The vast majority of information about substances, their properties and chemical transformations was obtained using chemical or physicochemical experiments. Therefore, the main method used by chemists should be considered a chemical experiment.
The traditions of experimental chemistry have evolved over the centuries. Even when chemistry was not an exact science, in ancient times and in the Middle Ages, scientists and artisans sometimes accidentally, and sometimes purposefully, discovered ways to obtain and purify many substances that were used in economic activity: metals, acids, alkalis, dyes and etc. Alchemists contributed a lot to the accumulation of such information (see Alchemy).
Thanks to this, by the beginning of the 19th century. chemists were well versed in the basics of experimental art, in particular the methods of purification of various liquids and solids, which allowed them to make many important discoveries. Nevertheless, chemistry began to become a science in the modern sense of the word, an exact science, only in the 19th century, when the law of multiple ratios was discovered and the atomic-molecular theory was developed. Since that time, the chemical experiment began to include not only the study of the transformations of substances and methods of their isolation, but also the measurement of various quantitative characteristics.
A modern chemical experiment includes many different measurements. The equipment for setting up experiments and chemical glassware have also changed. In a modern laboratory, you will not find homemade retorts - they have been replaced by standard glass equipment produced by industry and adapted specifically for performing a particular chemical procedure. Work methods have also become standard, which in our time no longer have to be reinvented by every chemist. Description of the best of them, proven by many years of experience, can be found in textbooks and manuals.
Methods for studying matter have become not only more universal, but also much more diverse. An increasing role in the work of a chemist is played by physical and physicochemical research methods designed to isolate and purify compounds, as well as to establish their composition and structure.
The classical technique for purifying substances was extremely labor intensive. There are cases when chemists spent years of work on the isolation of an individual compound from a mixture. Thus, salts of rare earth elements could be isolated in pure form only after thousands of fractional crystallizations. But even after that, the purity of the substance could not always be guaranteed.
The sophistication of technology has reached such a high level that it has become possible to accurately determine the rate of even "instantaneous", as previously believed, reactions, for example, the formation of water molecules from hydrogen cations H + and anions OH - . With an initial concentration of both ions equal to 1 mol/l, the time of this reaction is several hundred-billionths of a second.
Physicochemical research methods are also specially adapted for the detection of short-lived intermediate particles formed in the course of chemical reactions. To do this, the devices are equipped with either high-speed recording devices or attachments that ensure operation at very low temperatures. Such methods successfully capture the spectra of particles whose lifetime under normal conditions is measured in thousandths of a second, such as free radicals.
In addition to experimental methods, calculations are widely used in modern chemistry. Thus, the thermodynamic calculation of a reacting mixture of substances makes it possible to accurately predict its equilibrium composition (see Fig.
TOPIC 1. Forced slaughter, the procedure for its implementation and veterinary sanitary examination of forced slaughter meat
The goal is to learn the procedure for performing forced slaughter of animals, conducting a veterinary sanitary examination of slaughter products and their use.
1. To study and assimilate the procedure for performing forced slaughter of animals, conducting veterinary sanitary examination and using slaughter products, established by the "Rules for the veterinary inspection of slaughtered animals and the veterinary and sanitary examination of meat and meat products". Prepare and give answers to control questions:
1) What is meant by forced slaughter of animals, in what cases is slaughter not considered forced and when is it forbidden to subject animals to forced slaughter?
2) The procedure for registration and conduct of forced slaughter and veterinary sanitary examination of slaughter products.
3) The procedure for sampling and issuing an accompanying document when sending the material to a veterinary laboratory for bacteriological and other studies.
4) By what organoleptic characteristics are carcasses obtained from animals that died or were in an agonal state detected?
5) What laboratory research methods are used to detect meat obtained from animals that have died or were in a state of agony, and what is their essence?
6) The procedure for the delivery of forced slaughter meat to meat processing enterprises for neutralization and processing.
7) The procedure for acceptance, examination of forced slaughter meat at a meat processing enterprise, its neutralization and processing.
2. Perform laboratory tests on forced slaughter meat samples in order to identify the fact of obtaining meat from an animal that has died or was in a state of agony
a) Perform a reaction for peroxidase.
b) React with formalin.
c) Carry out a bacterioscopic examination of meat samples.
d) Determine the pH of meat by colorometric and potentiometric research methods.
e) Examine the meat samples of the cooking test.
f) On the basis of the performed studies, give a conclusion on the suitability or unsuitability of meat for food purposes.
The procedure for the forced slaughter of animals and the study of meat in accordance with the "Rules for the veterinary examination of slaughter animals and the veterinary and sanitary examination of meat and meat products"
In case of forced slaughter of animals at a meat processing plant, slaughterhouse, on farms due to illness or other reasons that threaten the life of an animal, as well as in cases requiring long-term, economically unjustified treatment, a veterinary and sanitary examination of meat and other slaughter products is carried out in the usual manner. . In addition, it is mandatory to carry out a bacteriological and, if necessary, physico-chemical examination, but with a mandatory test of cooking to identify foreign odors that are unusual for meat.
Forced slaughter of animals is carried out only with the permission of a veterinarian (paramedic).
Ante-mortem holding of animals delivered to a meat processing plant for forced slaughter is not carried out.
An act signed by a veterinarian must be drawn up on the reasons for the forced slaughter of animals on farms. This act and the conclusion of the veterinary laboratory on the results of a bacteriological examination of the carcass of a forcedly slaughtered animal, together with a veterinary certificate, must accompany the specified carcass upon delivery to the meat processing plant, where it is repeatedly subjected to bacteriological examination.
In case of suspected poisoning of an animal with pesticides and other pesticides, it is necessary to have a conclusion from a veterinary laboratory on the results of meat testing for the presence of pesticides.
Transportation of meat of forcedly slaughtered animals from farms to meat industry enterprises must be carried out in compliance with the current veterinary and sanitary rules for the transportation of meat products.
In order to ensure the correct examination of the meat of forcibly slaughtered sheep, goats, pigs and calves, it must be delivered to the meat processing plant in whole carcasses, and the meat of cattle, horses and camels - in whole carcasses, half carcasses and quarters and placed in a separate refrigerator. Half carcasses and quarters are tagged to establish that they belong to the same carcass.
The carcasses of pigs forcedly killed on farms must be delivered to the meat processing plant with their heads not separated.
When delivering the meat of animals forcedly killed on farms to the meat processing plant, in salted form, each barrel must contain corned beef from one carcass.
Carcasses of animals forcedly slaughtered en route without a pre-slaughter veterinary examination, delivered to a meat processing plant without a veterinary certificate (certificate), a veterinary act on the reasons for forced slaughter and a conclusion of a veterinary laboratory on the results of a bacteriological examination, are prohibited to be accepted to the meat processing plant.
If, according to the results of the examination, bacteriological and physico-chemical studies, meat and other products of forced slaughter are found suitable for use in food, then they are sent for boiling, as well as for the manufacture of meat loaves or canned food "Goulash" and "Meat Pate".
The release of this meat and other slaughter products in raw form, including in the public catering network (canteens, etc.), without prior disinfection by boiling is prohibited.
Note. Cases of forced slaughter do not include:
slaughter of clinically healthy animals that are not fattenable to the required conditions, lagging behind in growth and development, unproductive, barren, but having a normal body temperature; slaughter of healthy animals that are threatened with death as a result of a natural disaster (snow drifts on winter pastures, etc.), as well as those injured before slaughter at a meat processing plant, slaughterhouse, slaughterhouse; forced slaughter of livestock at meat-packing plants is carried out only at a sanitary slaughterhouse.
Sampling, packaging and sending samples to a veterinary laboratory According to the above-mentioned rules of veterinary sanitary examination, depending on the alleged diagnosis and the nature of pathological changes, the following are sent for bacteriological examination:
part of the flexor or extensor muscle of the fore and hind limbs of the carcass, covered with fascia at least 8 cm long, or a piece of other muscle at least 8x6x6 cm in size;
lymph nodes - from cattle - superficial cervical or actually axillary and external iliac, and from pigs - superficial cervical dorsal (in the absence of pathological changes in the head and neck) or axillary of the first rib and patella;
spleen, kidney, liver lobe with a hepatic lymph node (in the absence of a lymph node - a gallbladder without bile).
When taking part of the liver, kidney, and spleen, the surface of the incisions is cauterized until a scab forms.
When examining half carcasses or quarters of carcasses, a piece of muscle, lymph nodes and tubular bone are taken for analysis.
When examining the meat of small animals (rabbits, nutria) and poultry, whole carcasses are sent to the laboratory.
When examining salted meat in a barrel container, meat samples and existing lymph nodes are taken from the top, from the middle and from the bottom of the barrel, and also, if available, tubular bone and brine.
If erysipelas are suspected, in addition to muscles, lymph nodes and internal organs, a tubular bone is sent to the laboratory.
For bacteriological examination, the brain, liver lobe and kidney are directed to listeriosis.
If anthrax, emkar, malignant edema are suspected, the lymph node of the affected organ or the lymph node collecting lymph from the location of the suspicious focus, edematous tissue, exudate, and in pigs, in addition, the mandibular lymph node are sent for research.
The samples taken for research with the accompanying document are sent to the laboratory in a moisture-proof container, sealed or sealed. When sending samples for research to the production laboratory of the same enterprise where the samples were taken, there is no need to seal or seal them. The accompanying document indicates the type of animal or product, their identity (address), what material was sent and in what quantity, the reason for sending the material for research, what changes were found in the product, the proposed diagnosis and what kind of research is required (bacteriological, physico-chemical, etc.). .d.).
Methods for determining the meat of forced slaughter - sick, killed in agony or dead animals
Pathological anatomical and organoleptic examination When determining meat from a sick animal killed in an agonal state or a fallen animal, the following external signs must be taken into account: the state of the slaughter site, the degree of bleeding, the presence of hypostases and the color of the lymph nodes on the cut.
The state of the slaughter site . Under the slaughter is understood the place of transection of blood vessels during the slaughter of an animal. To create the appearance of a normally slaughtered animal, owners often make neck incisions in dead animals, rub blood into the incision site, hang them by their hind limbs for better blood flow, etc.
There are the following differences between the intravital and postmortem incisions: the intravital incision is uneven due to muscle contraction, the tissues in the area of the incision are infiltrated (impregnated) with blood to a greater extent, compared with deeper ones. The incision made after the death of the animal is more even, the blood almost does not impregnate the tissues, the blood on the surface of the tissues is easily washed off with water. The tissues do not differ in the degree of blood infiltration in the area of the incision from the tissues located deeper.
The degree of bleeding of the carcass . Carcasses obtained from sick animals, and especially from animals that were in an agonal state, or fallen, are poorly or very poorly bled. Carcasses are dark red in color, small and large blood vessels filled with blood are found on cuts. Intercostal vessels look like dark veins. If you separate the shoulder blade from the carcass, you can find vessels filled with blood.
If you put a strip of filter paper (10 cm long and 1.5 cm wide) into a fresh cut and leave it there for several minutes, then in case of poor bleeding, not only the part of the paper that comes into contact with the meat, but also the free its end (this method is not acceptable for thawed meat), adipose tissue has a pink or reddish color.
With good bleeding, the meat is crimson or red, the fat is white or yellow, there is no blood on the section of the muscles. The vessels under the pleura and peritoneum are not translucent, the intercostal vessels look like light strands.
The color of the lymph nodes on the cut. Lymph nodes on the cut in the carcasses of healthy animals and cut up in a timely manner have a light gray or yellowish color. In the meat of animals seriously ill, killed in an agonal state, or fallen, the lymph nodes on the cut have a lilac-pink color. In addition, depending on the diseases in the lymph nodes, their increase, various forms of inflammatory processes, hemorrhages, necrosis, hypertrophy will be detected.
The presence of hypostases . Under hypostases understand the post-mortem and pre-mortem with prolonged agony redistribution (drainage) of blood to the underlying parts of the body. The tissues on the side of the body on which the sick animal lay were saturated with blood to a greater extent. The same is observed on paired organs (kidneys, lungs). Hypostasis should not be confused with bruising. Bruising occurs in the subcutaneous tissue as a result of a violation of the integrity of blood vessels due to bruises. They are local and superficial in nature, and hypostases are diffuse (diffuse) and, with hypostases, the deep layers of tissues are also infiltrated with blood. Hypostases can form not only after the death of the animal, but even during life. They can form during prolonged agony, when the animal's cardiac activity is weakened and the blood gradually stagnates in the underlying parts of the body. Thus, the detection of hypostases indicates that the meat was obtained from a fallen animal that had lain uncut for a certain time, or from an animal that was in a state of prolonged agony. If the animal was in an agonal state for a short time and was slaughtered, then hypostases may be absent. Therefore, the absence of hypostases is not yet an indicator that the meat was obtained from a non-agonizing animal.
Finding out the fact of obtaining meat from animals that were in an agonal state or fallen is of fundamental importance, since such meat is dangerous to human health and, according to veterinary legislation, is not allowed for food and must be disposed of or destroyed.
Boil test . Meat obtained from seriously ill, in a state of agony or dead animals can be detected to a certain extent using an organoleptic method, the so-called boil test. For this 20 gr. chopped meat to the state of minced meat is placed in a 100 ml conical flask, pour 60 ml. distilled water, mix, cover with a watch glass, put in a boiling water bath and heat to 80-85ºС, until vapors appear. Then the lid is slightly opened and the smell and condition of the broth are determined. The broth from the meat of seriously ill, agonizing or fallen animals, as a rule, has an unpleasant or medicinal smell, it is cloudy with flakes. Conversely, the broth from the meat of healthy animals has a pleasant specific meat smell and is transparent. Tasting is not recommended.
Physical and chemical studies
According to the "Rules for the Veterinary Examination of Animals and the Veterinary and Sanitary Expertise of Meat and Meat Products", in addition to pathological, organoleptic and bacteriological analysis, the meat of forced slaughter, as well as if it is suspected that the animal was in a state of agony before slaughter or was dead, must be subjected to physical and chemical research.
Bacterioscopy . Bacterioscopic examination of smears of imprints from the deep layers of muscles, internal organs and lymph nodes is aimed at preliminary (before obtaining the results of bacteriological examination) detection of pathogens of infectious diseases (anthrax, emphysematous carbuncle, etc.) and contamination of meat with opportunistic microflora (E. coli, Proteus and etc.).
The technique of bacterioscopic research is as follows. Pieces of muscles, internal organs or lymph nodes are cauterized with a spatula or immersed twice in alcohol and set on fire, then a piece of tissue is cut out from the middle with sterile tweezers, a scalpel or scissors and smears are made on a glass slide. Air dry, flambé over a flame and Gram stain. The drug is stained through filter paper with a solution of carbolic gentian violet - 2 min., the filter paper is removed, the paint is drained and without washing the drug is treated with Lugol's solution - 2 min., discolored with 95% alcohol - 30 sec., washed with water, stained with Pfeiffer fuchsin - 1 min. ., washed again with water, dried and microscoped under immersion. There is no microflora in smears-imprints from the deep layers of meat, internal organs and lymph nodes of healthy animals.
In diseases, bacilli or cocci are found in smears-imprints. A complete definition of the detected microflora can be determined in a veterinary laboratory, for which they are sown on nutrient media, a pure culture is obtained and it is identified.
pH determination . The pH value of meat depends on the content of glycogen in it at the time of slaughter of the animal, as well as on the activity of the intramuscular enzymatic process, which is called meat maturation.
Immediately after slaughter, the reaction of the environment in the muscles is slightly alkaline or neutral - equal to - 7. Already a day later, the pH of meat from healthy animals decreases to 5.6-5.8 as a result of the breakdown of glycogen to lactic acid. In the meat of sick or agonized animals, such a sharp decrease in pH does not occur, since the muscles of such animals contain less glycogen (used as an energy substance during illness), and, consequently, less lactic acid is formed and the pH is less acidic, t .e. higher.
The meat of sick and overworked animals is in the range of 6.3-6.5, and the agonizing or fallen 6.6 and above, it approaches neutral - 7. It should be emphasized that the meat must be aged for at least 24 hours before the study.
These pH values do not have an absolute value, they are indicative, auxiliary in nature, since the pH value depends not only on the amount of glycogen in the muscles, but also on the temperature at which the meat was stored and the time elapsed after the slaughter of the animal.
Determine the pH by colorimetric or potentiometric methods.
Colorimetric method. To determine pH, the Michaelis apparatus is used, which consists of a standard set of colored liquids in sealed test tubes, a comparator (stand) with six test tube sockets and a set of indicators in vials.
First, an aqueous extract (extract) is prepared from muscle tissue in a ratio of 1: 4 - one weight part of the muscles and 4 - distilled water. To do this, weigh 20 gr. muscle tissue (without fat and connective tissue) is finely chopped with scissors, rubbed with a pestle in a porcelain mortar, to which a little water is added from a total of 80 ml. The contents of the mortar are transferred to a flat-bottomed flask, the mortar and pestle are washed with the remaining amount of water, which is poured into the same flask. The contents of the flask are shaken for 3 minutes, then for 2 minutes. defend and again 2 min. shake. The extract is filtered through 3 layers of gauze, and then through a paper filter.
First, approximately determine the pH to select the desired indicator. To do this, pour 1-2 ml into a porcelain cup, extracts and add 1-2 drops of a universal indicator. The color of the liquid obtained by adding the indicator is compared with the color scale available in the kit. With an acid reaction of the medium, the indicator paranitrophenol is taken for further research, with a neutral or alkaline reaction, metanitrophenol. Test tubes of the same diameter made of colorless glass are inserted into the nests of the comparator and filled as follows: 5 ml are poured into the first, second and third test tubes of the first row, 5 ml of distilled water are added to the first and third, 4 ml of water are added to the second and 1 ml, indicator, 7 ml of water is poured into the 5th test tube (middle of the second row), standard sealed test tubes with colored liquid are inserted into the fourth and sixth slots, selecting them so that the color of the contents in one of them is the same as the color of the middle tubes in the middle row. The pH of the studied extract corresponds to the figure indicated on the standard test tube. If the shade of the color of the liquid in the test tube with the test extract is intermediate between the two standards, then take the average value between the values of these two standard test tubes. When using the micro-Michaelis apparatus, the number of reaction components is reduced by 10 times.
Potentiometric method. This method is more accurate, but difficult to perform in that it requires constant adjustment of the potentiometer to standard buffer solutions. A detailed description of the determination of pH by this method is available in the instructions attached to devices of various designs, and the pH value can be determined using potentiometers both in extracts and directly in muscles.
Reaction to peroxidase. The essence of the reaction is that the peroxidase enzyme in the meat decomposes hydrogen peroxide with the formation of atomic oxygen, which oxidizes benzidine. In this case, paraquinone diimide is formed, which, with unoxidized benzidine, gives a blue-green compound, turning into brown. Peroxidase activity plays an important role in this reaction. In the meat of healthy animals, it is very active, in the meat of the sick and those killed in agony, its activity is significantly reduced.
The activity of peroxidase, like that of any enzyme, depends on the pH of the medium, although there is no complete correspondence between the benzidine reaction and pH.
Progress of the reaction: pour 2 ml of meat extract (at a concentration of 1:4) into a test tube, add 5 drops of a 0.2% alcohol solution of benzidine and add two drops of a 1% hydrogen peroxide solution.
The extract from the meat of healthy animals acquires a blue-green color, turning brown-brown after a few minutes (positive reaction). In the extract from the meat of a sick or animal killed in an agonal state, a blue-green color does not appear, and the extract immediately acquires a brown-brown color (negative reaction).
Formol test (test with formalin). In case of severe diseases, even during the life of the animal, intermediate and final products of protein metabolism - polypeptides, peptides, amino acids, etc. - accumulate in the muscles in a significant amount.
The essence of this reaction is the precipitation of these products with formaldehyde. To set up the sample, an aqueous extract from meat is required in a ratio of 1:1.
To prepare an extract (1:1), a meat sample is freed from fat and connective tissue and weighed 10 g. Then the sample is placed with a mortar, carefully crushed with curved scissors, 10 ml are added. physiological saline and 10 drops of 0.1 N. sodium hydroxide solution. The meat is rubbed with a pestle. The resulting slurry is transferred with scissors or a glass rod into a flask and heated to boiling to precipitate the proteins. The flask is cooled under a stream of cold water, after which its contents are neutralized by adding 5 drops of a 5% oxalic acid solution and filtered through filter paper. If the extract remains cloudy after filtration, it is filtered a second time or centrifuged. If you need to get more extract, take 2-3 times more meat and, accordingly, 2-3 times more other components.
Commercially produced formalin has an acidic environment, so it is preliminarily neutralized with 0.1 N. sodium hydroxide solution according to the indicator, consisting of an equal mixture of 0.2% aqueous solutions of neutrality and methylene blue until the color changes from purple to green.
Reaction course: 2 ml of extracts are poured into a test tube and 1 ml of neutralized formalin is added. The extract obtained from the meat of an animal killed in agony, seriously ill or fallen turns into a dense jelly-like clot. In the extract from the meat of a sick animal, flakes fall out. The extract from the meat of a healthy animal remains liquid and transparent or becomes slightly cloudy.
Sanitary assessment of meat
According to the Rules for Veterinary Inspection of Slaughter Animals and Veterinary and Sanitary Examination of Meat and Meat Products, meat is considered to be obtained from a healthy animal in the presence of good organoleptic indicators of the carcass and the absence of pathogenic microbes.
The organoleptic characteristics of the broth during the cooking test (color, transparency, smell) correspond to fresh meat.
The meat of sick animals, as well as those killed in a state of agony, has insufficient or poor bleeding, lilac-pink or bluish color of the lymph nodes. The presence of pathogenic microflora in the meat is possible. When cooking is sampled, the broth is cloudy, with flakes, it may have an extraneous smell that is not characteristic of meat. Additional indicators in this case can also be a negative reaction to peroxidase, pH - 6.6 and higher, and for cattle meat, in addition, positive reactions: formol and with a solution of copper sulfate, accompanied by the formation of flakes or a jelly-like clot in the extract. Moreover, before determining the pH, setting the reaction to peroxidase, formal and with a solution of copper sulfate, the meat must be subjected to maturation for at least 20-24 hours.
If, according to the results of the examination, bacteriological and physico-chemical studies, meat and other products of forced slaughter are found suitable for use in food, then they are sent for boiling, according to the regime established by the Rules, as well as for the manufacture of meat loaves or canned food "Goulash" and " Meat pate.
The release of this meat and other slaughter products in raw form, including into the public catering network (canteens, etc.), without preliminary disinfection by inspection, is prohibited.
The procedure for processing meat and meat products subject to disinfection
According to the Rules of Veterinary Sanitary Expertise, meat and meat products of forced slaughter are disinfected by boiling in pieces weighing no more than 2 kg, up to 8 cm thick in open boilers for 3 hours, in closed boilers at an excess steam pressure of 0.5 MPa for 2.5 hours.
The meat is considered disinfected if the temperature inside the piece reaches at least 80ºС; the color of pork on the cut becomes white-gray, and the meat of other animal species is gray, without signs of a bloody tint; juice flowing from the cut surface of a piece of boiled meat is colorless.
At meat-packing plants equipped with electric or gas ovens or having canning shops, meat subject to disinfection by boiling is allowed to be sent to the production of meat loaves. When processing meat into meat loaves, the mass of the latter should not exceed 2.5 kg. Bread baking should be carried out at a temperature not lower than 120ºС for 2-2.5 hours, and the temperature inside the product by the end of the baking process should not be lower than 85ºС.
For the manufacture of canned food, meat is allowed that meets the requirements for raw materials for canned food - "Goulash" and "Meat pate".
Chemical analysis of the studied substances is carried out using chemical, physical and physico-chemical methods, as well as biological ones.
Chemical methods are based on the use of chemical reactions accompanied by a visual external effect, such as a change in the color of the solution, dissolution or precipitation, gas evolution. These are the simplest methods, but not always accurate; based on one reaction, it is impossible to accurately determine the composition of a substance.
Physical and physico-chemical methods, in contrast to chemical ones, are called instrumental, since analytical instruments and apparatuses are used for analysis that record the physical properties of a substance or changes in these properties.
Physical analysis does not use a chemical reaction, but measures some physical property of a substance that is a function of its composition. For example, in spectral analysis, the emission spectra of a substance are studied and, by the presence in the spectrum of lines characteristic of these elements, their presence is determined, and their quantitative content is determined by the brightness of the lines. When a dry substance is introduced into the flame of a gas burner, the presence of some components can be established, for example, potassium ions will color a colorless flame purple, and sodium ions yellow. These methods are accurate but expensive.
When conducting an analysis by the physicochemical method, the composition of a substance is determined based on the measurement of a physical property using a chemical reaction. For example, in a colorimetric analysis, the concentration of a substance is determined by the degree of absorption of a light flux passing through a colored solution.
Biological methods of analysis are based on the use of living organisms as analytical indicators for determining the qualitative or quantitative composition of chemical compounds. The most famous bioindicator is lichens, which are very sensitive to the content of sulfur dioxide in the environment. Microorganisms, algae, higher plants, invertebrates, vertebrates, organs and tissues of organisms are also used for these purposes. For example, microorganisms whose vital activity can be changed by the action of certain chemicals are used to analyze natural or waste water.
Methods of chemical analysis apply in various areas of the national economy: in medicine, agriculture, food industry, metallurgy, production of building materials (glass, ceramics), petrochemistry, energy, forensics, archeology, etc.
For laboratory assistants, the study of analytical chemistry is necessary, since most biochemical analyzes are analytical: determination of the pH of gastric juice using titration, the level of hemoglobin, ESR, calcium and phosphorus salts in the blood and urine, the study of cerebrospinal fluid, saliva, sodium and potassium ions in blood plasma, etc.
2. The main stages in the development of analytical chemistry.
1. The science of the ancients.
According to historical data, even the emperor of Babylon (VI century BC) wrote about the evaluation of the gold content. The ancient Roman writer, scientist and statesman Pliny the Elder (1st century AD) mentions the use of tannin extract as a reagent for iron. Even then, several methods were known for determining the purity of tin, in one of them molten tin was poured onto papyrus, if it burned out, then the tin is pure, if not, then there are impurities in the tin.
Since ancient times, the first analytical instrument, the balance, has been known. The hydrometer, which was described in the writings of ancient Greek scientists, can be considered the second device in time of appearance. Many methods of processing substances used in ancient chemical crafts (filtering, drying, crystallization, boiling) have entered the practice of analytical research.
2. Alchemy - the realization by chemists of the desire of society to obtain gold from base metals (IV - XVI centuries). In search of the philosopher's stone, alchemists established the composition of sulfur compounds of mercury (1270), calcium chloride (1380), learned how to produce valuable chemical products such as essential oil (1280), gunpowder (1330).
3. Iatrochemistry or medical chemistry - during this period, the main direction of chemical knowledge was obtaining drugs (XVI-XVII centuries).
During this period, many chemical methods for detecting substances appeared, based on their transfer into solution. In particular, the reaction of a silver ion with a chloride ion was discovered. During this period, most of the chemical reactions that form the basis of qualitative analysis were discovered. The concept of "precipitation", "precipitation" was introduced.
4. The era of phlogiston: “phlogiston” is a special “substance” that allegedly determines the mechanism of combustion processes (in the 17th-18th centuries, fire was used in a number of chemical crafts, such as the production of iron, porcelain, glass, and paints). Using a blowtorch, the qualitative composition of many minerals was established. The greatest analyst of the 18th century, T. Bergman, opened the way for modern metallurgy by determining the exact carbon content in various samples of iron obtained using coal, and created the first scheme for qualitative chemical analysis.
R. Boyle (1627-1691) is considered to be the founder of analytical chemistry as a science. As indicators for the determination of acids and hydroxides, he used tinctures of violets, cornflowers.
Works by Lomonosov M.V. also belong to this time, he denied the presence of phlogiston, for the first time introduced into the practice of chemical research the quantitative accounting of the reagents of chemical processes and is rightfully considered one of the founders of quantitative analysis. He was the first to use a microscope in the study of qualitative reactions and, based on the shape of crystals, he drew conclusions about the content of certain ions in the substance under study.
5. The period of scientific chemistry (XIX-XX centuries) development of the chemical industry.
V.M. Severgin (1765-1826) developed colorimetric analysis.
The French chemist J. Gay-Lussac (1778-1850) developed a titrimetric analysis that is widely used to this day.
The German scientist R. Bunsen (1811-1899) founded gas analysis and, together with G. Kirchhoff (1824-1887), developed spectral analysis.
The Russian chemist F.M. Flavitsky (1848-1917) in 1898 developed a method for detecting ions by “dry way” reactions.
The Swedish chemist A. Werner (1866-1919) created the coordination theory, on the basis of which the structure of complex compounds is studied.
In 1903 M.S. Color developed the chromatographic method.
6. Modern period.
If in the previous period, analytical chemistry developed in response to the social demands of industry, then at the present stage, the development of analytical chemistry is driven by awareness of the environmental situation of our time. These are means of control over OS, agricultural products, pharmacy. Research in the field of cosmonautics, sea waters also suggests the further development of ACh.
Modern instrumental methods of ACh, such as neutron activation, atomic adsorption, atomic emission, infrared spectrometry, make it possible to determine the extremely low values of substances and are used to determine highly toxic pollutants (pesticides, dioxins, nitrosamines, etc.).
Thus, the stages of development of analytical chemistry are closely interrelated with the progress of society.
3. The main classes of inorganic compounds: oxides, classification, physical. and chem. Holy Island, receiving.
Oxides are complex substances consisting of oxygen atoms and an element (metal or non-metal).
I. Classification of oxides.
1) salt-forming, which, reacting with acids or bases, form salts (Na 2 O, P 2 O 5, CaO, SO 3)
2) non-salt-forming, which do not form salts with acids or bases (CO, NO, SiO 2, N 2 O).
Depending on what oxides react with, they are divided into groups:
acidic, reacting with alkalis to form salt and water: P 2 O 5, SO 3, CO 2, N 2 O 5, CrO 3, Mn 2 O 7 and others. These are oxides of metals and non-metals in a high degree of oxidation;
basic, reacting with acids to form salt and water: BaO, K 2 O, CaO, MgO, Li 2 O, FeO, etc. These are metal oxides.
amphoteric, reacting with both acids and bases to form salt and water: Al 2 O 3, ZnO, BeO, Cr 2 O 3, Fe 2 O 3, etc.
II. physical properties.
Oxides are solid, liquid and gaseous.
III. Chemical properties of oxides.
A. Chemical properties of acid oxides.
Acid oxides.
S +6 O 3 → H 2 SO 4 Mn +7 2 O 7 → HMn +7 O 4
P +5 2 O 5 → H 3 P +5 O 4 P +3 2 O 3 → H 3 P +3 O 3
N +3 2 O 3 → HN +3 O 3 N +5 2 O 5 → HN +5 O 3
Reaction of acidic oxides with water:
acid oxide + water = acid
SO 3 + H 2 O \u003d H 2 SO 4
The reaction of acid oxides with bases:
oxide + base = salt + water
CO 2 + NaOH = Na 2 CO 3 + H 2 O
In the reactions of acid oxides with alkalis, the formation of acid salts is also possible with an excess of acid oxide.
CO 2 + Ca (OH) 2 \u003d Ca (HCO 3) 2
Reaction of acidic oxides with basic oxides:
acidic oxide + basic oxide = salt
CO 2 + Na 2 O \u003d Na 2 CO 3
B. Chemical properties of basic oxides.
Bases correspond to these metal oxides. There is the following genetic relationship:
Na → Na2O → NaOH
Reaction of basic oxides with water:
basic oxide + water = base
K 2 O + H 2 O \u003d 2KOH
Oxides of only some metals react with water (lithium, sodium, potassium, rubidium, strontium, barium)
Reaction of basic oxides with acids:
oxide + acid = salt + water
MgO + 2HCl \u003d MgCl 2 + H 2 O
If in such a reaction the acid is taken in excess, then, of course, an acid salt will be obtained.
Na 2 O + H 3 PO 4 = Na 2 HPO 4 + H 2 O
Reaction of basic oxides with acidic oxides:
basic oxide + acid oxide = salt
CaO + CO 2 \u003d CaCO 3
B. Chemical properties of amphoteric oxides.
These are oxides, which, depending on the conditions, exhibit the properties of basic and acidic oxides.
Reaction with bases:
amphoteric oxide + base = salt + water
ZnO + KOH \u003d K 2 ZnO 2 + H 2 O
Reaction with acids:
amphoteric oxide + acid = salt + water
ZnO + 2HNO 3 \u003d Zn (NO 3) 2 + H 2 O
3. Reactions with acidic oxides: t
amphoteric oxide + basic oxide = salt
ZnO + CO 2 = ZnCO 3
4. Reactions with basic oxides: t
amphoteric oxide + acid oxide = salt
ZnO + Na 2 O \u003d Na 2 ZnO 2
IV. Obtaining oxides.
1. Interaction of simple substances with oxygen:
metal or non-metal + O 2 = oxide
2. Decomposition of some oxygen-containing acids:
Oxoacid \u003d acid oxide + water t
H 2 SO 3 \u003d SO 2 + H 2 O
3. Decomposition of insoluble bases:
Insoluble base = basic oxide + water t
Сu (OH) 2 \u003d CuO + H 2 O
4. Decomposition of some salts:
salt = basic oxide + acidic oxide t
CaCO 3 \u003d CaO + CO 2
4. Main classes of inorganic compounds: acids, classification, physical. and chem. Holy Island, receiving.
An acid is a complex compound containing hydrogen ions and an acid residue.
acid \u003d nH + + acid residue - n
I. Classification
Acids are inorganic (mineral) and organic.
anoxic (HCl, HCN)
According to the number of H + ions formed during dissociation, is determined basicity of acids:
monobasic (HCl, HNO 3)
dibasic (H 2 SO 4, H 2 CO 3)
tribasic (H 3 PO 4)
II. physical properties.
Acids are:
soluble in water
insoluble in water
Almost all acids taste sour. Some of the acids have an odor: acetic, nitric.
III. Chemical properties.
1. Change the color of indicators: litmus turns red;
methyl orange - red; phenolphthalein is colorless.
2. Reaction with metals:
The ratio of metals to dilute acids depends on their position in the electrochemical series of metal voltages. Metals to the left of hydrogen H in this row displace it from acids. Exception: when nitric acid interacts with metals, hydrogen is not released.
acid + metal \u003d salt + H 2
H 2 SO 4 + Zn \u003d ZnSO 4 + H 2
3. Reaction with bases (neutralization):
acid + base = salt + water
2НCl + Cu(OH) 2 = CuCl 2 + H 2 O
In reactions with polybasic acids or polyacid bases, there can be not only medium salts, but also acidic or basic ones:
Hcl + Cu(OH) 2 = CuOHCl + H 2 O
4. Reaction with basic and amphoteric oxides:
acid + basic oxide = salt + water
2HCl + CaO \u003d CaCl 2 + H 2 O
5. Reaction with salts:
These reactions are possible if they form an insoluble salt or a stronger acid than the original one.
A strong acid always displaces a weaker one:
HCl > H 2 SO 4 > HNO 3 > H 3 PO 4 > H 2 CO 3
acid 1 + salt 1 = acid 2 + salt 2
HCl + AgNO 3 = AgCl↓ + HNO 3
6. Decomposition reaction: t
acid = oxide + water
H 2 CO 3 \u003d CO 2 + H 2 O
IV. Receipt.
1. Anoxic acids are obtained by synthesizing them from simple substances and then dissolving the resulting product in water.
H 2 + Cl 2 \u003d Hcl
2. Oxygen-containing acids are obtained by the interaction of acid oxides with water:
SO 3 + H 2 O \u003d H 2 SO 4
3. Most acids can be obtained by reacting salts with acids.
2Na 2 CO 3 + Hcl \u003d H 2 CO 3 + NaCl
5. Main classes of inorganic compounds: salts, classification, physical. and chem. Holy Island, receiving.
Salts are complex substances, products of complete or partial replacement of hydrogen in acids with metal atoms or hydroxo groups in bases with an acid residue.
In other words, in the simplest case, the salt consists of metal atoms (cations) and an acid residue (anion).
Salt classification.
Depending on the composition of the salt, there are:
medium (FeSO 4, Na 2 SO 4)
acidic (KH 2 PO 4 - potassium dihydrogen phosphate)
basic (FeOH (NO 3) 2 - iron hydroxonitrate)
double (Na 2 ZnO 2 - sodium zincate)
complex (Na 2 - sodium tetrahydroxozincate)
I. Physical properties:
Most salts are white solids (Na 2 SO 4, KNO 3). Some salts are colored. For example, NiSO 4 - green, CuS - black, CoCl 3 - pink).
According to the solubility in water, salts are soluble, insoluble and slightly soluble.
II. Chemical properties.
1. Salts in solutions react with metals:
salt 1 + metal 1 = salt 2 + metal 2
CuSO 4 + Fe \u003d FeSO 4 + Cu
Salts can interact with metals if the metal to which the salt cation corresponds is in the voltage series to the right of the reacting free metal.
2. The reaction of salts with acids:
salt 1 + acid 1 = salt 2 + acid 2
BaCl 2 + H 2 SO 4 \u003d BaSO 4 + 2HCl
Salts react with acids:
a) whose cations form an insoluble salt with acid anions;
b) whose anions correspond to unstable or volatile acids;
c) whose anions correspond to sparingly soluble acids.
3. The reaction of salts with base solutions:
salt 1 + base 1 = salt 2 + base 2
FeCl 3 + 3KOH \u003d Fe (OH) 3 + 3KCl
Only salts react with alkalis:
a) whose metal cations correspond to insoluble bases;
b) whose anions correspond to insoluble salts.
4. The reaction of salts with salts:
salt 1 + salt 2 = salt 3 + salt 4
AgNO 3 + KCl = AgCl↓ + KNO 3
Salts interact with each other if one of the resulting salts is insoluble or decomposes with the release of gas or precipitate.
5. Many salts decompose when heated:
MgCO 3 \u003d CO 2 + MgO
6. Basic salts interact with acids to form medium salts and water:
Basic salt + acid \u003d medium salt + H 2 O
CuOHCl + HCl \u003d CuCl 2 + H 2 O
7. Acid salts interact with soluble bases (alkalis) to form medium salts and water:
Acid salt + acid \u003d medium salt + H 2 O
NaHSO 3 + NaOH = Na 2 SO 3 + H 2 O
III. Methods for obtaining salts.
Methods for obtaining salts are based on the chemical properties of the main classes of inorganic substances - oxides, acids, bases.
6. Main classes of inorganic compounds: bases, classification, physical. and chem. sv-va, receiving
Bases are complex substances containing metal ions and one or more hydroxo groups (OH -).
The number of hydroxo groups corresponds to the degree of oxidation of the metal.
According to the number of hydroxyl groups, bases are divided into:
single acid (NaOH)
diacid (Ca (OH) 2)
polyacid (Al (OH) 3)
By solubility in water:
soluble (LiOH, NaOH, KOH, Ba (OH) 2, etc.)
insoluble (Cu (OH) 2, Fe (OH) 3, etc.)
I. Physical properties:
All bases are crystalline solids.
A feature of alkalis is their soapiness to the touch.
II. Chemical properties.
1. Reaction with indicators.
base + phenolphthalein = raspberry color
base + methyl orange = yellow color
base + litmus = blue color
Insoluble bases do not change the color of indicators.
2. Reaction with acids (neutralization reaction):
base + acid = salt + water
KOH + HCl = KCl + H 2 O
3. Reaction with acid oxides:
base + acid oxide = salt + water
Ca (OH) 2 + CO 2 \u003d CaCO 3 + H 2 O
4. Reaction of bases with amphoteric oxides:
base + amphoteric oxide = salt + water
5. Reaction of bases (alkalis) with salts:
base 1 + salt 1 = base 2 + salt 2
KOH + CuSO 4 \u003d Сu (OH) 2 ↓ + K 2 SO 4
For the reaction to proceed, it is necessary that the reacting base and salt be soluble, and the resulting base and/or salt should precipitate.
6. Decomposition reaction of bases when heated: t
base = oxide + water
Cu (OH) 2 \u003d CuO + H 2 O
Alkali metal hydroxides are resistant to heat (with the exception of lithium).
7. Reaction of amphoteric bases with acids and alkalis.
8. The reaction of alkalis with metals:
Alkali solutions interact with metals, which form amphoteric oxides and hydroxides (Zn, Al, Cr)
Zn + 2NaOH \u003d Na 2 ZnO 2 + H 2
Zn + 2NaOH + H 2 O \u003d Na 2 + H 2
IV. Receipt.
1. You can get a soluble base by reacting alkali and alkaline earth metals with water:
K + H 2 O \u003d KOH + H 2
2. A soluble base can be obtained by reacting oxides of alkali and alkaline earth metals with water.
CHEMICAL ANALYSIS
Analytical chemistry. Tasks and stages of chemical analysis. Analytical signal. Classification of methods of analysisper. Identification of substances. Fractional analysis. Systematic analysis.
Main tasks of analytical chemistry
One of the tasks in carrying out environmental protection measures is the knowledge of the patterns of cause-and-effect relationships between various types of human activity and changes occurring in the natural environment. Analysis It is the main means of controlling environmental pollution. The scientific basis of chemical analysis is analytical chemistry. Analytical chemistry - the science of methods and means for determining the chemical composition of substances and materials. Method- this is a fairly universal and theoretically justified way to determine the composition.
Basic requirements for methods and techniques of analytical chemistry:
1) correctness and good reproducibility;
2) low detection limit- this is the lowest content at which the presence of the determined component with a given confidence probability can be detected using this method;
3) selectivity (selectivity)- characterizes the interfering influence of various factors;
4) range of measured contents(concentrations) using this method according to this method;
5) expressiveness;
6) simplicity in analysis, the possibility of automation, cost-effectiveness of determination.
Chemical analysis is a complex multi-stage about cess, which is a collection of ready-made techniques and related services.
Analysis tasks
1. Identification of the object, i.e. establishing the nature of the object (checking the presence of certain main components, impurities).
2. Quantitative determination of the content of one or another component in the analyzed object.
Stages of analysis of any object
1. Statement of the problem and choice of method and scheme of analysis.
2. Sampling (competent sampling of a part of the sample allows you to draw the correct conclusion about the composition of the entire sample). Try- this is a part of the analyzed material, representative of the a chewing its chemical composition. In some cases, the entire analytical material is used as a sample. Sample storage time should be kept to a minimum. eh nym. Storage conditions and methods should exclude uncontrolled loss of volatile compounds and any other physical and chemical changes in the composition of the analyzed sample.
3. Preparation of samples for analysis: transferring the sample to the desired state (solution, steam); separation of components or separation of interfering; concentration of components;
4. Obtaining an analytical signal. Analytical signal- this is a change in any physical or physico-chemical property of the determined component, functionally related to its content (formula, table, graph).
5. Analytical signal processing, i.e. separation of signal and noise. Noises- side signals arising in measuring instruments, amplifiers and other devices.
6. Application of the results of the analysis. Depending on the property of the substance underlying the definition, the methods of analysis are divided into:
On chemical methods analysis based on a chemical analytical reaction, which is accompanied by a pronounced effect. These include gravimetric and titrimetric methods;
- physical and chemical methods, based on the measurement of any physical parameters of a chemical system that depend on the nature of the components of the system and change during a chemical reaction (for example, photometry is based on a change in the optical density of a solution as a result of a reaction);
- physical methods analysis not involving the use of chemical reactions. The composition of substances is established by measuring the characteristic physical properties of the object (for example, density, viscosity).
Depending on the measured value, all methods are divided into the following types.
Methods for measuring physical quantities
|
Measured physical quantity |
Method name |
|
Gravimetry |
|
|
Titrimetry |
|
|
Equilibrium potential of the electrode |
Potentiometry |
|
Polarization resistance of the electrode |
Polarography |
|
The amount of electricity |
Coulometry |
|
Solution conductivity |
Conductometry |
|
Photon absorption |
Photometry |
|
Emission of photons |
Emission spectral analysis |
Substance identification is based on methods of qualitative recognition of elementary objects (atom, molecules, ions, etc.) that make up substances and materials.
Very often, the analyzed sample of a substance is converted into a form convenient for analysis by dissolving in a suitable solvent (usually water or aqueous acid solutions) or fusing with some chemical compound, followed by dissolution.
Chemical methods of qualitative analysis are based on using reactions of identifiable ions with certain substances - analytical reagents. Such reactions should be accompanied by precipitation or dissolution of the precipitate; the appearance, change or disappearance of the color of the solution; release of gas with a characteristic odor; the formation of crystals of a certain shape.
Reactions that take place in solutions by way of execution are classified into test-tube, microcrystalloscopic and drip. Microcrystalloscopic reactions are carried out on a glass slide. Observe the formation of crystals of a characteristic shape. Drop reactions are performed on filter paper.
Analytical reactions used in qualitative analysis, by area of application share:
1.) on group reactions- these are reactions for the precipitation of a whole group of ions (one reagent is used, which is called group);
2;) characteristic reactions:
a) selective (selective)- give the same or similar analytical reactions with a limited number of ions (2~5 pcs.);
b) specific (highly selective)- selective towards alone component.
There are few selective and specific reactions, so they are used in combination with group reactions and with special techniques to eliminate the interfering influence of the components present in the system along with the substance being determined.
Simple mixtures of ions are analyzed fractional method, without prior separation of interfering ions, individual ions are determined by means of characteristic reactions. M destroying ion- this is an ion that, under the conditions of detection of the desired one, gives a similar analytical effect with the same reagent or an analytical effect that masks the desired reaction. The detection of different ions in fractional analysis is carried out in separate portions of the solution. If it is necessary to eliminate interfering ions, use the following methods of separation and camouflage.
1. Conversion of interfering ions to precipitate. The basis is the difference in the magnitude of the solubility product of the resulting precipitates. In this case, the PR of the connection of the ion being determined with the reagent should be greater than the PR of the connection of the interfering ion.
2. Binding of interfering ions into a strong complex compound. The resulting complex must have the necessary stability in order to complete the binding of the interfering ion, and the desired ion must not react at all with the introduced reagent, or its complex must be unstable.
3. Change in the oxidation state of interfering ions.
4. The use of extraction. The method is based on the extraction of interfering ions from aqueous solutions with organic solvents and the separation of the system into its component parts (phases) so that the interfering and determined components are in different phases.
Advantages of fractional analysis:
Speed of execution, as the time for long-term operations of sequential separation of some ions from others is reduced;
Fractional reactions are easily reproducible; they can be repeated several times. However, if it is difficult to select selective (specific) reactions for detecting ions, masking reagents, calculating the completeness
removal of ions and other causes (complexity of the mixture) resort to performing a systematic analysis.
Systematic analysis- this is a complete (detailed) analysis of the object under study, which is carried out by dividing all the components in the sample into several groups in a certain sequence. The division into groups is based on the similarity (within the group) and differences (between groups) of the analytical properties of the components. In the selected analysis group, a series of successive separation reactions are used until only components that give characteristic reactions with selective reagents remain in one phase (Fig. 23.1).
Several analytical classifications have been developed ka thions and anions into analytical groups, which are based on the use of group reagents (i.e., reagents for isolating a whole group of ions under specific conditions). Group reagents in the analysis of cations serve both for detection and for separation, and in the analysis of anions - only for detection (Fig. 23.2).
Analysis of mixtures of cations
Group reagents in the qualitative analysis of cations are acids, strong bases, ammonia, carbonates, phosphates, alkali metal sulfates, oxidizing and reducing agents. The combination of substances into analytical groups is based on the use of similarities and differences in their chemical properties. The most important analytical properties include the ability of an element to form various types of ions, the color and solubility of compounds, the ability to enter in certain reactions.
The group reagents are selected from the general reagents because the group reagent is required to release a relatively large number of ions. The main method of separation is precipitation, i.e. the division into groups is based on the different solubility of cationic precipitates in certain media. When considering the action of group reagents, the following groups can be distinguished (Table 23.2).
In addition, three cations remain (Na + , K + , NH4) that do not form precipitates with the indicated group reagents. They can also be separated into a separate group.
Cation groups
In addition to the indicated general approach, when choosing group reagents, one proceeds from the values of precipitation solubility products, since, by varying the precipitation conditions, it is possible to separate substances from a group by the action of the same reagent.
The most widespread is the acid-base classification of cations. Advantages of the acid-base method of systematic analysis:
a) the basic properties of the elements are used - their relationship to acids, alkalis;
b) analytical groups of cations to a greater extent co correspond to the groups of the periodic system of elements D.I. Mendeleev;
c) the analysis time is significantly reduced in comparison with the hydrogen sulfide method. The study begins with preliminary tests, in which the pH of the solution is set by a universal indicator and NH 4 , Fe 3+ , Fe 2+ ions are detected by specific and selective reactions.
Division into groups. The general scheme of division into groups given in table. 23.3. In the analyzed solution, first of all, cations of groups I and II are separated. To do this, 10-15 drops of the solution are placed in a test tube and a mixture of 2M HCl and 1M H 2 S0 4 is added dropwise. The precipitate is left for 10 min, then it is centrifuged and washed with water acidified with HCl. A mixture of chlorides and sulfates Ag + , Pb 2+ , Ba 2+ , Ca 2+ remains in the precipitate. The presence of basic antimony salts is possible. In solution - cations III-VI groups.
Group III is separated from the solution by adding a few drops of 3% H 2 0 2 and an excess of NaOH while heating and stirring. Excess hydrogen peroxide is removed by boiling. In the sediment - hydroxides of cations of groups IV-V, in solution - cations of groups III and VI and partially Ca 2+, which may not completely precipitate in the form of CaS0 4 when separating groups I and II.
Group V cations are separated from the precipitate. The precipitate is treated with 2N Na 2 CO 3 and then with excess NH 3 while heating. Group V cations pass into solution in the form of ammonia, in the precipitate - carbonates and basic salts of group IV cations.
The virtue of systematic analysis- Obtaining sufficiently complete information about the composition of the object. Flaw- bulkiness, duration, laboriousness. Complete schemes of systematic qualitative analysis are rarely carried out. Usually they are used partially if there is information about the origin, approximate composition of the sample, a So in the course of analytical chemistry.
Magnesium hydroxide dissolves in a mixture of NH 3 + NH 4 C1. Thus, after dividing the cations into groups, four test tubes were obtained containing a) a precipitate of chlorides and sulfates of cations of I-II groups; b) a solution of a mixture of cations III and VI groups; c) a solution of ammonium cations of group V; d) sediment of carbonates and basic salts of group IV cations. Each of these objects is analyzed separately.
Analysis of anion mixtures
General characteristics of the studied anions. Anions are formed mainly by elements of groups IV, V, VI and VII of the periodic system. One and the same element can form several anions that differ in their properties. For example, sulfur forms anions S 2 -, S0 3 2 ~, S0 4 2 ~, S 2 0 3 2 ~, etc.
All anions are constituents of acids and corresponding branching salts. Depending on the composition of which substance the anion is included, its properties change significantly. For example, for the ion SO 4 2 "in the composition of concentrated sulfuric acid, oxidation-reduction reactions are characteristic, and in the composition of salts - precipitation reactions.
The state of anions in a solution depends on the medium of the solution. Some anions decompose under the action of concentrated acids with the release of the corresponding gases: CO 2 (anion CO 2-3), H 2 S (anion S 2 "), N0 2 (anion N0 3), etc. Under the action of dilute acids, anions MoO 4 2 -, W0 4 2 ~, SiO 3 2 "form water-insoluble acids (H 2 Mo0 4, H 2 W0 4 * H 2 0, H 2 SiO 3 ). Anions of weak acids (C0 3 2 ~, P0 4 ", Si0 3 2 ~, S 2") in aqueous solutions are partially or completely hydrolyzed, for example:
S 2 "+ H 2 0 →HS" + OH _.
Most of the elements that form anions have a variable valence and, under the action of oxidizing or reducing agents, change the oxidation state, while changing the composition of the anion. Chloride ion, for example, can be oxidized to C1 2, ClO", ClO 3, ClO 4. Iodide ions, for example, are oxidized to I 2, IO 4; sulfide ion S 2 ~ - to S0 2, SO 4 2- ; anions N0 3 can be reduced to N0 2, NO, N 2, NH 3.
Reducing anions (S 2 ~, I - , CI -) reduce Mn0 4 - ions in an acidic environment, causing their discoloration. oxidizing ions (NO3 , CrO 4 2 ", V0 3 -, Mn0 4 ~) oxidize iodide ions to acid oh medium to a free ion, color diphenylamine blue. These properties are used for qualitative analysis, the redox properties of chromate, nitrate, iodide, vanadate, molybdate, tungstate ions underlie them typical reactions.
Group reactions of anions. According to their action on anions, the reagents are divided into the following groups:
1) reagents that decompose substances with the release of gases. These reagents include dilute mineral acids (HC1, H 2 S0 4);
2) reagents that release anions from solutions in the form of slightly dissolved precipitates (Table 23.4):
a) ВаС1 2 in a neutral medium or in the presence of Ba (OH) 2 precipitates: SO 2-, SO, 2 ", S 2 0 3 2 ~, CO 3 2", PO 4 2 ", B 4 0 7 2 ~, As0 3 4 ", SiO 3 2";
b) AgNO 3 in 2n HNO 3 precipitates: SG, Br - , I - , S 2- (SO 4 2 only in concentrated solutions);
3) reducing agents (KI) (Table 23.5);
4) oxidizing reagents (KMn0 4, solution of I 2 in KI, HNO 3 (conc), H 2 S0 4).
Anions in the analysis basically do not interfere with the detection of each other, therefore, group reactions are used not for separation, but for preliminary verification of the presence or absence of a particular group of anions.
Systematic methods for the analysis of a mixture of anions, based nye on dividing them into groups, are rarely used, mainly zom for the study of simple mixtures. The more complex the mixture of anions, the more cumbersome the analysis schemes become.
Fractional analysis makes it possible to detect anions that do not interfere with each other in separate portions of the solution.
In semi-systematic methods, the separation of anions into groups using group reagents and the subsequent fractional detection of anions takes place. This leads to a reduction in the number of required sequential analytical operations and ultimately simplifies the scheme for analyzing an anion mixture.
The current state of qualitative analysis is not limited to the classical scheme. In the analysis of both inorganic, So and organic substances, instrumental methods are often used, such as luminescent, absorption spectroscopic, various electrochemical methods, “which are variants of chromatography, etc. However, in a number of cases (field, factory express laboratories, etc.), classical analysis has not lost its significance due to its simplicity, accessibility, and low cost.
1. Sampling:
A laboratory sample consists of 10-50 g of material, which is taken so that its average composition corresponds to the average composition of the entire lot of the analyte.
2. Decomposition of the sample and its transfer to the solution;
3. Carrying out a chemical reaction:
X is the component to be determined;
P is the reaction product;
R is a reagent.
4. Measurement of any physical parameter of the reaction product, reagent or analyte.
Classification of chemical methods of analysis
I By reaction components
1. Measure the amount of reaction product P formed (gravimetric method). Create conditions under which the analyte is completely converted into a reaction product; further, it is necessary that the reagent R does not give minor reaction products with foreign substances, the physical properties of which would be similar to the physical properties of the product.
2. Based on the measurement of the amount of the reagent consumed in the reaction with the analyte X:
– the action between X and R must be stoichiometric;
- the reaction must proceed quickly;
– the reagent must not react with foreign substances;
– a way to establish the equivalence point is needed, i.e. the moment of titration when the reagent is added in an equivalent amount (indicator, color change, potential island, electrical conductivity).
3. Records the changes that occur with the analyte X itself in the process of interaction with the reagent R (gas analysis).
II Types of chemical reactions
1. Acid-base.
2. Formation of complex compounds.
Acid-base reactions: used mainly for the direct quantitative determination of strong and weak acids and bases, and their salts.
Reactions for the formation of complex compounds: determined substances are converted into complex ions and compounds by the action of reagents.
The following separation and determination methods are based on complex formation reactions:
1) Separation by means of precipitation;
2) Extraction method (water-insoluble complex compounds often dissolve well in organic solvents - benzene, chloroform - the process of transferring complex compounds from aqueous phases to dispersed ones is called extraction);
3) Photometric (Co with nitrous salt) - measure the optimal density of solutions of complex compounds;
4) Titrimetric analysis method
5) Gravimetric method of analysis.
1) cementation method - reduction of metal Me ions in solution;
2) electrolysis with a mercury cathode - during the electrolysis of a solution with a mercury cathode, ions of many elements are reduced by electric current to Me, which dissolve in mercury, forming an amalgam. The ions of other Me remain in solution;
3) identification method;
4) titrimetric methods;
5) electrogravimetric - an el is passed through the test solution. a current of a certain voltage, while the Me ions are restored to the Me state, the released is weighed;
6) coulometric method - the amount of a substance is determined by the amount of electricity that must be spent for the electrochemical transformation of the analyzed substance. Analysis reagents are found according to Faraday's law:
M is the amount of the element being determined;
F is the Faraday number (98500 C);
A is the atomic mass of the element;
n is the number of electrons involved in the electrochemical transformation of a given element;
Q is the amount of electricity (Q = I ∙ τ).
7) catalytic method of analysis;
8) polarographic;
III Classification of separation methods based on the use of various types of phase transformations:
The following types of equilibria between phases are known:
Equilibrium L-G or T-G is used in the analysis when substances are released into the gas phase (CO 2 , H 2 O, etc.).
Equilibrium W 1 - W 2 is observed in the extraction method and in electrolysis with a mercury cathode.
Zh-T is typical for the processes of deposition and the processes of precipitation on the surface of the solid phase.
Analysis methods include:
1. gravimetric;
2. titrimetric;
3 optical;
4. electrochemical;
5. catalytic.
Separation methods include:
1. precipitation;
2. extraction;
3. chromatography;
4. ion exchange.
Concentration methods include:
1. precipitation;
2. extraction;
3. grouting;
4. stripping.
Physical methods of analysis
A characteristic feature is that they directly measure any physical parameters of the system associated with the amount of the element being determined without prior chemical reaction.
Physical methods include three main groups of methods:
I Methods based on the interaction of radiation with a substance or on the measurement of the radiation of a substance.
II Methods based on measuring the parameters of el. or magnetic properties of matter.
IIIMethods based on the measurement of density or other parameters of the mechanical or molecular properties of substances.
Methods based on the energy transition of the outer valence electrons of atoms: include atomic emission and atomic absorption methods of analysis.
Atomic emission analysis:
1) Flame photometry - the analyzed solution is sprayed into the flame of a gas burner. Under the influence of high temperature, the atoms go into an excited state. The outer valence electrons move to higher energy levels. The reverse transition of electrons to the main energy level is accompanied by radiation, the wavelength of which depends on the atoms of which element were in the flame. The intensity of radiation under certain conditions is proportional to the number of atoms of the element in the flame, and the wavelength of radiation characterizes the qualitative composition of the sample.
2) Emission method of analysis - spectral. The sample is introduced into the flame of an arc or a condensed spark, under high temperature the atoms go into an excited state, while the electrons go not only to the closest to the main, but also to more distant energy levels.
Radiation is a complex mixture of light vibrations of different wavelengths. The emission spectrum is decomposed into the main parts of the special. instruments, spectrometers, and photographing. Comparison of the position of the intensity of individual lines of the spectrum with the lines of the corresponding standard, allows you to determine the qualitative and quantitative analysis of the sample.
Atomic absorption methods of analysis:
The method is based on measuring the absorption of light of a certain wavelength by unexcited atoms of the element being determined. A special radiation source produces resonant radiation, i.e. radiation corresponding to the transition of an electron to the lowest orbital with the lowest energy, from the orbital closest to it with a higher energy level. The decrease in the intensity of light when it passes through the flame due to the transfer of the electrons of the atoms of the element being determined into an excited state is proportional to the number of unexcited atoms in it. In atomic absorption, combustible mixtures with temperatures up to 3100 ° C are used, which increases the number of elements to be determined, in comparison with flame photometry.
X-ray fluorescent and X-ray emission
X-ray fluorescent: the sample is exposed to x-rays. top electrons. The orbitals closest to the nucleus of the atom are knocked out of the atoms. Their place is taken by electrons from more distant orbitals. The transition of these electrons is accompanied by the appearance of secondary X-ray radiation, the wavelength of which is functionally related to the atomic number of the element. Wavelength - qualitative composition of the sample; intensity - the quantitative composition of the sample.
Methods based on nuclear reactions - radioactive. The material is exposed to neutron radiation, nuclear reactions occur and radioactive isotopes of elements are formed. Next, the sample is transferred into a solution and the elements are separated by chemical methods. After that, the intensity of radioactive radiation of each element of the sample is measured, and the reference sample is analyzed in parallel. The intensity of radioactive radiation of individual fractions of the reference sample and the analyzed material is compared and conclusions are drawn about the quantitative content of elements. Limit of detection 10 -8 - 10 -10%.
1. Conductometric - based on measuring the electrical conductivity of solutions or gases.
2. Potentiometric - there is a method of direct and potentiometric titration.
3. Thermoelectric - based on the occurrence of thermoelectromotive force, which arose when heating the place of contact of steel, etc. Me.
4. Mass spectral - is used with the help of strong elements and magnetic fields, gas mixtures are separated into components in accordance with the atoms or molecular weights of the components. It is used in the study of a mixture of isotopes. inert gases, mixtures of organic substances.
Densitometry - based on the measurement of density (determination of the concentration of substances in solutions). To determine the composition, viscosity, surface tension, sound speed, electrical conductivity, etc. are measured.
To determine the purity of substances, the boiling point or melting point is measured.
Prediction and calculation of physical and chemical properties
Theoretical foundations for predicting the physicochemical properties of substances
Approximate prediction calculation
Prediction implies an assessment of physicochemical properties based on the minimum number of readily available initial data, and may also assume the complete absence of experimental information about the properties of the substance under study (“absolute” prediction relies only on information about the stoichiometric formula of the compound).