Quantitative composition methods of quantitative analysis. detectable precipitant precipitated gravimetric
Quantitative analysis is a large section of analytical chemistry that allows you to determine the quantitative (molecular or elemental) composition of an object. Quantitative analysis has become widespread. It is used to determine the composition of ores (to assess the degree of their purification), the composition of soils, plant objects. In ecology, the content of toxins in water, air, and soil is determined. In medicine, it is used to detect fake drugs.
Tasks and methods of quantitative analysis
The main task of quantitative analysis is to establish the quantitative (percentage or molecular) composition of substances.
Depending on how this problem is solved, there are several methods of quantitative analysis. There are three groups of them:
- Physical.
- Physical and chemical.
- Chemical.
The former are based on measuring the physical properties of substances - radioactivity, viscosity, density, etc. The most common physical methods of quantitative analysis are refractometry, X-ray spectral and radioactivation analysis.
The latter is based on the measurement of the physicochemical properties of the analyte. These include:
- Optical - spectrophotometry, spectral analysis, colorimetry.
- Chromatographic - gas-liquid chromatography, ion-exchange, distribution.
- Electrochemical - conductometric titration, potentiometric, coulometric, electroweight analysis, polarography.
The third methods in the list of methods are based on the chemical properties of the substance under study, chemical reactions. Chemical methods are divided into:
- Weight analysis (gravimetry) - based on accurate weighing.
- Volumetric analysis (titration) - based on accurate measurement of volumes.
Methods of quantitative chemical analysis
The most important are gravimetric and titrimetric. They are called classical methods of chemical quantitative analysis.
Gradually, classical methods give way to instrumental ones. However, they remain the most accurate. The relative error of these methods is only 0.1-0.2%, while for instrumental methods it is 2-5%.
Gravimetry
The essence of gravimetric quantitative analysis is the isolation of the substance of interest in its pure form and its weighing. The isolation of a substance is most often carried out by precipitation. Sometimes the component to be determined must be obtained in the form of a volatile substance (distillation method). This way it is possible to determine, for example, the content of water of crystallization in crystalline hydrates. The precipitation method determines silicic acid in the processing of rocks, iron and aluminum in the analysis of rocks, potassium and sodium, organic compounds.
The analytical signal in gravimetry is mass.
The method of quantitative analysis by gravimetry includes the following steps:
- Precipitation of a compound that contains the substance of interest.
- Filtration of the resulting mixture to remove the precipitate from the supernatant.
- Washing the precipitate to eliminate the supernatant and remove impurities from its surface.
- Drying at low temperatures to remove water or at high temperatures to convert the precipitate into a form suitable for weighing.
- Weighing the resulting sediment.
The disadvantages of gravimetric quantitative analysis are the duration of the determination and non-selectivity (precipitating reagents are rarely specific). Therefore, a preliminary division is necessary.
Calculations with the gravimetric method
The results of the quantitative analysis carried out by gravimetry are expressed in mass fractions (%). For the calculation, it is necessary to know the mass of the sample of the test substance - G, the mass of the resulting sediment - m and its formula for determining the conversion factor F. The formulas for calculating the mass fraction and the conversion factor are presented below.
You can calculate the mass of the substance in the sediment, for this, the conversion factor F is used.
The gravimetric factor is a constant value for a given test component and gravimetric shape.
Titrimetric (volume) analysis
Titrimetric assay is the precise measurement of the volume of a reagent solution that is consumed in an equivalent interaction with a substance of interest. In this case, the concentration of the reagent used is pre-set. Given the volume and concentration of the reagent solution, the content of the component of interest is calculated.
The name "titrimetric" comes from the word "titer", which means one way of expressing the concentration of a solution. The titer shows how many grams of the substance are dissolved in 1 ml of the solution.
Titration is the process of gradually adding a solution of known concentration to a specific volume of another solution. It is continued until the moment when the substances react with each other completely. This moment is called the equivalence point and is determined by the change in color of the indicator.
- Acid-base.
- Redox.
- Precipitation.
- Complexometric.
Basic concepts of titrimetric analysis
The following terms and concepts are used in titrimetric analysis:
- A titrant is a solution that is added. Its concentration is known.
- A titratable solution is a liquid to which a titrant is added. Its concentration must be determined. The titratable solution is usually placed in the flask, and the titrant is placed in the burette.
- The equivalence point is the point in the titration when the number of equivalents of the titrant becomes equal to the number of equivalents of the substance of interest.
- Indicators - substances used to establish the equivalence point.
Standard and working solutions
Titrants are standard and working.
Standard ones are obtained by dissolving an exact sample of a substance in a certain (usually 100 ml or 1 l) volume of water or another solvent. So you can prepare solutions:
- Sodium chloride NaCl.
- Potassium dichromate K 2 Cr 2 O 7.
- Sodium tetraborate Na 2 B 4 O 7 ∙ 10H 2 O.
- Oxalic acid H 2 C 2 O 4 ∙2H 2 O.
- Sodium oxalate Na 2 C 2 O 4.
- Succinic acid H 2 C 4 H 4 O 4 .
In laboratory practice, standard solutions are prepared using fixanals. This is a certain amount of a substance (or its solution) in a sealed ampoule. This amount is calculated for the preparation of 1 liter of solution. Fixanal can be stored for a long time, since it is without air access, with the exception of alkalis that react with the glass of the ampoule.
Some solutions cannot be prepared with precise concentration. For example, the concentration of potassium permanganate and sodium thiosulfate changes already during dissolution due to their interaction with water vapor. As a rule, it is these solutions that are needed to determine the amount of the desired substance. Since their concentration is unknown, it must be determined before titration. This process is called standardization. This is the determination of the concentration of working solutions by their preliminary titration with standard solutions.
Standardization is necessary for solutions:
- Acids - sulfuric, hydrochloric, nitric.
- alkalis.
- Potassium permanganate.
- silver nitrate.
Indicator selection
To accurately determine the equivalence point, that is, the end of the titration, the correct choice of indicator is necessary. These are substances that change their color depending on the pH value. Each indicator changes the color of its solution at a different pH value, called the transition interval. For a properly selected indicator, the transition interval coincides with the change in pH in the region of the equivalence point, called the titration jump. To determine it, it is necessary to construct titration curves, for which theoretical calculations are carried out. Depending on the strength of the acid and base, there are four types of titration curves.
Calculations in titrimetric analysis
If the equivalence point is correctly determined, the titrant and the titratable substance will react in an equivalent amount, that is, the amount of the titrant substance (n e1) will be equal to the amount of the titrated substance (n e2): n e1 \u003d n e2. Since the amount of the equivalent substance is equal to the product of the molar concentration of the equivalent and the volume of the solution, the equality is true
C e1 ∙V 1 = C e2 ∙V 2, where:
C e1 - normal concentration of the titrant, a known value;
V 1 - the volume of the titrant solution, a known value;
C e2 - normal concentration of the titratable substance, it is necessary to determine;
V 2 - the volume of the solution of the titrated substance, is determined during the titration.
C e2 \u003d C e1 ∙ V 1 / V 2
Performing Titrimetric Analysis
The method of quantitative chemical analysis by titration includes the following steps:
- Preparation of a 0.1 N standard solution from a sample of the substance.
- Preparation of approximately 0.1 N working solution.
- Standardization of the working solution according to the standard solution.
- Titration of the test solution with a working solution.
- Carrying out the necessary calculations.
These are gravimetric and titrimetric methods. Although they are gradually giving way to instrumental methods, they remain unsurpassed in accuracy: their relative error is less than 0.2%, while instrumental ones are 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 is the molar concentration of the 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 at a certain pH value are used. 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 of 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.
Goals of quantitative analysis
Quantitative analysis allows you to establish the elemental and molecular composition of 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 elemental analysis, the task of which is to establish how many elements (ions) are contained in the analyzed object, into molecular and functional analyzes, which give an answer about the quantitative content of radicals, compounds, and functional groups of atoms in the analyzed object.
Methods of quantitative analysis
The classical methods of quantitative analysis are gravimetric (weight) analysis and titrimetric (volume) analysis.
For a complete classification of quantitative analysis methods, see the article Analytical chemistry.
Instrumental methods of analysis
For a classification of instrumental methods of analysis, see the article Instrumental methods of analysis
polarography
POLAROGRAPHY, a kind of voltammetry using an indicator microelectrode made of liquid metal, the surface of which is periodically or continuously updated. In this case, there is no long-term accumulation of electrolysis products on the electrode-solution interface in the electrolytic cell. The indicator electrode in polarography is most often a mercury dripping electrode. Also used are dripping electrodes from liquid amalgams and melts, jet electrodes from liquid metals, multi-drop electrodes, in which liquid metal or melt is forced through disks of porous glass, etc.
In accordance with the recommendations of IUPAC, there are several options for polarography: direct current polarography (investigates the dependence of the current I on the potential E of the indicator microelectrode), oscillopolarography (the dependence of dE / dt on t for a given I(t), where t is time), polarography with a scan I ( dependence of E on I), difference polarography (dependence of the current difference in two cells on E), polarography with a single or multiple sweep E during the lifetime of each drop, cyclic polarography with a triangular sweep E, step scan polarography E, decomp. types of alternating current and pulsed polarography, etc.
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:
- Atomic absorption spectroscopy
- Atomic emission spectroscopy.
- 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. Atomizers are used to atomize the sample. (flame, high voltage spark, inductively coupled plasma). Each atomizer has its pros and cons. For the decomposition of light, dispersants are used (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 light source, then in the atomic emission method, the sample itself serves as a source of radiation. Everything else is similar.
X-ray fluorescence analysis
Activation analysis
see also
Literature
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Books
- Analytical chemistry. Analytics 2. Quantitative analysis. Physico-chemical (instrumental) methods of analysis, Yury Yakovlevich Kharitonov. The textbook has been prepared in accordance with the federal state educational standard of the third generation. The book covers the basics of gravimetric, chemical titrimetric…
The task of quantitative analysis is to obtain information about the content of elements (ions), radicals, functional groups, compounds or phases in the analyzed object, as well as to develop methods by which this information is obtained. In quantitative analysis, the intensity of the analytical signal is measured, i.e. find the numerical value of the optical density of the solution, the consumption of the solution for titration, the mass of the calcined precipitate, etc. Based on the results of quantitative measurement of the signal, the content of the analyte in the sample is calculated. The results of the determinations are usually expressed in mass fractions,%.
With the help of quantitative analysis, mass ratios between elements in compounds are found, the amount of a dissolved substance in a certain volume of a solution is determined, sometimes the content of an element in a homogeneous mixture of substances is found, for example, carbon in oil or natural gas. In agricultural practice, the content of one or another component in heterogeneous substances is most often determined, for example: nitrogen, P 2 O 5 or K 2 O - in nitrogen, phosphorus or potassium fertilizers, trace elements - in the soil, sugars - in plant material, etc. .
Quantitative analysis is needed when evaluating mineral deposits, for metallurgy and the chemical industry, it is important for biology and agrochemistry, soil science, plant physiology, etc.
New problems for quantitative analysis are posed by the developing national economy - industry and agriculture; such, for example, are the development of methods for the separation and quantitative determination of "rare" or trace elements (uranium, titanium, zirconium, vanadium, molybdenum, tungsten, etc.); determination of negligibly small amounts of impurities of certain elements (arsenic, phosphorus, etc.) in many metals and trace elements in biological material, in soils.
Quantitative analysis allows biologists to obtain the necessary information about the composition of animal and plant organisms, to study the influence of individual elements on their growth, development and productivity.
The main objects of quantitative research in agriculture are soil, plants, fertilizers, agricultural poisons, feed, etc. Soils are analyzed in order to determine the degree of provision of plants with nutrients. The quantitative analysis of mineral fertilizers is used to check the content of components useful for crops (nitrogen, P 2 O 5 , K 2 O), and the analysis of agricultural poisons - to find the amount of the active principle. The composition of the feed must be known in order to correctly compose the diets of animals. They also analyze livestock and crop production.
Recently, due to the increased content of nitrates in soils, drinking water and crop products, it has become necessary to control food products. The content of nitrates is determined by ionometric or photometric methods.
Modern methods of quantitative analysis are classified according to measured properties, such as the mass of a substance, the volume of the reagent solution, the intensity of the spectral lines of the elements, the absorption of visible, infrared or ultraviolet radiation, the scattering of light by suspensions, the rotation of the polarization plane, the adsorption properties of sorbents, the electrical conductivity of the solution, the electrode potential , diffuse current strength, number of radioactive particles, etc.
Methods of quantitative analysis are divided into chemical, physical and physico-chemical.
Chemical methods include gravimetric, titrimetric and gas volumetric analyses.
Physical and physico-chemical methods of analysis are conditionally called instrumental.
In addition, there are so-called methods for separating mixtures of substances (or ions). These, in addition to various types of chromatography, include extraction with organic solvents, sublimation (and sublimation), distillation (i.e., distillation of volatile components), chemical methods of fractional precipitation and co-precipitation.
Of course, the above classification does not cover all the methods used by modern quantitative analysis; it lists only the most common of them.
2. DETERMINATION OF THE DISSOCIATION CONSTANT
Electrolytic dissociation is a reversible process that leads to equilibrium between undissociated molecules and ions, so the law of mass action applies to it. The ionization of a weak electrolyte proceeds according to the scheme
AB "A + + B -
If we denote the equilibrium concentration of non-dissociated molecules [AB], and the concentrations of ions - [A + ] and [B - ], respectively, then the equilibrium constant will take the form
[A + ][B —
]/[AB] = K (*)
The value of K is called electrolyte dissociation constant. It characterizes its tendency to ionization. How; the larger the value of K, the stronger the weak electrolyte dissociates and the higher the concentration of its ions in the solution at equilibrium. The value of the dissociation constant is calculated based on the molar concentration of the solution and the degree of ionization of a weak electrolyte (at a constant temperature).
This equation is a mathematical expression of the Ostwald dilution law, which establishes the relationship between the degree of dissociation of a weak electrolyte and its concentration.
For sufficiently weak electrolytes in not too dilute solutions, the degree of dissociation a is very small, and the value (1 - α) is close to unity. So for them
The regularities considered make it possible to calculate the dissociation constants of weak electrolytes from the degree of their dissociation found experimentally, and vice versa.
The dissociation constant, as well as the degree of dissociation, characterize the strength of -acids and bases. The greater the value of the constant, the more the electrolyte is dissociated in solution. Since the dissociation constant does not depend on the concentration of the solution, it better characterizes the tendency of the electrolyte to decompose into ions than the degree of dissociation. It has been experimentally proven that the dilution law is valid only for weak electrolytes.
In solutions of polybasic acids that dissociate in several steps, several equilibria are also established. Each such degree is characterized by its own dissociation constant.
Using the dissociation constants of the most important weak electrolytes, the degree of their dissociation is calculated.
a) Expression of the dissociation constant for potassium hydroxide
KOH« K + + OH -
b) Expression of the dissociation constant of acetic acid:
Dissociation equation
CH 3 COOH "H + + CH 3 COO -
Then the dissociation constant can be written
c) Expression of the dissociation constant
HCN « H + + CN -
3. ESSENCE AND METHODS OF VOLUME ANALYSIS. CALCULATIONS IN GRAVIMETRIC ANALYSIS. OPERATIONS OF THE GRAVIMETRIC METHOD OF ANALYSIS
The "classic" method is a titrimetric (volumetric) analysis. It is based on measuring the volumes of reacting solutions, and the concentration of the reagent solution must be precisely known. In volumetric analysis, the reagent is poured into the test solution until the moment when equivalent amounts of substances react. This moment is determined using indicators or other methods. Knowing the concentration and volume of the reagent used in the reaction, the result of the determination is calculated.
According to the type of chemical reactions used, the methods of titrimetric (volume) analysis are divided into three groups: 1) methods based on ion combination reactions; 2) methods based on oxidation-reduction reactions; 3) methods based on complex formation reactions. The first group includes methods of acid-base and precipitation titration, the second - various methods of redox titration, and the third - methods of complexometric (chelatometric) titration.
Acid-base titration method(or neutralization) is based on the interaction of acids with bases.
The method makes it possible to determine not only the concentration of acids or bases in solutions, but also the concentration of hydrolyzable salts.
To determine the concentration of bases or salts in solutions that give an alkaline reaction during protolysis, titrated acid solutions are used. These definitions are called acidimetry.
The concentration of acids or hydrolytic acid salts is determined using titrated solutions of strong bases. Such definitions refer to alkalimetry.
The neutralization equivalence point is determined by the change in color of the indicator (methyl orange, methyl red, phenolphthalein).
Precipitation titration method. The element to be determined, interacting with the titrated solution, can precipitate in the form of a poorly soluble compound. The latter, by changing the properties of the environment, allows one way or another to determine the equivalence point.
Titrimetric precipitation methods are given names depending on what serves as the titrant.
Method of complexometric titration combines titrimetric determinations based on the formation of low-ionizing complex ions (or molecules).
Using these methods, various cations and anions are determined that have the property of entering into complex formation reactions. Recently, methods of analysis based on the interaction of cations with organic reagents - complexones - have become widespread. This titration is called complexometric or chelatometric.
Redox Titration Methods(redox methods) are based on redox reactions between the analyte and the titrated solution.
They are used for quantitative determination in solutions of various reducing agents or oxidizing agents.
The gravimetric method determines, in addition, crystallization water in salts, hygroscopic water in soil, fertilizers, plant material. Gravimetrically determine the content of dry matter in fruits and vegetables, fiber, as well as "raw" ash in plant material.
In the course of gravimetric determination, the following operations are distinguished: 1) taking an average sample of a substance and preparing it for analysis; 2) taking a sample; 3) dissolution; 4) precipitation of the element to be determined (with a test for the completeness of precipitation); 5) filtering; 6) sediment washing (with a test for the completeness of washing); 7) drying and calcination of the precipitate; 8) weighing; 9) calculation of the results of the analysis.
Successful implementation of the definition requires, in addition to theoretical knowledge, a good command of the technique of individual operations.
The listed operations belong to the so-called sedimentation methods widely used in gravimetry.
But other methods are also used in gravimetry.
The isolation method is based on the isolation of the analyte from the analyte and its precise weighing (for example, solid fuel ash).
In the distillation method, the analyte is isolated as a volatile compound by the action of an acid or high temperature on the analyte. So, determining the content of carbon monoxide (IV) in a carbonate rock, its sample is treated with hydrochloric acid, the released gas is passed through absorption tubes with special reagents, and a calculation is made by increasing their mass.
Usually the results of gravimetric determinations are expressed in mass fractions (%). To do this, you need to know the sample size of the analyte, the mass of the resulting precipitate and its chemical formula.
Gravimetric determinations serve different purposes. In some cases, it is necessary to determine the content of an element in a chemically pure substance, for example, the content of barium in barium chloride BaCl 2 * 2H 2 O. In other cases, it is required to find the content of the active principle in some technical product or in general in a substance that has impurities. For example, it is necessary to determine the content of barium chloride BaCl 2 * 2H 2 O in commercial barium chloride. The technique of definitions in both cases may remain the same, but the calculations are different. Let's consider the course of calculations on examples.
Often, for calculations in gravimetric analysis, conversion factors, also called analytical factors, are used. The conversion factor (F) is the ratio of the molar mass (or Mg) of the analyte to the molar mass of the substance in the precipitate:
M of analyte ___
M of the substance in the sediment
The conversion factor shows how many grams of the analyte contains 1 g of sediment.
In the practice of technical and agricultural analysis, calculations are usually made according to ready-made formulas. For all calculations with complex numbers, a microcomputer should be used.
Records in the laboratory journal are of great importance. They are a document confirming the performance of the analysis. Therefore, the quantitative definition is briefly drawn up directly in the lesson. The date, the name of the analysis, the method of determination (with reference to the textbook), the data of all weighings or other measurements, and the calculation of the result are recorded in the journal.
BIBLIOGRAPHY
Kreshkov A.P. Fundamentals of Analytical Chemistry.–M.: Chemistry, 1991.
Classification of methods of quantitative analysis. Main steps of quantitative analysis
Quantitative Analysis- a set of methods of analytical chemistry, the task of which is to determine the quantitative content of individual constituents in the substance under study.
Depending on the object of study, inorganic and organic analysis are distinguished. In turn, they are divided into elemental analysis, the task of which is to establish how many elements are contained in the analyzed object, on molecular And functional analyzes that give an answer about the quantitative content of radicals, compounds, as well as functional groups of atoms in the analyzed object.
Methods of quantitative analysis are divided into chemical, physical and chemical And physical. Classical chemical methods of quantitative analysis include gravimetric And volumetric analysis.
Along with classical chemical methods, physical and physicochemical (instrumental) methods based on the measurement of optical, electrical, adsorption, catalytic and other characteristics of the analyzed substances, depending on their quantity (concentration), are widely used. Usually these methods are divided into the following groups: electrochemical(conductometry, polarography, potentiometry, etc.); spectral, or optical(emission and absorption spectral analysis, photometry, luminescent analysis, etc.); x-ray; chromatographic; radiometric; mass spectrometric. The listed methods, inferior to chemical ones in accuracy, significantly exceed them in sensitivity, selectivity and execution speed.
In this course, only classical chemical methods of quantitative analysis will be considered.
Gravimetric analysis is based on the exact measurement of the mass of the analyte in its pure form or in the form of its compound. Volume analysis includes titrimetric volumetric analysis- methods for measuring the volume of a solution of a reagent with a precisely known concentration consumed in a reaction with an analyte, and gas volumetric analysis- methods for measuring the volume of analyzed gaseous products.
In the course of quantitative analysis, the following main stages can be distinguished.
1. Sampling, averaging and sampling. Sampling often determines the overall error of the analysis and makes it meaningless to use high-precision methods. The purpose of sampling is to obtain a relatively small amount of the initial substance, in which the quantitative content of all components should be equal to their quantitative content in the entire mass of the analyzed substance. Primary sample is taken directly from the analyzed object by combining the required number of incremental samples. Sampling methods are determined by the following factors: aggregate state of the analyzed object (gas, liquid, solid); heterogeneity of the analyzed material; the required accuracy of assessing the content of the component over the entire mass of the analyzed object (the physiologically active component in the drug is more accurate than the component in the ore for assessing the profitability of the deposit), the possibility of changing the composition of the object over time. Liquid and gaseous materials are generally homogeneous and their samples are already averaged. Solid materials are heterogeneous in volume, therefore, for their analysis, parts of the substance are taken from different zones of the material under study. The primary sample is quite large - usually 1-50 kg, and for some objects (for example, for ore) it is 0.5-5 tons.
From the primary sample, by reducing it, they take average (representative) sample(usually from 25 g to 1 kg). To do this, the primary sample is crushed, mixed and averaged in composition, for example, quartering. When quartering, the crushed material is scattered in an even layer in the form of a square (or circle), divided into four sectors, the contents of two opposite sectors are discarded, and the other two are combined together. The quartering operation is repeated many times until the required amount of the average sample is obtained.
Samples are taken from the homogeneous material thus obtained for analysis, one part is kept for possible arbitration analyzes ( control sample), the other is directly used for analysis ( analyzed sample).
The part of the analyzed sample with the mass accurately measured on the analytical balance is called hinge. The sample to be analyzed must be large enough to obtain several samples.
2. Decomposition (opening) of the sample. This stage consists in transferring the analyzed sample to a state of aggregation or compound convenient for analysis. To transfer a sample into a solution in chemical methods, it is directly treated with liquid solvents (water, acids, alkalis) or, after sample destruction (by calcination, burning, fusion or sintering), it is converted into compounds that can dissolve.
3. Separation, isolation of the determined component and its concentration. Since most analytical methods are not selective enough, methods are used to separate the analyzed mixture or isolate the analyte from it. In the case when the concentration of the analyte is less than the detection limit of this method or less than the lower limit of its operating range, then the concentration of the analyte is used. Used for separation, isolation and concentration chemical(masking, precipitation and co-precipitation), physical(evaporation methods: distillation, distillation (distillation), sublimation (sublimation), etc.) and physical and chemical methods (extraction, sorption, ion exchange, chromatography and various electrochemical methods, such as electrolysis, electrophoresis, electrodialysis, etc.).
4. Carrying out quantification. All preliminary stages of the analysis should ensure that a reliable result is obtained during the analysis. The choice of method of analysis should be based on factors such as speed, convenience, correctness, availability of suitable equipment, number of analyses, size of the analyzed sample, content of the analyte. Comparing the sensitivity of various methods and evaluating the approximate content of the component in the sample, the chemist chooses one or another method of analysis. For example, to determine sodium in silicate rocks, a gravimetric method is used, which makes it possible to determine milligram and higher amounts of sodium; to determine microgram amounts of the same element in plants and biological objects - the method of flame photometry; for the determination of sodium in water of high purity (nano- and picogram quantities) - the method of laser spectroscopy.
5. Calculation of analysis results and evaluation of measurement results- the final stage of the analytical process. After calculating the results of the analysis, it is important to evaluate their reliability, taking into account the correctness of the method used and statistically processing numerical data.
Control questions
1. What is the purpose of quantitative analysis?
2. List the methods of quantitative analysis.
3. What is gravimetric analysis?
4. What is the essence of titrimetric analysis?
5. List the main stages of the analysis and describe them.
6. How is an average sample taken? What is sample quartering?
7. What is a hitch?
8. What methods are used to open the sample and isolate the analyte from it?
1. Vasiliev V.P. Analytical chemistry. Book. 1. Titrimetric and gravimetric methods of analysis. - M.: Bustard, 2005. - S. 16 - 24.
S.B. Denisova, O.I. Mikhailenko
