The vast majority of information about substances, their properties and chemical transformations was obtained through 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 centuries. Even when chemistry was not an exact science, in ancient times and in the Middle Ages, scientists and artisans, sometimes by accident, and sometimes purposefully, discovered ways to obtain and purify many substances that were used in economic activities: metals, acids, alkalis, dyes and etc. Alchemists contributed greatly 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, especially methods of purifying all kinds of liquids and solids, which allowed them to make many important discoveries. And yet, 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 atomic-molecular science was developed. Since that time, 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 involves many different measurements. Both the equipment for conducting experiments and chemical glassware have 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 In a modern laboratory you will not find homemade retorts - they have been replaced by standard glass equipment produced by industry and adapted specifically for performing a particular chemical procedure. Working methods have also become standard, which in our time no longer has to be reinvented by every chemist. A 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 increasingly important 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 of purifying substances was extremely labor intensive. There are cases where chemists spent years of work isolating 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 this, the purity of the substance could not always be guaranteed.
The perfection 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 specially adapted for the detection of short-lived intermediate particles formed during chemical reactions. For this purpose, devices are equipped with either high-speed recording devices or attachments that ensure operation at very low temperatures. These methods successfully record the spectra of particles whose lifespan under normal conditions is measured in thousandths of a second, for example, 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.
TOPIC 1. Forced slaughter, the procedure for its implementation and veterinary examination of forced slaughter meat
The goal is to learn the procedure for performing forced slaughter of animals, conducting veterinary examinations of slaughter products and their use.
1. Study and understand the procedure established by the “Rules for veterinary inspection of slaughter animals and veterinary and sanitary examination of meat and meat products” for performing forced slaughter of animals, conducting veterinary examinations and using slaughter 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 prohibited to subject animals to forced slaughter?
2) The procedure for registration and carrying out forced slaughter and veterinary examination of slaughter products.
3) The procedure for sampling and preparing an accompanying document when sending material to a veterinary laboratory for bacteriological and other studies.
4) What organoleptic characteristics are used to identify carcasses obtained from animals that have died or were in an agonal state?
5) What laboratory research methods are used to identify meat obtained from animals that have died or were in a state of agony and what is their essence?
6) The procedure for delivering forced slaughter meat to meat processing plants for neutralization and processing.
7) The procedure for acceptance, examination of meat from forced slaughter at a meat processing plant, its neutralization and processing.
2. Perform laboratory tests on samples of forced slaughter meat in order to identify the fact that meat was obtained from an animal that had died or was in a state of agony
a) Perform a peroxidase reaction.
b) Perform a reaction with formaldehyde.
c) Conduct a bacteriscopic examination of meat samples.
d) Determine the pH of meat using colorimetric and potentiometric research methods.
e) Examine meat samples by cooking test.
f) Based on the research performed, give a conclusion on the suitability or unsuitability of meat for food purposes.
The procedure for carrying out forced slaughter of animals and examination of meat in accordance with the “Rules for veterinary examination of slaughter animals and 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 threatening the life of the animal, as well as in cases requiring long-term, economically unjustified treatment, 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 bacteriological and, if necessary, physical and chemical research, but with a mandatory cooking test to identify extraneous odors that are unusual for meat.
Forced slaughter of animals is carried out only with the permission of a veterinarian (paramedic).
Pre-slaughter holding of animals delivered to a meat processing plant for forced slaughter is not carried out.
A report 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 bacteriological examination of the carcass of a forcibly killed animal, together with a veterinary certificate, must accompany the specified carcass when delivered to the meat processing plant, where it is re-subjected to bacteriological examination.
If an animal is suspected of being poisoned by pesticides or other toxic chemicals, it is necessary to have a conclusion from a veterinary laboratory on the results of testing the meat for the presence of toxic chemicals.
Transportation of meat from forcedly slaughtered animals from farms to meat industry enterprises must be carried out in compliance with current veterinary and sanitary rules for the transportation of meat products.
In order to ensure correct examination of the meat of forcedly killed 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 refrigeration chamber. Half-carcasses and quarters are tagged to determine whether they belong to the same carcass.
Carcasses of pigs forcedly killed on farms must be delivered to the meat processing plant with their heads intact.
When delivering salted meat from animals forcedly killed on farms to a meat processing plant, each barrel must contain corned beef from one carcass.
Carcasses of animals forcedly killed en route without a pre-mortem 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 from a veterinary laboratory on the results of a bacteriological examination, are prohibited from being accepted at the meat processing plant.
If, according to the results of examination, bacteriological and physico-chemical research, meat and other products of forced slaughter are found suitable for use as food, then they are sent for boiling, as well as for the production of meat loaves or canned goods “Goulash” and “Meat Pate”.
The release of this meat and other slaughter products in raw form, including into public catering networks (canteens, etc.), without prior disinfection by boiling is prohibited.
Note: Cases of forced slaughter do not include:
slaughter of clinically healthy animals that cannot be fattened to the required standards, are lagging in growth and development, are unproductive, are barren, but have a normal body temperature; slaughter of healthy animals that are at risk of 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 in meat processing plants is carried out only in a sanitary slaughterhouse.
Selection, packaging and forwarding of samples to the veterinary laboratory According to the above rules of veterinary examination, depending on the expected diagnosis and the nature of pathological changes, the following are sent for bacteriological examination:
a part of the flexor or extensor muscle of the front and hind limbs of the carcass, covered with fascia at least 8 cm long, or a piece of another muscle measuring at least 8x6x6 cm;
lymph nodes - from cattle - superficial cervical or axillary and external iliac, and from pigs - superficial cervical dorsal (in the absence of pathological changes in the head and neck area) or axillary of the first rib and patellar;
spleen, kidney, liver lobe with hepatic lymph node (in the absence of a lymph node - 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 or quarter carcasses, a piece of muscle, lymph nodes and tubular bone are taken for analysis.
When examining meat from small animals (rabbits, nutria) and poultry, whole carcasses are sent to the laboratory.
When examining salted meat in a barrel container, samples of meat and existing lymph nodes are taken from the top, middle and bottom of the barrel, as well as, if present, tubular bone and brine.
If erysipelas is suspected, in addition to muscles, lymph nodes and internal organs, tubular bone is sent to the laboratory.
For bacteriological examination, the brain, liver lobe and kidney are sent for listeriosis.
If anthrax, emcar, or malignant edema is suspected, a lymph node of the affected organ or a lymph node that collects lymph from the location of the suspicious focus, edematous tissue, exudate, and in pigs, in addition, the mandibular lymph node, is sent for examination.
Samples taken for research with an 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 them. The accompanying document indicates the type of animal or product, its affiliation (address), what material is sent and in what quantity, the reason for sending the material for research, what changes have been established in the product, the intended diagnosis and what kind of research is required (bacteriological, physico-chemical, etc.) .d.).
Methods for identifying forced slaughter meat - sick, killed in agony or dead animals
Pathoanatomical and organoleptic examination When determining meat from a sick animal killed in an agonal state or a dead animal, it is necessary to take into account the following external signs: the condition of the cutting site, the degree of bleeding, the presence of hypostases and the color of the lymph nodes on the cut.
Condition of the stabbing site . A cut refers to the place where blood vessels are cut during the slaughter of an animal. To create the appearance of a normally slaughtered animal, owners often make cuts in the neck of dead animals, rub blood into the cut site, hang them by the hind legs for better blood drainage, etc.
There are the following differences between the intravital and postmortem incision: the intravital incision is uneven due to muscle contraction, the tissues in the area of the incision are infiltrated (soaked) with blood to a greater extent compared to those lying deeper. The cut made after the death of the animal is more even, the blood almost does not permeate the tissue, and the blood present on the surface of the tissue is easily washed off with water. The tissues in the degree of blood infiltration in the area of the incision do not differ from the tissues located deeper.
Degree of carcass bleeding . Carcasses obtained from sick animals, and especially from animals that were in an agonal state or died, are poorly or very poorly bled. The carcasses are dark red in color; the cuts reveal small and large blood vessels filled with blood. Intercostal vessels appear as dark veins. If you separate the shoulder blade from the carcass, you can find vessels filled with blood.
If you insert 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 if bleeding is poor, not only the part of the paper that comes into contact with the meat will become saturated with blood, but also the free its end (this method is not acceptable for thawed meat), the fatty tissue is pink or reddish in color.
With good bleeding, the meat is crimson or red, the fat is white or yellow, and there is no blood on the cut muscle. 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 section. Lymph nodes when cut in carcasses of healthy animals and those that were dressed in a timely manner have a light gray or yellowish color. In the meat of animals that are seriously ill, killed in an agonal state, or dead, the lymph nodes on the cut have a lilac-pink color. In addition, depending on the disease in the lymph nodes, their enlargement, various forms of inflammatory processes, hemorrhages, necrosis, and hypertrophy will be detected.
Presence of hypostases . By hypostasis we understand the postmortem and premortem redistribution (draining) of blood into the underlying parts of the body during prolonged agony. The tissues on the side of the body on which the sick animal lay are 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 disruption of the integrity of blood vessels due to bruises. They are local and superficial in nature, and hypostases are diffuse (diffuse) and during hypostases, the deep layers of tissue are also infiltrated with blood. Hypostases can form not only after the death of an animal, but during life. They can form during prolonged agony, when the animal’s cardiac activity is weakened and the blood gradually stagnates in the underlying areas of the body. Thus, the detection of hypostases indicates that the meat was obtained from a dead 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 not obtained from a dying animal.
Determining the fact that meat was obtained from animals that were in an agonal state or died 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.
Cooking test . Meat obtained from seriously ill, moribund or dead animals can be identified to a certain extent using an organoleptic method, the so-called cooking test. For this purpose 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, place in a boiling water bath and heat to 80-85ºС until vapor appears. Then open the lid slightly and determine the smell and condition of the broth. Broth made from the meat of seriously ill, agonizing or dead animals, as a rule, has an unpleasant or medicinal smell, it is cloudy with flakes. Conversely, broth made from the meat of healthy animals has a pleasant, specific meaty smell and is transparent. Taste testing is not recommended.
Physico-chemical research
According to the “Rules for veterinary examination of animals and veterinary and sanitary examination of meat and meat products”, in addition to pathological, organoleptic and bacteriological analysis, meat from forced slaughter, as well as if there is a suspicion that the animal was in a state of agony before slaughter or was dead, must be subjected to physico-chemical research.
Bacterioscopy . Bacterioscopic examination of fingerprint smears from deep layers of muscles, internal organs and lymph nodes is aimed at preliminary (before receiving the results of bacteriological examination) detection of pathogens of infectious diseases (anthrax, emphysematous carbuncle, etc.) and contamination of meat with opportunistic microflora (Escherichia coli, Proteus etc.).
The bacterioscopic examination technique 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 using sterile tweezers, a scalpel or scissors, a piece of tissue is cut out from the middle and smears are made on a glass slide. Air-dried, flamed over a burner flame and Gram-stained. The preparation is stained through filter paper with a solution of carbolic gentian violet - 2 minutes, the filter paper is removed, the paint is drained and without washing the preparation it is treated with Lugol's solution - 2 minutes, decolorized with 95% alcohol - 30 seconds, washed with water, counterstained with Pfeiffer fuchsin - 1 minute ., washed again with water, dried and microscopically examined under immersion. In fingerprint smears from the deep layers of meat, internal organs and lymph nodes of healthy animals, there is no microflora.
In case of diseases, rods or cocci are found in fingerprint smears. A complete determination of the detected microflora can be determined in a veterinary laboratory, for which they inoculate on nutrient media, obtain a pure culture and identify it.
pH determination . The pH value of meat depends on the glycogen content 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 ripening.
Immediately after slaughter, the reaction of the environment in the muscles is slightly alkaline or neutral - equal to - 7. Within a day, the pH of meat from healthy animals, as a result of the breakdown of glycogen to lactic acid, decreases to 5.6-5.8. In the meat of sick or killed animals in an agonal state, such a sharp decrease in pH does not occur, since the muscles of such animals contain less glycogen (used during illness as an energy substance), and, therefore, less lactic acid is formed and the pH is less acidic, i.e. .e. higher.
Meat from sick and overworked animals is in the range of 6.3-6.5, and agonizing or dead animals are 6.6 and higher, it approaches neutral - 7. It should be emphasized that meat must be aged for at least 24 hours before examination.
The indicated pH values do not have an absolute meaning, 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.
pH is determined by colorometric or potentiometric methods.
Colorimetric method. To determine pH, a 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 part by weight of muscle and 4 parts of distilled water. To do this, weigh 20 grams. muscle tissue (without fat and connective tissue) is finely chopped with scissors, ground with a pestle in a porcelain mortar, to which a little water is added from a total amount 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. stand and again for 2 minutes. shaken. The extract is filtered through 3 layers of gauze and then through a paper filter.
First, the pH is approximately determined to select the desired indicator. To do this, pour 1-2 ml into a porcelain cup, extract 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. If the medium is acidic, use the indicator paranitrophenol for further research; if the medium is neutral or alkaline, use metanitrophenol. Test tubes of the same diameter made of colorless glass are inserted into the comparator sockets and filled as follows: 5 ml is poured into the first, second and third test tubes of the first row, 5 ml of distilled water is added to the first and third, and 4 ml of water is added to the second. and 1 ml of indicator, 7 ml of water is poured into test tube 5 (middle of the second row), standard sealed test tubes with colored liquid are inserted into the fourth and sixth sockets, selecting them so that the color of the contents in one of them is the same as the color of the middle middle row test tubes. The pH of the extract under study corresponds to the figure indicated on the standard test tube. If the color shade of the liquid in the test tube with the extract under study is intermediate between two standards, then take the average value between the indicators of these two standard test tubes. When using a micro-Michaelis apparatus, the number of reaction components is reduced by 10 times.
Potentiometric method. This method is more accurate, but difficult to implement because it requires constant adjustment of the potentiometer using standard buffer solutions. A detailed description of determining pH by this method is available in the instructions supplied with devices of various designs, and the pH value can be determined using potentiometers both in extracts and directly in muscles.
Peroxidase reaction. The essence of the reaction is that the peroxidase enzyme found in meat decomposes hydrogen peroxide to form atomic oxygen, which oxidizes benzidine. This produces paraquinone diimide, which, when combined with unoxidized benzidine, produces a blue-green compound that turns brown. During this reaction, peroxidase activity is important. In the meat of healthy animals it is very active; in the meat of sick animals and those killed in an agonal state, its activity is significantly reduced.
The activity of peroxidase, like any enzyme, depends on the pH of the medium, although complete correspondence between the benzidine reaction and pH is not observed.
Reaction progress: 2 ml of meat extract (at a concentration of 1:4) is poured into a test tube, 5 drops of a 0.2% alcohol solution of benzidine are added and two drops of a 1% hydrogen peroxide solution are added.
The extract from the meat of healthy animals acquires a blue-green color, turning into brownish-brown after a few minutes (positive reaction). In an extract from the meat of a sick animal or an animal killed in an agonal state, the 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, significant quantities of intermediate and final products of protein metabolism - polypeptides, peptides, amino acids, etc. - accumulate in the muscles.
The essence of this reaction is the precipitation of these products with formaldehyde. To perform the test, an aqueous extract from meat is required in a 1:1 ratio.
To prepare an extract (1:1), a meat sample is freed from fat and connective tissue and 10 grams are weighed. Then the sample is placed in a mortar, thoroughly crushed with curved scissors, and 10 ml is added. physiological solution and 10 drops of 0.1 N. sodium hydroxide solution. The meat is ground with a pestle. The resulting slurry is transferred using scissors or a glass rod into a flask and heated to boiling to precipitate the proteins. The flask is cooled under running 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 filtering, it is filtered a second time or centrifuged. If you need to get a larger amount of extract, take 2-3 times more meat and, accordingly, 2-3 times more other components.
Industrially produced formalin has an acidic environment, so it is first neutralized with 0.1 N. sodium hydroxide solution using an indicator consisting of an equal mixture of 0.2% aqueous solutions of neutralrot and methylene blue until the color changes from violet to green.
Reaction progress: 2 ml of extract is poured into a test tube and 1 ml of neutralized formaldehyde is added. The extract obtained from the meat of an animal killed in agony, seriously ill or dead turns into a dense jelly-like clot. When extracted 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 if there are good organoleptic characteristics 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 coloration of the lymph nodes. There may be pathogenic microflora in the meat. When you try cooking, the broth is cloudy and with flakes it may have an extraneous odor that is not typical for 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, staging the reaction with peroxidase, formol 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 examination, bacteriological and physical-chemical studies, meat and other products of forced slaughter are found suitable for use as food, then they are sent for boiling, according to the regime established by Privila, as well as for the production of meat loaves or canned goods “Goulash” and “ Meat pate."
The release of this meat and other slaughter products in raw form, including into public catering networks (canteens, etc.) without prior disinfection by inspection is prohibited.
Procedure for processing meat and meat products subject to disinfection
According to the Veterinary Sanitary Expertise Rules, meat and meat products of forced slaughter are disinfected by boiling 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.
Meat is considered disinfected if the temperature inside the piece reaches at least 80ºC; When cut, the color of pork becomes white-gray, and the meat of other types of animals becomes gray, without signs of a bloody tint; the juice flowing from the cut surface of a piece of boiled meat is colorless.
At meat processing plants equipped with electric or gas ovens or with canning shops, meat subject to disinfection by boiling is allowed to be sent for the production of meat loaves. When processing meat into meat loaves, the weight of the latter should be no more than 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.
Meat that meets the requirements for raw materials for canned food – “Goulash” and “Meat Pate” – is allowed for the production of canned food.
Chemical analysis of the substances under study is carried out using chemical, physical and physicochemical methods, as well as biological ones.
Chemical methods are based on the use of chemical reactions accompanied by a visible external effect, for example, a change in the color of a solution, dissolution or precipitation, or the release of gas. 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 apparatus are used to carry out analysis, recording the physical properties of a substance or changes in these properties.
When carrying out an analysis by the physical method, a chemical reaction is not used, but a physical property of a substance is measured, which is a function of its composition. For example, in spectral analysis, the emission spectra of a substance are studied and their presence is determined by the presence in the spectrum of lines characteristic of these elements, and their quantitative content is determined by the brightness of the lines. When a dry substance is added to the flame of a gas burner, the presence of certain components can be determined, for example, potassium ions will color the colorless flame violet, and sodium ions will color it yellow. These methods are accurate but expensive.
When carrying out analysis using the physicochemical method, the composition of a substance is determined based on the measurement of any physical property using a chemical reaction. For example, in colorimetric analysis, the concentration of the substance is determined by the degree of absorption of the light flux passing through a colored solution.
Biological methods of analysis are based on the use of living organisms as analytical indicators to determine 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 change under the influence of certain chemicals are used to analyze natural or waste water.
Chemical analysis methods apply in various spheres of the national economy: medicine, agriculture, food industry, metallurgy, production of building materials (glass, ceramics), petrochemistry, energy, forensics, archeology, etc.
For medical laboratory assistants, the study of analytical chemistry is necessary, since most biochemical tests are analytical: determination of the pH of gastric juice using titration, hemoglobin levels, ESR, calcium and phosphorus salts in the blood and urine, examination of cerebrospinal fluid, saliva, sodium and potassium ions in blood plasma, etc.
2. Main stages in the development of analytical chemistry.
1. Science of the ancients.
According to historical data, even the Emperor of Babylon (6th century BC) wrote about estimating the gold content. The ancient Roman writer, scientist and statesman Pliny the Elder (1st century AD) mentions the use of tanning nut 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 through, then the tin was pure; if not, it meant that there were impurities in the tin.
The first analytical instrument, the scale, has been known since ancient times. The second oldest device can be considered a hydrometer, which was described in the works of ancient Greek scientists. Many methods of processing substances used in ancient chemical crafts (filtration, drying, crystallization, boiling) have become part of the practice of analytical research.
2. Alchemy – the realization by chemists of society’s desire 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), and learned to produce valuable chemical products such as essential oil (1280), gunpowder (1330).
3. Iatrochemistry or medicinal chemistry - during this period the main direction of chemical knowledge was the production of medicines (XVI-XVII centuries).
During this period, many chemical methods for detecting substances appeared, based on transferring them into solution. In particular, the reaction of silver ion with chloride ion was discovered. During this period, most of the chemical reactions that form the basis of qualitative analysis were discovered. The concept of “deposition”, “sediment” was introduced.
4. The era of phlogiston: “phlogiston” is a special “substance” that supposedly 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, paints). Using a blowtorch, the qualitative composition of many minerals was established. The largest analyst of the 18th century, T. Bergman, opened the way to modern metallurgy, determining the exact carbon content in various samples of iron obtained using coal, and created the first scheme for qualitative chemical analysis.
The founder of analytical chemistry as a science is considered to be R. Boyle (1627-1691), who coined the term “chemical analysis” and used various reagents when conducting qualitative analysis, for example, silver nitrate to determine hydrochloric acid, and copper salts by adding excess ammonia. He used tinctures of violets and cornflowers as indicators for determining acids and hydroxides.
Works by Lomonosov M.V. also belong to this time, he denied the presence of phlogiston, was the first to introduce quantitative accounting of the reagents of chemical processes into the practice of chemical research, and is rightfully considered one of the founders of quantitative analysis. He was the first to use a microscope to study qualitative reactions and, based on the shape of the crystals, made 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” 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 a chromatographic method.
6. Modern period.
If in the previous period analytical chemistry developed in response to the social needs 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 environmental protection, agricultural products, and pharmacy. Research in the field of astronautics and sea waters also suggests further development of AH.
Modern instrumental methods of chromatography, such as neutron activation, atomic adsorption, atomic emission, and infrared spectrometry, make it possible to determine 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 interconnected with the progress of society.
3. Main classes of inorganic compounds: oxides, classification, physical. and chem. Saints, receiving.
Oxides are complex substances consisting of oxygen atoms and an element (metal or non-metal).
I. Classification of oxides.
1) salt-forming agents, which react with acids or bases to 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 the 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 oxidation state;
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.
Acidic 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 acid oxides with water:
acid oxide + water = acid
SO 3 + H 2 O = H 2 SO 4
Reaction of acid oxides with bases:
oxide + base = salt + water
CO 2 + NaOH = Na 2 CO 3 + H 2 O
In reactions of acidic oxides with alkalis, the formation of acidic salts is possible with an excess of acidic oxide.
CO 2 + Ca(OH) 2 = Ca(HCO 3) 2
Reaction of acidic oxides with basic oxides:
acidic oxide + basic oxide = salt
CO 2 + Na 2 O = Na 2 CO 3
B. Chemical properties of basic oxides.
These metal oxides correspond to bases. There is the following genetic relationship:
Na → Na 2 O → NaOH
Reaction of basic oxides with water:
basic oxide + water = base
K 2 O + H 2 O = 2 KON
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 = MgCl 2 + H 2 O
If in such a reaction the acid is taken in excess, then, of course, an acidic 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 + acidic oxide = salt
CaO + CO 2 = CaCO 3
B. Chemical properties of amphoteric oxides.
These are oxides that, depending on conditions, exhibit the properties of basic and acidic oxides.
Reaction with bases:
amphoteric oxide + base = salt + water
ZnO + KOH = K 2 ZnO 2 + H 2 O
Reaction with acids:
amphoteric oxide + acid = salt + water
ZnO + 2HNO 3 = Zn(NO 3) 2 + H 2 O
3. Reactions with acid oxides: t
amphoteric oxide + basic oxide = salt
ZnO + CO 2 = ZnCO 3
4. Reactions with basic oxides: t
amphoteric oxide + acidic oxide = salt
ZnO + Na 2 O = 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 = acid oxide + water t
H 2 SO 3 = SO 2 + H 2 O
3. Decomposition of insoluble bases:
Insoluble base = basic oxide + water t
Сu(OH) 2 = CuO + H 2 O
4. Decomposition of some salts:
salt = basic oxide + acidic oxide t
CaCO 3 = CaO + CO 2
4. Main classes of inorganic compounds: acids, classification, physical. and chem. Saints, receiving.
An acid is a complex compound containing hydrogen ions and an acidic residue.
acid = nH + + acid residue - n
I. Classification
Acids are inorganic (mineral) and organic.
oxygen-free (HCl, HCN)
Based on the number of H + ions formed during dissociation, it is determined acid basicity:
monobasic (HCl, HNO 3)
dibasic (H 2 SO 4, H 2 CO 3)
tribasic (H 3 PO 4)
II. Physical properties.
Acids are:
water soluble
insoluble in water
Almost all acids taste sour. Some of the acids have a smell: acetic, nitric.
III. Chemical properties.
1. Change the color of the 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 voltage series of metals. Metals to the left of hydrogen H in this row displace it from acids. Exception: when nitric acid reacts with metals, hydrogen is not released.
acid + metal = salt + H 2
H 2 SO 4 + Zn = ZnSO 4 + H 2
3. Reaction with bases (neutralization):
acid + base = salt + water
2HCl + 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 = CaCl 2 + H 2 O
5. Reaction with salts:
These reactions are possible if they result in the formation of 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 = 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 = HCl
2. Oxygen-containing acids are obtained by reacting acid oxides with water:
SO 3 + H 2 O = H 2 SO 4
3. Most acids can be obtained by reacting salts with acids.
2Na 2 CO 3 + HCl = H 2 CO 3 + NaCl
5. Main classes of inorganic compounds: salts, classification, physical. and chem. Saints, receiving.
Salts are complex substances, products of complete or partial replacement of hydrogen in acids with metal atoms or hydroxyl groups in bases with an acid residue.
In other words, in the simplest case, a salt consists of metal atoms (cations) and an acid residue (anion).
Classification of salts.
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 their solubility in water, salts are classified as 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 = FeSO 4 + Cu
Salts can interact with metals if the metal to which the salt cation corresponds is located in the voltage series to the right of the reacting free metal.
2. Reaction of salts with acids:
salt 1 + acid 1 = salt 2 + acid 2
BaCl 2 + H 2 SO 4 = BaSO 4 + 2HCl
Salts react with acids:
a) the cations of which form an insoluble salt with the anions of the acid;
b) anions of which correspond to unstable or volatile acids;
c) the anions of which correspond to slightly soluble acids.
3. Reaction of salts with solutions of bases:
salt 1 + base 1 = salt 2 + base 2
FeCl 3 + 3KOH = Fe(OH) 3 + 3KCl
Only salts react with alkalis:
a) metal cations of which correspond to insoluble bases;
b) the anions of which correspond to insoluble salts.
4. 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 sediment.
5. Many salts decompose when heated:
MgCO 3 = CO 2 + MgO
6. Basic salts react with acids to form medium salts and water:
Basic salt + acid = medium salt + H 2 O
CuOHCl + HCl = CuCl 2 + H 2 O
7. Acidic salts react with soluble bases (alkalis) to form medium salts and water:
Acid salt + acid = 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. saints, receiving
Bases are complex substances containing metal ions and one or more hydroxyl groups (OH -).
The number of hydroxyl groups corresponds to the oxidation state of the metal.
Based on the number of hydroxyl groups, bases are divided:
monoacid (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 special feature of alkalis is their soapiness to the touch.
II. Chemical properties.
1. Reaction with indicators.
base + phenolphthalein = crimson 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 = 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 = Cu(OH) 2 ↓ + K 2 SO 4
For the reaction to occur, it is necessary that the reacting base and salt be soluble, and the resulting base and/or salt precipitate.
6. Decomposition reaction of bases when heated: t
base = oxide + water
Сu(OH) 2 = СuO + 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. Reaction of alkalis with metals:
Alkali solutions interact with metals that form amphoteric oxides and hydroxides (Zn, Al, Cr)
Zn + 2NaOH = Na 2 ZnO 2 + H 2
Zn + 2NaOH + H 2 O = Na 2 + H 2
IV. Receipt.
1. A soluble base can be obtained by reacting alkali and alkaline earth metals with water:
K + H 2 O = 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. Classifications of analysis methodsfor. Identification of substances. Fractional analysis. Systematic analysis.
Main tasks of analytical chemistry
One of the tasks when carrying out environmental protection measures is to understand the patterns of cause-and-effect relationships between various types of human activity and changes occurring in the natural environment. Analysis- This 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 of 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) accuracy and good reproducibility;
2) low detection limit- this is the lowest content at which, using this method, it is possible to detect the presence of the analyte component with a given confidence probability;
3) selectivity (selectivity)- characterizes the interfering influence of various factors;
4) range of measured contents(concentrations) using this method using this method;
5) expressiveness;
6) ease of analysis, possibility of automation, cost-effectiveness of determination.
Chemical analysis- this is a complex multi-stage about process, which is a set of ready-made techniques and corresponding 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 a particular component in the analyzed object.
Stages of analysis of any object
1. Statement of the problem and choice of method and analysis scheme.
2. Sampling (proper selection of part of the sample allows one to draw a correct conclusion about the composition of the entire sample). Try- this is part of the analyzed material, representatively negative A compressing its chemical composition. In some cases, the entire analytical material is used as a sample. The storage time of the collected samples should be kept to a minimum. l nom. Storage conditions and methods must exclude uncontrolled losses 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 ones; concentration of components;
4. Obtaining an analytical signal. Analytical signal- this is a change in any physical or physico-chemical property of the component being determined, functionally related to its content (formula, table, graph).
5. Processing of the analytical signal, i.e. separation of signal and noise. Noises- spurious signals arising in measuring instruments, amplifiers and other devices.
6. Application of analysis results. Depending on the properties of the substance used as the basis for the definition, analysis methods 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 the chemical reaction (for example, photometry is based on a change in the optical density of a solution as a result of the reaction);
- physical methods analyzes not related to the use of chemical reactions. The composition of substances is determined by measuring the characteristic physical properties of an 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 electrode potential |
Potentiometry |
|
Electrode polarization resistance |
Polarography |
|
Amount of electricity |
Coulometry |
|
Electrical conductivity of the solution |
Conductometry |
|
Photon absorption |
Photometry |
|
Emission of photons |
Emission spectral analysis |
Identification of substances is based on methods of qualitative recognition of elementary objects (atoms, molecules, ions, etc.) of which substances and materials are composed.
Very often, the analyzed substance sample is converted into a form convenient for analysis by dissolving it in a suitable solvent (usually water or aqueous solutions of acids) or fusing it with some chemical compound and then dissolving it.
Chemical methods of qualitative analysis are based on using reactions of identified ions with certain substances - analytical reagents. Such reactions must be accompanied by precipitation or dissolution of a 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 occurring in solutions by method of execution classified into test-tube, microcrystalscopic and drop-type. Microcrystalloscopic reactions are carried out on a glass slide. The formation of crystals of a characteristic shape is observed. Droplet reactions are performed on filter paper.
Analytical reactions used in qualitative analysis are by area of application divided:
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 in relation to alone component.
There are few selective and specific reactions, so they are used in combination with group reactions and special techniques to eliminate the interfering influence of components present in the system along with the substance being analyzed.
Simple mixtures of ions are analyzed using the fractional method Without prior separation of interfering ions, individual ions are determined using characteristic reactions. M negative 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. 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. Transfer of interfering ions into sediment. 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 determined ion with the reagent must be greater than the PR of the compound of the interfering ion.
2. Binding of interfering ions into a stable complex compound. The resulting complex must have the necessary stability to achieve complete binding of the interfering ion, and the desired ion must not react at all with the introduced reagent or its complex must be fragile.
3. Change in the oxidation state of interfering ions.
4. Use of extraction. The method is based on the extraction of interfering ions from aqueous solutions with organic solvents and separation of the system into 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 lengthy operations of sequential separation of some ions from others is reduced;
Fractional reactions are easily reproducible, i.e. they can be repeated several times. However, if it is difficult to select selective (specific) reactions for detecting ions, masking reagents, and calculating the completeness
removal of ions and other reasons (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 components in the sample into several groups in a certain sequence. The division into groups is based on the similarities (within the group) and differences (between groups) of the analytical properties of the components. In a dedicated analysis group, a series of sequential separation reactions is used until only the 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 detection and separation, and in the analysis of anions they serve only detection (Fig. 23.2).
Analysis of cation mixtures
Group reagents in the qualitative analysis of cations are acids, strong bases, ammonia, carbonates, phosphates, alkali metal sulfates, oxidizing agents and reducing agents. The grouping 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 V certain reactions.
Group reagents are selected from the common reagents because the group reagent is required to release a relatively large number of ions. The main method of separation is precipitation, i.e. division into groups is based on the different solubility of cation precipitates in certain environments. When considering the action of group reagents, the following groups can be distinguished (Table 23.2).
In addition, three cations remain (Na +, K +, NH4), which do not form precipitation with the indicated group reagents. They can also be separated into a separate group.
Groups of cations
In addition to the indicated general approach, when choosing group reagents, they proceed from the values of the solubility products of precipitation, since by varying the precipitation conditions, it is possible to separate substances from a group by the action of the same reagent.
The most widely used is the acid-base classification of cations. Advantages of the acid-base method of systematic analysis:
a) the basic properties of elements are used - their relationship to acids and alkalis;
b) analytical groups of cations to a greater extent with correspond to groups of the periodic system of elements D.I. Mendeleev;
c) the analysis time is significantly reduced compared to the hydrogen sulfide method. The study begins with preliminary tests, in which the pH of the solution is established using a universal indicator and NH 4, Fe 3+, Fe 2+ ions are detected by specific and selective reactions.
Division into groups. 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 H2S04 is added dropwise. Leave the precipitate for 10 minutes, then it is centrifuged and washed with water acidified with HC1. A mixture of chlorides and sulfates Ag +, Pb 2+, Ba 2+, Ca 2+ remains in the sediment. The presence of basic antimony salts is possible. In solution there are cations of groups III-VI.
Group III is separated from the solution by adding a few drops of 3% H 2 0 2 and excess NaOH with heating and stirring. Excess hydrogen peroxide is removed by boiling. In the precipitate there are hydroxides of cations of groups IV-V, in the solution there are cations of groups III and VI and partially Ca 2+, which may not completely precipitate in the form of CaS0 4 during the separation of groups I and II.
Group V cations are separated from the sediment. The precipitate is treated with 2N Na 2 CO 3 and then with excess NH 3 while heating. Cations of group V pass into solution in the form of ammonia, in the sediment - carbonates and basic salts of cations of group IV.
The virtue of systematic analysis- obtaining sufficiently complete information about the composition of the object. Flaw- bulkiness, duration, labor intensity. Fully systematic qualitative analysis designs are rarely implemented. Usually they are used partially if there is information about the origin, approximate composition of the sample, a So same in training courses in analytical chemistry.
Magnesium hydroxide dissolves in a mixture of NH 3 + NH 4 C1. Thus, after separating the cations into groups, four test tubes were obtained containing a) a precipitate of chlorides and sulfates of cations of groups I-P; b) a solution of a mixture of cations of groups III and VI; c) a solution of ammonia 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. 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 ratio corresponding salts. Depending on what substance the anion is part of, its properties change significantly. For example, the SO 4 2 " ion in the composition of concentrated sulfuric acid is characterized by oxidation-reduction reactions, and in the composition of salts - precipitation reactions.
The state of anions in a solution depends on the solution environment. Some anions decompose under the action of concentrated acids with the release of the corresponding gases: CO 2 (CO 2-3 anion), H 2 S (S 2 "anion), N0 2 (N0 3 anion), etc. Under the action of dilute acids, MoO 4 2 anions - , W0 4 2 ~, SiO 3 2 "form water-insoluble acids (H 2 Mo0 4, H 2 W0 4 * H 2 0, H 2 SiABOUT 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 elements that form anions have variable valency and, when exposed to oxidizing or reducing agents, change the degree of oxidation, and the composition of the anion changes. 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 into acid Ouch medium to a free ion, diphenylamine is colored blue. These properties are used for qualitative analysis; the redox properties of chromate, nitrate, iodide, vanadate, molybdate, tungstate ions are the basis their characteristic reactions.
Group reactions of anions. Reagents based on their action on anions are divided into the following groups:
1) reagents that decompose substances with the release of gases. Such reagents include dilute mineral acids (HC1, H 2 S0 4);
2) reagents that release anions from solutions in the form of slightly dissolved precipitation (Table 23.4):
a) BaCl 2 in a neutral environment 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 reagents (KI) (Table 23.5);
4) oxidizing reagents (KMn0 4, solution of I 2 in KI, HNO 3 (conc), H 2 S0 4).
During analysis, anions generally 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 analyzing a mixture of anions, based on new on dividing them into groups, are rarely used, mainly zom for studying simple mixtures. The more complex the anion mixture, the more cumbersome the analysis schemes become.
Fractional analysis allows you to detect anions that do not interfere with each other in separate portions of the solution.
Semi-systematic methods involve the separation of anions into groups using group reagents and subsequent fractional detection of the anions. This leads to a reduction in the number of required sequential analytical operations and ultimately simplifies the analysis scheme for a mixture of anions.
The current state of qualitative analysis is not limited to the classical scheme. In analysis as 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, due to its simplicity, accessibility, and low cost, has not lost its significance.
1. Sampling:
A laboratory sample consists of 10–50 g of material, which is selected so that its average composition corresponds to the average composition of the entire batch of the analyzed substance.
2. Decomposition of the sample and transferring it into solution;
3. Carrying out a chemical reaction:
X – determined component;
P – reaction product;
R – reagent.
4. Measurement of any physical parameter of a 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). Conditions are created under which the analyte is completely converted into a reaction product; Further, it is necessary that the reagent R does not produce minor reaction products with foreign substances, the physical properties of which would be similar to the physical properties of the product.
2. Based on measuring the amount of reagent consumed for the reaction with the analyte X:
– the influence 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, electrical conductivity).
3. Records changes occurring with the analyte X itself during 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: The substances being determined are converted into complex ions and compounds by the action of reagents.
The following separation and determination methods are based on complexation reactions:
1) Separation by means of deposition;
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 is called extraction);
3) Photometric (Co with nitrous salt) - measure the optimal density of solutions of complex compounds;
4) Titrimetric method of analysis
5) Gravimetric method of analysis.
1) cementation method - reduction of Me metal ions in solution;
2) electrolysis with a mercury cathode - during 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 – electricity is passed through the solution being tested. a current of a certain voltage, while the Me ions are reduced 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 analyte. Analysis reagents are found according to Faraday's law:
M – quantity of the element being determined;
F – Faraday number (98500 C);
A is the atomic mass of the element;
n – the number of electrons taking part 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:
The L-G or T-G equilibrium is used in analysis when releasing substances into the gas phase (CO 2 , H 2 O, etc.).
The equilibrium Zh 1 – Zh 2 is observed in the extraction method and during electrolysis with a mercury cathode.
Liquid-T is characteristic of precipitation processes and processes of separation of the solid phase on the surface.
Analysis methods include:
1. gravimetric;
2. titrimetric;
3 optical;
4. electrochemical;
5. catalytic.
Separation methods include:
1. deposition;
2. extraction;
3. chromatography;
4. ion exchange.
Concentration methods include:
1. deposition;
2. extraction;
3. cementation;
4. distillation.
Physical methods of analysis
A characteristic feature is that they directly measure any physical parameters of the system related to the amount of the element being determined without first conducting a chemical reaction.
Physical methods include three main groups of methods:
I Methods based on the interaction of radiation with matter or on the measurement of radiation from matter.
II Methods based on measuring electrical parameters. or magnetic properties of a substance.
III Methods based on measuring 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 analysis methods.
Atomic emission analysis:
1) Flame photometry - the analyzed solution is sprayed into the flame of a gas burner. Under the influence of high temperature, atoms go into an excited state. The outer valence electrons move to higher energy levels. The return 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 the 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 condensed spark; at high temperature, the atoms go into an excited state, and electrons move not only to the energy levels closest to the main one, but also to more distant ones.
Radiation is a complex mixture of light vibrations of different wavelengths. The emission spectrum is decomposed into the main parts of the spec. instruments, spectrometers, and photographs. Comparing the position of the intensity of individual lines of the spectrum with the lines of the corresponding standard allows us to determine the qualitative and quantitative analysis of the sample.
Atomic absorption analysis methods:
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 closest orbital with a higher energy level. A decrease in the intensity of light as it passes through a flame due to the transfer of electrons of the atoms of the element being determined to an excited state is proportional to the number of unexcited atoms in it. In atomic absorption, flammable mixtures with temperatures up to 3100 o C are used, which increases the number of elements to be determined in comparison with flame photometry.
X-ray fluorescence and X-ray emission
X-ray fluorescence: the sample is exposed to x-ray radiation. 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 related functionally to the atomic number of the element. Wavelength – qualitative composition of the sample; intensity – quantitative composition of the sample.
Methods based on nuclear reactions - radioactivation. The material is exposed to neutron radiation, nuclear reactions occur and radioactive isotopes of the elements are formed. Next, the sample is transferred into solution and the elements are separated using chemical methods. Then 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. Detection limit 10 -8 – 10 -10%.
1. Conductometric – based on measuring the electrical conductivity of solutions or gases.
2. Potentiometric – there are direct and potentiometric titration methods.
3. Thermoelectric - based on the occurrence of thermoelectromotive force, which arises when the place of contact of steel, etc. is heated.
4. Mass spectral - used with the help of strong elements and magnetic fields, gas mixtures are separated into components in accordance with the atoms or molecular masses of the components. Used in the study of mixtures of isotopes. inert gases, mixtures of organic substances.
Densitometry is based on measuring density (determining the concentration of substances in solutions). To determine the composition, viscosity, surface tension, speed of sound, electrical conductivity, etc. are measured.
To establish 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 physical and chemical properties of substances
Approximate forecasting calculation
Prediction implies an assessment of physicochemical properties based on a minimum number of readily available initial data, and may even assume the complete absence of experimental information about the properties of the substance under study (“absolute” prediction is based only on information about the stoichiometric formula of the compound).