Methods for purifying substances vary and depend on the properties of the substances and their application. In chemical practice, the most common methods are: filtration, recrystallization, distillation, sublimation, salting out. Gas purification is usually carried out by absorbing gaseous impurities with substances that react with these impurities. Pure substances have their own characteristic physical and chemical properties. Therefore, the purity of a substance can be checked by both physical and chemical methods. In the first case, density, melting, boiling, freezing points, etc. are determined. Chemical testing methods are based on chemical reactions and are methods of qualitative analysis.
In accordance with the standard (GOST), according to the degree of purity, reagents are divided into:
a) chemically pure (reagent grade),
b) pure for analysis (analytical grade),
c) clean (h.) and others.
Substances marked with chemical grade are suitable for laboratory work in inorganic chemistry. and ch.d.a.
- Recrystallization
To recrystallize a substance, it is dissolved in distilled water or a suitable organic solvent at a certain 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 using a hot filtration funnel. The filtrate is collected in a glass placed in a crystallizer with cold water with ice or a cooling mixture. When cooled, small crystals fall out of the filtered saturated solution, since the solution becomes supersaturated at a lower temperature. The precipitated crystals are filtered on a Buchner funnel, then transferred to a sheet of filter paper folded in half. Using a glass rod or spatula, distribute the crystals in an even layer, cover with another sheet of filter paper and squeeze the crystals between the sheets of filter paper. The operation will be repeated several times. Then the crystals are transferred to a bottle. The substance is brought to constant mass in an electric drying oven at a temperature of 100-105 . The temperature in the cabinet should be increased gradually to this limit. To obtain a very pure substance, recrystallization is repeated several times.
- Sublimation (sublimation)
- Distillation (distillation)
- a) at atmospheric pressure (simple distillation),
- b) at reduced pressure (vacuum distillation),
- c) steam distillation.
- Salting out
Salting out is that under the influence of significant quantities of a saturated solution of a strong electrolyte, high-molecular natural compounds (proteins, gums, mucus, pectins) precipitate from the extracts. This occurs because when an electrolyte solution is added to the extract, the resulting electrolyte ions are hydrated, removing water from the biopolymer molecules. The protective hydration layer of biopolymer molecules disappears. Particle aggregation and biopolymer deposition are observed. Salting out is quite widely used for the purification of protein drugs, such as pepsin. The term “salting out” gets its name from the process of precipitation of proteins when sodium chloride is added to their solutions.
It must be borne in mind that different salts have different salting out properties, which is explained by the ability of anions and cations to hydrate. The salting out ability of electrolytes depends mainly on anions. Anions, according to their salting out power, are arranged in the following lyotropic series >>>>>.
For cations there is the same lyotropic series: > > > > .
However, sodium chloride is usually used for this purpose, which is cheaper.
- Sodium chloride
Sodium chloride is a chemical compound NaCl, sodium salt of hydrochloric acid, sodium chloride.
Sodium chloride is known in everyday life as table salt, of which it is the main component. Sodium chloride is found in significant quantities in sea water, creating its salty taste. It occurs naturally in the form of the mineral halite (rock salt).
Pure sodium chloride appears as colorless crystals. But with various impurities, its color can take on: blue, purple, pink, yellow or gray tint.
Moderately soluble in water, solubility depends little on temperature: the solubility coefficient of NaCl (in g per 100 g of water) is 35.9 at 21 °C and 38.1 at 80 °C. The solubility of sodium chloride is significantly reduced in the presence of hydrogen chloride, sodium hydroxide, and salts - metal chlorides. Dissolves in liquid ammonia and enters into exchange reactions.
- Sodium chloride called "table salt"
Table salt (sodium chloride, NaCl; also called " sodium chloride", "table salt", "rock salt", "edible salt" or simply "salt") is a food product. When ground it appears as small crystals white. Table salt of natural origin almost always contains admixtures of other mineral salts, which can give it shades of different colors (usually gray). Produced in different types: purified and unrefined (rock salt), coarse and finely ground, pure and iodized, sea salt, etc. Salt is obtained by industrial purification of halite (rock salt) deposits located on the site of dried up seas.
- Sodium chloride occurs naturally in the form of the mineral halite.
Halite (Greek ??? - salt) is rock salt, a mineral of the chloride subclass, a crystalline form of sodium chloride (NaCl). The raw material from which table salt is made. Halites can be found in layers of sedimentary rocks among other minerals - products of water evaporation - in drying estuaries, lakes, and seas. The sedimentary layer is up to 350 meters thick and extends over vast areas. For example, in America and Canada, underground salt deposits extend from the Appalachian Mountains west of New York through Ontario to the Michigan basin.
- Purification of sodium chloride by salting out method.
When recrystallizing substances whose solubility changes little with temperature, the salting out method is used. Substances are added to solutions of such substances to reduce their solubility.
- Experimental part
Equipment: technochemical scales, mortar, beaker, tile, folded and ordinary filters, beaker, glass rod, funnel, Petri dish.
Reagents: saturated sodium chloride solution, table salt, distilled water, concentrated hydrochloric acid (? = 1, 19 ) .
- Cleaning Method
- Conducting an experiment
I weighed out 20 g of table salt on a technochemical scale and poured it into a glass. Added 50 ml of distilled water there. Then she placed the glass on the stove and brought the contents to a boil. The salt has flaked off. I filtered the solution and placed it in a fume hood. There, slowly, while stirring, I began to add concentrated hydrochloric acid. At the same time, the solubility of the electrolyte decreases when another electrolyte with the same ion is introduced into the solution. With the introduction of chlorine ions Cl? into a saturated solution of sodium chloride NaCl(k) > +Cl? the equilibrium shifts to the left, resulting in the precipitation of salt crystals that do not contain impurities.
I waited until the solution cooled down. The cooled solution was filtered. The resulting crystals were placed in a Petri dish and left to dry.
After the crystals dried, I weighed them: m=5,200 g.
etc.............
Some solids, when heated, can actively evaporate before reaching their melting temperatures. The reverse transition of vapor to the solid state occurs immediately, bypassing the liquid phase. This process is called sublimation or sublimation and is used to purify substances.
Sublimation, even a single one, as a rule, leads to the obtaining of a completely pure product and often replaces several recrystallizations. It can be used both for final purification of the product and for preliminary separation of volatile compounds from non-volatile impurities. Sublimation also differs favorably from recrystallization in its higher yield of pure product (98-99%).
On the other hand, sublimation is a very long process, so it is usually used to purify small quantities of substances. The scope of this method is also limited by the fact that the ability of many solid compounds to sublimate is so insignificant that it cannot be used for preparative purposes.
Since the rate of evaporation is proportional to the total surface area of evaporation, the substance subjected to sublimation must be ground as finely as possible. You should also not allow the substance to melt during sublimation, since this leads to a drop in the rate of the process due to a sharp decrease in the surface of the substance.
The use of rarefaction, as well as during distillation, reduces the temperature at which substances begin to sublime, so under vacuum it is possible to sublimate many labor-volatile compounds.
When choosing devices for sublimation, preference should be given to designs in which the distance between the sublimated substance and the condensation surface is minimal. As this distance decreases, the rate of sublimation increases.
Rice. 81. Devices (a, b) for sublimation with condensation of vapors on cooled surfaces.
Rice. 82. The simplest device for sublimation: 1 - porcelain cup with a substance; 2 - glass funnel; 3- a circle of filter paper with holes; 4 - sand bath; 5 - cotton wool.
To sublimate small amounts of easily sublimated substances, a simple device can be used, consisting of a porcelain cup, a watch glass and an ordinary chemical funnel (Fig. 80). The sublimated substance is heated in a sand bath; the sublimate collects on the cold walls of the funnel, from where it should be periodically cleaned. To prevent the sublimation crystals from falling back into the cup, the substance is covered with a circle of filter paper or asbestos, with several holes punctured in it.
Rice. 82. A device for sublimation of small quantities of substances in a vacuum.
In many cases it is preferable to conduct condensation onto a cooled surface. Of all the devices proposed for this purpose, the simplest and at the same time providing the minimum distance to the condensation surface are the devices shown in Fig. 81.
A commonly used device for the sublimation of small quantities of substances in a vacuum is shown in Fig. 82. Its disadvantages include the need to periodically turn off the vacuum and disassemble the device to scrape off the sublimate.
In the vacuum sublimator shown in Fig. 83, the sublimate is collected in a horizontally located refrigerator with a fairly wide inner tube. To avoid premature condensation of the product, the flask with the sublimated substance is immersed up to the neck in a liquid bath heated to the required temperature. A small current of air or inert gas supplied to the flask through a capillary facilitates the effective removal of vapors from the evaporation surface, which dramatically increases the performance of the device.
To prevent the smallest particles of the substance from being carried away with the gas flow into the outlet neck of the flask, it is advisable to solder a porous glass partition, however, with a small gas flow, this measure is not necessary.
Depending on the properties of the substance being purified and its quantity, the design can be changed individual parts device without changing the principle of its operation. Thus, the shape of the flask and the method of heating it can be different. An externally cooled two-neck flask is very convenient as a condenser for the sublimation of large quantities of a substance.
Sublimation or sublimation is the process in which a crystalline substance, heated below its melting point, passes into a vapor state (bypassing the liquid state), and then settles on a cold surface in the form of crystals.
At atmospheric pressure at temperatures below T pl, only organic compounds with relatively high pressure vapor There are few of them; the vast majority of compounds sublimate only under reduced pressure.
Sublimation is an excellent method for purifying substances in cases where the contaminants have a different volatility than the compound itself (compounds with similar volatility will sublimate together) and replaces time-consuming and labor-intensive crystallization. Sublimation is easy to carry out even with very small amounts of substance with minimal losses. This method is especially convenient for the purification of quinones, polynuclear hydrocarbons, and substances that form solvates or hydrates.
The simplest device for sublimation at atmospheric pressure is a low glass without a spout with a thin layer of a substance intended for sublimation at the bottom. The glass is closed with a round-bottomed flask through which water flows. At high sublimation temperatures, the water in the flask may not be flowing.
Sublimation can also be carried out in a porcelain cup, closed with the wide end of a funnel, the diameter of which is slightly smaller than the diameter of the cup (Fig. 3a).
The narrow end of the funnel is loosely covered with cotton wool. To prevent the sublimate from getting back into the cup, it is covered with a sheet of filter paper with holes in it. The substance subjected to sublimation must be finely ground.
Even slight overheating can contribute to the rapid thermal decomposition of the sublimated substance.
This danger can be avoided by carrying out the sublimation in a vacuum. To create a vacuum, water-jet and oil pumps are used. The device for sublimation in vacuum is shown in Fig. 3b. The sublimated substance is placed at the bottom of a test tube into which a finger cooler is inserted. The distance between the bottom of the sublimator and the end of the refrigerator should be small, but sufficient so that the sublimated substance is not contaminated when the solid substance splashes.
Rice. 3. Device for sublimation (a), device for vacuum sublimation (b): 1 - glass with a ground section; 2 - a cap with a finger cooler, 3 - a tube for introducing water; 4 - pipe for water outlet; 5 - pipe for connection to a vacuum pump; 6 - sublimable substance
Usually it is about 1 cm. After evacuation, the sublimator is immersed in an oil bath and gradually heated until a film of sublimated substance forms on the surface of the refrigerator. Upon completion of sublimation, first turn off the vacuum and remove the refrigerator. The sublimated substance is scraped onto a watch glass.
Lesson 8 CHEMICAL REAGENTS AND METHODS FOR THEIR PURIFICATION
Meaning of the theme
Analysis in a laboratory is impossible without the use of chemicals called reagents. The number of different substances used in the analysis is huge. Knowledge of the properties of reagents, the rules for their storage and work with them is necessary in the everyday work of medical laboratory technician. The laboratory may not have a reagent of the required purity. In addition, many salts containing water of crystallization lose some of this water during storage. Hygroscopic substances absorb water vapor from the air during storage. Reagents such as alcohol, benzene, ether contain more or less water. In all these cases, the reagents are purified.
know:
Classification of chemical reagents;
Rules for storage and use of chemical reagents;
Methods for purifying chemical reagents from impurities;
Distiller structure, operating rules.
be able to:
Purify chemical reagents by sublimation and recrystallization;
Demonstrate the operation of the distiller.
Chemical reagents (chemical reagents, or chemical reagents) are chemical substances that are used for analysis in research, laboratory work. In theory, it would be great to use absolutely pure chemical reagents (consisting of one type of particle) for research, but in practice, a pure reagent is considered to be a substance that contains the smallest amount of impurities, which can be achieved with modern development science and technology. Thus, all chemical reagents can be classified according to their degree of purity.
CLASSIFICATION OF REAGENTS
By degree of purity
Characteristic
color of the stripe on the label
Technical
those.
The content of the main substance is from 70%. Such reagents contain many impurities and are used to perform auxiliary work.
light brown
Purified
h.
content of the main substance is from 98%. Such reagents contain only 2% impurities.
green
Clean for analysis
ch.d.a.
content of the main substance is about 99%, % depends on the scope of application. With the help of such reagents, precise analytical studies are carried out. Reagents contain 0.5-1% impurities.
blue
chemically pure
reagent grade
The content of the main component is 99% or higher. They contain no more than 0.001-0.00001% impurities.
red
For special purposes:
These include substances of high purity. The content of the main component is almost 100%. The impurity content is 10 -5 -10 -10 %.
Spectrally pure
s.p.h.
Reference purity
e.h.
Brown
Extra clean
o.ch.
Yellow
By use
Commonly used indicators
Dyes for microscopy,
Dyes for chromatography,
Reagents for disinfection.
III. By properties
A) Hygroscopic (moisture-sensitive) reagents. Moisture absorption can Moisture absorption can occur when the reagent is not hermetically sealed and can lead not only to wetting of the substance, but also to a change in its properties.
B)Photosensitive reagents. Some substances change under the influence of light, entering into reactions of oxidation, reduction, isomerization, etc.
IN)Flammable reagents. These include compounds that are capable of spontaneously igniting from short-term contact with an ignition source (spark, flame, filament).
G)Poisonous reagents. Many chemicals are more or less toxic. Particularly dangerous is the systematic entry into the human body over a long period of time of compounds that cause chronic poisoning (compounds of mercury, arsenic, hydrocyanic acid, menthol, etc.). Even compounds that are used in large quantities every day can be toxic. Work with such substances only in a fume hood.
Examples of reagents belonging to different groups
Reagent groupsExamples of reagents
Hygroscopic
reagents
potassium and sodium hydroxides, ammonium chloride, acid anhydrides, etc.
Photosensitive reagents
solution of iodine, hydrogen peroxide, silver compound.
Fire hazardous
reagents
highly flammable liquids (alcohol, acetone, benzene, ethers, etc.)
Poisonous reagents
compounds of mercury, arsenic, hydrocyanic acid, menthol, etc.
Chemical Reagent Labels
All chemicals in the laboratory must be labeled.
The substance cannot be stored without a label!
According to GOST 3885-73, reagents (drugs) must be packaged in appropriate consumer containers, hermetically sealed and provided with a standard label.
For reagents of each classification, the label on the container must be a certain color or have a colored stripe on it.
If the reagents have toxic, flammable, or explosive properties, a separate label is affixed with an inscription of a certain color.
Certain substances are marked on labels with the following drawings:
Ways to write labels:
Printed labels
Universal with adhesive tape
Temporary (pencil on glass)
Oil paints or varnish
Hydrofluoric acid vapors – “eternal labels”.
Rules for storing chemical reagents
Small stocks of chemical reagents should be stored in the laboratory room. They are kept in jars, flasks with ground glass stoppers or plastic polyethylene lids, and the most volatile ones (hydrochloric acid, ammonia solution, bromine) are kept on special shelves in a fume hood. The total supply of flammable liquids simultaneously stored in each working room of the laboratory should not exceed daily requirements. Bottles containing more than 50 ml. Flammable liquids should be stored in iron fuel boxes with a tight-fitting lid, with walls and bottom lined with asbestos. Light-sensitive reagents are stored in dark bottles or jars wrapped in black paper. Strong poisons should be stored in sealed cabinets and safes. Reagents may only be stored in specially equipped and well-ventilated rooms, in strict order. The joint storage of reagents that can interact with each other, for example, oxidizing and reducing agents, acids and alkalis, is not permitted.
The following groups of reagents should be stored separately:
Explosives,
Flammable and liquefied gases,
Spontaneously combustible or self-igniting substances,
Poisons.
Reagents that do not require special conditions storage, placed on racks. Inorganic substances are arranged according to the well-known classification: simple substances(metals, non-metals), oxides, bases, salts. It is better to arrange salts by cations. Acids are stored separately. It is convenient to arrange organic substances alphabetically. Norms and rules for storing reagents are developed and approved separately in each organization, depending on the specifics of the work, the availability of equipment and storage facilities.
When storing chemicals, the choice of stopper is important. . The following should be remembered about traffic jams and their handling:
The choice of stopper for chemical glassware depends on the reagent. Select a plug:
First you need to select a stopper for the vessel, and only then place substances into it. Plugs from different vessels should not be confused; Each vessel must have its own stopper, this especially applies to glass stoppers.
If a vessel with a ground-in stopper is empty, then be sure to put a piece of paper between the neck and the stopper.
If you need to close a vessel with an acid or alkali with a cork stopper, then the stopper should first be processed.
It is impossible to store alkalis in vessels with ground-in stoppers, since in this case the stopper will inevitably “get stuck”.
Rules for using reagents
1. The main requirement for reagents is their purity. The reagent must be protected from contamination.
2. Do not pour or drain the reagent from the vessel in which the reaction is carried out back into the storage vessel.
3. Do not confuse container caps with different reagents, or store reagents without caps. It is necessary to strictly consider which stopper to close the bottles or flasks with. Rubber stoppers should not be used to close bottles containing reagents such as gasoline, kerosene, benzene, toluene and other liquid hydrocarbons, as well as dichloroethane, ether, etc., the vapors of which cause the rubber to swell and soften.
4. Do not take the reagent with your hands.
5. Cans with volatile substances must be opened at the time of direct use.
6. Work with toxic, bad-smelling, flammable substances is carried out in a fume hood.
7. If it is necessary to determine the smell, carefully direct the vapor of the substance with your hand from the vessel towards yourself.
8. Poisonous and caustic reagents should be poured into special bottles after work.
Rules for working with acids and alkalis
All concentrated solutions must be stored in special bottles with ground-in stoppers, over which a ground-in cap must be placed. It is recommended to store alkalis in wide-mouth dark orange glass jars, closed with cork or polyethylene stoppers and filled with a layer of paraffin.
Acids and alkalis should be stored on the lower shelves of cabinets separately from reagents and paints.
Dishes for storing toxic substances, alkalis and acids must have clear inscriptions (in ink on glass or in another way).
Bixes, jars, bottles with volatile substances must be opened only at the time of direct use.
5. Open vessels with concentrated acidsand alkalis and volatile substances and preparing solutions from them is allowed only in a fume hood with forced ventilation turned on.
Alkalis should be taken from the jar with a spatula.
Bottles with acids, alkalis and other caustic substances should be carried by two people in special boxes or baskets or transported on a special trolley.
When diluting strong acids, the acid should be poured into water, and not vice versa.
When working with acids and alkalis, do not suck liquid into the pipette with your mouth. Rubber bulbs with tubes should be used to collect liquid.
Solutions for neutralizing concentrated acids and alkalis should be on the rack (shelf) throughout the working day.
To avoid hand burns, dishes containing solutions of caustic substances should be washed using rubber gloves.
Safety precautions when working with chemicals
Experiments with toxic and bad-smelling substances are carried out in a fume hood.
To determine the smell of a gas or liquid, carefully inhale the air, slightly directing the fumes with your hand from the vessel towards you.
When pouring reagents, do not lean over the vessel to avoid splashes on your face and clothes.
All experiments with flammable substances are carried out in a fume hood.
METHODS FOR PURIFYING CHEMICAL REAGENTS
If a chemical reagent of a certain purity is not available in the laboratory, it must be further purified. The most common cleaning methods are:
filtration,
centrifugation,
recrystallization,
distillation (distillation),
sublimation (sublimation),
absolutization (drying).
Cleaning by decanting
Decantation - This is the settling of solid particles contained in a liquid under the influence of gravity. After decanting, the clarified liquid is separated from the sediment of solid particles; this removes impurities. The advantage of the method is its simplicity, but the disadvantage is the slow settling of small particles. The separation of a mixture of liquid and solid particles by centrifugation occurs much faster.
Cleaning by centrifugation
Centrifugationbased on the use of centrifugal force that occurs during rapid rotation. In ordinary laboratory centrifuges, the rotation speed is about 1000 rpm, and in special ones (ultracentrifuges) – up to 6000 rpm. The artificial force of gravity in centrifuges exceeds gravity tens to hundreds of thousands of times, as a result of which solid particles settle in a few minutes.
Cleaning by filtration
Filtration consists of passing the suspension through a porous partition - a filter that retains particulate matter. The filter can be special paper, fabric, porous ceramics, porous glass, a layer of sand and other porous materials. Under normal conditions, filtration is slow. To speed it up, it is carried out under vacuum: a vacuum is created in the liquid receiver using a pump, as a result of which atmospheric pressure begins to act on the liquid above the filter, and the greater the pressure difference (atmospheric and in the receiver), the faster the filtration occurs.
Purification by recrystallization
Recrystallization is used to purify various soluble salts and many organic solids. Recrystallization is one of the most common methods for purifying and separating crystalline substances. This method is based on the different solubility of a substance in a cold and hot solvent and on the different solubility of the components of a mixture in the same solvent.
The recrystallization process includes several stages:
1. Choice of solvent. The choice is made empirically. The solvent must meet the following requirements:
1) do not interact with the substance,
2) do not dissolve the substance at room temperature and dissolve well when heated,
3) when the hot solution is cooled, crystals should fall out,
4) dissolve impurities well at room temperature or do not dissolve them when boiling,
5) the boiling point of the solvent should be 10-15ºС lower than the melting point of the substance,
6) the solvent should be easily removed from the surface of the crystals by washing and drying.
2. Cooking rich at the boiling point of the solventsolution .
3. Filtration of hot solution through a folded filter to get rid of mechanical impurities.
4. Cooling the solution causing crystallization . Cooling is carried out at such a speed that medium-sized crystals fall out. Typically the solution is left to stand at room temperature for 20-30 minutes and then placed in an ice bath. If the crystals do not fall out, then you can place a crystal of this substance (“seed”) into the solution or rub it with a glass rod on the inner wall of the glass with the solution.
5. Separation of crystals from mother liquor (filtration under reduced pressure).
6. Washing crystals with cold solvent . If the substance is practically insoluble at room temperature, then the crystals can be washed with a solvent at room temperature.
7. Drying crystals . Crystals are usually dried in air or in a vacuum crystallizer.
Installations for carrying out all stages of recrystallization are shown in Figure No. 4
1- Installation for preparing a saturated solution (a – round-bottomed flask, b – reflux condenser, c – hotplate)
2- Installation for hot filtration (a – glass, b – chemical funnel, c – folded filter)
3- Installation for filtration under reduced pressure (a – Schott filter, b – Bunsen flask)
Purification by distillation or distillation
Distillation or distillation - one of the most important methods for purifying liquids. During distillation, a liquid is converted into a vapor state by heating, then condensed again, i.e., turned into a liquid. In this case, all solid impurities and higher-boiling liquid impurities remain in the flask, and lower-boiling impurities are distilled off before the main liquid. Distillation purifies water and other liquids. A funnel with a long tube is inserted into the Wurtz flask (1) and the liquid to be distilled is carefully poured, several capillaries with one sealed end are thrown in (the sealed end must be above the liquid), this is necessary for uniform boiling. Close the neck of the flask with a stopper with a thermometer (2). After this, place the distillate receiver (5) and begin heating.
Care must be taken during distillation. So that the liquid boils evenly and does not boil. The distillation should not take place too quickly. As soon as the liquid boils, carefully monitor the thermometer readings. The first small portion of distillate is impurities. When the thermometer readings correspond to the boiling point of the substance being distilled, another receiver is substituted into which the substance being distilled is collected. The distillation is completed when a small amount of liquid remains in the Wurtz flask. Dry distillation is not permitted.
In the laboratory, great importance is attached to the distillation of water, since all solutions are prepared only with distilled water. It is consumed in large quantities for other purposes. Distillers are used in laboratories to obtain distilled water.
Purification by sublimation method.
Some solids, such as iodine, have the ability to turn into a solid state when heated without melting. This phenomenon is called sublimation or sublimation. Sublimation is used to purify substances from non-volatile impurities. This method can be used to purify iodine, ammonium chloride, sulfur, etc. To purify small quantities of a substance by sublimation, use two watch glasses of the same size, ground to each other. A sublimated substance is placed on the bottom glass, and a circle of filter paper, perforated in several places, is clamped between the glasses, the purpose of which is to prevent the formation of crystals from falling onto the lower heated glass. The bottom glass is heated in a sand bath or very carefully, with a small flame, on an asbestos mesh; The top glass is cooled with a piece of damp filter paper.
Sublimation of large quantities of the substance is carried out in a glass heated in an oil or air bath. Cooled from the inside, the glass is lowered running water a flask on the surface of which crystals of a sublimated substance settle.
Dehydration of organic reagents.
When working in a laboratory, you often have to purify various solvents (alcohol, ether, benzene, etc.). All these reagents contain water in varying quantities, the presence of which can interfere with work. Therefore, these reagents are dried before distillation. Liquids purified in this way are called absolute. Since organic reagents have different properties, the methods for drying them are different.
Absolutization of alcohol.
To dry the alcohol, place dehydrated copper sulfate CuSO4 into a round-bottomed flask and pour in the alcohol. The flask is connected to a reflux condenser, which is closed with a stopper with a calcium chloride tube. Calcined calcium chloride is placed in a calcium chloride tube to absorb water vapor from the air. The device is installed in a water bath and boiled for 6-8 hours. At the end of boiling, the reflux condenser is replaced with a Liebig condenser and the alcohol is distilled into a clean flask. During distillation, the device is carefully protected from air moisture.
Absolutization of benzene.
Calcined calcium chloride is placed in benzene, sealed and allowed to stand for 24 hours. Filter and add finely chopped, well cleaned from kerosene and oxide film sodium metal. Assemble the device with a reflux condenser and boil for 3-4 hours in a sand bath. After this, benzene is distilled over sodium, carefully protecting it from air moisture. It is strictly forbidden to heat benzene with sodium metal in a water bath or gas burner. Absolutization of the ether. Ether that has been stored for a long time may contain impurities of dioxethyl peroxide. Therefore, first of all, the ether is shaken vigorously in a separating funnel with a concentrated solution of sodium or potassium hydroxide. The ether separated from the alkali is shaken in a separating funnel with an equal portion of water and separated from the water. After washing the ether with water, calcined calcium chloride is added to it and allowed to stand for 24 hours. Then the ether is filtered, finely chopped sodium metal is added, refluxed, as when dehydrating benzene, and distilled by heating in a sand bath.
Questions for self-study:
1. What groups are chemical reagents divided into according to their properties? Give examples.
2. Features of storage of various groups of chemical reagents?
3. Name the basic rules for using chemical reagents.
4. How should you select stoppers for storing different chemical reagents?
5. What do the symbols on chemical reagent labels mean?
6. Methods for purifying chemical reagents.
Prepare a summary on the topic “Types of distillation. Conditions"
Test “Chemical reagents. Cleaning methods."
1. Brand of reagent in which the impurity content does not exceed 0.5-1%
a) reagent b) reagent grade c) ch.d.a. d) tech.
2. Sodium hydroxide, potassium hydroxide, calcium oxide belong to the group of substances
a) hygroscopic b) photosensitive c) flammable d) poisonous
3. Properties of compounds of mercury, arsenic, hydrocyanic acid, methanol
a) hygroscopic b) photosensitive c) fire hazardous d) poisonous
4. Toxic substances are stored
a) in a fume hood b) in a sealed cabinet or safe c) in an iron box along with flammable liquid d) on shelves in a laboratory cabinet
5. Method for purifying potassium iodide from iodine crystals
a) distillation b) sublimation c) distillation d) recrystallization
6. Method for purifying sodium nitrite from soluble chemical impurities
a) recrystallization b) sublimation c) filtration d) distillation
7.Purification method for obtaining distilled water
a) distillation b) sublimation c) recrystallization d) filtration
8. Flask used for distillation of liquids
a) conical b) Wurtz c) round-bottomed d) dimensional e) flat-bottomed
9. Method of purification of solid reagents
a) recrystallization b) filtration c) distillation d) centrifugation e) precipitation
10. Method of purification of liquid reagents
a) recrystallization b) sublimation c) distillation d) centrifugation e) precipitation
Situational tasks
1. During general cleaning, the laboratory assistant accidentally spilled potassium iodide reagent and iodine. Create a method for purifying potassium iodide containing mechanical impurities and iodine crystals.
2. Create a method for purifying sodium chloride containing mechanical impurities and sodium sulfate.
3. The laboratory received the potassium hydroxide reagent (tech.). For laboratory research, it is necessary to purify the reagent. Create a method for purifying KOH, which contains mechanical impurities.
4. Create a method for purifying sodium nitrate containing mechanical and chemical impurities.
Sample answers to problems
1. First, iodine crystals are isolated from the mixture by sublimation. The potassium iodide reagent and mechanical impurities are dissolved in water (a saturated solution is prepared), the mechanical impurities are filtered off and the potassium iodide crystals are evaporated.
2. A mixture of sodium chloride, sodium sulfate and mechanical impurities are dissolved in water. To do this, prepare a hot saturated solution of sodium chloride with impurities, filter and cool.
3. To purify the potassium hydroxide reagent from mechanical impurities, filtration is used. To do this, the mixture is dissolved in water, filtered and then the crystals are evaporated.
4. A mixture containing mechanical and chemical impurities is dissolved in water. To do this, prepare a hot saturated solution, filter and then cool.
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 tap, 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 cabinet 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 are condensed, and impurities that cannot sublimate are separated. Substances such as crystalline iodine, ammonium chloride (ammonia), and naphthalene easily sublimate. 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. Often used for purification of organic substances various types distillation: 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 the receiver through the refrigerator.
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 cleaning the connection, you can begin qualitative analysis. To determine the composition of an organic substance, 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.