Quantitative Analysis allows you to install elemental and molecular composition the object under study or the content of its individual components.
Depending on the object of study, inorganic and organic analysis are distinguished. In turn, they are divided into elementary analysis, whose task is to establish how many elements (ions) are contained in the analyzed object, on the molecular and functional analysis s, giving an answer about the quantitative content of radicals, compounds, as well as functional groups atoms in the analyzed object.
Methods of quantitative analysis
The classical methods of quantitative analysis are gravimetric (weight) analysis and titrimetric (volume) analysis.
Instrumental methods of analysis
Photometry and spectrophotometry
The method is based on the use of the basic law of light absorption. A=elc. Where A is the absorption of light, e is the molar coefficient of light absorption, l is the length of the absorbing layer in centimeters, c is the concentration of the solution. There are several methods of photometry:
1. Atomic absorption spectroscopy
2. Atomic emission spectroscopy.
3. Molecular spectroscopy.
Atomic absorption spectroscopy
A spectrometer is required to perform analysis with this method. The essence of the analysis is to illuminate an atomized sample with monochrome light, then decompose the light that has passed through the sample with any light disperser and a detector to fix the absorption.
Various atomizers are used to atomize the sample. In particular: flame, high voltage spark, inductively coupled plasma. Each atomizer has its pros and cons. Various dispersants are also used to decompose light. This is diffraction grating, prism, light filter.
Atomic emission spectroscopy
This method is slightly different from the atomic absorption method. If in it a separate source of light was a source of light, then in the atomic emission method, the sample itself serves as a source of radiation. Everything else is similar.
Chromatography
Chromatography (from Greek chroma, Genitive chromatos - color, paint and ... graphics), a physicochemical method for separating and analyzing mixtures, based on the distribution of their components between two phases - stationary and mobile (eluent), flowing through a stationary one.
History reference. The method was developed in 1903 by M. Tsvet, who showed that when a mixture of plant pigments is passed through a layer of a colorless sorbent, individual substances are arranged in separate colored zones. The colored column of sorbent thus obtained in this way was called Tsvet chromatogram, and the method - X. Subsequently, the term "chromatogram" began to be attributed to different ways fixing the results of many types of X. However, up to the 40s. H. did not receive proper development. It was not until 1941 that A. Martin and R. Sing discovered the distributive chromatography method and demonstrated its broad possibilities for studying proteins and carbohydrates. In the 50s. Martin and the American scientist A. James developed the gas-liquid X-ray method.
The main types of Ch. Depending on the nature of the interaction that determines the distribution of components between the eluent and the stationary phase, the following main types of Ch. are distinguished - adsorption, distributive, ion-exchange, exclusion (molecular sieve), and sedimentary. Adsorption chlorine is based on the difference in the sorbability of the substances to be separated by the adsorbent ( solid with a developed surface); distributive chemistry - on the different solubility of the components of the mixture in the stationary phase (high-boiling liquid deposited on a solid macroporous carrier) and eluent (it should be borne in mind that with the distributive separation mechanism, the movement of component zones is also partially affected by the adsorption interaction of the analyzed components with a solid sorbent ); ion-exchange X. - on the difference in the constants of ion-exchange equilibrium between the stationary phase (ion exchanger) and the components of the mixture being separated; exclusion (molecular sieve) Ch. - on the different permeability of the molecules of the components into the stationary phase (highly porous non-ionic gel). Size exclusion chromatography is subdivided into gel filtration (GPC), in which the eluent is a non-aqueous solvent, and gel filtration, in which the eluent is water. Sedimentary X is based on the different ability of the separated components to precipitate on the solid stationary phase.
In accordance with the state of aggregation of the eluent, gas and liquid X are distinguished. Depending on the state of aggregation stationary phase Gas chlorine is either gas-adsorption (the stationary phase is a solid adsorbent) or gas-liquid (the stationary phase is liquid), while liquid chlorine is liquid-adsorption (or solid-liquid) and liquid-liquid. The latter, like gas-liquid, is a distributive chemist. Solid-liquid chemistries include thin-layer and paper.
There are column and planar X. In the column, special tubes - columns are filled with the sorbent, and the mobile phase moves inside the column due to the pressure drop. A variety of columnar X. - capillary, when a thin layer of sorbent is applied to the inner walls capillary tube. Planar cold is subdivided into thin-layer and paper. In thin-layer chemistries, a thin layer of granular sorbent or a porous film is applied to glass or metal plate; in the case of paper chromatography, special chromatographic paper is used. In planar chemistry, the movement of the mobile phase occurs due to capillary forces.
During chromatography, it is possible to change the temperature, composition of the eluent, its flow rate, and other parameters according to a given program.
Depending on the method of moving the mixture to be separated along the sorbent layer, the following types of Xing are distinguished: frontal, developing, and displacement. In the frontal version, a separated mixture is continuously introduced into the sorbent layer, consisting of a carrier gas and separated components, for example 1, 2, 3, 4, which itself is a mobile phase. Some time after the start of the process, the least sorbed component (for example, 1) is ahead of the rest and exits as a zone of pure substance before all, and behind it, in the order of sorption, the zones of mixtures of components are sequentially located: 1 + 2, 1 + 2 + 3, 1 + 2 + 3 + 4 (Fig., a). In the developing variant, an eluent flow continuously passes through the sorbent layer, and a mixture of substances to be separated is periodically introduced into the sorbent layer. Through certain time division occurs initial mixture on pure substances located in separate zones on the sorbent, between which there are eluent zones (Fig., b). In the displacement variant, the mixture to be separated is introduced into the sorbent, and then the carrier gas flow containing the displacer (eluent), during the movement of which the mixture is divided into zones after a certain period of time pure substances, between which there will be zones of their mixture (Fig., c). A number of types of chromatography are carried out using instruments called chromatographs, in most of which the developing variant of chromatography is used. Chromatographs are used for analysis and for the preparative (including industrial) separation of mixtures of substances. In the course of analysis, the substances separated in the chromatograph column, together with the eluent, enter at various time intervals into a detection device installed at the outlet of the chromatographic column, which records their concentrations over time. The resulting output curve is called a chromatogram. For a qualitative chromatographic analysis, the time from the moment of sample injection to the exit of each component from the column at a given temperature and using a certain eluent is determined. For quantitative analysis, the heights or areas of chromatographic peaks are determined, taking into account the sensitivity coefficients of the detection device used to the analyzed substances.
Gas chromatography, in which helium, nitrogen, argon, and other gases, are used as the eluent (carrier gas), is most widely used for the analysis and separation of substances that pass into the vapor state without decomposition. Silica gels, aluminum gels, molecular sieves, porous polymers, and other sorbents with a specific surface area of 5–500 m2/g are used as a sorbent (particles with a diameter of 0.1–0.5 mm) for the gas-adsorption variant of X. For gas-liquid chemistry, the sorbent is prepared by applying a liquid in the form of a film (high-boiling hydrocarbons, esters, siloxanes, etc.) several microns thick on a solid carrier with a specific surface area of 0.5-5 m2/g or more. The operating temperature limits for the gas-adsorption version of X. are from -70 to 600 °C, for the gas-liquid version from -20 to 400 °C. Gas chlorine can separate several cm3 of gas or mg of liquid (solid) substances; analysis time from several seconds to several hours.
In liquid column chemistry, highly volatile solvents (for example, hydrocarbons, ethers, and alcohols) are used as the eluent, and silica gels (including silica gels with various functional groups, such as ether, alcohol, and others, chemically grafted to the surface) are used as the stationary phase. ), aluminum gels, porous glasses; the particle size of all these sorbents is several microns. By supplying the eluent under pressure up to 50 MN/m2 (500 kgf/cm2), it is possible to reduce the analysis time from 2-3 hours to several minutes. To increase the efficiency of separation of complex mixtures, a time-programmed change in the properties of the eluent is used by mixing solvents of different polarity (gradient elution).
Liquid molecular sieve chemistry is distinguished by the use of sorbents that have pores of a strictly defined size (porous glasses, molecular sieves, including dextran and other gels). In thin-layer and paper chlorine, the liquid mixture under investigation is applied to the starting line (the beginning of a plate or strip of paper) and then separated into components by an ascending or descending eluent flow. The subsequent detection (development) of separated substances on a chromatogram (as in these cases they call a plate with a sorbent applied to it or chromatographic paper on which the mixture under study was separated into components) is carried out using ultraviolet (UV) spectroscopy, infrared (IR) spectroscopy or processing reagents that form colored compounds with the analyzed substances.
Qualitatively, the composition of mixtures is characterized with the help of these types of chlorine by a certain rate of movement of spots of substances relative to the rate of movement of the solvent under given conditions. Quantitative analysis is carried out by measuring the color intensity of the substance on the chromatogram.
Ch. is widely used in laboratories and industry for the qualitative and quantitative analysis of multicomponent systems, production control, especially in connection with the automation of many processes, and also for the preparative (including industrial) isolation of individual substances (for example, noble metals), separating rare and trace elements.
Gas chemistry is used for separation gases, determination of impurities harmful substances in air, water, soil, industrial products; determining the composition of products of the main organic and petrochemical synthesis, exhaust gases, medicines, as well as in forensics, etc. Equipment and methods for gas analysis in spaceships, analysis of the atmosphere of Mars, identification organic matter in lunar rocks, etc.
Gas chemistry is also used to determine the physicochemical characteristics of individual compounds: the heat of adsorption and dissolution, enthalpy, entropy, equilibrium constants, and complex formation; for solids this method allows you to measure the specific surface area, porosity, catalytic activity.
Liquid chemistry is used to analyze, separate, and purify synthetic polymers, drugs, detergents, proteins, hormones, and other biologically important compounds. The use of highly sensitive detectors makes it possible to work with very small amounts of substances (10-11-10-9 g), which is extremely important in biological research. Often used molecular sieve X. and X. by affinity; the latter is based on the ability of molecules biological substances selectively communicate with each other.
Thin-layer and paper chemistries are used to analyze fats, carbohydrates, proteins, and so on. natural substances and inorganic compounds.
In some cases, chromatography is used to identify substances in combination with other physicochemical and physical methods, such as mass spectrometry, IR and UV spectroscopy, and others. Computers are used to interpret chromatograms and select experimental conditions.
Lit .: Zhukhovitsky A. A., Turkeltaub N. M., Gas chromatography, M., 1962; Kiselev A. V., Yashin Ya. I., Gas-adsorption chromatography, M., 1967; Sakodynsky K. I., Volkov S. A., Preparative gas chromatography, M., 1972; Golbert K. A., Vigdergauz M. S., Course of gas chromatography, M., 1974; Chromatography on paper, trans. from Czech., M., 1962; Determan G., Gel chromatography, trans. from German., M., 1970; Morris C. J. O., Morris P., Separation methods in biochemistry, L., 1964.
RFA
Activation analysis
see also
Literature
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See what "Quantitative Analysis" is in other dictionaries:
QUANTITATIVE ANALYSIS, identification of quantities chemical substances included in the material or mixture. For analysis, chemical methods such as neutralization and oxidation are used, during which the concentration of components is determined ... ... Scientific and technical encyclopedic Dictionary
- (a. quantitative analysis; n. Quantitatsanalyse; f. analyse quantitative; i. analisis cuantitativo) determination of content or quantities. ratios of elements, functional groups, compounds or phases in the analyzed object. K.a.… … Geological Encyclopedia
Determination of the content or quantitative ratios of components in the analyzed object. Chapter analytical chemistry. An important characteristic of quantitative analysis methods, in addition to specificity and detection limit (see Qualitative analysis), ... ... Big Encyclopedic Dictionary
quantitative analysis- - analysis, the purpose of which is to establish the amount in the sample of certain chemical elements, atomic groupings or structures. Dictionary of Analytical Chemistry ... Chemical terms
QUANTITATIVE ANALYSIS- a section of analytical chemistry, whose task is to determine the amount (content) of elements (ions), radicals, functional groups, compounds or phases in the analyzed object. K. a. allows you to determine the elemental and molecular composition ... ... Great Polytechnic Encyclopedia
The task of quantitative analysis is to determine the quantitative
All methods of quantitative analysis are divided into chemical, physico-chemical and physical. Chemical methods include gravimetric, titrimetric and gas analyses, physicochemical methods include photometry, electrochemical and chromatographic analyses, and physical methods spectral analysis, luminescent.
1. Gravimetric analysis is based on determining the mass of a substance released in pure form or as a compound of known composition. For example, to determine the amount of barium in its compounds, the Ba 2+ ion is precipitated with dilute sulfuric acid. The BaSO 4 precipitate is filtered, washed, calcined and accurately weighed. By the mass of the precipitate BaSO 4 and its formula, calculate how much it contains
barium. The gravimetric method gives high accuracy results, but it is very labor intensive.
2. Titrimetric analysis based on precise measurement of reagent volume,
spent on the reaction with a certain component. The reagent is taken in the form of a solution of a certain concentration - a titrated (standard) solution. The moment when the reagent will be added in an amount equivalent to the content of the analyte, i.e. the end of the reaction is determined different ways. During titration, the reagent is poured in an amount equivalent to the amount of the test substance. Knowing the volume and exact concentration of the solution that reacted with the substance to be determined, its amount is calculated.
Titrimetric analysis gives less accurate results than gravimetric analysis, but its important advantage is high speed analysis. Depending on the type of reactions occurring during the titration, titrimetric analysis includes acid-base titration methods, oxidimetry methods, and precipitation and complex formation methods.
3. Methods of photometry are based on the measurement of absorption, transmission and scattering of light by a solution. For most photometric methods, so-called color reactions are used, i.e. chemical reactions accompanied by a change in the color of the solution. A method based on determining the content of a substance by color intensity is called colorimetry. The color intensity of the solution is assessed visually or with the help of appropriate instruments.
Sometimes the component to be determined is converted into a sparingly soluble compound, and its content is judged by the intensity of the turbidity of the solution. A method based on this principle is called nephelometry. Methods of colorimetry and nephelometry are used to determine the components that make up the analyte in very small quantities. The accuracy of this method is lower than gravimetric or titrimetric.
4. Electrochemical methods. These methods include electrogravimetric analysis, conductometry, potentiometry and polarography. Electrogravimetric method used to determine the concentration of metals. The element to be determined is deposited by electrolysis on an electrode whose mass is known. Conductometry and Potentiometry related to electrotitrimetry. The end of the reaction during titration is established either by measuring the electrical conductivity of the solution or by measuring the potential of the electrode immersed in the test solution. The potentiometric method is also used to determine the pH of a solution. Definition based on measurement electromotive force solution (emf), which depends on the concentration of hydrogen ions. In the polarographic method i.e., the amount of the ion being determined is judged by the nature of the current-voltage curve (polarogram) obtained by electrolysis of the test solution with a drop mercury cathode in a special device - a polarograph. This method is different high sensitivity. Using the polarographic method, it is possible to qualitatively and quantitatively determine in the same solution various elements without resorting to chemical reactions.
Analytical chemistry methods can be classified based on different principles. Depending on the measured property of the substance, the following methods are distinguished: chemical; physical and chemical; physical (Table 14). The basis of chemical methods are analytical chemical reactions. Physical and chemical methods are based on the measurement of any physical parameters chemical system, depending on the nature of the components of the system and changing in the course of a chemical reaction. These parameters include, for example, the values of potentials in potentiometry, optical densities in spectrophotometry, etc. Physical methods are not related to the application chemical reactions. The composition of a substance is established by changing any physical properties object (density, viscosity, radiation intensity, etc.). There are no clear boundaries between chemical and physicochemical and physicochemical and physical methods. Physical and physical and chemical methods often referred to as instrumental. AT recent times use the so-called "hybrid" methods, combining two or more methods. For example, chromato-mass spectrometry.
Methods of quantitative analysis
|
Analysis Methods |
||
|
Chemical |
Physico-chemical |
Physical |
|
gravimetry titrimetry |
electrochemical spectroscopic (optical) fluorescent kinetic thermometric chromatographic |
spectroscopic (not optical) nuclear physics radiochemical |
|
Analytical signal (value functionally related to the content of the analyte) |
||
|
change in the color of the indicator, the release of gas, sediment, etc. |
|
|
The analysis of a substance consists in obtaining empirically information about its chemical composition. Regardless of the methods used, the following requirements are imposed on the analysis:
- 1. The accuracy of the analysis is a collective characteristic of the method, including their correctness and reproducibility.
- 2. The correctness of the results of the analysis - obtaining results close to the real ones.
- 3. Reproducibility - obtaining the same or similar results with repeated determinations.
- 4. Expressivity - the speed of the analysis.
- 5. Sensitivity - minimal amount substance that can be determined by this method.
- 6. Versatility - the ability to define many components. It is especially important to determine them simultaneously in one sample.
- 7. Automation of analysis. When conducting mass homogeneous analyzes, one should choose a method that allows automation, which reduces labor intensity, errors, increases speed, and reduces the cost of analysis.
- 21. Characteristic analysis method
Quantitative analysis, a combination of chemical, physico-chemical and physical methods definitions quantitative ratio components that make up the analyte. Along with the qualitative analysis To. and. is one of the main branches of analytical chemistry. According to the amount of the substance taken for analysis, macro-, semi-micro-, micro- and ultra-micro methods are distinguished. K. a. In macromethods, sample weight is usually >100 mg, solution volume > 10 ml; in ultramicromethods - 1-10-1 mg and 10-3-10-6 ml, respectively (see also Microchemical analysis, Ultramicrochemical analysis). Depending on the object of study, inorganic and organic K. a. are distinguished, which, in turn, are divided into elemental, functional n molecular analysis. Elemental analysis allows you to determine the content of elements (ions), functional analysis - the content of functional (reactive) atoms and groups in the analyzed object. Molecular K. a. provides for the analysis of individual chemical compounds, characterized by a certain molecular weight. Importance has a so-called phase analysis - a set of methods for separating and analyzing individual structural (phase) components heterogeneous systems. In addition to specificity and sensitivity (see Qualitative Analysis), important characteristic methods K. and. - accuracy, that is, the value of the relative error of determination; accuracy and sensitivity in K. a. expressed as a percentage.
To the classical chemical methods of K. a. include: gravimetric analysis, based on an accurate measurement of the mass of the analyte, and volumetric analysis. The latter includes volumetric titrimetric analysis - methods for measuring the volume of a reagent solution consumed in a reaction with an analyte, and gas volume analysis - methods for measuring the volume of analyzed gaseous products (see Titrimetric analysis, Gas analysis).
Along with classical chemical methods, physical and physicochemical (instrumental) methods of CA are widely used, based on the measurement of the optical, electrical, adsorption, catalytic, and other characteristics of analyzed substances, which depend on their quantity (concentration). Usually these methods are divided into the following groups: electrochemical (conductometry, polarography, potentiometry, etc.); spectral or optical (emission and absorption spectral analysis, photometry, colorimetry, nephelometry, luminescent analysis, etc.); X-ray (absorption and emission X-ray spectral analysis, X-ray phase analysis, etc.); chromatographic (liquid, gas, gas-liquid chromatography, etc.); radiometric (activation analysis, etc.); mass spectrometric. Listed Methods, inferior to chemical ones in accuracy, significantly surpass them in sensitivity, selectivity, speed of execution. Accuracy of chemical methods K. a. is usually in the range of 0.005-0.1%; errors in the determination by instrumental methods are 5-10%, and sometimes much more. Sensitivity of some methods To. and. is given below (%):
Volume................................................. ......10-1
Gravimetric ................................................... 10-2
Emission Spectral..............................10-4
Absorption X-ray spectral ...... 10-4
Mass spectrometric ...............................10-4
Coulometric .............................................. 10-5
These are gravimetric and titrimetric methods. Although they are gradually giving way to instrumental methods, they remain unsurpassed in accuracy: their relative error less than 0.2%, while instrumental - 2-5%. They remain standard for evaluating the correctness of the results of other methods. Main application: precision determination of large and medium quantities of substances.
gravimetric method consists in isolating a substance in its pure form and weighing it. Most often, the isolation is carried out by precipitation. The precipitate should be practically insoluble. The component to be determined should precipitate almost completely, so that the concentration of the component in the solution does not exceed 10 -6 M. This precipitate should be as coarse as possible so that it can be easily washed out. The precipitate must be a stoichiometric compound of a certain composition. During precipitation, impurities are captured (co-precipitation), so it must be washed. The precipitate must then be dried and weighed.
Application of gravimetric methods:
You can determine most of the inorganic cations, anions, neutral compounds. For precipitation, inorganic and organic reagents are used; the latter are more selective. Examples:
AgNO 3 + HCl \u003d AgCl + HNO 3
(determination of silver or chloride ions),
BaCl 2 + H 2 SO 4 \u003d BaSO 4 + 2HCl
(determination of barium or sulfate ions).
Nickel cations are precipitated by dimethylglyoxime.
Titrimetric methods use reactions in solutions. They are also called volumetric, as they are based on measuring the volume of a solution. They consist in the gradual addition to a solution of an analyte with an unknown concentration of a solution of a substance reacting with it (with a known concentration), which is called a titrant. Substances react with each other in equivalent quantities: n 1 =n 2 .
Since n \u003d CV, where C - molar concentration equivalent, V is the volume in which the substance is dissolved, then for stoichiometrically reacting substances it is true:
C 1 V 1 \u003d C 2 V 2
Therefore, it is possible to find an unknown concentration of one of the substances (for example, C 2), if the volume of its solution and the volume and concentration of the substance that reacted with it are known. Knowing the molecular weight of the equivalent of M, you can calculate the mass of the substance: m 2 \u003d C 2 M.
In order to determine the end of the reaction (called the equivalence point), the color change of the solution is used or some physical-chemical property of the solution is measured. All types of reactions are used: neutralization of acids and bases, oxidation and reduction, complexation, precipitation. The classification of titrimetric methods is given in the table:
|
Titration method, type of reaction |
Method subgroups |
Titrants |
|
Acid-base |
Acidimetry | |
|
Alkalimetry |
NaOH, Na 2 CO 3 |
|
|
redox |
permanganatometry | |
|
Iodometry | ||
|
dichromatometry | ||
|
Bromatometry | ||
|
Iodatometry | ||
|
Complexometric |
Complexometry | |
|
Precipitation |
Argentometry |
Titration is either direct or reverse. If the reaction rate is low, a known excess of titrant is added to bring the reaction to completion, and then the amount of unreacted titrant is determined by titration with another reagent.
Acid-base titration is based on the neutralization reaction, during the reaction the pH of the solution changes. A plot of pH versus volume of titrant is called a titration curve and usually looks like this:
To determine the equivalence point, either pH-metry or indicators that change color when certain value pH. The sensitivity and accuracy of a titration are characterized by the steepness of the titration curve.
Complexometry is based on the reaction of complex formation. The most commonly used is ethylenediaminetetraacetic acid (EDTA).
(HOOC)(OOC-H2C)NH-CH2CH2-NH(CH2COO)(CH2COOH)
or her) disodium salt. These substances are often called complexones. They form strong complexes with many metal cations, so titration applications require separation.
Redox titration is accompanied by a change in the potential of the system. The course of the titration is usually controlled by the potentiometric method, see later.
Precipitation titration - argentometry is most often used as a method for determining halide ions. The latter form an almost insoluble precipitate with silver cations.
Methods titrimetric analysis have high accuracy (relative error of determination - 0.1 - 0.3%), low labor intensity, ease of instrumentation. Titrimetry is used for rapid determination of high and medium concentrations of substances in solutions, including non-aqueous ones.
QUANTITATIVE ANALYSIS
Classical chemical methods of analysis
Gravimetry (weight analysis).
The method is based on measuring the mass (weight) of a poorly soluble compound (precipitate) formed as a result of a chemical reaction between determined component and reagent(precipitator). The measurement is carried out by weighing on an analytical gravimetric balance.
Determined component + precipitant = sediment weighed form
(determined form) (reagent, (precipitated (gravimetric
reagent) form) form)
Titrimetry (titrimetric or volumetric analysis).
The method is based on accurate measurement of the volume of a solution of a known reagent that reacted with the component being determined. used in titrimetry. titrated solutions, whose concentration is known. These solutions are called titrants (working solutions). The process of gradually pouring (adding dropwise) a titrant solution to a solution of the analyte is called titration. During the titration, the amount of titrant is added equivalent to quantity the substance being determined.
The end of the reaction is called the stoichiometric point or the equivalence point.
Experimentally, the end of the titration is determined by the appearance or disappearance of the color of the solution, the cessation of precipitation, or with the help of indicators. This point, called the end point of the titration
Reaction requirements that form the basis of the methods
quantitative analysis
The interaction between the component to be determined and the reagent must proceed in certain stoichiometric ratios according to the reaction equation. The reaction should go almost to completion. The reaction product must be of a certain exact composition and formula.
The reaction must be fast high speed, which is especially important for direct titration. It is difficult to accurately fix the equivalence point for slow reactions. Adverse or competing reactions should be kept to a minimum.
There must be a satisfactory way to find (determine) the equivalence point and the end of the titration.
Titrimetry
Classification of titrimetric analysis methods
By types of chemical reactions
1. Acid - basic titration (neutralization method)
For example.
HCl + NaOH = NaCl + H2O
strong strong salt
acid base
indicator
HCl + NH 4 OH \u003d NH 4 Cl + H 2 O
weak salt
base
titrant determined
component
2. Redox titration
For example.
2 KMnO 4 + 10 FeSO 4 + 8 H 2 SO 4 = 2 MnSO 4 + 5 Fe 2 (SO 4) 3 + K 2 SO 4 + 8 H 2 O
oxidizing agent reducing agent acidic environment
titrant determined
substance
Titration methods
1. Direct titration method
The titrant is added in small portions (dropwise) to the solution of the component to be determined until the equivalence point.
The method of direct reverse titration: to the exact volume of the titrant in a conical flask, add in small portions (drop by drop) a solution of the analyte from the buret.
2.Back titration or Residue titration
In this case, two titrants with known exact concentrations are used. In a conical flask, an exact volume of the first titrant V 1 with an exact concentration C 1 is added in excess to the solution of the analyte. Since the first titrant is added in excess, part of it reacts with the analyte, and the unreacted part of the first titrant remains in solution and is titrated with the second titrant, and this consumes the volume V 2 of the second titrant with a concentration of C 2 .
If the concentrations of titrants are equal to each other (C 1 \u003d C 2), then the amount of the solution of the first titrant V that reacted with the component being determined is determined by the difference between the added V 1 and the titrated V 2 volume:
If the concentrations of titrants are not equal, then calculate the number of mole equivalents (n) of the first titrant that reacted with the analyte, by the difference between the number of mole equivalents of the first titrant C 1 V 1 and the number of mole equivalents of the second titrant C 2 V 2:
n \u003d C 1 V 1 - C 2 V 2
The back titration method is used when no suitable indicator is available or when the main reaction is not proceeding very rapidly.
For example. Determination of the amount of sodium chloride NaCl.
An excess volume of the first AgNO 3 titrant is added to the NaCl solution. Part of this titrant reacts with the analyte according to the equation
AgNO 3 + NaCl = AgCl + NaNO 3
Titrant 1 white
The remainder of titrant 1 (AgNO 3), which did not react with NaCl, is then titrated with a second NH 4 SCN titrant.
AgNO 3 + NH 4 SCN = AgSCN + NH 4 NO 3
Titrant 1 Titrant 2 red-brown
3. substitution titration method
This method is used when for some reason it is difficult to determine the equivalence point, especially when working with unstable substances that are easily oxidized by atmospheric oxygen, etc., or substances that are difficult to determine by direct titration, or the reaction is going on slowly.
The method consists in adding an auxiliary reagent to the substance to be determined, upon interaction with which quantitatively the reaction product is released. This liberated reaction product is called deputy and then titrated with the appropriate titrant.
For example.
K 2 Cr 2 O 7 + 6 KI + 7 H 2 SO 4 \u003d 3 I 2 + 4 K 2 SO 4 + Cr 2 (SO 4) 3 + 7 H 2 O
determined auxiliary acidic product
substance reagent reaction medium
deputy
I 2 + 2 Na 2 S 2 O 3 \u003d 2 NaI + Na 2 S 4 O 6
Deputy titrant indicator
Calculations in titrimetry
Law of Equivalents: Substances react with each other in equivalent amounts. AT general view for any reacting substances according to the law of equivalents
where n is the number of mole equivalents of reactants.
where C e is the molar concentration of the equivalent, mol / l.
C 1 V 1 = C 2 V 2
At the same concentration of solutions of the reacting substances, the reactions proceed between their equal volumes.
For example. For 10.00 ml of an acid solution, 10.00 ml of an alkali solution is consumed if their concentrations are 0.1 mol / l.
Titer(T) solution is the mass of a substance contained in 1 ml of a solution (or 1 cm 3), the dimension is g / ml.
T \u003d m (substance) / V (solution)
T \u003d C e M e / 1000
For example. T (HCl / HCl) = 0.0023 g / ml reads: titer of hydrochloric acid(or hydrochloric acid) for HCl is 0.0023 g / ml. This means that each 1 ml of this hydrochloric acid solution contains 0.0023 g of HCl or 2.3 mg in 1 ml.
NEUTRALIZATION METHOD
Single weight method
For example. A certain sample is taken into a conical flask m(chemically pure) oxalic acid H 2 C 2 O 4 2H 2 O (weighed on an analytical balance to the nearest 0.0001 g). Dissolved in water and fully titrated with NaOH solution with methyl orange indicator. Volume used for titration V ml of NaOH solution. Calculate the concentration of NaOH.
To calculate the concentration of NaOH, we use the formula:
m (H 2 C 2 O 4 2H 2 O) \u003d C (NaOH) x V (NaOH) x M (1/2 H 2 C 2 O 4 2H 2 O)
From this formula we derive C (NaOH), all other data are known.
QUANTITATIVE ANALYSIS
METHODS OF QUANTITATIVE ANALYSIS
In quantitative analysis, chemical, physical and physicochemical methods are distinguished. The assignment of a method to one group or another depends on the extent to which the definition chemical composition substances by this method is based on the use of chemical or physical processes, or a combination of both processes.
Developed analytical methods, which are based on the use of almost all known chemical and physical properties of atoms and molecules. It should be taken into account that analytical technique, as a rule, consists of several stages, each of which is based on a particular property.
According to three aggregate states matter - solid, liquid, gaseous - quantitative measurements can be carried out by determining the mass (by weighing) and by determining the volumes of liquid or gaseous substances.
Chemical Methods
Chemical methods are based on the following transformations: the formation of a precipitate or the dissolution of a precipitate, the formation of a colored compound or a change in the color of a solution, the formation of gaseous substances.
Chemical methods are used in analyzes that are called "classical". They are well tested, they consist of several stages, each of which introduces its own error, and requires the analyst to be attentive, accurate, and have great patience.
