What is centrifugation? What is the method used for? The term "centrifugation" means the separation of liquid or solid particles of a substance into various fractions using centrifugal forces. This separation of substances is carried out through the use of special devices - centrifuges. What is the principle of the method?

Centrifugation principle

Let's look at the definition in more detail. Centrifugation is the effect on substances through ultra-high-speed rotation in a specialized apparatus. The main part of any centrifuge is the rotor, which contains nests for installing test tubes with material that is subject to separation into separate fractions. When the rotor rotates at high speeds, the substances placed in the test tubes are separated into different substances according to the density level. For example, centrifuging groundwater samples separates the liquid and precipitates the solid particles it contains.

Author of the method

For the first time it became known what centrifugation is after experiments conducted by scientist A.F. Lebedev. The method was developed by a researcher to determine the composition of soil water. Previously, for these purposes, settling of liquid with subsequent separation of solid samples from it was used. The development of the centrifugation method made it possible to cope with this task much faster. Thanks to this separation, it became possible to extract the solid portion of substances from a liquid in dry form within a matter of minutes.

Centrifugation steps

Differential centrifugation begins with the settling of substances that are subject to research. This material processing occurs in settling devices. During settling, particles of matter are separated under the influence of gravity. This allows you to prepare substances for better separation using centrifugal forces.

Next, the substances in the test tubes undergo filtration. At this stage, so-called perforated drums are used, which are intended to separate liquid particles from solid ones. During the presented activities, all sediment remains on the walls of the centrifuge.

Advantages of the method

Compared to other methods aimed at separating individual substances, such as filtration or sedimentation, centrifugation makes it possible to obtain a sediment with a minimum moisture content. The use of this separation method allows the separation of fine suspensions. The result is the production of particles with a size of 5-10 microns. Another important advantage of centrifugation is the ability to perform it using equipment of small volumes and dimensions. The only drawback of the method is the high energy consumption of the devices.

Centrifugation in biology

In biology, the separation of substances into individual substances is resorted to when it is necessary to prepare preparations for examination under a microscope. Centrifugation here is carried out using complex devices - cytorotors. In addition to slots for test tubes, such devices are equipped with sample holders and all kinds of slides of complex design. The design of the centrifuge when conducting research in biology directly affects the quality of the materials obtained and, accordingly, the amount of useful information that can be gleaned from the analysis results.

Centrifugation in the oil refining industry

The centrifugation method is indispensable in oil production. There are hydrocarbon minerals from which water is not completely released during distillation. Centrifugation makes it possible to remove excess liquid from the oil, increasing its quality. In this case, oil is dissolved in benzene, then heated to 60 o C, and then subjected to centrifugal force. Finally, measure the amount of remaining water in the substance and repeat the procedure if necessary.

Blood centrifugation

This method is widely used for medicinal purposes. In medicine, it allows you to solve the following number of problems:

1. Obtaining purified blood samples for plasmapheresis. For these purposes, the formed elements of blood are separated from its plasma in a centrifuge. The operation makes it possible to rid the blood of viruses, excess antibodies, pathogenic bacteria, and toxins.
2. Preparing blood for donor transfusion. After the body fluid is separated into separate fractions by centrifugation, the blood cells are returned to the donor, and the plasma is used for transfusion or frozen for later use.
3. Isolation of platelet mass. The substance is obtained from the resulting mass and is used in surgical and hematological departments of medical institutions, in emergency therapy, and operating rooms. The use of platelet mass in medicine makes it possible to improve blood clotting in victims.
4. Synthesis of red blood cells. Centrifugation of blood cells occurs through delicate separation of its fractions according to a special technique. The finished mass, rich in red blood cells, is used for transfusion during blood loss and operations. Red blood cells are often used to treat anemia and other systemic blood diseases.

In modern medical practice, many new generation devices are used, which make it possible to accelerate a rotating drum to a certain speed and stop it at a certain moment. This allows blood to be more accurately separated into red blood cells, platelets, plasma, serum and clots. Other bodily fluids are examined in a similar way, in particular, substances in urine are separated.

Centrifuges: main types

We figured out what centrifugation is. Now let's find out what devices are used to implement the method. Centrifuges can be closed or open, mechanically or manually driven. The main working part of hand-held open instruments is a rotating axis located vertically. In its upper part there is a perpendicularly fixed bar where movable metal sleeves are located. They contain special test tubes that are narrowed at the bottom. Cotton wool is placed at the bottom of the sleeves, which avoids damage to the glass test tube when it comes into contact with metal. Next, the apparatus is set in motion. After some time, the liquid separates from the suspended solids. After this, the manual centrifuge is stopped. A dense, solid sediment concentrates at the bottom of the test tubes. Above it is the liquid part of the substance.

Closed-type mechanical centrifuges have a large number of sleeves to accommodate test tubes. Such devices are more convenient compared to manual ones. Their rotors are driven by powerful electric motors and can accelerate to 3000 rpm. This makes it possible to carry out better separation of liquid substances from solid ones.

Features of preparing tubes for centrifugation

Test tubes used for centrifugation must be filled with the test material of identical mass. Therefore, special high-precision scales are used for measurements here. When it is necessary to balance numerous tubes in a centrifuge, the following technique is used. After weighing a couple of glass containers and achieving the same mass, one of them is left as a standard. Subsequent tubes are equilibrated with this sample before being placed into the apparatus. This technique significantly speeds up work when it is necessary to prepare a whole series of tubes for centrifugation.

It is worth noting that too much of the test substance is never placed in test tubes. Glass containers are filled in such a way that the distance to the edge is at least 10 mm. Otherwise, the substance will flow out of the test tube under the influence of centrifugal force.

Supercentrifuges

To separate the components of extremely thin suspensions, it is not enough to use conventional manual or mechanical centrifuges. In this case, a more impressive effect on substances from centrifugal forces is required. When implementing such processes, supercentrifuges are used.

The devices of the presented plan are equipped with a blind drum in the form of a tube of small diameter - no more than 240 mm. The length of such a drum significantly exceeds its cross-section, which makes it possible to significantly increase the number of revolutions and create a powerful centrifugal force.

In a supercentrifuge, the substance being tested enters the drum, moves through the tube and hits special reflectors, which throw the material onto the walls of the device. There are also chambers designed for separate discharge of light and heavy liquids.

The advantages of supercentrifuges include:

• absolute tightness;
• the highest intensity of substance separation;
• compact dimensions;
• the ability to separate substances at the molecular level.

Finally

So we found out what centrifugation is. Currently, the method finds its application when it is necessary to isolate precipitates from solutions, purify liquids, and separate components of biologically active and chemical substances. Ultracentrifuges are used to separate substances at the molecular level. The centrifugation method is actively used in the chemical, oil, nuclear, food industries, as well as in medicine.

In addition to filtration, separation of a mixture of liquid and solid substances is also possible by centrifugation, that is, the separation of substances in devices called centrifuges.

The use of a centrifuge is based on the use of centrifugal force. During rapid rotation (centrifugation), solid particles suspended in a liquid (with a higher density than the density of the liquid) are thrown away from the center under the influence of the centrifugal force developing during rotation and are thus separated from the liquid.

Rice. 407. Thiessen apparatus for microanalytical work

Rice. 408. Manual centrifuge

Centrifuges are available: open and closed, manually and mechanically driven. The main part of an open manual centrifuge (Fig. 408) is a vertically mounted rotating axis, perpendicular to which at its upper end there is attached a bar with two (or four) movably fixed metal sleeves. Special tubes, narrowed downward, are inserted into these sleeves (Fig. 409) with liquid from which suspended particles need to be removed,

A piece of cotton wool is placed at the bottom of the sleeve “to avoid direct contact of glass with metal. When the tubes are inserted into the sleeves, the centrifuge is set in motion and after some time (depending on the viscosity of the liquid, the size of the suspended particles and the density difference), the suspended solids are separated from the liquid, after which the centrifuge is stopped. A dense sediment of solid substance collects at the bottom of the test tube, above which there is a clear liquid.

Z covered centrifuges(Fig. 410) depending on the size, contain a different number of sleeves, from 2 to 12 or more, located symmetrically at the same distance from each other and from the axis of the centrifuge.

Mechanical closed centrifuges(Fig. 410, b) are more convenient than manual ones (Fig. 410, a). They usually give 2000-3000 rpm, allowing for more perfect separation of liquid and solid matter.

Centrifuge tubes must have the same mass after filling with liquid. Where the centrifuge has to be used frequently, it is recommended to have special scales adapted for weighing (or rather, taring) test tubes. In these scales, the cups are suspended from a rocker using rods attached to the center of the cups. These rods have rings into which test tubes are inserted.

Having strengthened the test tubes, first pour the liquid to be centrifuged into one test tube (using, for example, a pipette), and then into the second, ensuring that the cups are balanced.

You should never put too much liquid into test tubes; The tubes are filled so that the distance from the edge to the liquid level is at least 10 mm.

When you need to balance many test tubes, it is advisable to use the following technique. Having balanced the first pair of test tubes, one of them is removed and placed in the centrifuge nest, and the other is left on the scales; this last test tube will serve as a standard for the rest; another test tube is inserted into the space vacated on the scales, balanced with the standard and removed. It is also advisable to pre-fill the test tubes (taking a slightly less amount of liquid than required) and then add the required amount of liquid during equilibration. This technique speeds up the work.

Rice. 409. Centrifuge tubes.

Balanced tubes are inserted into the centrifuge slots.

The centrifuge should not be started at full speed immediately, but gradually. This applies to both manual and mechanical centrifuges.

Rice. 410. Closed centrifuges: a - with manual drive; b - with an electric motor.

Mechanical centrifuges have appropriate devices for speed control. Thus, electric centrifuges are equipped with rheostats for gradual activation at full speed. For centrifuges driven by a water turbine, a gradual increase in speed is achieved by regulating the water jet. The more carefully the activation was carried out, the more reliably the centrifuge operates.

The centrifuge should be constantly monitored; Contamination of it, especially moving parts, is unacceptable. Metal sleeves should turn easily and freely. The gears that drive the centrifuge must move smoothly; They should not be lubricated with lubricants that may thicken. The centrifuge axis must also be in order and always clean.

If you handle centrifuges carelessly, especially manual ones, you can bend the axle and thereby disable the centrifuge.

After turning off the centrifuge, allow it to stop and only then remove the test tubes.

Recently, so-called supercentrifuges, producing up to 40,000 rpm, have become increasingly widespread (Fig. 411).

Rice. 411 supercentrifuge

Such centrifuges are especially convenient for centrifuging all kinds of viscous solutions, such as varnishes, thin dispersions, and emulsions.

The liquid to be supercentrifuged enters pipe 1 located in the lower part of the apparatus. Then the liquid is poured into working cylinder 2, rotating at a speed of up to 40,000 rpm, in which the separation of heavier particles suspended in the liquid occurs. The liquid gradually rises along the cylinder 2 up to the separator 5, and if the emulsion is destroyed, the lighter liquid flows out through drain 8, and the heavier liquid through drain 4. When solid particles with a density greater than one are separated, the liquid flows out through drain 3. On the inner wall a separated solid sediment is deposited in the working cylinder. Supercentrifuge. From time to time, the supercentrifuge is stopped, working cylinder 2 is removed, cleared of sediment and, having been put back in place, work continues. The entire process of cleaning the working cylinder, from the moment of stopping to the moment of restarting the supercentrifuge, takes no more than 15 minutes. If it is necessary to purify relatively large quantities of liquid, then they use three8 supercentrifuges: one is working, the other is being purified, the third is in reserve,

The centrifugation method is based on the different behavior of particles in the centrifugal field created by the centrifuge. The sample, located in the centrifugation vessel, is placed in a rotor, which is driven by a centrifuge drive. To separate a mixture of particles, it is necessary to select a set of conditions, such as rotation speed, centrifugation time and rotor radius. For spherical particles, the rate of deposition (sedimentation) depends not only on acceleration, but also on the radius and density of the particles, as well as on the viscosity of the medium in which the sample is deposited.

Centrifugation can be divided into two types: preparative and analytical. Preparative centrifugation is used when it is necessary to isolate part of the sample for further research. This method is used to isolate cells from suspension, biological macromolecules, etc.

Analytical centrifugation is used to study the behavior of biological macromolecules in a centrifugal field. This method allows one to obtain data on the mass, shape and size of molecules located in relatively small volumes of the sample. In everyday laboratory practice, preparative centrifugation is most often encountered.

Preparative laboratory centrifuges, in turn, are divided into groups according to their purpose: preparative ultracentrifuges, general-purpose centrifuges and high-speed centrifuges. General purpose centrifuges have the greatest practical application in medical laboratories and have a maximum speed of up to 6 thousand rpm. The main feature of this type of device is its relatively large capacity - up to 6 liters, which allows the use for centrifugation not only of centrifuge tubes with a volume of up to 100 ml, but also containers of up to 1.25 liters. In all general purpose centrifuges, the rotors are rigidly mounted on the drive shaft, so the centrifuged containers must be fairly accurately balanced. To avoid breakage, do not load an odd number of tubes into the rotor; if the load is incomplete, the containers should be placed opposite each other.

High-speed centrifuges have a maximum speed of 25 thousand rpm and acceleration up to 89 thousand g. The chamber containing the rotor and centrifuged samples is equipped with a cooling system to prevent heating caused by friction when the rotor rotates at high speeds. Typically, such centrifuges can operate with a volume of up to 1.5 liters and are equipped with angular rotors or rotors with replaceable bowls.

Preparative ultracentrifuges accelerate to 75,000 rpm and have a maximum centrifugal acceleration of 510 thousand g. They are equipped with refrigeration and vacuum units to prevent the rotor from overheating from friction with the air. The rotors for these centrifuges are made of high-strength titanium or aluminum alloys. The shaft of ultracentrifuges, unlike high-speed and preparative centrifuges, is made flexible to reduce vibration when the rotor balance is disturbed. The containers in the rotor must be carefully balanced to within one tenth of a gram.

Filtration is the process of separating suspended solids in liquids or gases. A liquid or gas with solid particles in it is passed through a porous material (filter), the pore sizes of which are so small that solid particles do not pass through the filter. The pore sizes determine the ability of the filter to retain solid particles of various sizes, as well as its productivity, i.e. the amount of liquid that can be separated per unit of time.

The filtration process is affected by the viscosity of the liquid and the pressure difference on both sides of the filter. The higher the viscosity of a liquid, the more difficult it is to filter. Since the viscosity of a liquid decreases with increasing temperature, hot liquids are easier to filter than cold ones. Filtration of viscous liquids can often be made easier by diluting them with a solvent, which can be easily distilled off after filtration is complete. The greater the pressure difference, the higher the filtration rate. Therefore, filtration is often carried out at reduced or excess pressure. When filtering under pressure, gelatinous sediments adhere tightly to the filter, the pores of which easily become clogged, and filtration stops.

If the particle size of the solid phase is smaller than the pore size of the filter, the suspension cannot be filtered. Thus, ordinary paper filters do not retain fine particles of many colloidal solutions. In such cases, before filtering, the colloidal solution is heated or an electrolyte is added to it, which leads to coagulation (coarsening of particles and formation of precipitate).

When the purpose of filtration is to obtain a transparent filtrate rather than a pure precipitate, to better separate fine particles from the liquid, a small amount of powdered active carbon is added to the latter, shaken and filtered.

Filtration of mixtures containing substances that clog the pores of the filter and form viscous layers on it is often facilitated by the addition of fine quartz sand, infusor soil, asbestos fiber, and cellulose (paper) pulp.

Filtration can be carried out in various ways, depending on the nature of the liquids being filtered and the properties of the solid phase (precipitate) that needs to be separated from the liquid or gas.

If the solid phase of the mixture settles easily, then most of it can be removed before filtering itself by decanting. Decantation - the simplest method of separating solid and liquid phases - is based on the fact that in the absence of stirring, the solid substance settles to the bottom of the vessel and the clear liquid can be separated by draining from the settled sediment. Sometimes decanting can also be used to separate two solids of different densities. Decantation using a siphon is often used to rinse poorly soluble solids (Fig. 118). Decantation rinsing is significantly more effective than filter cake rinsing, where the liquid typically does not penetrate evenly between the solid particles.

Filtration by the liquid's own weight

This filtration method is usually used in cases where the filtered solid phase is not needed (removing mechanical impurities from solutions), or when the liquid phase can be completely removed by repeated treatment of the sediment with an appropriate solvent.

Conventional filtration is used when it is necessary to filter hot concentrated solutions or solutions of crystalline substances in volatile solvents. When such solutions are filtered in a vacuum, the solvent evaporates under the filter, which cools sharply and becomes clogged with the released crystals.

Various types of filter paper, ready-made fat-free and ash-free paper filters are mainly used as filter material.

Filter paper for direct use is available in two grades: FNB - fast filtration with a pore size of 3.5-10 microns and FNS - medium-speed filtration with a pore size of 1-2.5 microns. The ash content of these grades of paper is up to 0.2%.

For the production of ash-free and fat-free paper filters, three grades of filter paper are produced: FOB - fast filtration; FOS - medium filtration; FOM - slow filtration.

Ready-made round paper filters, fat-free (with yellow tape) and ash-free, are available in various diameters in packs of 100 pcs. The choice of filter size depends on the mass of solids being separated, not the volume of liquid being filtered.

Ashless filters for laboratory work vary in separation (retention) ability. This difference is determined by the color of the paper tape used to cover the packaging.

The following designations are accepted: white tape - fast filtering, red - medium filtering, blue - slow filtering, intended for filtering fine-grained sediments (such as BaSO4).

The choice of filter brand in each individual case depends on the properties of the separated solid. Very dense filters should only be used when really necessary.

Filter paper and ready-made filters cannot be used to filter concentrated solutions of strong acids or alkalis, as this reduces the mechanical strength of the filters.

Paper filters can be simple or folded (flattened). To make a simple smooth filter, a round piece of filter paper of a certain size is folded four times and cut with scissors to form a sector of the circle. The dependence of the filter diameter on the diameter of the glass filter funnel is presented below:

The smooth filter should fit snugly against the walls of the funnel, especially in the upper part. To do this, it is recommended that when folding the filter, bend the semicircle not along the middle line, but along a parallel line close to it.

The folded filter is placed in a funnel (you can fill it no more than 1/3 or 1/2 with sediment), moisten it with distilled water and fill the spout (tube) of the funnel with water. To do this, the filter is raised and quickly lowered. The edges of the filter should be 5-10 mm below the edge of the funnel. The wet filter is carefully pressed against the funnel. Filtration begins immediately to ensure the funnel spout remains filled with liquid. Do not fill the funnel with solution more than 3/4 of the volume. The tip of the spout should touch the inside wall of the glass containing the filtrate to prevent splashing.

Simple smooth filters are commonly used in analytical laboratories to filter dilute solutions.

Filtration is significantly faster when using pleated filters. These filters are easy to make (Fig. 119). The folds of the filter should not come close to its center, otherwise the paper in the center of the filter may tear. The finished filter is inserted into the funnel so that it is adjacent to its walls. If the funnel has an angle greater or less than 60°, the filter is adjusted to it by changing the position of the second bend. It is necessary that the filter has a sufficiently sharp end so that the filter paper is not damaged by repeated folding.

Before placing the prepared filter in the funnel, it is unfolded and bent so that the outer side of the filter paper is on the inside of the filter. The filter, correctly placed in the funnel, is moistened with the filtered liquid or distilled water.

When filtering hot solutions and using large diameter funnels, the top of the filter may burst. To eliminate this danger, a small or special perforated porcelain insert is inserted into a large funnel, and it is best to filter through two folded filters placed together.

Equipment for filtration at atmospheric pressure and room temperature is simple and consists of a funnel, filter, receiver and stand. To filter hot saturated solutions of solids, wide, shortened funnels are used, and for rapid filtration of large volumes of liquids, corrugated funnels are used, the uneven walls of which, in combination with smooth filters, increase the effective filtering surface. The funnel is secured in a ring attached to a laboratory stand, or it is inserted directly into the neck of the flask - the filtrate receiver. In the latter case, it is necessary to place a strip of filter paper under the funnel so that the air displaced by the filtrate can escape from the flask.

Filtration is often difficult if a layer of air (an air pocket) forms between the paper filter and the wall of the funnel. To avoid this, a slight excess pressure is created inside the funnel: the funnel is covered with a piece of filter paper moistened at the edges and an inverted funnel of the same diameter. Air is pumped through the upper funnel tube using a rubber bulb and thereby eliminating the air pocket.

To speed up filtering, lengthen the funnel tube: a glass tube of the same (or slightly smaller) internal diameter is connected to the spout with a rubber tube. After some time, the entire tube is filled with a column of filtrate, creating a vacuum.

It is recommended to filter highly alkaline solutions and solutions of hydrofluoric acid through a funnel made of porous polyethylene. To make such a funnel (Fig. 120), two glass funnels are used, the outer one is closed at the narrowing point with a stopper, and the inner one is melted in the same place. A mixture of polyethylene powder and finely ground sodium chloride in a mass ratio of 1:4 is placed between the walls of the funnels and kept in an oven at 130-150 °C. From time to time, the inner funnel is turned with pressure to evenly apply the semi-liquid mass to the inner surface of the outer funnel. After cooling, the inner funnel is removed, the plug is removed from the outer funnel tube, and the sintered mass is washed with warm water to remove sodium chloride.

The filtration speed is directly proportional to the hydrostatic pressure of the filtered liquid, therefore, when filtering large volumes of liquids, it is beneficial to maintain a constant liquid level on the filter. In Fig. 121 shows simple homemade devices for automatically adding liquid to the filter. The container with the liquid is closed with a clean rubber stopper equipped with a tube for liquid intake and a tube for air intake. The level of the lower end of the air intake tube determines the liquid level on the filter. If the level drops, air enters the vessel and squeezes the liquid onto the filter. As a result, the liquid level on the filter rises, and access to air inside the vessel is closed.

Filtration when heating or cooling

Heating filtration is carried out when it is necessary to clean hot concentrated solutions from impurities, to filter viscous solutions, as well as solutions containing substances that easily crystallize at normal temperatures.

First of all, you need to carefully select the type of filter paper, filter size and funnel size in order to speed up the process. Before pouring the hot solution onto the filter, the funnel with the inserted filter is heated by passing through the filter a certain amount of hot pure solvent or solvent vapor if it is heated in a bath to a boil. In the latter case, the funnel is covered with a watch glass. Before filtration, the solvent is poured out of the receiver so that it does not dilute the filtrate. The filter should have a high fluid level to speed up filtration.

A funnel with a filter can also be heated with a metal funnel for hot filtering (Fig. 122, a) or a funnel, between the double walls of which hot water, water steam or hot air is passed (Fig. 122, b). Heating can also be accomplished by immersing the electric heater in a filtered solution, if the latter does not contain substances that react with the metal.

For uniform heating of laboratory glassware, knitted covers (caps) with electrical heating are also used. They are usually made from a thin strand of glass and contain a flexible heating element in the form of a thin wire or coil.

Cooling filtration can be carried out in an ice-cooled funnel or in a funnel through which cooled brine is passed between the double walls.

Reduced pressure filtration

Filtration under reduced pressure allows for more complete separation of solids from liquids and increases the speed of the process.

Vacuum filtration equipment consists of a filtration device, a receiver, a water-jet pump and a safety bottle.

When filtering large quantities of substances, perforated porcelain or slit-shaped glass cylindrical Buchner funnels inserted into conical flasks for filtering under vacuum with a tube are most often used; the latter are connected to the water-jet pump through a safety bottle. It is necessary that the size of the funnel corresponds to the amount of filtered solid, which should completely cover the surface of the filter. However, too thick a layer of sediment makes suction and subsequent rinsing difficult.

A filter for Buchner funnels is a round sheet of filter paper placed on the perforated partition of the funnel. The diameter of the filter should be slightly smaller than the diameter of the partition. Large Buchner funnels usually have two filters stacked on top of each other. In order for the fitted paper filter to fit tightly enough to the perforated partition of the funnel, it is first moistened with a solvent on the funnel and pressed evenly against it. Then, after removing the solvent, the filtered mixture is poured into the funnel and sucked off.

In the case of aqueous solutions, the small amounts of water used to wet the filter are not significant. In those cases where the presence of water is unacceptable, the wet filter, having achieved a tight fit, is washed with ethyl alcohol or acetone, and then with a solvent whose presence in the filtrate is acceptable. Filter paper moistened with an organic solvent does not stick to the funnel as well as when moistened with water.

Buchner funnels are secured in conical flasks using rubber stoppers or thick flat pieces of rubber covering the top of the neck of the flask; the latter are convenient in that they cannot be drawn into the flask during suction during filtration.

To completely separate the mother liquor, the precipitate on the filter is squeezed out with the flat surface of a glass stopper or a thick-walled cylinder with a flat bottom until the liquid stops dripping. In this case, it is necessary to ensure that cracks do not form on the surface of the thick layer of sediment, as this leads to incomplete suction of the mother liquor and contamination of the sediment. To remove the remaining mother liquor, the precipitate is washed on the filter with small portions of solvent at atmospheric pressure. When the filter cake is saturated with solvent, the vacuum is turned on again for suction.

When filtration with suction, synthetic fiber filters are used as filter materials in addition to conventional paper filters. Thus, filters made of polyvinyl chloride or polyester fiber are resistant to acids and alkalis, but are destroyed by organic solvents.

To separate sticky sediments that are difficult to filter, an asbestos mass is often used, which can be compacted on a suction funnel or Gooch crucible. The asbestos mass is prepared as follows: in a porcelain mortar, the asbestos is ground with conc. HCl, transfer the mass into a glass and boil for 20-30 minutes in a fume hood. Then the mass is diluted with a 20-30-fold volume of distilled water, filtered on a Buchner funnel and washed with water until the acidic reaction in the filtrate disappears. Then the mass is dried at 100-120 °C and calcined in a muffle. Calcined asbestos is shaken with water until a homogeneous mass is obtained, transferred to the filter plate of a funnel or Gooch crucible, sucked off and compacted.

Funnels, crucibles and gas filters with a soldered plate of sintered glass powder are extremely convenient for filtering. Glass filters are used to separate solids from liquids during filtration and extraction, to remove mist particles from gases, and to bubbling (distribute) gases in liquids. Glass filters, however, are inconvenient in cases where quantitative separation of sediment is required, since it is difficult to completely remove the sediment from the filter. They are not suitable for filtering very concentrated hot solutions of alkalis and alkali metal carbonates.

The porosity of glass filter plates and their designations changed frequently. According to GOST 9775-69, the filter class depends on the pore size (Table 8).

Types of glass funnels and crucibles with porous filters are shown in Fig. 123.

In addition to glass products with filters for liquids, products with filters for filtering and washing gases are also manufactured.

Filter funnels with a thermostated tube and a thermostated jacket are also available (Fig. 124). Electrically heated funnels are designed for heated filtering of solutions and suspensions that are crystallizing and viscous at room temperature. Heating the filter funnel to 130°C prevents the solution from solidifying, and filtration proceeds quickly.

The main element of an electrically heated filter funnel with a thermostated tube is a glass filter with a diameter of 40 mm with a soldered thin-walled glass tube, which contains a 30 W electric heater. Funnels are available with filters with sizes of 40, 100, 160 microns.

In a heated filter funnel, temperature control is ensured by flowing coolant. The volume of the funnel above the filter with a thermostated tube is 80 ml, with a thermostated jacket - 58 ml.

To separate liquids from solids, a reverse submersible filter funnel is used (Fig. 123, d). The filter is immersed in the liquid, and the filtrate enters the receiver to which the filter is connected. With this device it is convenient to carry out filtering at a low temperature, maintaining a low temperature of the filtered mixture through a cooling bath.

To separate small amounts of substances, use a funnel with a glass “nail”, which is covered with a round piece of filter paper. To do this, the end of a glass rod is softened in the flame of a burner and then flattened, pressing it against the flat horizontal surface of a metal plate. It is necessary that the filter fits tightly to the nail, and the edges of the filter are bent 1-2 mm along the wall of the funnel. The filtrate receiver is a filter tube (with a side outlet).

To filter substances with a low melting point or highly soluble at room temperature, use vacuum during cooling. In the case of small quantities of sediment, the funnel and solution are pre-cooled in a refrigerator. In other cases, a Buchner funnel is built into a flask with the bottom cut off, filling the latter with ice or a cooling mixture.

When filtering in an inert gas atmosphere, use the installations shown in Fig. 125.

Analytical aerosol filters AFA

AFA filters are used to study and control aerosols contained in air or other gases. AFA filters consist of a separate filter element or a filter element glued to a support ring and protective paper rings with protrusions.

The filter element used is FP filter material (Petryanov filter) made of ultra-thin polymer fibers (cellulose acetate, perchlorovinyl, polystyrene). The working surface of the round filter is 3, 10, 20 and 160 cm2.

Centrifugation

Centrifugation is one of the methods for separating heterogeneous systems (liquid - liquid, liquid - solid particles); in rotors under the influence of centrifugal forces. Centrifugation is advantageous if the filtered substances clog the pores of the filter, deteriorate upon contact with the filter material, or are finely dispersed.

Centrifugation is carried out in special devices called centrifuges. The main part of the centrifuge is the rotor, which rotates at high speed.

Types of centrifuges are numerous; They are divided primarily according to the size of the separation factor. It is equal to the ratio of the acceleration of the centrifugal field developed in a centrifuge to the acceleration of gravity. The separation factor is a dimensionless quantity. The separating effect of the centrifuge increases in proportion to the separation factor.

The separation factor of electrically driven centrifuges produced by domestic industry varies from 1,600 to 300,000, and the rotor speed ranges from 1,000 to 50,000 rpm.

Heterogeneous systems in centrifuges are separated either by settling or filtration. Depending on this, centrifuges are available with a solid rotor or with a perforated rotor covered with filter material.

Centrifugation by settling is carried out to clarify a liquid containing suspended solids, or to sediment the solid phase. It consists of the sedimentation of the solid phase, compaction of the sediment and the release of the supernatant.

In laboratory practice, various types of centrifuges are used: manual or electric, tabletop (portable), mobile and stationary. Based on the separation factor, centrifuges are divided into conventional (with a separation factor of less than 3500), supercentrifuges and ultracentrifuges (with a separation factor of at least 3500). Conventional centrifuges are used primarily for separating low-disperse (size greater than 10-50 microns) suspensions of various concentrations. Supercentrifuges are mainly used for separating emulsions and highly dispersed suspensions (size less than 10 microns). For the separation and study of highly dispersed systems and high-molecular compounds, analytical and preparative ultracentrifuges with a separation factor of more than 100,000 are common. Analytical centrifuges are used to determine the molecular weight and degree of polymerization of high-molecular compounds, preparative centrifuges are used to isolate substances from solutions that are under normal conditions in a colloidal state or in the form of inseparable suspensions (proteins, nucleic acids, polysaccharides).

The ultracentrifuge rotor rotates, as a rule, in a vacuum chamber during cooling (refrigerated centrifuges). The speed and time of rotation of the rotor, as well as the temperature conditions of centrifugation, are controlled by electronic devices.

The solution to be treated is placed in a special vessel, which is then rotated at high speed on a centrifuge rotor. In this case, the components of the mixture, under the influence of centrifugal force, are distributed in layers to different depths (in accordance with the masses of the particles); the heaviest particles are pressed to the bottom of the vessel.

When using small-sized portable tube centrifuges with manual or electric drive, the suspension is placed in glass or plastic tubes, which rotate around the main axis, suspended on axles. Tube centrifuges for periodic separation of small quantities of a substance can be of two types. In some, the test tubes are held by pins on the rotor and take a horizontal position during rotation, in others they are rigidly fixed at a certain angle to the axis of rotation (angular rotors).

In Fig. 126 shows the position of the tubes during centrifugation in an angular rotor and in a rotor with swinging cups.

After stopping the centrifuge, the clear liquid phase (centrate) is drained or collected using a pipette. The precipitate is washed and centrifuged again. If the maximum amount of sediment needs to be extracted from the test tube, then the centrate is discarded, and the sediment is dried in a vacuum desiccator without removing it from the centrifuge glass tube.

When using tube centrifuges, test tubes made of thick-walled glass or synthetic material are inserted into protective metal cups. The bottom of glass tubes is protected with rubber gaskets. Glass test tubes can be filled to half the volume, and test tubes made of synthetic materials at high rotor speeds (5000 rpm) should be filled almost to the top so that they do not deform under the influence of centrifugal force. To ensure safe operation, it is necessary to very accurately balance the tubes with the centrifuged suspension. Unbalance at high speeds can lead to rotor damage. Considering that volatile solvents can evaporate during centrifugation, it is better to seal the tubes with stoppers.

The rotors of laboratory test tube centrifuges, with the exception of manual ones, are placed in protective metal cases (lids) so that there is no threat of danger to workers if a test tube with a beaker falls off the hangers.

It is necessary to strictly follow the instructions given in the factory instructions for this centrifuge; the rotor speed specified in the instructions must not be exceeded. The centrifuge can only be set in motion with the safety cover closed; The lid may only be opened after the centrifuge has completely stopped.

Manual centrifuge RC-4. This centrifuge is designed to separate liquids of different densities or to separate suspended or agitated particles from liquids. The main parts of the centrifuge: a cast-iron body, inside of which gears (worm gear), a test tube holder, a handle and a clamp are mounted. There are four sleeves made of carbolite on the hinged suspension of the test tube holder. Liquids and solids of different densities are distributed in different places in the tube when rotated. Separation can be carried out simultaneously in four tubes. For one revolution of the handle, the test tube holder makes eight revolutions. To operate, the centrifuge is mounted with a clamp on the lid of the laboratory table or on a special stand.

Laboratory tabletop centrifuge TsLN-2. The TsLN-2 centrifuge operates with an RU 6x10 angle-type rotor. The maximum volume of centrifuged material is 60 cm3. Rotor speed 3000-8000 rpm; The rotation speed interval, adjusted by the switch, is 1000 revolutions. The separation factor reaches 5,500. The rotor acceleration time to maximum speed is 10 minutes; braking time no more than 8 minutes. Continuous operation time 60 min; minimum mandatory break 15 minutes. The working chamber of the centrifuge is closed with a lid with a self-closing device. Centrifuge weight 8 kg.

When working with the TsLN-2 centrifuge, it is prohibited to: work without grounding; increase the rotation speed above 8000 rpm; work with open rotor and centrifuge covers; work with glass test tubes at rotor speeds above 4000 rpm; place tubes filled with centrifuged material not diametrically opposite.

The difference in mass of diametrically located test tubes filled with the material to be centrifuged should not exceed 0.5 g. The density of the liquid separated in test tubes made of polymeric materials should not be more than 2 g/cm3, in glass test tubes - no more than 1.5 g/cm3.

Angular small-sized centrifuge TsUM-1. The centrifuge has a cross-rotor for simultaneous centrifugation of liquids in four tubes with a capacity of 25 ml, four of 10 ml and eight of 5 ml. The rotor speed from 2000 to 8000 rpm is adjusted in steps. The separation factor reaches 6000. The rotor acceleration time is 8-10 minutes. The centrifuge is equipped with an electric clock, which makes it possible to set the centrifugation time from 0 to 60 minutes, followed by automatic braking. Centrifuge weight 16 kg.