Drying It is called the process of removing water residues from the substance. Drying methods in the laboratory are quite a lot, the most commonly used absorption of water vapor by hygroscopic substances, evaporation of water during heating and drying outdoors at normal temperature.

Drying the absorption of water vapor by hygroscopic substances. The method is based on the ability of some substances to absorb water or its pairs, forming crystallization compounds with it. These substances include sulfuric acid, calcium chloride, calcium oxide, phosphorus oxide (V) and others. If over a hygroscopic substance in a closed vessel (most often in the desiccator), put the dried substance, then it will begin to absorb water from the dried substance.

Drying outdoors at normal temperature. Many substances can be dried outdoors. For this, the substance is placed on a clean sheet of filter paper and distributed on it a loose layer with a thickness of no more than 3-5 mm, covered with a different sheet of filter paper to protect it from dust, and leave for 12 hours. After 12 hours, the substance is stirred so that the lower layers are upstairs. After another 12 o'clock, the substance tends to dry out, it is collected by a clean spatula in the jar and closed. Drying in air - the operation is quite long, is used only if the substance is non-hygroscopic and decomposes when heated.

Drying when heated. Drying is very widely used at elevated temperature in the drying cabinet. The maximum temperature to which the drying cabinet is heated - 300 o C. The substance to be dried is placed on the cabinet shelf in the evaporation cup or on paper. The temperature inside the cabinet to remove the waters should be 100-105 o C. The temperature must be raised gradually to avoid formation on the surface of the substance dense "crust". Duration of drying depends on the amount of substance, the thickness of its layer and heating temperature.

Organic liquids usually dried with solid inorganic dryers, while you should take a small number of latter to avoid losses from the adsorption of the substance by a dryer. First, the organic fluid is shaken with a small amount of drying agent (up to 3% of the mass of the solution), after a while there is a small layer of aqueous solution of the drainer, if taken for drying the substances that form hydrates with water (calcium chloride, sodium sully acid, caustic sulfate, sulphate magnesium). The liquid is poured, a fresh portion of the desiccant is again made and they are repeated until the desiccant will stop absorbing water, for example, calcium chloride will not break out, phosphoric anhydride will not stick together, etc. After such treatment, the organic fluid is placed in the flask, which Close the plug with a chlocalcium tube and leave stand at night with a new portion of the dryer. Before distillation, the dried organic liquid is filtered or most often decanted.

2.5.3. Drying of solids

Drying outdoors at normal temperature

Many substances as inorganic and organic can be dried outdoors. The drying occurs due to the natural evaporation of the moisture contained in the substance until the pressure of water vapor in the air and over the body will not come to the equilibrium state.

The substance to be dried, for example, wet crystals, pour on a sheet of clean filter paper, distributing their layer with a thickness of no more than 3 - 5 mm. It should not be understood in this case, since the loose layer will be, the sooner the drying will be better. To protect against dust or contamination, the dried substance is covered on top with another sheet of clean filter paper and leave for several hours. Then the dried substance is stirred by a spatula so that more wet lower layers are above; The mass should remain loose. The product is closed with a sheet of filter paper and leave it to dry for another 12 hours. Sometimes the substance has to be stirred several times, especially if the layer thickness was significant. The dried salt is folded with a spatula in the jar and close it tightly. If, when standing in a tightly closed jar on the walls, water drops will appear, this means that the substance was dried not fully and the drying should be repeated.

Drying in air - the operation is quite long and is resorted to it only when the dried substance decomposes when heated or when they want to obtain a substance in the form of a loose, bulk powder without lumps. In this way, it is possible to dry substances not hygroscopic, that is, not absorbing moisture from the ambient air.

Drying with reduced pressure (vacuum drying)

For drying substances, easily decomposing or changing when heated even under normal pressure; Apply drying with reduced pressure (under vacuum).

For this purpose, the so-called vacuum-drying cabinets with electrical heating are used. Maximum heating temperature of them - 200 ° C.:

Drying in the excicator

Highly hygroscopic, spacing substances in the air it is convenient to dry without heating in ordinary and vacuum-excitators. The latter have a hole in which the tube with a crane is inserted on the rubber stopper. This makes it possible to connect the exicor in the violated pump, between which the pressure gauge and the safety flask are placed.

Sometimes under vacuum, the excryators explode, so before turning on the pump they must be wrapped with a towel. When opening the vacuum desiccator to avoid spraying the dried substance by air, it should be very careful and slowly rotating the crane. Only after the pressure is equal, you can open the Vacuum Excitulator's Pretty Cover.

A substance absorbing moisture is placed in the desiccator. The substance to be dried is placed in bures or a cup, put an excitator open to the porcelain liner and leave in the last one or more.

The draining agent is selected depending on the chemical properties of the drying substance. Most often, calcium chloride, intronic lime, caustic shed, caustic, phosphoric anhydride, concentrated sulfuric acid are used as desiccators for excitators. It should be remembered that sulfuric acid cannot be used to dry in vacuo, it is used only in ordinary excitators for moisture absorption, alcohol residues, ether, acetone, aniline, pyridine. For adsorption of hydrocarbons, especially hexane, ligroine, benzene and its homologs, paraffin is used as a semicircular aggregate; To remove acidic substances, the caustic sterry or caustic potassium is used. Water and alcohols are well absorbed by phosphoric anhydride, natron lime.

Basic dryers

Anhydrous sodium chloride is a cheap widely used desiccant, has a high drainage. However, it dries slowly and unsuitable for drying alcohols, phenols, amines, amino acids, amides, acid nitriles, esters, some ketones and aldehydes, as it forms compounds with them. In addition, calcium chloride contains lime as an impurity, therefore, it cannot be used for drying of acids. It is used for pre-drying of limit, ethylene hydrocarbons, acetone, ethers and other compounds from water.

Anhydrous Magnesium sulfate - one of the best drying neutral agents, which has a high speed of water absorption and good absorption capacity; Used for drying the greatest number connections.

Anhydrous sodium sulfate - a cheap neutral desiccant, which is used to pre-remove large amounts of water, but it acts slowly and does not bind all the water. It can not be used for drying benzene, toluene, chloroform.

The caustic soda and caustic potatoes are good and fast dryers, but they only find very limited use, exclusively for amines and ethers, hygroscopic wool, previously dried in a drying cabinet at 100 ° C, is an excellent drying agent and is used in chlocalcium tubes.

Table - Dehumidifiers for organic compounds

Organic compounds	Desiccants
Hydrocarbons	CAC1 2, CASQ 4, P 2 O 5, NA
Halogen derivatives	CAC1 2, Na 2 SO 4, MgSO 4 P 2 O 5
	MGSO 4, CASO 4, K 2 CO 3, CAO
	CAC1 2, CASO 4, NA
Aldehydes	CAC1 2, MgSO 4, Na 2 SO 4
	MgSO 4, Na 2 SO 4, K 2 CO 3
Organic acids	MgSO 4, Na 2 SO 4 J Caso 4
	Kon, Naoh, K 2 CO 3, Cao
Nitro compound	CAC1 2, NA 2 SO 4

Drying when heated and ordinary atmospheric pressure

The highest distribution is drying when heated and ordinary atmospheric pressure. Distinguish the following drying methods when heated: 1) outdoors; 2) in drying cabinets.

The choice of drying method depends on the properties of the substance and conditions.

When drying outdoors, the dried substance is placed on a pan or in a porcelain cup and heated on any bath (sand, oil, water) or on an electric stub. In this case, the substance is stirred with a glass stick or blade, not allowing to form a crust. In this way, many substances can be dried, mainly inorganic, withstanding heating.

The disadvantage of this drying method is that it is almost impossible to adjust the temperature of the drying during it and therefore overheating, and sometimes accompanied by the melting of the dried substance.

It is more convenient to dry the substance in the drying cabinets. In laboratories, you can meet several types of drying cabinets for drying under normal atmospheric pressure: with electric, gas or other heating. They are asbestile or metal (most often copper).

Duration of drying depends on the amount of dried substance, the thickness of the layer of it, the temperature of the drying and humidity of the substance.

Rules of drying

1. The substance to be dried must be pre-pressed from excess water.

2. The substance layer when dried both in air, and when heated should not exceed 10 mm.

3. The dried layer of time from time to time you need to mix and apart again,

4. When drying, overheating should be avoided in simple drying cabinets. In most cases, the drying temperature should not exceed 105 - 110 ° C.

5. Solids containing organic solvents, dry in a drying cabinet with electrical heating is dangerous.

6. When used as a drying agent of concentrated sulfuric acid, it is necessary to pour it into the absorption flasks so that the fluid is not transferred.

Drying - process of removal from gaseous, liquid or solids Mused to them fluid, more often water (dehydration).

V. is widely used in Chem. and the food industry, in Galenovo farm. Production, in the treatment of medicinal plant materials, etc. V. is used when conducting various types of biochemical. Analyzes, when preserving blood plasma and its individual fractions, tissues for transplantation, for morphol, or histochim. The study of tissues, upon receipt of drugs for electron microscopy, etc. V. is used as an auxiliary means when disinfection. Some types of microbes (a stick of influenza, meningococcus, gonococcus, cysts of dysenteric amuba, etc.) with V. Quickly die. The causative agents of the abdominal typhoids and paratyphones, brucellosis, tuberculosis, diphtheria, natural smallpox, etc. Healing V. for a long time. Disputes of microbes retain the viability and virulence in the dried state for many years.

The basis of existing methods V. Lies Him. binding, or sorption, removed fluid, evaporation of it at low, high temperatures Or in vacuum when heated (see evaporation) or in a frozen state - freeze drying (see lyophilization).

In Laboratories, V. Gazov produce through them through conc. Sulfur K-Tu in the flasks of Tishchenko, Drexel or Wulf, through solid absorbers, for example, calcined calcium chloride, phosphoric anhydride, etc., k-fish and fill the absorbing speakers or special vessels.

Dehydration of liquids is carried out in them by hygroscopic substances - pieces of melted calcium chloride or caustic potassium, calcined sulfuric acid or calcium sulfur, etc.. In this case, the desiccant should not chemically interact with the dried liquid. The final dehydration of many organic liquids is carried out with a metal sodium.

Solid bodies are dried by heating them in porcelain cups, in the roasting in the open air or in drying cabinets, withstanding in the desiccator over hygroscopic substances, usually above the conc. Sulfur Calcision Calcision, Calcium, Phosphoric Anhydride, when removing water, over calcium chloride when removing alcohol, over paraffin when removing ether, heating in vacuum-excitators or vacuum drying cabinets, heating with infrared rays.

B. leads to a noticeable change in fiz.-chemical. properties of substances, for example, boiling and melting temperatures, electrical conductivity, reactivity, etc. V. substances that are subjected to even during moderate heating in a wet or dissolved state of denaturation and other irreversible changes, produce a lyophilization method. The choice of method and conditions V. depends on the properties of the dried material and its subsequent purpose.

Bibliography: Resurrection P. I. Technique laboratory work, M., 1973; Application of freezing - drying in biology, ed. R. Harris, Per. from English, M., 1956, Bibliogr.

Drying is the process of release of substance in any aggregate state from any fluid impurity. Most often, under drying, the exemption from moisture or organic solvents is understood.

Many reactions in organic chemistry are conducted in the absence of moisture, in such cases it is necessary to dry the starting materials, apply absolute solvents and protect the reaction medium from moisture from the air. The desiccant must act quickly, not dissolve in organic fluids, do not interact with the dried substance.

Drying gases. Most of the gases obtained in the laboratory, as well as many compressed gases from cylinders can be drained by concentrated sulfuric acid or solid dryers, such as calcium chloride, natron lime, phosphoric anhydride. Sulfuric acid can be dried by air and the following most frequently used gases: oxygen, hydrogen, nitrogen, dioxide and carbon oxide, chlorine, chloropodo-genus, sulfur gas. For drying, the gas is passed through the flushing flasks of Drexel (Fig. 20), Tishchenko or Alifano-VA, in which a third of the capacity of the nutrition concentrated sulfuric acid. Usually, the washing flask is connected to the gas source and the device by means of two empty safety flasks, the role of which is performed by drakels or Tishchenko. Gas drying with solid dryers are carried out in drying columns, and to protect the gas from moisture of air, the device is closed with a chlocalcium tube.

Fig. 20. Flask Drexel

Drying organic liquids. The drying of liquid organic compounds or their solutions in organic solvents is usually produced by solid inorganic dryers. The sequel selection is determined by a number of conditions, and a good desiccant must meet the following basic requirements:

Should not chemically interact with the dried organic compound;

should not catalytically contribute to self-examination,
polymerization and condensation of dried organic compounds;

should not be significantly dissolved in the organic fluid;

must dry quickly and efficiently;

must be an available substance.

The relative effectiveness of dryers depends on the pressure of vapors in the water system - dryer.

When drying liquid organic compounds or solutions in organic solvents, you should always take a small amount of dryer in order to avoid losses from the adsorption of the substance by the desiccant. It is best to shake the liquid with the dryer until its action stops. If the volume of water removed from the liquid is large and as a result of this, a small layer of aqueous desiccant solution is released (for example, with calcium chloride, sodium hydroxide, or other dryers), then this aqueous solution should be separated, and the liquid should be processed further than a new portion of the dryer when shaking. Even in the case when after such a treatment with a dryer, the liquid will seem dry, should be filtered and left overnight with a new portion of the dryer.

Before distillation, the dried liquid is usually filtered from the dryer through a fold filter. This is especially necessary in cases where driers were used, the action of which is based on the ability to form hydrates (anhydrous sodium sodium, sulfate magnesium, calcium chloride); At elevated temperatures, the pressure of steam above the salt becomes noticeable, and if the salt was not filtered, then most of the water, if not all water, can again be in the distillate obtained during distillation.

Some dryers (metallic sodium, calcium oxides, barium, phosphorus (V)) when reaction interacts with water, are completely stable hydrates as the reaction products, and therefore the filtering of the liquid dried with them is not mandatory.

Drying of solids. The volatile impurities can be removed from non-fragic solids drying on filter paper, thermally stable substances can be dried in drying cabinets. Common and vacuum desiccators are often used for drying solids. The latter in the lid have a hole in which the tap with a crane is inserted on the rubber stopper. This makes it possible to combine the desiccator with a water jet pump, between which the pressure gauge and the safety flask are placed.

Under vacuum, the excryators can explode, so before turning on the pump they must be wrapped with a towel. When opening the vacuum desiccator to avoid spraying the dried substance by air, it should be very careful and slowly rotating the crane. Only after the pressure is equal, you can open the Vacuum Excitulator's Pretty Cover.

A discriminant agent is selected depending on the chemical stem, the drying substance. Most often, calcium chloride, intronic lime, sodium hydroxide, potassium hydroxide, phosphoric anhydride, concentrated sulfuric acid are used for desiccators. It should be remembered that sulfuric acid cannot be used to dry in vacuo, it is used only in ordinary excitators for moisture absorption, alcohol residues, ether, acetone, aniline, pyridine. For adsorption of hydrocarbons, especially hexane, ligroine, benzene and its homologs, paraffin is used as a semicircular aggregate; To remove acidic substances, sodium hydroxide or potassium hydroxide is used. Water and alcohols are well absorbed by phosphoric anhydride, natron lime.

Basic dryers

We describe the description of usually used dryers with the indication of their desiccitation and cases of their use.

Anhydrous calcium chloride (CAC1 2). Due to its availability, cheapness, ease of preparation and high drainage ability is widely used as a desiccant. It adsorbs water very well, since at temperatures not exceeding 30 ° C, forms CAC1 2. 6H 2 O. However, calcium chloride does not apply to the number of high-speed dryers and for drying it takes for a long time. The slowness of the action is due to the fact that the surface of solid calcium chloride is covered with a thin layer of its solution in the extracted water; When standing, water is absorbed in the formation of a solid lower hydrate, which in turn is also desiccant.

In the process of cooking anhydrous calcium chloride (evaporation of a saturated solution and subsequent calcination), although at a slight degree, salt hydrolysis occurs. As a result, the desiccant can always contain a small amount of calcium hydroxide or the main calcium salt. Therefore, it is impossible to use calcium chloride to dry acids or acidic liquids.

Calcium chloride forms compounds with alcohols, phenols, amines, amino acids, amides and nitriles of acids, ketones, some aldehydes and esters, and therefore it cannot be used to dry such substances.

Anhydrous sulfate magnesium (MGSO 4). It is a very good neutral desiccant. He dries quickly, chemically inert, and therefore can be used to dry the greatest number of compounds, including those for which calcium chloride.

Granulated magnesium sulfate is obtained by cautious heating MGSO 4. 7H 2 O first at 150-175 ° C in a muffle or any other furnace until most of the hydrate water is removed, and then during red combation.

You can get anhydrous magnesium sulfate and more quickly, but with less drainage, heating in a cup on a bare burner flame thin layer of crystalline salt. The substance at the same time is partially melting and abundantly, it allocates pairs of water.

The solid residue (pieces and powder) is triturated into the mortar into the powder and store in a tightly closed jar. If when calcined, stir the crystal salt with a glass daddy, then only dry powder is immediately obtained.

Anhydrous sodium sulfate NA 2 SO 4). This is a neutral, a cheap dryer, which has a high ability to adsorption of water: at temperatures below 32.4 ° C, it forms NA 2 SO 4 hydrate. 10N 2 O. It can be used in almost all cases, but he dries slowly and not to the end. Anhydrous sodium sulfate should be used to pre-remove large volumes of water. It is not suitable as a dryer for solvents such as benzene and toluene, the solubility of which is small in water; In these cases, it is better to use anhydrous copper sulfate. Anhydrous sodium sulfate cannot be used as a dryer at temperatures above 32.4 ° C - decomposition temperatures of the decahydrate (NA 2 SO 4. 10N 2 O).

Anhydrous potassium carbonate (K2CO 3). It has a moderate drainage effect, it forms dihydrate to 2 C 3. 2N 2 O. It is used to dry ketones, nitriles, esters of some acids. Sometimes, for example, when drying amines, they replace potassium hydroxide and sodium hydroxide, in order to avoid strong alkali. Potassium carbonate can not be consumed for drying acids, phenols and other acidic compounds.

Anhydrous potassium carbonate is often used to plant alcohols dissolved in water, glycols, ketones, simple 1 fins and amines. In many cases, the anhydrous potassium carbonate can be replaced with anhydrous magnesium sulfate.

Sodium hydroxide (NAON) and potassium hydroxide (Kon). They are used mainly to dry amines (for this purpose you can also use calcium oxide, barium oxide and natron lime). Sometimes it is better to use potassium hydroxide than sodium hydroxide. Most of the water can be first removed by shaking with a concentrated solution of potassium hydro-kola. Sodium hydroxide and potassium hydroxide react in the presence of water with many organic compounds (acids, phenols, esters, amides) and dissolve in some organic fluids, so they find only very limited use as dryers.

Calcium oxide (SAO). It is usually used for drying alcohols with low molecular weight. Calcium oxide action can be enhanced by pre-heating it to 700-900 ° C. Calcium oxide and the forming calcium hydroxide are insoluble in the dried liquid, resistant to heating and almost non-volatile, so there is no need to separate the dryer before distillation. Calcium oxide (due to its strong alkalinity) can not be used to dry acidic compounds and esters; The latter would have been loosening. Alcohols dried by distillation over the natron lime or calcium oxide, yet not completely dry; Last traces of moisture from them can be removed by distillation over metallic calcium, amalgam magnesium or aluminum, or by treating small amounts of sodium and high-boiling ester.

Aluminium oxide (A1 2 O 3), prepared from aluminum hydroxide, can adsorb water to 15-20% of its mass. The activity of the aluminum oxide used may be

it is becoming heating at 175 ° C for 7-8 hours and noticeably decreased when reused. It is used as a dryer in the excitators.

Phosphorus oxide (V) (p 2 o 5). Exceptionally effective and high-speed dryer. However, phosphorus oxide is an expensive drug and besides uncomfortable in circulation; When using it, the surface is quickly covered with thick syrup. Therefore, it is necessary to pre-dry the liquid with anhydrous magnesium sulfate or another similar desiccant. Phosphorus oxide should be used only in cases where it is required exclusively high degree Drying. It is used, for example, to dry hydrocarbons, ethers, alkyl and arylgargenides and nitriles, but not used for drying alcohols, acids, amines and ketones. Phosphorus oxide is sometimes used as a desiccant in the excitators.

Metal sodium (NA). It is used to dry paraffin, cycloparaffin, ethylene and aromatic hydrocarbons, as well as ethers. Previously most of the water from the liquid or solution are removed by anhydrous calcium chloride or magnesium sulfate. Sodium use is most effectively in the form of a thin wire, which is squeezed directly into the liquid with a special press; In this way, a large surface is created for contact with liquid. It is impossible to use sodium to dry the compounds with which it reacts and which can be harmful to the resulting alkali or when the dried compound can be restored by hydrogen released during dehydration. Therefore, it is impossible to use sodium to dry alcohols, acids, esters. Organic halides, aldehydes, ketones and some amines.

When working with sodium, special caution should be taken.

Concentrated sulfuric acid (H 2 SO 4). It is used, for example, to dry the bromine, with which it does not mix. To dry the bromine, ethyl bromide and some other halide alkyls, they are shaken in a dividing funnel with small amounts of concentrated acid until its action stops.

Concentrated sulfuric acid is widely used as a dryer in the excitators.

Gigroscopic Vata. - an excellent desiccant for use in the so-called "chlocalcium tubes", i.e. Drainage tubes covering drip funnels, reverse refrigerators to protect them from moisture. Gigroscopic wool is more convenient for this purpose than calcium chloride. Before use, cotton should be dried in a drying cabinet at 100 ° C.

Filtration

In laboratory practice, for the mechanical separation of solid and liquid components, any mixture is usually used by filtration. However, in the simplest case, a liquid drain can be used with a gravity precipitate, i.e. Decantation. It is recommended to use both receptions: first separating the liquid and rinse several times with decoration decoration, and then apply filtration.

Washing with the use of decantation is that the precipitate is poured with water or specially prepared washing liquid, shackled with a glass stick and give it to stand. Then the liquid is careful, in order to avoid splashing, merge from the sediment along the glass stick to the filter in the funnel, while the precipitate should remain in the vessel. Flushing sediment repeat several times. By decantation, it is possible to more fully wash the precipitate from the mother liquor; When filtering, it is not always possible to do this, due to the fact that the precipitate is easily joined. Washing should be made possible by a small amount of fluid, since there are absolutely insoluble substances, and each time it was washed with fresh portions of the liquid, the precipitate, although minor, passes into the solution. When washing the sediment, pour fluid to the filter in such a quantity so that it completely covers the precipitate and did not reach the edges of the filter by 3-5 mm; In addition, it is necessary to pour a new portion of liquid to the filter after the previous one will be completely filtered.

The following factors affect the filtering efficiency: viscosity (the higher the viscosity of the solution, the more difficult to filter); Temperature (the higher the temperature of the solution is the easier
filtration); pressure (the higher the pressure, the faster the fluid filtration); The size of the particles of the solid (the greater the particle size of the substance compared with the pore size of the filter, the easier it is filtration).

From the filtering agents in the laboratory, filter paper, fabric, porous glass, asbestos, and the like are most often used. Filtering under normal pressure. This filtering method is the most simple and frequently used. The filtering under normal pressure is the process in which the liquid passes through the filter material only under the pressure of the filter fluid.

In the ordinary glass funnel, simple or folded filter paper filters are investing. For the manufacture of a simple filter, a square piece of filter paper is folded in four, the free angle of the resulting square is cut into scissors on the dotted line. Separating one paper layer, lay the finished filter that takes the type of cone.

Filming is significantly accelerated when using a folded filter (Fig. 22), since its filtering surface is greater than that of a simple filter. However, the folding filter is used only if the precipitate remains on the filter is not needed or a bit.

Fig. 22. Filtering on a folded filter (hot filtering)

The filter should be selected in such a way that its size comes with a precipitate volume, with the edge of the filter in the funnel should always be lower than the edge of the funnel for 3-5 mm. The filter should fit tightly to the walls of the funnel, and when investing it is necessary to follow, so that its top is not broken. Before filtering the filter must be moistened in the funnel with a clean solvent. The level of the filtered fluid in the funnel must be below the edge of the paper.

The speed filtering condition is the presence of fluid in the funnel tube. For this, with wetting, the solvent is poured into a funnel above the edge of the filter, and then slightly lifted the filter and quickly lowered, while the fluid column is formed in the tube.

In cases where the liquid has a greater viscosity, as well as in the case of recrystallization, filtering is carried out when heated. Usually for this purpose, funnels for hot filtering are used. To filter substances with low melting point (for example, acetic acid, benzene) use special funnels with cooling. In the presence of strong alkalis and acids, anhydrides, oxidizing agents and other substances that destroy filter paper, precipitation is filtered through porous glass filters.

Filtering under vacuum. The essence of filtration under vacuum is that the receiver creates reduced pressure, as a result of which the liquid is filtered under the pressure of atmospheric air. This speeds up the filtering process. The device for suction consists of a porcelain funnel of the Buchner, Bunzen Flasks, Safety Flat and Water Journey Pump (Fig. 23).

The bureau funnel size must correspond to the amount of filtered substances, crystals must completely cover the surface of the filter, however, too thick layer makes it difficult to suck and washing. A safety flask is placed between the Bunzen flask and the vacuum pump, since when the pressure falls in the water supply network, water from the pump in the absence of a safety flask falls into the flask of the Bunzen. The safety flask is connected to a water jet pump with a thick-walled rubber tube, the walls of which are not compressed in the presence in the cutting tube.

Fig. 23. Installation for filtering in vacuo (1 - Bühner funnel; 2 - Bunzen flask; 3 - safety flask; 4 - Vacuum fitting with a manomemetrom)

In the chemical laboratory, water-based vacuum pumps are most often used, which operate on the principle of passionating gas particles with a stream of liquid. They are glass and metal. They are attached to the water tap using the nozzle.

To the upper end of the pump, a thick-walled rubber tube or a rubberized 10 cm long hose, which is fixed with a soft iron wire so that water does not succeed. The other end of the tube or hose is connected to the tap nozzle and also tighten with wire. Then check the pump. To do this, operate a tap crane, and the hole of the lateral pump of the pump is closed with a finger. If the finger is embarrassed, it means that the pump for work is suitable. A thick-walled rubber tube, which is connected to the safety flask on the lateral producer of the water jet pump, which is connected.

Purely washed bubber funnel is inserted into the flask on the re-zero stopper (the cortical plugs are not recommended due to their porosity). On the mesh partition, the funnels are placed a circle of filter paper, the diameter of which on the i mm is less than the inner diameter of the funnel. To cut such a circle, take half a folded sheet of filter paper, superimpose on the funnel and slightly press the palm. On paper, the upper diameter circle is imprint; Then scissors customize the circle to the desired size. Putting a paper filter into a funnel, wets it with a solvent and include a pump so that the filter sucks for the bottom of the funnel. In the case of a well-known filter, a calm noisy sound is heard, if the filter is laid unpretentious and there is air seats - a whistling sound. After checking the filter, without turning off the pump, it is poured into a funnel to half a height filtered mixture.

When filtering, it is necessary to ensure that cracks are not formed on the surface of the precipitate, since it leads to uneven, incomplete suction and contamination of precipitate as a result of solvent evaporation. In addition, you need to follow so that the flask is not going too much filtrate, otherwise it will be drawn into the pump. When filtering flammable liquids, appropriate precautions must be observed. To remove the remains of the uterine solution, the crystals are washed on the filter with small portions of the solvent. To do this, the precipitate on the filter is impregnated with solvent, and then include the pump.

The washed crystals on the filter are pressed with a flat part of the glass plug until the mother liquor stops. Then the funnel along with the plug is removed from the flask and shake the filter along with the sediment on the filter paper. Cleaning the paper circle and the walls of the funnel with a spatula from the attractive crystals, press the resulting product in the filter paper and dried in air or other methods.

The drying of solids can be carried out in the open air at a conventional temperature, when heated and a conventional atmospheric pressure, at a low temperature under reduced pressure, in an atmosphere with a low pressure of water vapor (in the desiccator), in an atmosphere of inert gas.

Drying outdoors at normal temperature. Many substances (both inorganic and organic) can be dried outdoors. Evaporation will occur until the equilibrium occurs between the pressure of water vapor in the air and the moisture content in the solid.

In this way, you can dry, for example, chloride barium. For this, the barium chloride, pressed on the bureau funnel after recrystallization, is poured onto a blank sheet of filter paper and distribute a layer with a thickness of no more than 3-5 mm on it. Umbrella salt in this case is impossible: the loose it will be decomposed, the sooner the drying will be better. The salt from above is covered with another sheet of filtered paper to protect it from dust, and leave for 12 hours. During this period of time, the salt will increase significantly. To get a completely dry salt, it should be mixed with a clean spatula after 12 hours so that the lower (more wet) layers get up and that the mass remains loose. Leaving it to stand for another 12 hours, get a dry salt, which is folded with a spatula in the jar and closed. If when it stands in a tightly closed jar on the walls, water drops appear, it means the salt was dried not enough and the XE should be heard again.

Drying in air - the operation is rather long, and it is resorted only when the dried substance is non-fragrance and wish to obtain a substance loose, bulk, without lumps or when the substance decomposes when heated.

Drying when heated and ordinary atmospheric pressure. Drying during heating and ordinary atmospheric pressure is widespread. In this case, use the drying cabinet.

There are several types of laboratory drying cabinets for "stewing in conventional atmospheric pressure.

1. Copper or asbestos drying cabinets with gas or other heated.

2. Copper drying cabinets with water jacket and gas heating.

3. Electric drying cabinets.

Copper or asbestos (ordinary) Drying cabinets (Fig. 471) with gas heating usually represent a box with a side door. Inside there is a copper shelf with a round hole cut in it with a diameter of approximately 1 cm. At the top of the cabinet there are two holes, one of which is intended for the thermometer, another - for air circulation. The drying cabinet is suspended on the wall near the table or put on the table on the iron stand. The cabinet is heated below the gas burner.

The lack of such a cabinet is that it is difficult to precisely adjust the drying temperature in it.

Overheating is always possible, and therefore, when working with such a wardrobe, constant control is necessary.

The substance to be dried is put on the cabinet shelf in the evaporation cup or paper. If drying is haunting the goal to remove water and the substance is "not afraid of" heating, i.e., disintegrates and the PS varies when heating to 100-105 ° C, then drying leads at this temperature. However, you should not immediately bring the temperature to this limit, but to raise it gradually. This is necessary because if the temperature is to lift up to 105 ° C, the upper layer of the substance constructs and the resulting crust will prevent uniform drying.

Duration of drying depends on the amount of substance, the thickness of the layer of it, temperature, the correctness of the conduct.

The smaller the substance and the thinner the layer of it, the sooner it goes dry. It is more profitable for a large game to split small ones than to dry immediately a large amount of thick layer.

11m the temperature rises evenly, the more correctly, it will be driving.

All the time it is necessary to take care that the drying clock is not overheated, since it can be a speaker-drying substance. In some cases, relatively constant temperature regime can be created, opening the door of the cabinet and changing the width of the slit.

Significantly more convenient drying cabinets with double wall or shirt (Fig. 472). In the space between the walls through a special hole in one of the upper corners of the cabinet poured water. To observe the water level in the shirt, these cabinets are equipped with water tubes. Drying cabinets of such a design are heated by gas burners. The advantage of such cabinets is that they can create a constant temperature, PS exceeding 100 ° C. Adjusting the burner flame, it is possible to obtain a pretty constant temperature below 100 ° C.

When working with such a wardrobe, you only need to take care that water constantly was constantly in the cabinet shirt. The shirt is filled with water not completely, so that when boiling water is not pulled.

A wardrobe with double walls can be used for drying at temperatures above 100 ° C. For this, any liquid, boiling at temperatures above 100 ° C, is poured into the space between the walls, and the refrigerator is strengthened in the fluid hole.

Fig. 472. Drying cabinets: A -C Water shirt; BC Water shirt and refrigerator.

The most convenient electrical drying cabinets. In laboratories you can meet different types them. There are several types of simple drying cabinets with electrical heating. In fig. 473, but is shown the drying cabinet No. 0. It consists of a metal hull with a thermal insulation gasket inside the cabinet. At the bottom of the latter, inside, placed on a ceramic plate heating elements - spirals, as on a conventional electric stub. The cabinet has two shelves. Under the door of the cabinet, at the bottom of the front wall, a vent plate is made. On the top, the ceiling part of the wardrobe wall has a hole to strengthen the thermometer. The maximum temperature that can be reached inside the cabinet is about 1250C warming time to this temperature about 30-60 minutes.

Power of heating elements is made from the power supply.

In fig. 473, B is shown the drying cabinet Sh-005. GH consists of a housing in which the cylindrical working chamber is located. The cabinet is heated using a heating wire, wound on the heat-resistant, the well-strained plate located on the outer surface of the chamber. The space between the walls of the housing and the chamber is filled with thermal insulation material,

The cabinet has a thermostat, the control knob and the signal lamp are placed in the front panel. The maximum temperature to which you can heat the cabinet, compile 250 C. The time required to warm the cabinet to this temperature, about 60 minutes.

In fig. 474, and the drying cabinet with the thermostat and the signal lamp is shown. The cabinet consists of a metal housing and an inner plug-in chamber, between which the electric heating device is located. Walls and doors of the cabinet - from asbestos cardboard. Three lattice shelves are embedded inside the cabinet .. on the upper wall of the cabinet, ceiling, there are two holes for strengthening thermometers and ventilation valve. Temperature control error ± 10 ° C.

The drying cabinet No. 3 is more perfect, in which the temperature is controlled automatically in the range of up to 200 ° C with an accuracy of ± 3 ° C. By appearance This cabinet is similar to the S-005 drying cabinet described above. The drying cabinet number 3 has three shelves. To achieve maximum heating to 200 ° C, about 2 hours are required.

Very convenient. Electric drying cabinet "(Fig. 474, b) with automatic heating adjustment. The main advantage of this cabinet is to be heated at the required temperature, changing it in the range from 50 to 220 ° C, which is difficult to achieve when using the drying cabinets described above.

Fig. 474. Electric drying cabinets: A-with thermostat and signal lamp; b-with automatic thermostat.

For rapid drying of the substance, special electrical drying cabinets are very convenient (Fig. 475), through which hot air passes with continuous current; The latter, passing over the dried substance, takes a pair of remote liquid.

Drying at low temperature and reduced pressure (vacuum drying). For drying substances, easily decomposing or changing with heating to 100 0C, dried "in vacuum. For this purpose, the so-called vacuum drying cabinets use. Usually they are a cylindrical shape with a hermetically closing round side door. Inside them there are two shelves, in Some cases are one. Vacuum drying cabinet (Fig. 476) Double-sided, with a shirt, into which a liquid coolant is poured. Heating is carried out with a gas burner or electricity.

PA top of the cabinet is fridge Soxlet For condensation of steams of heating fluid, a tap for a connection with a vacuum pump, a thermometer for measuring the temperature inside the cabinet and a pressure gauge for measuring vacuum in the closet.

Fig. 475. Electrical cabinet for rapid drying.

Fig. 476. Vacuum drying cabinet.

Observation, dried through the glass window, existing in the door. Drying in the desiccator. Highly hygroscopic, the substances spreading in the air can not dry outdoors. In the same way, they are difficult to dry in the closet. Such substances are convenient to dry in the desiccator containing a substance that is vigorously absorbing moisture. The latter include: calcium chloride, concentrated sulfuric acid, phosphorus five-poin and others.

The substance to be dried is placed in the bures or a cup, put the excirator open to the porcelain liner and leave in the last one or more depending on the need.

Drying with irradiation with infrared lamps. For cautious and rapid drying of many precipitations, it is very convenient to use heating with the help of lamps. Infrared radiation. The device is a metal tripod with a reinforced packet with a reflector with a lamp of infrared radiation, which can be moved up and down by setting it to the desired distance from the dried material. The irradiation usually continues from 3 to 15 minutes, depending on the properties and type of materials, the amount of moisture contained in them or volatile substances, the values \u200b\u200bof the sample and the distance between the lamp and the irradiated surface.

A asbestos leaf is put on the table under the lamp to protect the surface of the table from overheating. The suspension of the dried substance is evenly distributed along the bottom of the aluminum or porcelain cuvette, the cups of koch, or Petri, or subscribing molds. Initially, the lamp includes the required temperature (in the center of the illuminated circle, by placing the thermometer or thermocouple tankway there, and the height of the reflector is adjusted. After that, the vessel with the dried substance is placed in the center of the illuminated circle. If the drying was carried out in the unit, after the completion of the operations, the binches are closed with a lid, cooled, as usual, and weighed.

Instead of infrared lamps, conventional electrollamps of 200 W can be used. The reflector can be made of a white tin coated with an asbestos layer. Infrared ka * border dryers were also proposed, allowing you to dry up to 8 hits at the same time.

Drying in the jet of inert gas. This method is used in cases where the substance in air is oxidized or destroyed. Drying is carried out in special devices like those described above. The specified method is particularly important for drying easily exploding substances. For this purpose, it is recommended as an inert gas, helium with high thermal conductivity.

Drying precipitation with organic solvents. For rapid drying of precipitation, in some cases, organic solvents are used, well-soluble water, for example - acetbn, methyl or ethyl alcohol. Naturally, only such organic solvents can be used, which do not dissolve the dried solid. When working with solvents, the pairs of which can ignore, it is necessary to take care that there are no active heating devices near the place of work.

IN out of wet oathstwo can be held.

1. The dried substance is placed in the conical flask, then the drying fluid is poured in such a quantity so that it is a layer of several centimeters above the sediment. The flask is closed and shaken about 1 min, after which they leave to stand for another 15 minutes, and then carefully and possibly fully poured the drying fluid and replace it with fresh. The drying fluid changes at least 3-4 times, each time merging it is possible fully.

Crystals during drying are impregnated with drying fluid, which is removed by evaporation. For this, the dried substance, if it is not hygroscopic, poured onto a sheet of pure filter paper, are covered with another as the same sheet and leave under a pull to completely evaporate organic solvent or placed in a closet for quick drying (see Fig. 475).

2. Crystals to be dried with an organic solvent are placed on a bureau funnel grid, closed by one layer of filter paper, inserted into the Bunsen flask. Initially, the dried substance is gradually dodged drying fluid that flows into the flask. When dehydration is completed, the organic solvent is poured from the Bunzen flask to the prepared dishes, join the Bunsen flask to the vacuum pump and include it. Thus, air, breathing a pair of an organic solvent, is pulled through a layer of dried substance. Suction is carried out until the smell of solvent ceases to feel. When this is achieved, stop the operation of the vacuum pump and move the dried solid in some container.

This method of drying can be applied when working with substances easily oxidizing in air. In this case, you need to use special funnels to filter in an inert gas jet.