Solids purification. Purification of substances from impurities is carried out using various methods.

Purification of soluble salts by recrystallization . The recrystallization method is based on the different dependence of the solubility of substances and contaminants on temperature. The purification of a substance by recrystallization is carried out according to the following scheme: a saturated solution of the substance being purified is prepared at elevated temperatures, then, to remove insoluble impurities, the solution is filtered through a hot filter funnel and cooled to a low temperature. As the temperature decreases, the solubility of the substance decreases and the main part of the purified substance precipitates; soluble impurities remain in the solution, since the solution remains unsaturated relative to them. The precipitated crystals are separated from the mother liquor and dried.

Depending on the properties of the substance being purified, various recrystallization techniques are possible.

Recrystallization without solvent removal. The method is used for salts whose solubility strongly depends on temperature (for example, sodium nitrate, potassium alum, copper (II) sulfate, etc.). After hot filtration, the solution is cooled in air to a low temperature, and the precipitated crystals are filtered off. It is also possible to carry out recrystallization without removing the solvent for salts whose solubility depends little on temperature. In this case, the salting out method is used. To do this, the solution after hot filtration is cooled to room temperature and an equal volume solution of concentrated hydrochloric acid is added, and the substance to be purified precipitates.

Recrystallization with solvent removal. The method is used for salts whose solubility depends little on temperature (for example, sodium chloride, etc.). The solution, after hot filtration, is transferred to a weighed porcelain cup and evaporated in a water bath to approximately half the volume. The solution is then cooled to room temperature. The precipitated crystals are filtered off.

The recrystallized substance (with the exception of ammonium chloride and crystalline hydrates) is dried in an oven to constant weight. Ammonium chloride and crystal hydrates are dried in air. Dry salts are placed in sealed bottles.

Purification of volatile substances by sublimation (sublimation) . The method is used for the purification of solid substances that, when heated, can pass directly from the solid phase into the gaseous phase, bypassing the liquid phase. The resulting gas is condensed by the cooled part of the device. Sublimation is usually carried out at a temperature close to the melting point of the substance. The method is applicable for purification from impurities that are not capable of sublimation. Sublimation can purify iodine, sulfur, and ammonium chloride.

Purification of liquids by distillation . The method is based on the fact that each substance has a certain boiling point. The simplest version of distillation is distillation at normal pressure, which consists of heating a liquid to a boil and condensing its vapors. Distillation is carried out in an apparatus consisting of a Wurtz flask (or a round-bottomed flask with a gas outlet tube), a straight condenser, a receiver flask, an allonge, a thermometer and a heating device. The contaminated liquid is heated in a distillation flask to boiling point, the vapors are removed to the refrigerator and the condensed liquid is collected in a receiver.

Introduction

Boron is mainly used in the form of borax.

BOROX - sodium salt of tetraboric acid. It is widely used in the production of fusible glaze for earthenware and porcelain products and, especially for cast iron cookware (enamel); In addition, it is used for preparing special types of glass.

The use of borax in soldering metals is based on the dissolution of metal oxides. Since only clean metal surfaces can be soldered, to remove oxides, the soldering area is sprinkled with borax, solder is placed on it and heated. Borax dissolves oxides, and the solder adheres well to the metal surface.

Boron plays an important role in plant life. the presence of a small amount of boron compounds in the soil is necessary for the normal growth of agricultural crops, such as cotton, tobacco, sugar cane, etc.

In nuclear engineering, boron and its alloys, as well as boron carbide, are used for the manufacture of reactor rods. Boron and its compounds are used as materials that protect against neutron radiation.

This work is devoted to methods for purifying borax as the main substance - a source of boron.

Borax and its properties

Sodium tetraborate (“borax”) - Na 2 B 4 O 7, a salt of a weak boric acid and a strong base, a common boron compound, has several crystalline hydrates, and is widely used in technology.

Chemistry

Structure of anion 2− in borax

The term “borax” is used in relation to several related substances: it can exist in anhydrous form, in nature it is more often found in the form of pentahydrate or decahydrate crystalline hydrate:

Anhydrous borax (Na 2 B 4 O 7)

Pentahydrate (Na 2 B 4 O 7 5H 2 O)

Decahydrate (Na 2 B 4 O 7 10H 2 O)

However, the word borax most often refers to the compound Na 2 B 4 O 7 10H 2 O.

Natural springs

Borax, "cottonball"

Sodium tetraborate (Borax) is found in salt deposits formed by the evaporation of seasonal lakes.

Borax (sodium tetraborate decahydrate, Na 2 B 4 O 7 · 10H 2 O) are transparent crystals that completely lose water when heated to 400°C.

Ordinary borax (hydrate decahydrate) forms large, colorless, transparent prismatic crystals; base-centered monoclinic lattice, a = 12.19 Å, b = 10.74 Å, c = 11.89 Å, ß = 106°35´; density 1.69 - 1.72 g/cm3; In dry air, the crystals erode from the surface and become cloudy.

Borax hydrolyzes in water, its aqueous solution has an alkaline reaction.

With the oxides of many metals, borax, when heated, forms colored compounds - borates (“borax pearls”). Occurs in nature as the mineral tincal.

Tinkal, or “Borax” (sodium tetraborate decahydrate, Na 2 B 4 O 7 · 10H 2 O) is a mineral of the monoclinic system, prismatic. “Tinkal” is a word of Sanskrit origin, which is synonymous with the more commonly used name for the mineral - “Borax” (from the Arabic “burak” - white).

White color, glass luster, Mohs hardness 2 - 2.5.

Density 1.71.

Cleavage is average in (100) and (110).

It forms short-prismatic crystals, shaped like pyroxene crystals, as well as solid granular masses and veinlets in clayey rocks.

A typical evaporite mineral.

In air it collapses, losing crystallization water and becomes covered with a crust of tincalconite or kernite, over time turning into them completely.

The so-called Jewelry Borax is sodium tetraborate pentahydrate Na 2 B 4 O 7 5H 2 O.

Borax is used:

· in the production of enamels, glazes, optical and colored glasses;

· when soldering and melting as a flux;

· in the paper and pharmaceutical industries;

· in the production of building materials as an antiseptic component for the production of cellulose insulation “Ekovata”

· as a disinfectant and preservative;

· in analytical chemistry:

o as a standard substance for determining the concentration of acid solutions;

o for the qualitative determination of metal oxides (by the color of pearls);

· in photography - in the composition of slow-acting developers as a weak accelerating substance;

· as a component of detergents;

· as a component of cosmetics;

· as a raw material for boron production;

· as an insecticide in poisoned baits to kill cockroaches.

In dry air, the crystals erode from the surface and become cloudy. When heated to 80°C, the decahydrate loses 8 water molecules; at 100 degrees, slowly, and at 200°C, another water molecule is quickly split off; in the range of 350 - 400°C, complete dehydration occurs.

Solubility of borax (in anhydrous salt per 100 g of water): 1.6 (10°C), 3.9 (30°C), 10.5 (50°C). The saturated solution boils at 105°C.

Borax hydrolyzes in water, so its solution has an alkaline reaction.

The alkaline reaction of the sodium tetraborate solution is due to the fact that a hydrolysis reaction occurs in an aqueous solution with the formation of boric acid B(OH) 3 in the solution:

Na 2 B 4 O 7 = 2Na + + B 4 O 7 2– ;

B 4 O 7 2– + 7H 2 O 2OH – + 4B(OH) 3,

and the release of ammonia upon interaction with NH4Cl corresponds to the equation:

Na 2 B 4 O 7 + 2NH 4 Cl + H 2 O = 2NH 3 + 2NaCl + 4B(OH) 3

Borax dissolves in alcohol and glycerin.

Completely decomposes with strong acids:

Na 2 B 4 O 7 + H 2 SO 4 + 5H 2 O = Na 2 SO 4 + 4H 3 BO 3.

This is exactly how the Dutch alchemist Wilhelm Gomberg, by heating borax with sulfuric acid H 2 SO 4, isolated boric acid B(OH) 3.

With the oxides of some metals, borax produces colored borates (“borax pearls”):

Na 2 B 4 O 7 + CoO = 2NaBO 2 + Co(BO 2) 2,

which is used in analytical chemistry to discover these metals.

When a solution of ordinary borax is slowly cooled at 79°C, octahedral borax Na 2 B 4 O 7 begins to crystallize. 5H 2 O (or “jewelry borax”), density 1.815 g/cm 3, stable in the range 60 - 150 ° C. The solubility of this borax is 22 g in 100 g of water at 65°C, 31.4 at 80°C and 52.3 at 100°C.

Borax is the most important flux that facilitates the smelting process. When cooled, molten borax forms a glaze on the walls of the crucible, protects the melt from oxygen and dissolves metal oxides.

With the slow thermal dehydration of ordinary borax, a pyroborax with a density of 2.371 g/cm 3 and a melting point of 741 ° C is obtained. Borax melts and breaks down into sodium metaborate and boron trioxide, which mix in a liquid state:

Na 2 B 4 O 7 → 2NaBO 2 + B 2 O 3 .

Boron oxide, combining with metal oxides, forms metaborates in the same way as boric acid. Sodium metaborate easily mixes with newly formed metaborates and quickly removes them from the molten metal zone, and new active boron oxide molecules take their place.

Borax has a greater ability to dissolve oxides than boric acid, and is used not only as a melting reducing flux, but also as the most important flux for brazing.

Ordinary borax is obtained from boric acid, from tincal, kernite and some other minerals (by recrystallization), as well as from salt lake water (by fractionated crystallization).

Borax is widely used in the preparation of enamels, glazes, in the production of optical and colored glasses, in welding, cutting and soldering of metals, in metallurgy, electroplating, dyeing, paper, pharmaceutical, leather production, as a disinfectant and preservative and fertilizer.

Purification of substances by recrystallization

Recrystallization is a method of purifying a substance based on the difference in solubility of a substance in a solvent at different temperatures (usually the temperature range from room temperature to the boiling point of the solvent, if the solvent is water, or to some higher temperature).

Recrystallization implies poor solubility of a substance in a solvent at low temperatures, and good solubility at high temperatures. When the flask is heated, the substance dissolves. After the stage of adsorption of impurities (if necessary) with activated carbon, hot filtration (if necessary) and cooling, a supersaturated solution is formed, from which the dissolved substance precipitates. After passing the mixture through a Bunsen flask and a Buchner funnel or centrifugation, we obtain a purified solute.

· Advantage of the method: high degree of purification.

· Disadvantage of the method: strong losses of substance during recrystallization: always part of the dissolved substance will not precipitate, losses during recrystallization often amount to 40-50%.

The solvent can be water, acetic acid, ethanol (95%), methanol, acetone, hexane, pentane - depending on the conditions.

If the solvent is water, then heating is carried out in a water bath. Cooling of the supersaturated solution is carried out using a water cooler if the boiling point of the solvent is below 130 degrees, if higher - using an air cooler.

The solubility of most solids increases with increasing temperature. If you prepare a hot, concentrated (almost saturated) solution of such a substance, then when this solution is cooled, crystals will begin to precipitate, since the solubility of the substance is less at a lower temperature. The formation of a cold saturated solution, the concentration of which is less than the initial (hot) one, will be accompanied by crystallization of the “excess” substance.

Dissolving a substance containing soluble impurities in hot water and then precipitating it from solution when cooled sufficiently is a method of purifying a substance from soluble impurities, which is called recrystallization. In this case, impurities, as a rule, remain in the solution, since they are present there in negligible (“trace”) quantities and upon cooling cannot form their saturated solution.

Some part of the substance being purified also remains in a cold saturated solution, which in laboratory practice is called uterine, and such inevitable (planned) losses of a substance can be calculated from the solubility of the substance at this temperature.

The more the solubility of a substance decreases when the solution is cooled, the higher the yield of recrystallized substance will be.

Many solids form crystalline hydrates when crystallized from an aqueous solution; for example, from an aqueous solution, copper (II) sulfate crystallizes in the form of CuSO 4 · 5 H 2 O. In this case, the calculation must take into account the water that is part of the crystalline hydrate.

Recrystallization is of great importance in chemistry and chemical technology, since the vast majority of solids - chemical products, reagents, chemicals, drugs, etc. are obtained from aqueous and non-aqueous solutions, and the final stage of this preparation is crystallization (or recrystallization in order to increase the purity of the product). Therefore, it is very important to carry out these processes efficiently, with minimal losses and high quality indicators.

To carry out recrystallization, special chemical glassware and laboratory equipment are used.

The recrystallization process is carried out in several stages:

Choice of solvent;

Preparation of a saturated hot solution;

- “Hot” filtration;

Cooling the solution;

Separation of formed crystals;

Washing the crystals with a clean solvent;

Drying.

Solvent selection

The correct choice of solvent is a condition for recrystallization.

There are a number of requirements for the solvent:

A significant difference between the solubility of a substance in a particular solvent at room temperature and when heated;

The solvent should dissolve only the substance when heated and not dissolve impurities. The efficiency of recrystallization increases with increasing difference in solubility of the substance and impurities;

The solvent must be indifferent to both the substance and impurities;

The boiling point of the solvent must be 10 - 15°C lower than the melting point of the substance, otherwise when the solution is cooled, the substance will not be released in crystalline form, but in the form of an oil.

Experimentally, the solvent is chosen as follows: a small sample of the substance is placed in a test tube, adding a few drops of solvent to it. If a substance dissolves without heating, such a solvent is not suitable for recrystallization.

The choice of solvent is considered correct if the substance dissolves poorly in it without heating, well - when boiling, and when the hot solution is cooled, its crystallization occurs.

Water, alcohols, benzene, toluene, acetone, chloroform and other organic solvents or mixtures thereof are used as solvents for recrystallization.

The substance for recrystallization is placed in a flask (1), a small portion of the solvent is added and heated under reflux (2) until the solution boils. If the initial amount of solvent is not enough to completely dissolve the substance, the solvent is added in small portions using a funnel through a reflux condenser.

Effective purification of heavily contaminated substances is possible using various adsorbents (activated carbon, silica gel, etc.). In this case, prepare a hot saturated solution of the substance, cool it to 40 - 50 ° C, add an adsorbent (0.5 - 2% by weight of the substance) and reflux it again for several minutes.

"Hot" filtration

To separate mechanical impurities and adsorbent, the hot solution is filtered. To prevent the release of substances on the filter, various methods are used.

A simple “hot” filtration installation (Fig. 3.2) consists of a special “hot” filtration funnel (1), heated by steam, a chemical funnel (2) with a pleated filter (3), which is placed in it.

The hot, saturated solution of the substance is quickly poured onto a paper filter placed in a glass funnel, which is heated using a hot filter funnel. The filtrate is collected in a beaker or conical flask. When substance crystals form on the filter, they are washed with a small amount of hot solvent.

Cooling the solution

When the filtrate is cooled to room temperature, the crystallization process begins. To speed it up, the filtrate is cooled under running cold water. In this case, the solubility of the substance decreases, and final crystallization occurs.

Separation of formed crystals

The separation of crystals from the solvent is carried out by filtration, while suction or creating a vacuum in the receiver is often used to speed up the filtration process. To do this, use a vacuum pump (water jet, oil or Kamovsky).

Filtration is carried out in an installation that consists of a Buchner funnel (1) with a paper filter, a Bunsen flask or a special test tube (2), an intermediate beaker (3) and a vacuum pump. The size of the paper filter must exactly match the area of the bottom of the Buchner funnel.

The paper filter is moistened with solvent, placed in a funnel and the vacuum pump is turned on. When the pump operates, a reduced pressure is created under the filter - a characteristic sound occurs, which indicates the presence of vacuum in the system and the possibility of filtration. The cooled crystalline product together with the solvent, while shaking, is transferred in small portions from the conical flask to a paper filter.

During the filtration process, the solvent passes through the filter and the precipitate remains on it. Care should be taken that the filtrate does NOT fill the flask to the level of the tube connected to the intermediate glass. Filtration is continued until the filtrate stops dripping. After this, the precipitate is squeezed out on the filter with a wide glass stopper or a special glass rod, the pump is turned off, the precipitate is washed with a clean solvent, the pump is turned on and squeezed out again. The installation is disconnected from the vacuum, the funnel is removed. The filter along with the substance is carefully transferred to a Petri dish or a special container for drying.

Drying the solid

The solid can be dried in air at room temperature. Hygroscopic substances are dried in desiccators; resistant to air and temperature - in a drying cabinet, where the temperature should be 20 - 50 ° C below the melting point of the substance. For the recrystallized and dried product, the mass, yield and melting point are determined.

Melting point determination

The melting point of a substance is the temperature interval from the beginning to the complete melting of this substance. The purer the substance, the shorter this interval. The difference between the temperature at which the formation of the liquid phase begins and the temperature of complete melting for pure compounds does not exceed 0.5°C.

The presence of a small amount of impurities in a substance reduces its melting point and accordingly increases the melting range. This property is used to establish the identity of two substances, if one of them is known: equal amounts of substances are thoroughly mixed and the melting point of the mixture is determined (mixed sample). If the melting point of the mixed sample is the same as that of the pure substance, it is concluded that both substances are identical.

The melting point of a crystalline organic substance is determined in a capillary. The capillary is removed from the glass tube by heating it on a burner flame. One end of the capillary is sealed.

The recrystallized substance is thoroughly ground on a watch glass or in a mortar. A small amount of the substance is collected with the open end of the capillary and thrown, sealed end down, into a glass tube ≈ 60 - 80 cm long, placed vertically on the laboratory table. The operation of filling the capillary is repeated several times until a solid column of substance 2 - 3 mm high is formed in it.

The filled capillary (1) is secured with rubber rings (2) on the thermometer (3) so that the sample of the substance is at the level of the thermometer balls. The heating of the device is adjusted so that the temperature increases at a rate of 1°C per minute. At the same time, they carefully monitor the state of the column of substance in the capillary, noting all changes - changes in color, decomposition, sintering, wetting, etc. The beginning of melting is considered to be the appearance of the first drop in the capillary (T 1), and the end is the end of melting of the last crystals of the substance ( T 2). The temperature range (T 2 - T 1) is called the melting point of a given substance (T pl).

Practical part

Cleaning Methods

1 way. 25 g of borax at 60 0 C are dissolved in 50 ml of water. The solution is quickly filtered through a pleated filter into a porcelain cup or glass cooled with snow. The filtrate is continuously stirred with a glass rod.

Sodium tetraborate precipitates in the form of small crystals, they are sucked off, washed with a small amount of cold water and recrystallization is repeated. The crystals are dried in air for 2 – 3 days. The resulting preparation has the formula Na 2 B 4 O 7 *10H 2 O and is suitable for setting the titer.

Method 2. 25 g of borax at 65 - 70 0 C are dissolved in 75 ml of water. The resulting solution is quickly filtered through a pleated filter inserted into a funnel with a cut end, or through a hot filter funnel. The filtrate is first cooled slowly to 25 - 30 0 C, and then quickly in ice water or snow, enhancing crystallization by stirring with a stick. The precipitated crystals are sucked off, washed with a small amount of ice water and dried between sheets of filter paper for 2 - 3 days. The dried borax crystals should easily come off the dry stick.

The percentage of practical yield of borax is calculated.

Recrystallized borax is stored in a jar with a well-ground stopper.

To analyze a substance, it must first be isolated, i.e. clean, because the properties of a substance depend on its purity. When isolating a substance from a mixture of substances, their different solubilities in water or organic solvents are often used.

Recrystallization– purification of solids, based on increasing the solubility of solids with increasing temperature in a given solvent. The substance is dissolved in distilled water or a suitable organic solvent at a specified elevated temperature. A crystalline substance is introduced into a hot solvent in small portions until it stops dissolving, i.e. a solution saturated at a given temperature is formed. The hot solution is filtered on a hot filter funnel through a paper filter or, if the solvent is an aggressive liquid, through a Schott filter (funnels with a sealed porous glass plate). In this case, the solution is freed from suspended small solid particles.

The filtrate is collected in a glass placed in a crystallizer with cold water with ice or a cooling mixture. When cooled, small crystals of the dissolved substance fall out of the filtered saturated solution, because the solution becomes supersaturated at a lower temperature. The precipitated crystals are filtered using a Buchner funnel. To speed up filtration and more completely free the precipitate from the solution, vacuum filtration is used. For this purpose, a device for filtering under vacuum is assembled (Fig. 15.1). It consists of a Bunsen flask (1), a porcelain Buchner funnel (2), a safety bottle (4) and a water-jet vacuum pump (10). In this case, soluble impurities go into the filtrate, which do not crystallize together with the main substance, because the solution was not oversaturated with respect to impurities.

Rice. 15.1. Installation for filtration under vacuum. 1 – Bunsen flask, 2 – Buchner funnel, 3 – rubber stopper with a hole, 4 – flask, 5 – connecting valve, 6 – glass gas outlet pipe, 7 – rubber stopper with three holes, 8, 11 – rubber hose, 9 – hose PVC, 10 – water jet pump

The filtered crystals, together with the filter from the Buchner funnel, are transferred to a sheet of filter paper folded in half and squeezed between the sheets of filter paper. I repeat the operation several times, then the crystals are transferred to a bottle. The substance is brought to constant weight in an electric drying oven at a temperature of 100–105°C.

Sublimation – The method is used to purify substances that, when heated, can transform from a solid state to a gaseous state, bypassing the liquid state. Next, the vapors of the substance being purified condense, and impurities that cannot sublimate are separated. Substances such as crystalline iodine, ammonium chloride (ammonia), and naphthalene easily sublime. However, this method of purifying substances is limited, because few solids can sublimate.

Separation of two immiscible liquids, having different densities and not forming stable emulsions, can be done using a separating funnel (Fig. 15.2). This way you can separate, for example, a mixture of benzene and water. A layer of benzene (density r = 0.879 g/cm3) is located above a layer of water, which has a higher density (r = 1.0 g/cm3). By opening the separatory funnel tap, you can carefully drain the bottom layer and separate one liquid from another.

Rice. 15.2. Separating funnel.

To separate liquid substances (most often organic), their solubility in immiscible solvents is used. After settling in a separating funnel, the layers of solvents are separated by draining one by one. Then the solvent is evaporated or distilled off. To purify organic substances, various types of distillation are often used: fractional, with steam, under low pressure (in vacuum).

Fractional distillation(Fig. 15.3) is used to separate mixtures of liquids with different boiling points. A liquid with a lower boiling point boils faster and passes through the fractionation column (or reflux condenser). When this liquid reaches the top of the fractionation column, it enters fridge, cooled with water and through allonge going to receiver(flask or test tube).

Rice. 15.3 Installation for fractional distillation: 1 – thermometer; 2 – reflux condenser; 3 – refrigerator; 4 – long; 5 – receiver; 6 – distillation flask; 7 – capillaries; 8 – heater.

Fractional distillation can be used to separate, for example, a mixture of ethanol and water. The boiling point of ethanol is 78°C, and that of water is 100°C. Ethanol evaporates more easily and is the first to enter through the refrigerator into the receiver.

Chromatography (adsorption)– a method for separating mixtures, proposed in 1903 by M.S. Color. Being a generally accepted physicochemical method, chromatography makes it possible to separate, as well as carry out qualitative and quantitative analysis of a wide variety of mixtures. Chromatographic methods are based on a wide range of physicochemical processes: adsorption, distribution, ion exchange, diffusion, etc. The separation of the analyzed mixture is often carried out on columns filled with silica gel, aluminum oxide, ion exchangers (ion exchange resins) or on special paper. Due to the different sorbability of the determined components of the mixture (mobile phase), their zonal distribution occurs over the sorbent layer (stationary phase) - a chromatogram appears, which makes it possible to isolate and analyze individual substances.

After purification of the compound, qualitative analysis can begin. To determine the composition of organic matter, it is determined which elements are included in its composition. To do this, elements from the composition of this substance are converted into well-known inorganic substances and discovered by methods of inorganic and analytical chemistry.

Additional material for teachers

8th grade on the topic “Purification of substances”

annotation

The proposed additional material describes special purification methods: dialysis, complexation, formation of volatile compounds, chromatography and ion exchange, distillation and rectification, extraction, zone melting.

Separation and purification of substances are operations that are usually related to each other. 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 which substance should be considered pure has not yet been finally established, since the requirements for the purity of a substance are changing. Currently, methods for producing chemically pure substances have acquired particular importance.

The separation and purification of substances from impurities is based on the use of their specific physical, physicochemical 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 corresponding 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.

Dialysis can be used to separate and purify substances dissolved in water or an organic solvent. This technique is most often used to purify high molecular weight substances dissolved in water from low molecular weight impurities or inorganic salts.

For purification by dialysis, so-called semi-permeable partitions, or “membranes” are required. Their peculiarity is that they have pores that allow substances whose molecules or ions are smaller in size to pass through them, and retain substances whose molecules or ions are larger in size membrane pore sizes. Thus, dialysis can be considered as a special case of filtration.

Films made from many high-molecular and high-polymer substances can be used as semi-permeable partitions or membranes. Films from gelatin, from albumin, parchment, films from cellulose hydrate (such as cellophane), from cellulose ethers (acetate, nitrate, etc.), from many polymerization and condensation products are used as membranes. Inorganic substances are used: unglazed porcelain, tiles made from certain types of fired clay (such as colloidal clays, such as bentonite), pressed finely porous glass, ceramics, etc.

The main requirements for membranes are: 1) insolubility in the solvent in which the dialyzed solution is prepared; 2) chemical inertness with respect to both the solvent and dissolved substances; 3) sufficient mechanical strength.

Many membranes are capable of swelling in water or other solvents, thereby losing mechanical strength. The swollen film can be easily damaged or destroyed. In such cases, the film for dialysis is made on some durable base, for example, on a fabric inert to the solvent (cotton, silk, fiberglass, synthetic fiber, etc.), or on filter paper. Sometimes, to give membranes mechanical strength, they are reinforced with metal mesh (reinforcement) made of the appropriate metal (bronze, platinum, silver, etc.).

To obtain different porosity for membranes made from cellulose ethers or some other high-polymer substances, different amounts of water are introduced into the corresponding varnishes. When the varnish film dries, a milky-colored membrane with a given porosity is obtained. For dialysis, devices called dialyzers are used. The rate of dialysis varies for different substances and depends on a number of conditions and properties of the substance that is being purified. Increasing the temperature of the solution and updating the solvent help speed up dialysis. In many cases, instead of conventional dialysis, electrodialysis The use of electric current during dialysis speeds up the process and creates a number of other advantages.

Precipitation of poorly soluble substances. This technique is widely used for analytical purposes, obtaining sediments containing only one, inorganic or organic, substance. The resulting precipitate can be further purified. The equipment used to carry out this method depends on the properties of the substances and the properties of the solvents.

Complexation is one of the methods for isolating pure substances, especially inorganic ones. Complex compounds can be either sparingly soluble in water, but readily soluble in organic solvents, or vice versa. In the first case, the sediments are processed as described above. If a complex compound is easily soluble in water, it can be extracted in pure form from an aqueous solution by extraction with a suitable organic solvent, or the complex can be destroyed in one way or another. Complexation can be used to isolate metals in very pure form. This is especially true for rare and trace metals, which can be isolated in the form of complexes with organic substances.

Formation of volatile compounds. This technique can be used if a volatile compound is formed only of the substance being released, for example, a metal. In the event that volatile compounds of impurities are simultaneously formed, this technique is not recommended, since getting rid of volatile impurities may be difficult. In many cases, the formation of volatile halides (chlorides or fluoride compounds) of certain substances can be very effective as a purification method, especially when combined with vacuum distillation. The lower the sublimation or boiling point of a substance of interest to us, the easier it is to separate it from others and purify it by fractional distillation or diffusion. The rate of diffusion of gaseous substances through semi-permeable partitions depends on the density and molecular weight of the substance being purified and is almost inversely proportional to them.

Chromatography and ion exchange. These methods are based on the use of the phenomenon of sorption to extract substances contained in solutions. The chromatography method is especially important for concentrating substances whose content in the original solution is very small, as well as for obtaining pure preparations. Using this method, rare earth elements of high purity were obtained. Many pharmaceutical and organic drugs are purified and obtained in pure form using this method. In almost all cases where the task is to purify or separate a substance from a mixture in solution, chromatography and ion exchange can be reliable methods.

For ion exchange, so-called ion exchangers are used, which are inorganic or organic adsorbents (mainly resins of different brands). According to their chemical properties, they are divided into the following groups: cation exchangers, anion exchangers and ampholytes. Cation exchangers exchange cations. Anion exchangers have the ability to exchange anions. Ion exchangers are capable of ion exchange until they are completely saturated with the absorbed ion.

Recrystallization. Of all the methods for purifying salts and other solid electrolytes and organic compounds, recrystallization should be placed in first place in terms of applicability. This is due to both the simplicity of the process and its efficiency (at least for rough cleaning). Taking advantage of the increase in the solubility of salts when heated, you can prepare a solution saturated at the boiling point, filter it from mechanical impurities and cool; in this case, it is often possible to obtain crystals of fairly pure salt. This is due to the fact that upon cooling the solution becomes supersaturated only with respect to the main substance, while impurities present in fractions of a percent remain in the mother solution. This is an elementary diagram of the recrystallization process. In reality, recrystallization is much more complicated, since it can be accompanied by a number of processes that significantly reduce the efficiency of purification during crystallization. Thus, ions or molecules of impurities can be mechanically captured by the resulting crystals of the main substance (occlusion, inclusion). A greater or lesser adsorption of impurity ions on the surface of crystals is also inevitable, although in the formation of large crystals with a small specific surface area, the role of adsorption is small. The formation of solid solutions (isomorphism) can occur when the ions of the main salt and the impurity ions differ in size by no more than 10-15% and both substances crystallize in the same system. Then some of the main salt ions can be replaced by impurity ions during crystal growth. The capture of foreign ions of any size may also occur, associated with crystal growth around the adsorbed ions. Such ions, since they do not enter the solid solution, represent defects in the crystal lattice.

It is quite clear that the separation of isomorphic substances by crystallization is impossible in principle. In these cases, sometimes you have to resort to special techniques. Thus, when purifying aluminum-ammonium alum intended for the production of laser rubies, it is not possible to get rid of Fe 3+ impurities by recrystallization, since aluminum-ammonium and iron-ammonium alum are isomorphic. At pH 2, the purification coefficient (the purification coefficient is the ratio of the impurity content in the crude product to the impurity content in the preparation after purification) does not exceed 10. But if Fe 3+ is reduced to Fe 2+, then the isomorphism is eliminated, and the purification coefficient reaches 100. Efficiency purification of a substance by recrystallization also depends on its solubility. When the solubility of the substance is in the range of 5-30%, purification occurs much more completely than with a solubility of 75-85%. It follows that recrystallization is impractical for the purification of very easily soluble substances.

Distillation and rectification. Purification of substances by distillation is based on the fact that when a mixture of liquids evaporates, the vapor usually results in a different composition and is enriched with a low-boiling component of the mixture. Therefore, it is possible to remove easily boiling impurities from many mixtures or, conversely, distill the main substance, leaving difficult-boiling impurities in the distillation apparatus. We often encounter systems in which during distillation all components are distilled off in a constant ratio (azeotropic mixtures). In this case, no separation occurs and purification by distillation is impossible. Examples of azeotropic mixtures include aqueous solutions of HCl (20.24% HCl) and ethyl alcohol (95.57% C 2 H 5 OH).

To obtain pure substances (especially during deep purification), instead of simple distillation, they prefer to use rectification, i.e. a process in which the automatic combination of distillation and condensation occurs. Without going into the theory of rectification, we will only point out that in the distillation column, steam meets various fractions of condensate, with part of the less volatile component condensing from steam into liquid, and part of the more volatile component passing from liquid to steam. Passing through many shelves (“plates”) of the distillation column, the steam manages to become so enriched in the more volatile component that at the exit from the column it practically contains only this component (or an azeotropic mixture).

The degree of separation depends on how much the vapor is depleted of impurities compared to the liquid phase. Calculations show that in modern laboratory distillation columns with a height of 1-2 m, it is possible to carry out purification by 10 5 times or more, even if the impurity content in the equilibrium vapor is only 10% less than in the liquid. This explains the widespread use of distillation and rectification in the production of pure substances.

Rectification used for purification not only of liquid preparations. The use of rectification for the separation of liquefied gases (oxygen, nitrogen, inert gases, etc.) is well known.

In recent years, rectification has begun to purify many solid substances that evaporate relatively easily. It was possible to successfully purify aluminum chloride (from Fe), sulfur (from Se), SiCl 4, Zn, Cd, SbCl 3. The impurity content decreases to 10 -4 and even 10 -7%. Thus, rectification can be classified as an extremely effective deep cleaning method. Rectification purification processes are especially effective at low temperatures; As the temperature rises, contamination of the substance being purified by the equipment material increases sharply.

Extraction. The extraction method of separating substances has been used for many decades, especially in analytical chemistry, but only recently has it become very important for the production of pure and ultrapure substances. The method is based on the extraction of one of the components of the solution using an organic solvent that is immiscible with the solution.

The advantages of the extraction method are as follows:

 extraction can be carried out from extremely dilute solutions (with a sufficiently large distribution coefficient)

 no coprecipitation occurs during extraction, and the extracted substance can be quantitatively isolated in pure form

 the method allows you to separate substances that cannot be separated by other methods, for example, when purifying uranyl salts from impurities of Fe, B, Mo, etc.

Zone melting. This purification method is based on the difference in solubility of the impurity in the solid and in the melt. A sample of a solid substance is slowly moved through a narrow heating zone, and gradual melting of individual sections of the sample currently located in the heating zone occurs. Impurities contained in the sample accumulate in the liquid phase, move along with it along the sample and, upon completion of melting, end up at the end of the sample. As a rule, zone melting is repeated many times. Often the sample moves through several heated zones, which allows the cleaning time to be reduced several times.

The advantages of zone melting are the simplicity of the equipment, relatively low process temperatures (compared to rectification) and high cleaning efficiency. In this way, for example, germanium is purified to an impurity content of about 10 -8%. Every year, an increasing number of substances intended for the most critical purposes are purified using the zone melting method. Inorganic and organic products can be cleaned with equal success. True, zone melting cannot always be used successfully. For example, zone melting cannot separate Au from Ag.

Document

... « Cleaning contaminated table salt" annotation Additional material provides a classification of the main separation methods substances... and other precision industries. For cleaning substances Various methods are used to separate mixtures...

Abstract of the main professional educational program in the specialty 240705. 01 operator-operator in biotechnology

Document

annotation The main professional educational... are being developed by the Federal State Autonomous Institution “FIRO”. Annotations placed according to discipline cycles. Generally professional... and harmful substances Topic 1.2.7 Storage conditions Topic 1.2.8 Instructions for cleaning and storage...

Abstract of the approximate program of the academic discipline “Ecology” Goals and objectives of the discipline

Document

Parts of the cycle “Ecology” annotation approximate program of the academic discipline “... . Interaction between living and bio-bone substances. Energy balance of the biosphere. Biogeochemical... emissions. Modern technologies cleaning and reducing emissions of pollutants...

Introduction

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

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

Purification of substances

Recrystallization

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

Solubility refers to 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 shown in Fig. 1, as well as in the solubility table.

According to these data, if, for example, you prepare a solution of potassium nitrate by taking 100 g of water, saturated at 45°, and then cool it to 0°, 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 achieved at the specified temperature decrease, and therefore they will not precipitate along with the salt crystals. Minor amounts of impurities, often undetectable by conventional analytical methods, can only be carried away by sediment 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 less of the mother liquor containing impurities of other substances. The reduction of impurities is facilitated by washing the crystals with a solvent after separating them from the mother liquor.

Thus, recrystallization comes down to dissolving a substance in a suitable solvent and then isolating it from the resulting solution in the form of crystals. This is one of the common methods of purifying substances from impurities.

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 without melting turns 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 state diagram of the substance shown in Fig. 2. The abscissa axis shows the temperature t (in degrees Celsius) and the ordinate axis shows the saturated vapor pressure p (in m/cm3). The state diagram of water has a similar appearance, so that its TV curve is inclined to the ordinate axis, since the freezing temperature of water decreases as the pressure increases.

The TA curve expresses the relationship between temperature and pressure of saturated vapor above a liquid. All points of the TA curve determine the conditions of equilibrium between the liquid and its saturated vapor. For example, at 100° 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 pressure of saturated vapor over a solid. The vapor pressure of solids is usually low and largely depends on the nature of the body and temperature. Thus, the vapor pressure of iodine at 16° is 0.15 mm Hg. Art., ice at - 15є is equal to 1.24 mm Hg. Art. The TB curve lies below the melting point of the substance. All points of this curve determine the conditions of equilibrium between a solid and its saturated vapor.

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

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

The TA, TB and TV curves divide the state diagram of a substance into three regions: 1 - the region of existence of the solid phase, 2 - the liquid phase and 3 - the vapor phase.

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

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

Let point 1 represent the solid state of a substance 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 intersect 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 move from state 3 to state 4. Cooling of the steam will repeat the processes considered 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 turn 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 steam is cooled at the desired temperature, crystallization of the substance will occur at point 6 (also without the formation of 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 sublimate;

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

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

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