the height of the layer respectively the standard and the investigated solution.
The devices serving for determination of the concentration of the studied solutions by the method of adjustment color intensity are called colorimeters.
Distinguish between visual and photoelectric colorimeters. The visual colorimetric definitions, the intensity of the color measured by direct observation. Photovoltaic methods based on the use of photocells-of the photocolorimeters. Depending on the intensity of the incident beam of light in the solar cell creates an electrical current. The strength of the current caused by the light exposure is measured by the galvanometer. The deflection of the needle shows the intensity of the color.
Spectrophotometry.
Macedonian on the photometric measurement of the absorption of the analyte light is not strictly monochromatic radiation.
If the photometric method of the analysis of the use of monochromatic radiation (radiation of one wavelength), such a method is called spectrophotometry.The degree of monochromaticity of the stream of electromagnetic radiation is defined by the minimum wavelength interval, which is released used by the monochromator (filter, diffraction grating or prism) of the continuous flow of electromagnetic radiation.
To spektrofotometriya field measuring equipment combining spectrometry, photometry and Metrology and working on system development methods and devices for quantitative measurements of spectral coefficients of absorption, reflection, radiation, spectral brightness as the characteristics of environments, coatings, surfaces, emitters.
Stage spectrophotometric study:
1) carrying out chemical reactions to obtain the system, convenient for carrying out spectrophotometric analysis.
2) measure absorbance of the resulting solutions.
The essence of the method of spectrophotometry
The dependence of the absorption solution of the substance of the wavelength on the graph depicted in the form of the absorption spectrum of the substance, which is easy to allocate a maximum absorption located at a wavelength of light maximally absorbed by the substance. Measurement of the optical density of the solutions of substances on the spectrophotometers is carried out at the wavelength of maximum absorption. This allows you to analyze one solution of the substance, absorption maxima which are located at different wavelengths.
In spectrophotometry in the ultraviolet and visible regions using electronic absorption spectra.
They characterize the highest energy transitions, which are capable of a limited range of compounds and functional groups. In inorganic compounds, electronic spectra associated with the high polarization of the atoms within the molecule, and usually occur in complex compounds. Organic compounds the appearance of electronic spectra is caused by transition of electrons between the main and excited levels.
The position and intensity of absorption bands is strongly influenced by ionization. During ionization for the acid type in the molecule, an additional unshared pair of electrons, which leads to additional bath-romname shift (shift to the longwave region of the spectrum) and increase the intensity of the absorption bands.
In the spectrum of many substances have multiple absorption bands.
For spectrophotometric measurements in the ultraviolet and visible regions is used two types of instruments — geregistreerde(see the result on the scale of the instrument visually) and recording spectrophotometers.
Fluorescent method of analysis.
Luminescence — the ability to self-illumination arising under various influences.
The classification of processes causing luminescence:
1)photoluminescence (excitation of visible or ultraviolet light);
2)chemiluminescence (excitation by energy from chemical reactions);
3)cathodoluminescence (excitation by electron impact);
4)thermoluminescence (excitation by heating);
5)triboluminescence (excitation of mechanical action).
In chemical analysis have a value of the first two types of luminescence.
Classification of luminescence by the presence of the afterglow.It may stop immediately with the disappearance of excitement — fluorestsentsiya to continue for a certain time after the cessation of the exciting impact is the phosphorescence.Basically, using the phenomenon of fluorescence, therefore the method is called fluorimetry.
The use of fluorimetry:the analysis of traces of metals, organic (aromatic) compounds, vitamin D, vitamin B6. Fluorescent indicators used in the titration in turbid or dark-colored environments (titration carried out in the dark, lighting the titrated solution added to the indicator, fluorescent lamp).
Nephelometric analysis.
Nephelometrically F. Kober in 1912 and is based on measuring the intensity of light scattered by a particle suspension, using solar cells.
With the help of nephelometry to measure the concentration of substances that are insoluble in water, but form a persistent suspension.
For conducting nephelometric measurements are used nephelometry,similar in principle to the colorimeters, the only difference is that with nephelometry
When conducting fotodefolladasgratis first results of determination of the series of standard solutions to build the calibration curve, and then analyze the investigated solution and on schedule define the concentration of the analyte. To stabilize the resulting suspension is added a protective colloid — a solution of starch, gelatin, etc.
Polarimetric analysis.
Electromagnetic oscillations in natural light occur in all planes perpendicular to the beam direction. The crystal lattice has the ability to transmit rays of only a specific direction. At the output of the crystal oscillations of the beam occur only in one plane. Ray, the vibrations of which are in one plane, called polarized. The plane in which the vibrations occur, is called the plane of oscillations of polarized beam and the plane perpendicular thereto, the plane of polarization.
Polarimetric analysis method based on the study of polarized light.
Refractometric method of analysis.
Based on the refractometric method of analysis is the determination of the refractive index of analyte, because the individual substance is characterized by a particular refractive index.
Technical products always contain impurities which affect the magnitude of the refractive index. Therefore, the refractive index can in some cases be used to estimate the purity of the product. For example, varieties of turpentine differ in refractive indices. Thus, the refractive index of turpentine at 20° for yellow, denoted by n20D (entry indicates that the refractive index measured at 20°C, the wavelength of the incident light is equal to 598 CMI), is equal to:
First grade Second grade Third grade
1,469 – 1,472 1,472 – 1,476 1,476 – 1,480
Refractometric method of analysis can be applied to binary systems, for example to determine the concentration of substances in aqueous or organic solutions. In this case, the analysis is based on the dependence of the refractive index of the solution from the concentration of a solute.
For some solutions there is a table of the dependence of the refractive indices of their concentrations. In other cases analyzed by the method of calibration curve: prepare a series of solutions of known concentrations, measure their refractive index and build a graph of the refractive index on the concentration, ie, build a calibration curve. It determines the concentration of the investigated solution.