Basic methods of quantitative analysis. Calculations in titrimetric analysis. Qualitative analysis methods

20.09.2019

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.

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 a 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.

Chromatography

Chromatography (from the 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 the form of separate colored zones. Tsvet called the layer-by-layer colored sorbent column obtained in this way a chromatogram, and the method - X. Subsequently, the term "chromatogram" began to refer to different methods of fixing the results of many types of X. 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 chemistry - 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 chemistries are distinguished. Depending on the state of aggregation of the stationary phase, gaseous chromatography can be gas-adsorption (the stationary phase is a solid adsorbent) and gas-liquid (the stationary phase is a liquid), while liquid chlorine is liquid-adsorption (or solid-liquid) and liquid-liquid. The latter, like gas-liquid, is distributive chemo. Solid-liquid chemistries include thin-layer and paper chemistries.

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 variation of column chlorine is capillary, when a thin layer of sorbent is applied to the inner walls of the capillary tube. Planar cold is subdivided into thin-layer and paper. In thin-layer chlorine, a thin layer of granular sorbent or a porous film is applied to glass or metal plates; 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, there are the following options X .: frontal, developing and displacing. 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. After a certain time, the initial mixture is divided into pure substances, which are 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 which the mixture after a certain period of time is divided into zones of pure substances, between which there are 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 sorbents (particles with a diameter of 0.1–0.5 mm) for the gas-adsorption variant of X. For gas-liquid chemistry, a sorbent is prepared by applying a liquid in the form of a film (high-boiling hydrocarbons, esters, siloxanes, etc.) several microns thick onto a solid support 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 chlorine, 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 with pores strictly certain size(porous glasses, molecular sieves, including dextran and other gels). In thin-layer and paper chlorine, the liquid mixture under study 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 and for determining impurities of harmful substances in air, water, soil, and 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 for the analysis, separation, and purification of 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 of biological substances to selectively bind to each other.

Thin-layer and paper chlorine are used to analyze fats, carbohydrates, proteins, and other natural substances and inorganic compounds.

In some cases, chlorine is used to identify substances in combination with other physicochemical and physical methods, for example, with mass spectrometry, IR, UV spectroscopy, etc. A computer is 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

Literature

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Quantitative analysis. Classification of methods. gravimetric analysis. Precipitated and gravimetric forms of sediments. Calculations in gravimetric analysis.

Quantitative Analysis is designed to establish the quantitative composition of the components in the analyzed sample. It is preceded qualitative analysis, which establishes which components (elements, ions, molecules) are present in the analyzed sample.

There are three types of quantitative analysis: complete, partial, general. With a complete quantitative analysis, the complete quantitative composition of all components present in the analyzed sample is established. For example, for a complete quantitative blood test, it is necessary to determine the content of 12 components: sodium, potassium, calcium, glucose, bilirubin, etc. A complete analysis requires a lot of time and labor.

When performing a partial analysis, the content is determined only for

component data. General analysis sets the content of each element in the analyzed sample, regardless of the composition of which compounds they are included. Such analysis is usually called elemental.

CLASSIFICATION OF QUANTITATIVE ANALYSIS METHODS

Methods of quantitative analysis can be divided into three large groups: chemical, physical, physico-chemical.

Chemical Methods based on the use of quantitatively flowing chemical reactions of various types: exchange, precipitation, redox and complexation reactions. Chemical methods include gravimetric and titrimetric (volumetric) methods of analysis.

gravimetric method The analysis is based on measuring the mass of the determined component after its isolation in the form of a gravimetric form. The method is characterized by high accuracy, but is lengthy and laborious. In pharmaceutical analysis, it is mainly used to determine the moisture and ash content of drugs.

Titrimetric method analysis is based on the introduction of a precisely measured volume of a solution of a known concentration - a titrant - into a precisely measured volume of a solution of the analyte. The titrant is injected until the analyte has completely reacted with it. This moment is called the end point of the titration and is set using special chemical indicators or instrumental methods. Among

chemical methods of quantitative analysis - this is the most common method.

Chemical methods of analysis, although they are currently the main ones in chemical laboratories, in many cases do not meet the increased requirements for analysis, such as high sensitivity, rapidity, selectivity, automation, etc. These shortcomings are not instrumental methods analysis, which can be divided into three large groups: optical, electrochemical, chromatographic .

GRAVIMETRIC ANALYSIS

gravimetric method is based on an accurate measurement of the mass of a substance of known composition, chemically associated with the component being determined and isolated as a compound or as a simple substance. The classical name of the method is weight analysis. Gravimetric analysis is based on the law of conservation of the mass of a substance during chemical transformations and is the most accurate of the chemical methods of analysis: the detection limit is 0.10%; accuracy (relative method error) ±0.2%.

In gravimetric analysis, methods of precipitation, distillation (direct and indirect), isolation, thermogravimetry, and electrogravimetry are used.

AT precipitation method the determined component enters into a chemical reaction with the reagent, forming a poorly soluble compound. After a series of analytical operations (Scheme 1.1), a solid precipitate of a known composition is weighed and the necessary calculations are carried out.

The sequence of analytical operations in the gravimetric precipitation method

1Calculation of the weighed portion of the analyte and its weighing

2 Sample dissolution

3 Deposition conditions

4 Precipitation (obtaining a deposited form)

5Separation of precipitate by filtration

6 Washing the precipitate

7 Obtaining a gravimetric form (drying, calcining to constant weight)

8 Weighing a gravimetric form

9 Calculation of analysis results

Stripping methods may be direct or indirect. In method direct distillation the component to be determined is isolated from the sample in the form of a gaseous product, captured, and then its mass is determined. In methods indirect distillation the mass of the gaseous product is determined by the difference between the masses of the analyzed component before and after heat treatment. In the practice of pharmaceutical analysis, this method is widely used in determining the moisture content of drugs, plant materials. For some drugs, the determination of the mass loss ∆m on drying (drying temperature t sushi ) is one of the required pharmacopoeial tests, for example: analgin - t sushi = 100...105˚С, Δm< 5,5 %; пиридоксина гидрохлорид (витамин В6) - t sushi = 100...105 ˚s, Δm< 0,5 %; парацетамол - t dry = 100...105 ˚, Δ m< 0,5 % и т. п.

AT thermogravimetric analysis they fix the change in the mass of the substance during heating, which makes it possible to judge the transformations taking place and to establish the composition of the resulting intermediate products. Thermogravimetric analysis is carried out using derivatograph instruments. In the course of the experiment, the change in the mass of the analyzed sample (ordinate axis) depending on time or temperature (abscissa axis) is fixed and presented in the form of a thermogravimetric curve - thermo-ravigram. Thermogravimetry is widely used to study changes in the composition of a substance and to choose the conditions for drying or calcining sediments.

Electrogravimetric analysis based on the electrolytic separation of metals and weighing the precipitate obtained on the electrode. The main physical condition for the electrolytic separation of metals is a certain voltage at which some metals are deposited and no other metals are separated.

In analytical practice, the most wide application finds gravity

metric precipitation method, which will be discussed in more detail.

SEDIMENT FORMATION MECHANISM AND SEDIMENT CONDITIONS

The formation of a precipitate occurs when the product of the concentrations of the ions that make up its composition exceeds the value of the solubility product ETC (KA)sparingly soluble electrolyte:

K + + Aˉ ↔ KA; [K + ] [Аˉ] > PR (KA),

i.e. when a local (relative) supersaturation of the solution occurs, which is calculated by the formula:

(Q - S) /S,

where Q is the concentration of the solute at any point in time, mol/cm 3 ; S - solubility of the substance at the moment of equilibrium, mol/cm 3 In this place, the germ of the future crystal appears (the process of nucleation). This requires certain time called the induction period. With further addition of the precipitant, the process of crystal growth becomes more likely, rather than the further formation of crystallization centers, which combine into larger aggregates consisting of tens and hundreds of molecules (aggregation process). In this case, the particle size increases, and larger aggregates precipitate under the action of gravity. At this stage, the individual particles, being dipoles, orient themselves with respect to each other so that their oppositely charged sides approach each other (orientation process). If the orientation rate is greater than the aggregation rate, then a regular crystal lattice is formed, if vice versa, an amorphous precipitate precipitates. The lower the solubility of the substance, the faster the precipitate forms and the smaller the crystals. The same poorly soluble substances can be isolated both in the crystalline and in the amorphous state, which is determined by the conditions of precipitation.

Based on the concept of relative supersaturation of the solution, it follows that the lower the solubility of the precipitate S and the higher the concentration of reactants Q, the more nuclei are formed and the greater the rate of aggregation. And vice versa: the smaller the difference (Q - S), that is, the higher the solubility of the precipitate and the lower the concentration of the precipitated substance, the higher the orientation rate. Therefore, to obtain large crystals that can be easily filtered and washed, it is necessary to carry out precipitation from dilute solutions by slowly adding a precipitant and heating (Table 1.1).

Conditions for the deposition of crystalline and amorphous precipitates

Influencing factor	Sediment character
Influencing factor	crystal	amorphous
Concentration of solutions of substance and precipitant	A dilute solution of the precipitant is added to a dilute solution of the test substance.	A concentrated solution of a precipitant is added to a concentrated solution of the test substance.
Settling rate	The precipitant solution is added dropwise	The precipitant solution is added quickly
Temperature	Precipitation is carried out from hot solutions (70 - 80 ° C) with a hot solution of the precipitant	Precipitation is carried out from hot solutions (70 - 80˚С)
Mixing	Precipitation is carried out with continuous stirring
Presence of foreign matter	Solubilizers are added (usually strong acids)	Add coagulant electrolytes
Settling time	For a long time withstand the sediment in the mother liquor for "ripening" ("aging")	Filtered immediately after precipitation

Table 1.1

Purity of crystalline precipitates. The specific surface area of crystalline precipitates (the area of the precipitate per unit mass, cm 2 /d) is usually small, so coprecipitation due to adsorption is negligible. However, other types of codeposition associated with contamination within the crystal can lead to errors.

There are two types of co-precipitation in crystalline sediments:

1) inclusion - impurities in the form of individual ions or molecules are homogeneously distributed throughout the crystal;

2) occlusion - uneven distribution of numerous ions or impurity molecules that have entered the crystal due to the imperfection of the crystal lattice.

An effective way to reduce occlusion is the "aging" ("maturation") of the sediment, during which spontaneous growth of larger crystals occurs due to the dissolution of small particles, the crystal structure of the sediment is improved, its specific surface is reduced, as a result of which impurities of previously absorbed particles are desorbed and transferred into solution. substances. The "ripening" time of the precipitate can be shortened by heating the solution with the precipitate.

Purity of amorphous precipitates significantly decreases as a result of the adsorption process, since the amorphous precipitate consists of particles with a disordered structure, forming a loose porous mass with a large surface. Most effective way decrease as a result of the adsorption process is reprecipitation. In this case, the filter cake is dissolved and precipitated again. Reprecipitation significantly lengthens the analysis, but it is unavoidable for hydrated iron ( III ) and aluminum oxides, zinc and manganese hydroxides, etc. The reverse process of coagulation of an amorphous precipitate is its peptization– a phenomenon in which a coagulated colloid returns to its original dispersed state. Peptization is often observed when amorphous precipitates are washed with distilled water. This error is eliminated by choosing the right wash liquid for the amorphous precipitate.

SEDIMENTED AND GRAVIMETRIC FORMS.

REQUIREMENTS TO THEM.

In the gravimetric method of sedimentation, there are concepts of precipitated

and gravimetric forms of matter. besieged form is a compound in the form of which the component to be determined precipitates from solution. Gravimetric (weight) form name the compound being weighed. Otherwise, it can be defined as the precipitated form after appropriate analytical treatment of the precipitate. Let us present the schemes of gravimetric determination of ions SO 4 2 -, Fe 3+, Mg 2+

S0 4 2 - + Ba 2+ ↔ BaS0 4 ↓ → BaS0 4 ↓

detectable precipitant precipitated gravimetric

ion form form

Fe3+ + 3OH‾ ↔ Fe(OH) 3 ↓ → Fe 2 O 3 ↓

detectable precipitant precipitated gravimetric

ion form form

Mg 2+ + HPO 4 2 - + NH 4 ∙H 2 O ↔ Mg NH 4 P0 4 ↓ + H 2 O → Mg 2 P 2 O 7 determined. precipitant precipitated form gravimetric the form

It can be seen from the given examples that the gravimetric form does not always coincide with the precipitated form of the substance. The requirements for them are also different.

besieged form must be:

· sparingly soluble enough to provide almost complete

Isolation of the analyte from the solution. In case of precipitation

Binary electrolytes ( AgCl; BaS0 4 ; SaS 2 O 4 etc.) is achieved

Virtually complete precipitation, since the solubility product of these

Precipitation less than 10 - 8 ;

· the resulting precipitate should be clean and easily filterable (which determines the advantages of crystalline precipitates);

· the precipitated form should easily transform into the gravimetric form.

After filtering and washing the precipitated form, it is dried or calcined until the mass of the precipitate becomes constant, which confirms the completeness of the transformation of the precipitated form into a gravimetric one and indicates the completeness of the removal of volatile impurities. Precipitates obtained by precipitation of the determined component with an organic reagent (diacetyldioxime, 8-hydroxyquinoline, α-nitroso-β-naphthol, etc.) are usually dried. Precipitates of inorganic compounds are usually calcined

The main requirements for the gravimetric form are:

· exact correspondence of its composition to a certain chemical formula;

· chemical stability in a fairly wide temperature range, lack of hygroscopicity;

· as high a molecular weight as possible the smallest content

In it, the determined component to reduce the influence of errors

When weighed on the analysis result.

CALCULATION OF RESULTS

IN THE GRAVIMETRIC METHOD OF ANALYSIS

Gravimetric analysis includes two experimental measurements: determination of sample massm nof the analyte and the mass of the product of known composition obtained from this sample, that is, the mass of the gravimetric formm gr.fanalyte.

Based on these data, it is easy to calculate the mass percentage w, % of the determined component in the sample:

w, % = m gr.ph ∙ F ∙ 100 / m n ,

where F- the gravimetric factor (conversion factor, analytical factor) is calculated as the ratio of the molecular weight of the analyte to the molecular weight of the gravimetric form, taking into account stoichiometric coefficients.

The value of gravimetric factors, calculated with high accuracy, is given in the reference literature.

Example 1. How many grams of Fe 2 O 3 can be obtained from 1.63 g of Fe 3 O 4? Calculate the gravimetric factor.

Solution.It must be admitted that Fe 3 O 4 quantified into Fe 2 O 3 and for this there is enough oxygen:

2 Fe 3 O 4 + [O] ↔ 3 Fe 2 O 3

From each mole of Fe 3 O 4, 3/2 moles of Fe 2 O 3 are obtained. Thus, the number of moles of Fe 2 O 3 is 3/2 times greater than the number of moles of Fe 3 O 4, that is:

nM (Fe 2 O 3) = 3/2 nM (Fe 3 O 4);

m (Fe 2 O 3) / M (Fe 2 O 3) \u003d 3/2 m (Fe 3 O 4) / M (Fe 3 O 4)

where n - the number of moles of the determined component, from which one mole of the gravimetric form is obtained; m - mass of substance, g; M- molar mass of the substance, g/mol.

From the formula m (Fe 2 O 3) \u003d 3/2 (m (Fe 3 O 4) ∙ M (Fe 2 O 3)) / M (Fe 3 O 4)

we get

m (Fe 2 O 3) \u003d m (Fe 3 O 4) ∙ 3M (Fe 2 O 3) / 2M (Fe 3 O 4)

and substitute numerical values into it:

m (Fe 2 O 3) \u003d 1.63 ∙ (3 ∙ 159.7) / (2 ∙ 231.5) \u003d 1.687 ≈ 1.69 g.

Gravimetric factor F equals:

F \u003d 3M (Fe 2 O 3) / 2M (Fe 3 O 4) \u003d 1.035.

Therefore, in the general case, the gravimetric factor is determined by the formula:

F = (a ∙ M def. in-in) / ( b ∙ M gr.f),

where a and bare small integers by which molecular weights must be multiplied so that the number of moles in the numerator and denominator is chemically equivalent.

However, these calculations are not applicable in all cases. In the indirect determination of iron in Fe 2 (SO 4) 3, which consists in the precipitation and weighing of BaSO 4 (gravimetric form), when calculating the analytical factor, there is no common element in the numerator and denominator of the formula. Here another way of expressing the chemical equivalence between these quantities is needed:

2 M(Fe 3+ ) ≡≡ l M(Fe 2 (SO 4) 3) ≡≡ 3 M(SO 4 2-) ≡≡ 3 M(BaSO 4).

The gravimetric factor for the mass percentage of iron will be expressed as:

F \u003d 2M (Fe 3+ ) / 3M (BaSO 4) .

Example 2. A solution of the drug Na 3 PO 4 (m n = 0.7030 g) was precipitated in the form of MgNH 4 PO 4 ∙ 6H 2 O. After filtering and washing, the precipitate was calcined at 1000 ˚C. The mass of the resulting precipitate Mg 2 P 2 O 7 was 0.4320 g. Calculate the mass percentage of phosphorus in the sample

Solution.

m gr.f (Mg 2 P 2 O 7) = 0.4320 g;

F \u003d 2M (P) / M (Mg 2 P 2 O 7) \u003d 0.2782; m n \u003d 0.7030 g;

W ,% = m gr.f ∙ F ∙ 100 / m n

w,% (P) = 0.4320 ∙ 0.2782 ∙ 100 / 0.7030 = 17.10%.

Example 3. When calcining the contaminated preparation of sodium oxalate m n = 1.3906 g, a residue was obtained with a mass m gr.f = 1.1436 g. Determine the degree of purity of the sample. t

Na 2 C 2 O 4 → Na 2 CO 3 + CO

Solution. It should be assumed that the difference between the initial and final masses corresponds to the loss of carbon oxide during calcination. The analysis is based on the measurement of this quantity:

n (CO) \u003d n (Na 2 C 2 O 4),

Consequently,

w,% (Na 2 C 2 O 4) \u003d (m n - m gr.f) ∙ F ∙ 100 / m n;

F \u003d M (Na 2 C 2 O 4) / M (CO) \u003d 4.784;

w,% (Na 2 C 2 O 4) \u003d (1.3906 - 1.1436) ∙ 4.784 ∙ 100 / 1.3906 \u003d 84.97%.

CHOICE OF WEIGHT IN GRAVIMETRY

As is known, the accuracy of the analysis depends both on the weight of the sample and on the weight of the gravimetric form obtained from it. If the sample is taken with great accuracy, and the gravimetric form obtained from it is a small value measured with a large error, then the entire analysis will be performed with an error made when weighing the gravimetric form. Therefore, such a sample must be taken so that when weighing it and when weighing the gravimetric form obtained from it, the error does not exceed ± 0.2%. To do this, it is necessary to determine the minimum mass that can still be weighed with an accuracy of ± 0.2% on an analytical balance with an absolute weighing error of ± 0.0001 g, and the minimum error, taking into account the possible spread (±), in this case will be equal to 2 ∙ ( ±0.000 1) = ±0.0002 g.

100 g - ± 0.2 g

x - ± 0.0002 g

x = 0.1 g

Therefore, such a minimum massmminis 0.1 g. If the value is less than 0.1 g, the error will exceed 0.2%. When calculating the mass of a sample in gravimetric analysis, the mass of the gravimetric form of the component is equated to the minimum mass of the substance:

m gr.f \u003d m min, m n \u003d m min ∙ F ∙ 100 / w, %.

If the value of the mass of the sample calculated according to the indicated formula turns out to be less than 0.1 g, then the sample should be increased to 0.1 g. and for crystalline from 0.1 to 0.5 g.

Calculation of the amount of precipitant carried out taking into account the possible content of the determined component in the analyzed sample. A moderate excess of the precipitant is used to complete the separation of the precipitate. If the precipitant is volatile (for example, a solution of hydrochloric acid), a two- or three-fold excess is taken, which is subsequently removed by heating the precipitate. If the precipitant is non-volatile (solutions of barium chloride, ammonium oxalate, silver nitrate, etc.), a one and a half times excess is sufficient.

ANALYTICAL SCALES. RULES FOR HANDLING THEM

Analytical balance - it's accurate physical device, the use of which is allowed with strict observance of the rules that ensure the necessary reproducibility and accuracy of weighing.

Rules for Handling Analytical Balances include the following basic requirements:

1. The balance must be placed on a rigid surface,

protecting them from various shocks, and in a specially equipped room - the weight room.

2. Sharp fluctuations in temperature, exposure to direct sunlight, as well as exposure to analytical balances of chemicals are unacceptable.

3. The maximum allowable load of the analytical balance should be no more than 200 g.

4. When weighing objects on an analytical balance, it is necessary that they have the temperature of the weighing room.

5. The substance to be weighed is placed on the left scale pan in a special container (bottle bottles, crucibles, watch glass). The weights of the analytical weight are placed on the right scale pan.

6. The weighed items and weights are brought in through the side doors of the scales (curtains). Weighing is carried out only with the doors of the scales closed.

7. Weights of analytical weight are taken only with specially designed tweezers. All operations with weight change are performed with full caging of scales.

8. Before and after each weighing, check the balance zero point.

9. Place the weights and objects to be weighed in the center of the pans to avoid tilting the pans.

10. The recording of the weighing results is carried out according to the empty nests of the analytical weight and according to the data of the drums with tenths and hundredths of a gram. The third and fourth decimal places are removed from the luminous display.

11. Upon completion of weighing, make sure that the scales are caged, completely unloaded and the doors of the case are tightly closed.

12. To reduce the weighing error, it is necessary to use an analytical weight intended for strictly defined analytical balances.

It should be noted that even if all the above rules are observed

Weighing errors may occur depending on various reasons:

· caused by the imbalance of the balance beam;

· due to changes in body weight during the weighing process;

· due to weighing in air, not in a vacuum;

· caused by the discrepancy between the weights (weights) of their nominal

mass.

APPLICATION OF GRAVIMETRIC METHOD OF ANALYSIS

The use of inorganic precipitants makes it possible to obtain either salts or oxides of analytes in the form of a gravimetric form. Inorganic reagents do not differ in specificity, but the most commonly used in the analysis are: NH 4 OH(Fe 2 O 3, SnO 2); H 2 S(C u S, ZnS or ZnSO 4 , As 2 S 3 or As 2 S 5 , Bi 2 S 3); (NH4)2S(HgS); NH 4 H 2 PO 4(Mg 2 P 2 O 7, Al 3 PO 4, Mn 2 P 2 O 7); H 2 SO 4(PbSO 4 , BaSO 4 , SrSO 4); H 2 C 2 O 4(CaO); NS l(AgCl, Hg 2 Cl 2 , Na as NaCl from butanol); AgNO 3(AgCl, AgBr, AgI); BaCl2(BaSO 4), etc.

Sometimes the gravimetric definitions are based on the restoration of the determined component to an element that serves as a gravimetric form.

For the gravimetric determination of inorganic substances, a number of organic reagents have been proposed, which, as a rule, have greater selectivity. Two classes of organic reagents are known. The former form sparingly soluble complex (coordination) compounds and contain at least two functional groups having a pair of unshared electrons. They are also called chelating agents, for example, 8-hydroxyquinoline precipitates more than twenty cations:

The solubility of metal oxyquinolates varies widely depending on the nature of the cation and the pH value of the medium.

In 1885, l-nitroso-2-naphthol was proposed - one of the first selective organic reagents, which is widely used for the determination of cobalt in the presence of nickel, as well as for the determination of bismuth (3), chromium (III), mercury (II), tin (IV), etc.:

Diacetyldioxime (dimethylglyoxime) is highly selective and is widely used for the gravimetric determination of low nickel concentrations:

CH 3 ─ C ─ C ─ CH 3

│ │

OH-NN-OH

GRAVIMETRY ERRORS

The gravimetric method of analysis gives the most correct result, and, despite the duration and laboriousness, it is very often used as a verification method in arbitration analyses. Systematic methodological errors in gravimetry can be taken into account and reduced in the course of performing the corresponding operations ( tab. 1.2).

Methodological errors of gravimetry

Gravimetric operation	Absolute error
Gravimetric operation	positive (inflated result)	negative (low result)
The choice of precipitator: a) the nature of the precipitant b) amount of precipitant	Non-volatile, non-specific precipitant Slight excess of precipitant, co-precipitation of foreign ions	High solubility of the precipitated form, colloid formation The lack of a precipitator. Too much excess of the precipitant, increased solubility of the precipitate as a result of complexation or salt effect
precipitation	Coprecipitation of foreign ions	Insufficient ripening time (crystalline precipitation). Colloidal formation (amorphous precipitates)
Filtration		Incorrect filter selection - sediment particles passing through the filter
Washing	Washing with a non-volatile washing liquid	Excess washing liquid: peptization of the amorphous precipitate; hydrolysis of the crystalline precipitate. Losses due to solubility
Obtaining a gravimetric form	Ignition temperature: obtaining a compound of a different composition, hygroscopicity, absorption of CO 2 from the air	Exceeding the drying temperature for sediments of organic nature. Exceeding the calcination temperature (obtaining a compound of a different chemical composition)

Table 1.2

The correctness of the method is explained by a small systematic measurement error associated with the accuracy of weighing on an analytical balance:

S x / x = √(S a / a ) 2 + 1/n (S m / m ) 2 ,

where S a– weighing accuracy on analytical balances (0.0002 g for balances ADV-200; 0.00005 g for semi-microbalances, etc.); a– weighed portion of the analyzed substance, g; t - weight of the gravimetric form, g; P - the number of calcinations or drying to obtain a constant mass.

The analysis of the given data shows that it is possible to identify the type of error by considering the method of determination, taking into account the mechanism of precipitation formation, the properties of the substances used and obtained during the analysis.

At present, the importance of gravimetric methods of analysis has somewhat decreased, but one should not forget that, having advantages and disadvantages, gravimetric analysis is optimal for solving a large number analytical tasks.

Methods of quantitative analysis. Quantitative analysis is designed to determine the quantitative composition of the analyte. There are chemical, physical and physico-chemical methods of quantitative analysis. The basis of any quantitative research is measurement. Chemical methods of quantitative analysis are based on the measurement of mass and volume. Quantitative Research allowed scientists to establish such basic laws of chemistry as the law of conservation of mass of matter, the law of constancy of composition, the law of equivalents, and other laws on which chemical science is based. The principles of quantitative analysis are the basis for chemical-analytical control of production processes various industries industry and constitute the subject of the so-called. technical analysis. There are 2 main methods of quantitative chemical analysis: weight or gravimetric and volumetric or titrimetric.

Weight analysis is a method of quantitative analysis in which only mass is accurately measured. Volumetric analysis - based on the precise measurement of the mass of substances and the volume of a solution of a reagent of known concentration, reacting with a certain amount of the analyte. special kind count analysis is the analysis of gases and gas mixtures, the so-called. gas analysis, also performed by measuring the volume or mass of the analyzed mixture or gas. The determination of the same substance can be performed by weight or volumetric methods of analysis. When choosing a method of determination, the analyst must take into account the required accuracy of the result, the sensitivity of the reaction and the speed of the analysis, and in the case mass definitions- availability and cost of reagents used. In connection with this, macro-, micro-, semi-micro-, ultra-micro methods of number analysis are distinguished, with the help of which it is possible to analyze the minimum amounts of the analyte. At present, simple chemical methods are increasingly being replaced by physical and physicochemical methods, which require expensive instruments and equipment.

Optical, electrochemical, chromatographic, various spectro- and photometric studies (infrared, atomic adsorption, flame, etc.), potentiometry, polarography, mass spectrometry, NMR studies. On the one hand, these methods speed up obtaining results, increase their accuracy and sensitivity of measurements: detection limit (1-10 -9 μg) and limiting concentration (up to 10 -15 g / ml), selectivity (it is possible to determine the constituent components of a mixture without separating them and selection), the possibility of their computerization and automation. But on the other hand, they are increasingly moving away from chemistry, reducing the knowledge of chemical methods of analysis among analysts, which led to a deterioration in teaching chemistry in schools, a lack of good chemistry teachers equipped with school chemical laboratories, and a decrease in knowledge of chemistry among schoolchildren.

The disadvantages include a relatively large error in the determination (from 5 to 20%, while chemical analysis gives an error usually from 0.1 to 0.5%), the complexity of the equipment and its high cost. Requirements for reactions in quantitative analysis. Reactions should proceed quickly, to the end, if possible, at room temperature. The initial substances that enter into the reaction must react in strictly defined quantitative ratios (stoichiometrically) and without side processes. Impurities should not interfere with the quantitative analysis. Errors, errors in measurements and calculations are not ruled out during measurements. To eliminate errors, reduce them to a minimum, the measurement is carried out in repetitions (parallel determinations), at least 2, and a metrological evaluation of the results is carried out (meaning the correctness and reproducibility of the analysis results).

The most important characteristics of analysis methods are their sensitivity and accuracy. The sensitivity of an analysis method is the smallest amount of a substance that can be reliably determined by this method. The accuracy of the analysis is the relative error of determination, which is the ratio of the difference between the found (x 1) and true (x) content of the substance to the true content of the substance and is found by the formula:

Rel. osh. = (x 1 -x) / x, for expression as a percentage, multiply by 100. The arithmetic mean content of the substance found in the analysis of the sample in 5-7 definitions is taken as the true content.

Method Sensitivity, mol/l Accuracy,%

Titrimetric 10 -4 0.2

Gravimetric 10 -5 0.05

Weight (gravimetric) analysis is a method of quantitative analysis, in which quantitative composition the analyte is determined on the basis of mass measurements, by accurately weighing the mass of a stable end substance of a known composition, into which this analyte is completely converted. For example, gravimetric determination of sulfuric acid in an aqueous solution is carried out using an aqueous solution of barium salt: ВаС1 2 + Н 2 SO 4 > ВаSO 4 v +2 HCl. Precipitation is carried out under conditions in which almost the entire sulfate ion passes into the precipitate BaSO 4 with the greatest completeness - quantitatively, with minimal losses, due to the insignificant, but still existing, solubility of barium sulfate. Next, the precipitate is separated from the solution, washed to remove soluble impurities, dried, calcined to remove sorbed volatile impurities, and weighed on an analytical balance in the form of pure anhydrous barium sulfate. And then calculate the mass of sulfuric acid. Classification of gravimetric analysis methods. Methods of precipitation, distillation, isolation, thermogravimetric methods (thermogravimetry).

Precipitation methods - the component to be determined is quantitatively bound into a chemical compound in the form of which it can be isolated and weighed. The composition of this compound must be strictly defined; be precise chemical formula, and it should not contain any foreign impurities. The compound in which the component to be determined is weighed is called the weight form. xH 2 O followed by its separation and calcination to oxide Fe 2 O 3 (weight form). Distillation methods. The component to be determined is isolated from the analyzed sample in the form of a gaseous substance and either the mass of the distilled off substance (direct method) or the mass of the residue (indirect method) is measured.

The direct method is widely used to determine the water content of analytes by distilling it from a weighed sample and condensing it, and then measuring the volume of condensed water in the receiver. By density, the volume of water is recalculated per mass and, knowing the mass of the sample and water, the water content in the analyzed sample is calculated. The indirect distillation method is widely used to determine the content volatile substances(including weakly bound water) by changing the mass of the sample before and after drying to a constant weight in a thermostat (in an oven) at a constant temperature. The conditions for conducting such tests (temperature, drying time) are determined by the nature of the sample and are specifically indicated in the methodological manuals.

Isolation methods are based on the isolation of the analyte from a solution by electrolysis on one of the electrodes (electrogravimetric method). Then the electrode with the released substance is washed, dried and weighed. By increasing the mass of the electrode with the substance, the mass of the substance released on the electrode is found (gold and copper alloys are transferred into solution).

Thermogravimetric methods are not accompanied by the separation of the test substance, but the sample itself is examined, therefore, these methods are conditionally referred to as gravimetric methods of analysis. The methods are based on measuring the mass of the analyte during its continuous heating in a given temperature range on special devices - derivatographs. According to the obtained thermogravigrams, when deciphering them, it is possible to determine the content of moisture and other components of the analyte.

The main stages of gravimetric determination: calculation of the weighed weight of the analyzed sample and the volume (or mass) of the precipitant; weighing (taking) a portion of the sample; dissolution of a weighed sample of the analyzed sample; precipitation, i.e. obtaining a deposited form of the component to be determined; filtration (separation of the precipitate from the mother liquor); sediment washing; drying and (if necessary) calcining the precipitate to constant weight, i.e. obtaining a gravimetric form; weighing gravimetric form; calculation of analysis results, their statistical processing and presentation. Each of these operations has its own characteristics.

When calculating the optimal weight of the sample of the analyte, the possible mass fraction of the analyte in the analyzed sample and in the gravimetric form, the mass of the gravimetric form, the systematic error of weighing on an analytical balance (usually 0.0002), the nature of the resulting precipitate - amorphous, finely crystalline, coarsely crystalline, are taken into account. The calculation of the initial sample is based on the fact that the mass of the gravimetric sample must be at least 0.1 g. In the general case, the lower limit of the optimal mass m of the initial sample of the analyte (in grams) is calculated by the formula: m = 100m (GF) F / W (X), where m(GF) is the mass of the gravimetric form in grams; F - gravimetric factor, conversion factor, analytical factor); W(X) - mass fraction (in%) of the determined component in the analyzed substance. The gravimetric factor F is numerically equal to the mass of the determined component in grams, corresponding to one gram of the gravimetric form.

The gravimetric factor is calculated by the formula as the ratio of the molar mass M(X) of the determined component X to molar mass gravimetric form M(GF), multiplied by the number n moles of the analyte, from which one mole of the gravimetric form is obtained: F = n M(X) / M (GF). So, if one mole of the gravimetric form Fe 2 O 3 is obtained from 2 moles of Fe C1 3 6H 2 O, then n \u003d 2. If one mole of the gravimetric form BaCrO 4 is obtained from one mole of Ba(NO 3) 2, then n \u003d one.

Methods of quantitative analysis. Quantitative analysis is designed to determine the quantitative composition of the analyte. There are chemical, physical and physico-chemical methods of quantitative analysis. The basis of any quantitative research is measurement. Chemical methods of quantitative analysis are based on the measurement of mass and volume. Quantitative research allowed scientists to establish such basic laws of chemistry as the law of conservation of mass of matter, the law of composition constancy, the law of equivalents, and other laws on which chemical science is based. The principles of quantitative analysis are the basis for the chemical-analytical control of production processes in various industries and are the subject of the so-called. technical analysis. There are 2 main methods of quantitative chemical analysis: weight or gravimetric and volumetric or titrimetric.

Weight analysis is a method of quantitative analysis in which only mass is accurately measured. Volumetric analysis - based on the precise measurement of the mass of substances and the volume of a solution of a reagent of known concentration, reacting with a certain amount of the analyte. A special type of number analysis is the analysis of gases and gas mixtures, the so-called. gas analysis, also performed by measuring the volume or mass of the analyzed mixture or gas. The determination of the same substance can be performed by weight or volumetric methods of analysis. When choosing a determination method, the analyst must take into account the required accuracy of the result, the sensitivity of the reaction and the speed of the analysis, and in the case of mass determinations, the availability and cost of the reagents used.

In connection with this, macro-, micro-, semi-micro-, ultra-micro methods of number analysis are distinguished, with the help of which it is possible to analyze the minimum amounts of the analyte. At present, simple chemical methods are increasingly being replaced by physical and physicochemical methods, which require expensive instruments and equipment. Optical, electrochemical, chromatographic, various spectro- and photometric studies (infrared, atomic adsorption, flame, etc.), potentiometry, polarography, mass spectrometry, NMR studies. On the one hand, these methods speed up obtaining results, increase their accuracy and sensitivity of measurements: detection limit (1-10 -9 μg) and limiting concentration (up to 10 -15 g / ml), selectivity (it is possible to determine the constituent components of a mixture without separating them and selection), the possibility of their computerization and automation.

But on the other hand, they are increasingly moving away from chemistry, reducing the knowledge of chemical methods of analysis among analysts, which led to a deterioration in teaching chemistry in schools, a lack of good chemistry teachers equipped with school chemical laboratories, and a decrease in knowledge of chemistry among schoolchildren. The disadvantages include a relatively large error in the determination (from 5 to 20%, while chemical analysis gives an error usually from 0.1 to 0.5%), the complexity of the equipment and its high cost. Requirements for reactions in quantitative analysis. Reactions should proceed quickly, to the end, if possible, at room temperature. The initial substances that enter into the reaction must react in strictly defined quantitative ratios (stoichiometrically) and without side processes. Impurities should not interfere with the quantitative analysis. Errors, errors in measurements and calculations are not ruled out during measurements. To eliminate errors, reduce them to a minimum, the measurement is carried out in repetitions (parallel determinations), at least 2, and a metrological evaluation of the results is carried out (meaning the correctness and reproducibility of the analysis results).

Classification of chemical methods of quantitative analysis:

Titrimetric method. Measurement of the volume of a reagent solution of exactly known concentration consumed in a reaction.

Gravimetric. Measurement of the mass of the analyte or its constituents, isolated in the form of the corresponding compounds.

The most important characteristics of analysis methods are their sensitivity and accuracy. The sensitivity of an analysis method is the smallest amount of a substance that can be reliably determined by this method. The accuracy of the analysis is the relative error of the determination, which is the ratio of the difference found (x 1) and the true (x) content of the substance to the true content of the substance and are found by the formula:

Weight (gravimetric) analysis is a method of quantitative analysis, in which the quantitative composition of the analyte is determined on the basis of mass measurements, by accurately weighing the mass of a stable final substance of a known composition, into which this analyte is completely converted. For example, gravimetric determination of sulfuric acid in an aqueous solution is carried out using an aqueous solution of barium salt: ВаС1 2 + Н 2 SO 4 > ВаSO 4 v +2 HCl. Precipitation is carried out under conditions in which almost the entire sulfate ion passes into the precipitate BaSO 4 with the greatest completeness - quantitatively, with minimal losses, due to the insignificant, but still existing, solubility of barium sulfate.

Next, the precipitate is separated from the solution, washed to remove soluble impurities, dried, calcined to remove sorbed volatile impurities, and weighed on an analytical balance in the form of pure anhydrous barium sulfate. And then calculate the mass of sulfuric acid. Classification of gravimetric analysis methods. Methods of precipitation, distillation, isolation, thermogravimetric methods (thermogravimetry). Precipitation methods - the component to be determined is quantitatively bound into a chemical compound in the form of which it can be isolated and weighed. The composition of this compound must be strictly defined; be accurately expressed by a chemical formula, and it must be free of any foreign matter. The compound in which the component to be determined is weighed is called the weight form.

Example, determination of H 2 SO 4 (above), determination of the mass fraction of iron in its soluble salts, based on the precipitation of iron (111) in the form of Fe (OH) 3 x H 2 O hydroxide, followed by its separation and calcination to oxide Fe 2 O 3 (weight form). Distillation methods. The component to be determined is isolated from the analyzed sample in the form of a gaseous substance and either the mass of the distilled off substance (direct method) or the mass of the residue (indirect method) is measured. The direct method is widely used to determine the water content of analytes by distilling it from a weighed sample and condensing it, and then measuring the volume of condensed water in the receiver. By density, the volume of water is recalculated per mass and, knowing the mass of the sample and water, the water content in the analyzed sample is calculated. The indirect distillation method is widely used to determine the content of volatile substances (including weakly bound water) by changing the mass of a sample before and after drying to constant weight in a thermostat (in an oven) at a constant temperature.

The conditions for conducting such tests (temperature, drying time) are determined by the nature of the sample and are specifically indicated in the methodological manuals. Isolation methods are based on the isolation of the analyte from a solution by electrolysis on one of the electrodes (electrogravimetric method). Then the electrode with the released substance is washed, dried and weighed. By increasing the mass of the electrode with the substance, the mass of the substance released on the electrode is found (gold and copper alloys are transferred into solution). Thermogravimetric methods are not accompanied by the separation of the test substance, but the sample itself is examined, therefore, these methods are conditionally referred to as gravimetric methods of analysis. The methods are based on measuring the mass of the analyte during its continuous heating in a given temperature range on special devices - derivatographs.

According to the obtained thermogravigrams, when deciphering them, it is possible to determine the content of moisture and other components of the analyte. The main stages of gravimetric determination: calculation of the weighed weight of the analyzed sample and the volume (or mass) of the precipitant; weighing (taking) a portion of the sample; dissolution of a weighed sample of the analyzed sample; precipitation, i.e. obtaining a deposited form of the component to be determined; filtration (separation of the precipitate from the mother liquor); sediment washing; drying and (if necessary) calcining the precipitate to constant weight, i.e. obtaining a gravimetric form; weighing gravimetric form; calculation of analysis results, their statistical processing and presentation. Each of these operations has its own characteristics. When calculating the optimal weight of the sample of the analyte, the possible mass fraction of the analyte in the analyzed sample and in the gravimetric form, the mass of the gravimetric form, the systematic error of weighing on an analytical balance (usually 0.0002), the nature of the resulting precipitate - amorphous, finely crystalline, coarsely crystalline, are taken into account. The calculation of the initial sample is based on the fact that the mass of the gravimetric sample must be at least 0.1 g.

In the general case, the lower limit of the optimal mass m of the initial sample of the analyte (in grams) is calculated by the formula:

m = 100m (GF) F/ W(X),

where m(GF) is the mass of the gravimetric form in grams; F - gravimetric factor, conversion factor, analytical factor); W(X) - mass fraction (in%) of the determined component in the analyzed substance. The gravimetric factor F is numerically equal to the mass of the determined component in grams, corresponding to one gram of the gravimetric form.

The gravimetric factor is calculated by the formula as the ratio of the molar mass M(X) of the determined component X to the molar mass of the gravimetric form M(GF), multiplied by the number n of moles of the determined component, from which one mole of the gravimetric form is obtained:

F = n M(X) / M (GF).

So, if one mole of the gravimetric form Fe 2 O 3 is obtained from 2 moles of Fe C1 3 6H 2 O, then n \u003d 2. If one mole of the gravimetric form BaCrO 4 is obtained from one mole of Ba(NO 3) 2, then n \u003d one.

The task of quantitative analysis is to determine the quantitative content of individual constituents in the test substance or mixture. The results of quantitative determination are usually expressed as a percentage. Quantitative analysis is used in biology, physiology, medicine, biochemistry, food chemistry, etc.

All methods of quantitative analysis can be divided into three main groups.

1. Gravimetric (weight) analysis. Gravimetric analysis is the determination of the amount of a component (element or ion) by the mass of the substance obtained as a result of the analysis. In the methods of this group, the determined part of the analyte is isolated in pure form or in the form of a compound of known composition, the mass of which is determined.

For example, to determine the amount of barium in its compounds, the Ba 2+ ion is precipitated with dilute sulfuric acid:

ВаС1 2 + H 2 S0 4 = BaS0 4 | + 2HC1.

The precipitate of BaSO 4 is filtered, washed, calcined and accurately weighed. Knowing the mass of the precipitate BaS0 4 and its formula, calculate how much barium it contains. The gravimetric method gives high accuracy results, but it is very labor intensive.

2. Titrimetric (volumetric) analysis. Titrimetric analysis is based on the precise measurement of the amount of reagent used in the reaction with the analyte.
component. The reagent is taken in the form of a solution of a certain concentration - titrated solution. Moment,
when the reagent is added in an amount equivalent to the content of the component being determined, i.e., the moment of completion of the reaction is determined in various ways. During titration, an amount of reagent is added that is equivalent to the amount of the analyte. Knowing the volume and exact concentration of the solution that reacted with the analyte, the amount of the analyte is calculated.

Titrimetric analysis gives less accurate results than gravimetric analysis, but its important advantage is the high speed of analysis. Depending on the type of reactions occurring during the titration, titrimetric analysis is divided into three groups: acid-base titration methods, redoximetry methods, and precipitation and complex formation methods.

3. Methods of photometry. In this method, the amount of a substance is determined by the color intensity of the solution. To do this, use the so-called color reactions, i.e., reactions accompanied by a change in the color of the solution. For example, when determining the amount of iron, the reaction is used

FeCl3 + 3KSCN 7-Fe(SCN)3 + 3KCI,

leading to the formation of a red solution. 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 poorly soluble compound, and the content of the analyte is judged by the intensity of the solution turbidity. A method based on this principle is called nephelometry. Photometry and nephelometry methods 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.

In addition to these methods, there are others: gas analysis, spectral analysis, electrochemical and chromatographic methods. This tutorial does not cover these methods.

All methods of quantitative analysis are divided into chemical and physico-chemical. Chemical methods include gravimetric, titrimetric and gas analysis, physicochemical methods include photometry and nephelometry, electrochemical, spectral, chromatographic methods of analysis

In quantitative analysis, macro-, micro- and semi-micro methods are distinguished. This tutorial covers only the macro method. When performing macro determinations, relatively large (0.01-0.1 g) amounts of a substance are determined. The exception is photometric and nephelometric methods, in which the amount of the analyte is a fraction of a milligram.