Introduction

The separation and purification of substances are operations that are usually linked. The separation of a mixture into components most often pursues the goal of obtaining pure, if possible, without impurities, substances. However, the very concept of what substance should be considered pure has not yet been finally established, since the requirements for the purity of a substance are changing. At present, methods for obtaining chemically pure substances have acquired special significance.

The separation and purification of substances from impurities are based on the use of their specific physical, physico-chemical or chemical properties.

The technique of the most important methods of separation and purification of substances (distillation and sublimation, extraction, crystallization and recrystallization, salting out) is described in the relevant chapters. These are the most common techniques, most often used not only in laboratory practice, but also in technology.

In some of the most difficult cases, special cleaning methods are used.

Purification of substances

Recrystallization

Purification by recrystallization is based on the change in the solubility of a substance with a change in temperature.

Solubility is understood as the content (concentration) of a solute in a saturated solution. It is usually expressed either as a percentage or as grams of solute per 100 g of solvent.

The solubility of a substance depends on temperature. This dependence is characterized by solubility curves. Data on the solubility of some substances in water are given in fig. 1, as well as in the solubility table.

According to these data, if, for example, a solution of potassium nitrate is prepared by taking 100 g of water, saturated at 45º, and then cooling it to 0º, then 60 g of KNO 3 crystals should fall out. If the salt contained small amounts of other water-soluble substances, saturation with respect to them will not be reached at the indicated decrease in temperature, and therefore they will not fall out with the salt crystals. Negligible amounts of impurities, often not detectable by conventional methods of analysis, can only be carried away by precipitate crystals. However, with repeated recrystallizations, an almost pure substance can be obtained.

The saturated salt solution that remains after filtering out the precipitated crystals, the more pure they are, since in this case they capture the mother liquor containing impurities of other substances less. The reduction of impurities is facilitated by washing the crystals with a solvent after separating them from the mother liquor.

Thus, recrystallization is reduced to the dissolution of a substance in a suitable solvent and its subsequent isolation from the resulting solution in the form of crystals. This is one of the most common methods for removing impurities from substances.

Sublimation

Sublimation, or sublimation, is the direct conversion of a solid into vapor without the formation of a liquid. Having reached the sublimation temperature, the solid passes without melting into vapor, which condenses into crystals on the surface of cooled objects. Sublimation always occurs at a temperature below the melting point of the substance.

Using the property of a number of substances (iodine, naphthalene, benzoic acid, ammonia, etc.) to sublimate, it is easy to obtain in pure form if the impurity is devoid of this property.

For a deeper study of the phenomenon of sublimation, it is necessary to get acquainted with the diagram of the state of matter, shown in Fig. 2. The temperature t (in degrees Celsius) is plotted on the abscissa axis; the saturated vapor pressure p (in m / cm 3) is plotted on the ordinate axis. The state diagram of water has a similar form, so that its TB curve is inclined to the y-axis, since the freezing point of water decreases with increasing pressure.

The TA curve expresses the relationship between temperature and saturation vapor pressure over a liquid. All points of the TA curve determine the equilibrium conditions between the liquid and its saturated vapor. For example, at 100º water and steam can only exist at a pressure of 760 mm Hg. Art. If the pressure is more than 760 mm Hg. Art., then the steam condenses into water (the area above the TA curve); if the pressure is less than 760 mm Hg. Art., then all the liquid turns into vapor (the area below the TA curve). The TA curve lies above the melting point of the substance. The TB curve expresses the relationship between temperature and saturated vapor pressure over a solid. The vapor pressure of solids is usually low and largely depends on the nature of the solid and temperature. Thus, the vapor pressure of iodine at 16º is 0.15 mm Hg. Art., ice at - 15º equals 1.24 mm Hg. Art. The TB curve lies below the melting point of the substance. All points of this curve determine the equilibrium conditions between the solid body and its saturated vapor.

The TB curve is called the melting curve and expresses the relationship between the melting point of a substance and pressure.

All points on this curve define the conditions (temperature and pressure) under which the solid and liquid are in equilibrium.

Curves TA, TB and TV divide the state diagram of matter into three areas: 1 - the area of ​​existence of the solid phase, 2 - the liquid phase and 3 - the vapor phase.

The point T, where all three regions converge, indicates the temperature and pressure at which all three phases of matter - solid, liquid and vapor - can be in equilibrium. It is called triple point(T).

By changing the temperature or pressure, you can change the state of matter.

Let point 1 represent the solid state of matter at a pressure above the triple point. When a substance is heated at constant pressure, point 1 will move along the dotted line 1-4 and at a certain temperature will cross the melting curve TB at point 2. When all the crystals have melted, further heating at constant pressure will lead to point 3 on the TA curve, where the liquid begins to boil , the substance will go into a vapor state. With a further increase in temperature, the body will pass from state 3 to state 4. Cooling of the vapor will repeat the considered processes in the opposite direction along the same dotted curve from state 4 to state 1.

If we take a substance at a pressure below the triple point, for example, at point 5, then by heating the substance at constant pressure, we will reach point 6, at which the solid will pass into vapor without the preliminary formation of a liquid, i.e. sublimation or sublimation will take place (see dotted line 5-7). On the contrary, when the vapor is cooled at the same pressure, crystallization of the substance will occur at point 6 (also without the formation of a liquid).

From the foregoing, the following conclusions can be drawn:

1) As a result of heating a solid at a pressure above the triple point, it will melt;

2) As a result of heating a solid at a pressure below the triple point, it will sublime;

3) If heating is carried out at atmospheric pressure, then sublimation will occur if the pressure of the triple point of a given substance is higher than atmospheric. So, for example, at p \u003d 1 atm, carbon dioxide sublimates at - 79º, but it will melt, provided that the heating is carried out at a pressure higher than the pressure of the triple point.

Keep in mind that solids can vaporize at pressures above the triple point (because all solids and liquids partially vaporize at any temperature). So, crystalline iodine at atmospheric pressure below the melting point turns into a violet vapor, which easily condenses into crystals on a cold surface. This property is used to purify iodine. However, since the pressure of the triple point of iodine is below atmospheric pressure, it will melt with further heating. Therefore, crystalline iodine at atmospheric pressure cannot be in equilibrium with its saturated vapor.

Only solids that are under pressure below the triple point can be in equilibrium with their saturated vapor. But at such a pressure, these substances cannot melt. Sublimable substances can be converted into a liquid state by heating them at a certain pressure.

Some chemical reagents for analytical work have to be cleaned in the laboratory. Purification is carried out by filtration, distillation, recrystallization, extraction, chromatography and iontophoresis methods.

Filtration

Filtration is performed to separate solid particles from a liquid, such as insoluble impurities from a reagent solution. Filtration is based on passing a liquid-solid mixture through a porous filter, such as filter paper. The pores (holes) in the paper are so small that only liquid passes through them, and all solid particles remain on the filter. Both the filtration rate and the degree of purification depend on the pore size of the filter. The filtration rate is greatly influenced by the viscosity of the liquid and its temperature. Hot liquids always filter faster than cold liquids.

For filtering, a glass funnel is used (see Fig. 4), which is fixed in a tripod ring or in a special plate with a hole for filtering. Sometimes a special glass hook is made for small funnels, with which you can attach the funnel directly to the glass.

Filter paper, unlike ordinary paper, is not glued, more fibrous, uniform and clean. There are also ready-made round filters made of ashless paper.

To make a filter, a square sheet of filter paper is folded in half, then four times and the outer edges are rounded with scissors. Separate one layer of paper, forming an angle, and adjust the filter to the funnel. The edges of the filter should be 3-5 mm below the edge of the funnel. The spatial angle of the funnel should be equal to 60°, but sometimes the mouth of the funnel deviates somewhat from 60° up or down, and then the filter does not fit snugly against the walls of the funnel. In this case, slightly changing the angle of the inflection of the filter in one direction or another, tightly fit the filter to the walls of the funnel. After fitting the filter to the funnel, it is moistened with a pure solvent, for aqueous solutions - with water and, stroking with a clean finger, press the filter against the walls of the funnel so that there are no air bubbles under it.

Filtration proceeds fairly quickly when a column of liquid forms in the funnel tube. If a liquid column has not formed in the tube, then water is poured into the funnel above the edges of the filter, then the filter is slightly lifted with a finger and lowered, the flowing liquid almost always forms a column in the funnel tube. For the same purpose, the glass tube of the funnel is sometimes extended with a rubber tube.

The filtering liquid is poured into a funnel along a glass rod, leaning the glass spout against it. The stick is held vertically above the filter, without leaning against the filter. If there is a precipitate in the solution, then you need to let it settle, carefully filter most of the liquid, and only at the end pour the solution along with the precipitate. This is done so that the sediment does not clog the pores of the filter at the beginning of the filtration and that it does not last too long.

Pleated (folded) filters are often used to purify reagent solutions, filtration through which occurs much faster. A pleated filter is also made from a square sheet of filter paper. First, it is folded and cut like a normal filter (Fig. 41). Then the half is unscrewed and the right quarter is bent in half inward, the top eight is bent and folded in half, the resulting sixteenth share is again folded in half. On this slice (1/32 of the filter), the entire filter is folded with an accordion. The finished filter is deployed and put into the funnel. If the filter is large, it may break during filtration, to prevent this, first put a small ordinary filter into the funnel and fit it tightly to the funnel. It is also necessary when folding the filter to ensure that the folds do not come close to the center of the filter.

Never pour liquid up to the edge of the filter. The end of the funnel tube must be leaned against the wall of the glass so that there is no splashing of the filtrate. If the filtrate is cloudy, it is filtered again through the same filter.

Concentrated solutions of acids and alkalis, as well as permanganate solutions, cannot be filtered through paper, since these substances destroy it. They are usually filtered through glass wool. To do this, cotton wool is first treated with heating with hydrochloric acid, and then washed well with water. Such cotton wool is stored in a glass of distilled water, and for filtering it is put into the corner of the funnel. After the end of filtration, it is washed with water and placed in the same glass for storage. Concentrated solutions can also be filtered through sintered glass filter funnels using suction.

Suction filtration is used to filter a large mass of solid from a liquid. To do this, use the Bunsen flask and Buchner funnel (see Fig. 6 and 29). The funnel is inserted into the hole of a rubber stopper, matched to the neck of the Bunsen flask - a thick-walled conical flask with a process for suction; a rubber tube from a water jet pump is put on the process (Fig. 42).

Two paper filters of the appropriate diameter are placed on the funnel partition, moistened with distilled water and pressed tightly against the partition, trying to remove all air bubbles from under the filters. Having opened the water jet pump, check whether the filters are well attached. If the filters lie well, then a calm noisy sound will be heard. If there is air leakage, then a whistling sound is heard. In this case, the filters are pressed with a finger against the mesh partition until the whistle is replaced by a calm noisy sound.

Without closing the water jet pump, immediately pour the filtered liquid into the funnel (up to half the funnel height) and periodically add it, preventing the filters from being exposed. Due to the vacuum created in the Bunsen flask, the liquid flows quite quickly through the filters. The precipitate is usually transferred to the filters simultaneously with the liquid, stirring the mixture well with a glass rod. The loose sediment is compacted in the funnel with a flat glass stopper from the bottle. Suction continues until the complete cessation of the appearance of drops from the spout of the funnel. It is necessary to ensure that the flask is not filled with filtrate to the very process.

To stop suction, the rubber tube coming from the water jet pump is disconnected from the Bunsen flask, and then the pump is turned off. If the water jet pump is started to close immediately, without disconnecting from the “flutter”, then water from the pump may enter the filtrate due to a decrease in pressure inside the pump. The funnel is removed from the flask, the substance is shaken out on filter paper and dried. Suction filtration is used in the recrystallization of substances.

Sometimes it is necessary to filter hot solutions so that they do not cool down during filtration. For this, hot filter funnels are used.

Distillation

Distillation (distillation) is used to purify liquid substances (for example, water, hydrochloric acid, alcohols, ether) from non-volatile impurities. Distillation is based on the fact that when a liquid is heated to a certain temperature, depending on the composition of the liquid and atmospheric pressure, it begins to boil - it rapidly turns into a gaseous state (steam). If this vapor is cooled by venting through a gas outlet pipe, it will turn into a liquid. The distillation apparatus consists of a distillation flask 1 (Fig. 43), a refrigerator 2 and a receiver 4. All non-volatile impurities that are in the liquid in a dissolved state remain in the distillation flask.

To assemble a liquid distillation apparatus, a Wurtz flask is used - a round-bottom flask with a long neck, from which a long narrow outlet tube extends. The neck of the Wurtz flask is closed with a rubber or cork stopper with a thermometer; the stopper must be tightly fitted to the neck of the flask. The thermometer is placed so that its reservoir of mercury is opposite the opening of the outlet tube and does not touch the walls of the neck of the flask. The end of the outlet tube is passed through a fitted cork into a Liebig refrigerator for 3-4 cm. This joint must also be airtight. At the other end of the refrigerator, allonge 3 is strengthened (see Fig. 43) - a curved glass tube, with its wide end placed on a cork put on the end of the refrigerator, which is passed through the cork by 2-3 cm. The narrowed end of the allonge is lowered into the receiver, which can be any dishes (flask, flask).

Sometimes a Liebig refrigerator consists of separate parts that are not soldered together: a refrigeration tube and a refrigeration jacket. To assemble such a refrigerator, the tube is passed into a shirt and fastened to it by means of segments (rings) of a rubber tube. The rubber tube is picked up to the sleeves of the shirt and put on them, then a refrigerating (gas outlet) tube is passed through them, well lubricated with Vaseline and turning all the time.

When turning on the refrigerator, always connect the lower end of its shirt, which faces the receiving flask, to the water tap with a rubber tube. From the upper end, a drain is made into the sink. It is necessary to ensure that the refrigerator jacket is always filled with water.

The Wurtz flask is fixed in the leg of a tripod so that it can be heated. The foot should wrap around the neck of the flask above the outlet tube. Connect the flask to a refrigerator mounted on a second stand. Carefully remove the cork with a thermometer, insert a funnel into the neck of the flask with a tube descending below the opening of the outlet tube, and pour into the flask 2/3 of its volume the liquid to be distilled. Several glass capillaries sealed at one end are placed in the flask to ensure uniform boiling of the liquid. Violent boiling of the liquid during distillation is unacceptable, as this can lead to droplets entering the outlet tube and contaminating the distillate.

After closing the flask with a stopper with a thermometer and checking the reliability of the assembly of the device, water is supplied to the refrigerator and then the heating is turned on. Heating can be carried out on a gas burner through a grid, in a water bath or by other means. After boiling the liquid, the heating is reduced so that even boiling occurs.

The liquid should never be evaporated completely, it should remain in the distillation flask 10-15% of the originally taken volume. For a new filling of the flask, the heating is turned off, the flask is allowed to cool slightly, the stopper with the thermometer is carefully removed and the liquid is added through the funnel. From time to time, residues with impurities should be removed from the distillation flask.

Distillation apparatuses are also made entirely of glass. Such an apparatus consists of distillation and receiving flasks and a condenser with polished stoppers. There is a special pocket for a thermometer in the stopper of the distillation flask. The bent end of the cooler tube in front of the section to the receiving flask has a process for removing excess gases.

Many liquids have their own characteristic features that must be taken into account when distilling. Therefore, before proceeding with the distillation of any substance, you need to familiarize yourself well with the features of its implementation according to the manual.

In some cases, a special device is used for distillation. It is a cylindrical vessel with a capacity of 1 liter, equipped with a screw cap with an inner cone (Fig. 44). A tripod and a cup are placed inside the cylinder. All details are made of PTFE-4.

This device is used, for example, to obtain highly pure hydrofluoric acid for the spectral analysis of silicon and its compounds.

500-600 ml of purified hydrofluoric acid is poured into a cylindrical vessel, 0.2 g of spectrally pure coal powder is added and thoroughly mixed with a fluoroplastic spatula. An empty cup is placed on the tripod - the receiver. The cylindrical vessel is closed with a lid and placed in a boiling water bath. The lid of the vessel is cooled from the outside with dry ice (solid CO2). Acid vapors, cooling on the conical side of the lid, condense and flow down from the top of the cone into the cup. Distillation is carried out at a rate of 15-20 ml/h. The first fraction and VAT residue (10% of the loaded acid) are discarded. The average fraction is used for analysis. The purified acid is stored in a fluoroplastic can with a well screw cap.

In the described device, in addition to hydrofluoric acid, hydrochloric and nitric acids can be distilled, as well as ammonia solutions, ethyl alcohol, and water can be purified.

Recrystallization

The essence of recrystallization is that the substance to be purified is dissolved in the smallest possible volume of hot water, the solution is filtered from insoluble impurities, and the filtrate is rapidly cooled. Due to the decrease in solubility upon cooling, part of the substance is released from the solution in the form of crystals. Dissolved contaminants, which are present in much smaller quantities than the main substance, do not crystallize, but remain in the mother liquor. Separating the crystals from the mother liquor by filtration, the substance is obtained in a fairly pure state.

Sometimes it is not possible to purify the substance by a single recrystallization, then it is repeated 2-3 times. Recrystallization cannot purify a substance from contaminants involved in the construction of the crystal lattice of the substance being purified, i.e. forming with it the so-called mixed crystals.

Recrystallization of oxalic acid. Recrystallized oxalic acid of the composition H2C2O4-2H2O is used to set the titer of solutions of potassium permanganate KMnO4 or solutions of alkalis NaOH or KOH.

Take in a glass with a capacity of 300 ml on a laboratory chemical balance 100 g of commercial oxalic acid; then measure with a graduated cylinder and pour 150 ml of hot distilled water into a glass. It is heated on a gas burner (on an asbestos grid) until the sample is completely dissolved, stirring the contents of the glass with a glass rod. Only a slight white amorphous insoluble residue may remain at the bottom.

The hot solution is filtered all at once through a pleated filter inserted into a funnel with a short tube. In the long tube of the funnel, crystallization of oxalic acid may occur and the tube will become clogged with crystals. To avoid crystallization during filtration, it is advisable to use a hot filtration funnel. The filtrate is collected in a beaker placed in a crystallizer with cold water. After the end of filtration, the filtrate is well stirred for 10 min with a glass rod.

The separated crystals are filtered off on a Buchner funnel with suction. Two filters are put into the funnel, moistened and firmly pressed to the bottom of the funnel, and the water jet pump is turned on. The entire solution, together with the crystals, is poured into a funnel. The remains of the crystals are cleaned with a glass rod from the walls of the glass into the funnel. Suction is carried out until the appearance of drops at the tip of the funnel tube stops, and the crystals acquire a snow-white color. After suction, first disconnect the flask from the pump, and then close the tap of the water jet pump.

The funnel is removed from the flask and the crystals are shaken out of it onto a sheet of filter paper folded in half. Spread the crystals evenly with a glass rod, cover with another sheet folded in half, and squeeze the crystals between the sheets. If the paper becomes wet, take new sheets and squeeze the crystals again until the paper is no longer wet. The crystals are “sorted out” with a glass rod, and if they do not stick to it or completely lag behind it with slight shaking, then the drying is considered complete. The crystals are left in the air for another half an hour, spreading them in a thin layer on a sheet of filter paper, then poured into a jar or bottle with a good cork. Exit around 70.

Owen proposed a convenient device for the recrystallization of organic substances for microanalysis (Fig. 45). In such a device, but only in a larger size, it is possible to recrystallize small samples of substances for routine analysis.

The device consists of two identical crystallization tubes 1 and 5 and the central part 3. The tightness of the connections is created by flanges 6 and 8, compressed by a spring clamp. Each part is made from a glass tube with a diameter of 10 mm with soldered conventional flanges. It is convenient to have several crystallization tubes 1 and 5. Filtration is carried out through one or two dense paper filters 7 with a diameter of 2 cm. To remove moisture, the tubes are pre-dried well. Drying can be carried out by blowing warm air through branches 2 or 4, placing pieces of cotton wool in them to protect against atmospheric dust.

To separate insoluble impurities in tube 5, a sample of a solid substance is dissolved in an appropriate solvent, filling the tube 1 cm below the exit of process 4. The tube is closed with a stopper, fixed on a tripod, and heated until the sample is completely dissolved. The instrument is then assembled as shown in Fig. 45, inserting a paper filter between the flanges, carefully turn over and filter the hot solution into the receiver tube 1. To speed up the filtration, weak suction through the process 2 or weak pressure through the process 4 can be used.

The receiver tube 1 containing the pure filtrate is used to crystallize the substance by cooling or evaporating the solvent with suction. For crystallization, the central part with the tube is detached and replaced with a stopper (flange 8). After separating the crystals, the stopper is removed, a paper filter is applied to the flanges, the central part is attached (tube up) to another receiving tube 5. Then the device is turned over and the mother liquor is filtered with suction. The receiver 5 is separated, the filtrate is poured into the collection, and the tube is rinsed with solvent. The receiver is again attached to the central part and the device is turned over. For washing, a washing liquid is introduced into the tube with crystals through the process 4 and the contents are shaken. The apparatus is inverted and the washing liquid is filtered off with suction. Washing can be repeated many times.

After washing, the main part of the crystals is on the filter. The central part of the device is separated. The crystals, together with the filter, are shaken off by tapping on a clean sheet of filter paper. The crystals are filtered and dried with the filter in an oven. Hygroscopic substances are dried directly in the tube, while the central part is removed and replaced with a glass cover. Suction is carried out through the tube 4.

Extraction

The word extraction means extracting. Purification of liquids by extraction is based on the different solubility of individual substances in different solvents. Extraction cleaning is carried out by shaking the solution with a water-immiscible liquid in which contaminants dissolve better than in water. Extraction is carried out in a separating funnel (Fig. 46).

The solution to be purified is poured up to no more than half of the separating funnel. A suitable solvent is also added there, immiscible with water, in an amount of not more than half of the solution taken for cleaning. Having closed the separating funnel and holding the stopper with one hand and the tap with the other, turn the funnel up and down several times with a smooth movement. Do not vigorously shake the contents of the funnel, as this may form a stable emulsion, which will take a long time to separate. Stirring should be carried out for 15-20 minutes so that the layers of the liquid seem to slide one over the other. From time to time, shaking is stopped and in an inverted state (when the valve is raised up), the valve is carefully opened to equalize the pressure of the gases.

At the end of the extraction, the separating funnel is allowed to stand in the rack until the liquids are completely separated and a sharp boundary is established between them. After that, the cork is opened, and then, carefully opening the tap, the lower layer of liquid is poured into the glass. To reduce the rate of outflow of liquid at the end of the expiration, the valve is slightly covered. Then the tap is closed and the remaining liquid is poured through the neck of the funnel into another glass. For complete purification, the extraction is repeated several times.

Purification of dithizone. For the photometric determination of zinc, a 0.02% solution of purified dithizone in chloroform is prepared. To do this, 0.2 g of dithizone is dissolved in 20 ml of chloroform and the solution is purified by extraction. The solution is placed in a separating funnel with a capacity of 600 ml, add 200 ml of 2% (by volume) ammonia solution and shake well. Dithizone thus passes into the ammonia layer. The chloroform layer is separated and discarded. Add another 5 ml of chloroform, mix again and pour off the chloroform layer. Washing with portions of 5 ml of chloroform is continued until the chloroform layer no longer turns red.

50 ml of chloroform, 4 ml of hydrochloric acid (1:1) are poured into a funnel to an ammonia solution of dithizone, and an excess of it is added dropwise until an acidic reaction, then mixed well. Dithizone goes into chloroform; the solution turns green. The chloroform layer is washed twice with water. Pour the dithizone solution into a 100 ml volumetric flask, dilute to the mark with chloroform and mix well.

In laboratory practice, the following methods of purification of substances are most often used: recrystallization, sublimation and absorption. Recrystallization and sublimation are used to purify solids, and absorption of impurity gases by various substances is used to purify gases.

Recrystallization

Purification by recrystallization is based on the change in the solubility of a substance with a change in temperature. Solubility is the content (concentration) of a solute in a saturated solution. It is usually expressed either as a percentage or as grams of solute per 100 g of solvent. Data on the solubility of some compounds in water at different temperatures are shown in Fig. . 2.1 and in the application. Small amounts of impurities, often not detectable by conventional methods of analysis, cannot be mechanically entrained in precipitate crystals. With repeated recrystallization, an almost pure substance can be obtained. The saturated salt solution that remains after filtering out the precipitated crystals is called the mother. The smaller the precipitated crystals, the more pure they are, since in this case they capture the mother liquor containing impurities of other substances less. The reduction of these impurities is facilitated by washing the crystals with a solvent after separating them from the mother liquor.

Rice. 2.1. Solubility Curves

Sublimation

Sublimation or sublimation is the direct transformation of a solid into vapor without the formation of a liquid. Having reached the sublimation temperature, the solid substance passes without melting into vapor, which condenses into crystals on the surface of cooled objects. Sublimation always occurs at a temperature below the melting point of the substance.

Using the ability of a number of substances (iodine, naphthalene, benzoic acid, ammonia, etc.) to sublimate, it is easy to obtain them in their pure form (if the impurity does not sublimate).

In engineering and laboratories, sublimation is carried out not only at atmospheric pressure, but also at reduced pressure (vacuum).

Distillation

Distillation or distillation is based on the conversion of a liquid into vapor, followed by the condensation of the vapor into a liquid. This method separates a liquid from dissolved solids or less volatile liquids. So, for example, with the help of distillation, water is purified from the salts that it contains. The result is distilled water.

For the distillation of small amounts of liquid in the laboratory, a distillation apparatus is used (Fig. 2.2).

A liquid boils when its vapor pressure becomes equal to the external pressure (usually atmospheric). A pure substance at constant pressure boils at a strictly defined temperature. The mixtures boil at different temperatures depending on the composition. Therefore, the boiling point is a characteristic of the purity of a substance. The purer the substance, the smaller the difference between the boiling point of the substance and the distillation temperature at which it is distilled.

Rice. 2.2. Distillation plant:

1 - Wurtz flask, 2 - Liebig refrigerator, 3 - allonge, 4 - receiver

Distillation, when the distillate is taken at different temperature ranges and in different receivers, is called fractional or fractional distillation. Liquids in receivers, taken in certain temperature ranges, are called fractions. By repeating fractional distillation several times, it is possible to almost completely separate a mixture of liquids and obtain the components of the mixture in pure form.

A more complete and faster separation of a mixture of liquids by fractional distillation is favored by the use of reflux condensers or distillation columns. Reflux distillation, as well as other distillation techniques such as reduced pressure distillation, are covered in manuals and workshops on organic chemistry.

Gas cleaning

Purification of gases from impurities is achieved by passing it through substances that absorb these impurities. For example, to obtain carbon dioxide in the Kipp device, along with CO 2, impurities come out: hydrogen chloride (from hydrochloric acid) and water vapor. If carbon dioxide with impurities is passed first through a wash bottle with water (to absorb hydrogen chloride), and then through sulfuric acid (to absorb water vapor), then it will turn out to be practically pure.

To determine the degree of purity of a substance, physical and chemical research methods are used. The former include: for liquid substances - determination of density, boiling point, refractive index; for solids - determination of the melting point and a number of others, the second methods include chemical qualitative and quantitative analyzes for the content of impurities.

There are no absolutely pure substances. Substances used in laboratory workshops have different degrees of purity. The maximum allowable amount of impurities in a substance is established by the state standard (GOST).

Substances labeled chemically pure are suitable for laboratory work in general chemistry and qualitative analysis. and h.d.a.

Sublimation or sublimation is a process in which a crystalline substance, heated below its melting point, passes into a vapor state (bypassing the liquid state), and then settles on a cold surface in the form of crystals.

At atmospheric pressure, at temperatures below Tm, only organic compounds with a relatively high vapor pressure can sublimate. There are few of them, the vast majority of compounds sublimate only under reduced pressure.

Sublimation is an excellent method of purifying substances in cases where contaminants have a different volatility than the compound itself (compounds of similar volatility will sublimate together) and replaces long and laborious crystallization. Sublimation is easy to carry out even with very small quantities of substance with minimal losses. This method is particularly suitable for the purification of quinones, polynuclear hydrocarbons, substances that form solvates or hydrates.

The simplest device for sublimation at atmospheric pressure is a low glass without a spout with a thin layer of a substance intended for sublimation at the bottom. The beaker is closed with a round bottom flask through which water flows. At high sublimation temperatures, the water in the flask may not be running.

Sublimation can also be carried out in a porcelain cup, closed with a wide end of a funnel, the diameter of which is slightly smaller than the diameter of the cup (Fig. 3a).

The narrow end of the funnel is loosely covered with cotton wool. In order to prevent the sublimate from getting back into the cup, it is covered with a sheet of filter paper with holes in it. Substance subjected to sublimation must be finely divided.

Already a slight overheating can contribute to the rapid thermal decomposition of the sublimated substance.

This danger can be avoided by carrying out sublimation in a vacuum. To create a vacuum, water-jet, oil pumps are used. The device for sublimation in vacuum is shown in Fig. 3b. The substance to be sublimated is placed at the bottom of a test tube into which a finger cooler is inserted. The distance between the bottom of the sublimator and the end of the cooler should be small, but sufficient so that the sublimated substance is not contaminated by splashing of the solid.

Rice. 3. A device for sublimation (a), a device for vacuum sublimation (b): 1 - a glass with a thin section; 2 - a cap with a finger cooler, 3 - a tube for water inlet; 4 - pipe for water outlet; 5 - pipe for connection to a vacuum pump; 6 - sublimable substance

Usually it is about 1 cm. After evacuation, the sublimator is immersed in an oil bath and gradually heated until a film of sublimated substance forms on the surface of the refrigerator. Upon completion of the sublimation, the vacuum is first turned off and the refrigerator is removed. The sublime substance is scraped off on the watch glass.



The substances used for work in the laboratory must be sufficiently pure, because the true properties of individual substances appear only when they are purified from impurities that accompany them in natural materials, as well as from impurities that enter them during the production process.

Each pure substance has certain physical properties: color, melting point, boiling point, density, etc., so the purity of a substance can be determined by studying these properties. The properties most suitable for evaluating the purity of a substance are those that can be quantified. The data obtained are compared with the data in the tables for the test substance. In practice, the melting point, boiling point and density are most often determined. Impurities for the most part lower the melting point, and the latter does not remain from the beginning of melting until the complete melting of the substance, as in the case of a pure substance. The boiling point of a liquid in the presence of impurities rises and does not remain constant during boiling.

The concept of the purity of a substance is of fundamental importance in modern inorganic chemistry. Absolutely pure substances do not exist in nature. Therefore, there are no absolutely insoluble substances and, therefore, any substance is contaminated with impurities. Impurities radically affect the properties of a substance.

The problem of obtaining pure substances has three main aspects. 1. The properties of a substance can only be determined by obtaining it in the required degree of purity. Comparison of the same properties of different substances is permissible only if they are of the same purity. 2. Selection of suitable methods to purify the substance to the required purity. 3. Ensuring sufficiently sensitive and selective purity control methods. (see Ya.A. Ugay Inorganic Chemistry, 1989, pp. 46-47).

With the development of science and technology, the problem of obtaining ever more pure substances arises. The advances in chemistry in recent decades have been exceptionally great, and no less significant is the technical progress in the field of pure substances. Over the past 40-50 years, the very concept of a pure substance has changed (in particular, of “chemically pure”), and the requirements for laboratory reagents have increased. The production of pure substances is the reduction of the content of impurities from 0.1-1% to hundredths of a percent. Further purification is a much more complex and time-consuming task. When working with reagents, one must always remember that a decrease in the content of impurities, even by one order of magnitude, leads to a sharp increase in the price of the reagent. Therefore, high-purity preparations should not be used for low-responsibility work.

According to the current situation, the reagents are classified as “pure” (pure), “pure for analysis” (analytical grade), “chemically pure” (chemically pure) and “extra pure” (specially pure) , the latter, in turn, is divided into several brands. Reagents of “pure” qualification can be successfully used in a variety of laboratory work, both educational and industrial. “Pure for analysis” reagents, as the name suggests, are designed for analytical work performed with great precision. The content of impurities in the preparations of analytical grade. so small that it usually does not introduce noticeable errors in the results of the analysis. These reagents may well be used in research work. Finally, reagents of the “chemically pure” qualification are intended for responsible scientific research; they are also used in analytical laboratories as substances for which titers of working solutions are established. These three qualifications cover all general purpose reagents. Preparations of higher purity (“special purity”) are intended only for special purposes, when even millionths of a percent of an impurity are completely unacceptable. Such highly pure substances can only be obtained with the help of special physicochemical purification methods based on different distributions of impurities in coexisting phases. Methods of sublimation, extraction, chromatography, directional crystallization, zone melting make it possible to obtain substances that are qualified as “extra pure”. It is absolutely unacceptable and pointless to use expensive substances of high purity for ordinary analytical and scientific work.