Currently there are many specialists who have devoted themselves to the physical or chemical sciences, and sometimes both. Indeed, most phenomena can be logically explained precisely through such experiments. We will consider physical research methods in more detail.
Methods of analysis in analytical chemistry
Analytical chemistry is the science of detecting, separating and identifying chemicals. To carry out certain operations with compounds, chemical, physical and physico-chemical methods of analysis are used. The latter method is also called instrumental, since its application requires modern laboratory equipment. It is subdivided into spectroscopic, nuclear physics and radiochemical groups.
Besides, in chemistry there can be problems of different types that require individual solutions. Depending on this, there are methods of qualitative (determining the name and form of a substance) and quantitative (determining how much of a given substance is contained in an aliquot or sample) analysis.
Quantitative analysis methods
They allow you to determine the content of the original substance in the sample. In total, there are chemical, physico-chemical and physical methods of quantitative analysis.
Chemical methods of quantitative analysis
They are divided into:
- Weight analysis that allows you to determine the content of a substance by weighing on an analytical balance and carrying out further operations.
- Volume analysis, which involves measuring the volume of substances in different aggregate states or solutions.
In turn, it is divided into the following subsections:
- volumetric titrimetric analysis is used at a known concentration of the reagent, the reaction with which the required substance is consumed, and then the volume consumed is measured;
- volumetric gas method is to analyze gas mixtures where the original substance is absorbed by another.
- volumetric sedimentation (from the Latin sedimentum - "settlement") is based on stratification by a dispersed system as a result of gravity. This is accompanied by precipitation, the volume of which is measured using a centrifuge tube.
Chemical methods are not always convenient to use, since it is often necessary to separate the mixture in order to isolate the desired component. To perform such an operation without the use of chemical reactions, physical methods of analysis are used. And to observe the change in the physical properties of the compound as a resultcarrying out reactions - physical and chemical.
Physical methods of quantitative analysis
They are used during many laboratory studies. Physical methods of analysis include:
- Spectroscopic - based on the interaction of atoms, molecules, ions of the studied compound with electromagnetic radiation, as a result of which photons are absorbed or released.
- The nuclear-physical method consists in exposing a sample of the substance under study to a neutron flux, by studying which, after the experiment, it is possible to determine the quantitative content of the elements contained in the sample by measuring radioactive radiation. This works because the amount of particle activity is directly proportional to the concentration of the element under study.
- The radiochemical method is to determine the content in the substance of radioactive isotopes formed as a result of transformations.
Physico-chemical methods of quantitative analysis
Since these methods are only part of the physical methods of analyzing a substance, they are also divided into spectroscopic, nuclear physical and radiochemical methods of research.
Qualitative analysis methods
In analytical chemistry, in order to study the properties of a substance, determine its physical state, color, taste, smell, methods of qualitative analysis are used, which, in turn, are divided into the same chemical, physical and physico-chemical (instrumental). Moreover, physical methods of analysis are preferred in analytical chemistry.
Chemical methods are carried out in two ways: reactions in solutions and reactions in dry way.
Wet way reactions
Reactions in solutions have certain conditions, one or more of which must be met:
- Formation of an insoluble precipitate.
- Changing the color of the solution.
- Evolution of a gaseous substance.
Precipitate formation can occur, for example, as a result of the interaction of barium chloride (BaCl2) and sulfuric acid (H2SO4). The products of the reaction are hydrochloric acid (HCl) and a water-insoluble white precipitate - barium sulfate (BaSO4). Then the necessary condition for the occurrence of a chemical reaction will be fulfilled. Sometimes the products of the reaction may be a couple of substances, which must be separated by filtration.
Changing the color of the solution as a result of chemical interaction is a very important feature of the analysis. This is most often observed when working with redox processes or when using indicators in the acid-base titration process. Substances that can color the solution with the appropriate color include: potassium thiocyanate KSCN (its interaction with iron III s alts is accompanied by a blood-red coloration of the solution), ferric chloride (when interacting with chlorine water, the weak green color of the solution turns yellow), potassium dichromate (when reduced and under the action of sulfuric acid, it changes from orange todark green) and others.
Reactions that proceed with the release of gas are not basic and are used in rare cases. The most commonly produced carbon dioxide in laboratories is CO2.
Dry reactions
Such interactions are performed to determine the content of impurities in the analyzed substance, in the study of minerals, and it consists of several stages:
- Fusibility test.
- Flame color test.
- Volatility test.
- The ability to redox reactions.
Usually, mineral substances are tested for melting ability by preheating a small sample of them over a gas burner and observing the rounding of its edges under a magnifying glass.
To check how the sample is capable of coloring the flame, it is applied on a platinum wire first to the base of the flame, and then to the place that is heated the most.
The volatility of the sample is checked in the assay cylinder, which is heated after the introduction of the test element.
Reactions of redox processes are most often carried out in dry balls of fused borax, in which the sample is placed and then subjected to heating. There are other ways to carry out this reaction: heating in a glass tube with alkali metals - Na, K, simple heating or heating on charcoal, and so on.
Use of chemical indicators
Sometimes chemical analysis methods use differentindicators that help determine the pH of the medium of a substance. The most commonly used are:
- Litmus. In an acidic environment, indicator litmus paper turns red, and in an alkaline environment it turns blue.
- Methylorange. When exposed to an acidic ion, it turns pink, alkaline - turns yellow.
- Phenolphthalein. In an alkaline environment, it is characteristic of a red color, and in an acidic environment it has no color.
- Curcumin. It is used less often than other indicators. Turns brown with alkalis and yellow with acids.
Physical methods of qualitative analysis
Currently, they are often used in both industrial and laboratory research. Examples of physical methods of analysis are:
- Spectral, which has already been discussed above. It, in turn, is divided into emission and absorption methods. Depending on the analytical signal of the particles, atomic and molecular spectroscopy are distinguished. During emission, the sample emits quanta, and during absorption, the photons emitted by the sample are selectively absorbed by small particles - atoms and molecules. This chemical method uses such types of radiation as ultraviolet (UV) with a wavelength of 200-400 nm, visible with a wavelength of 400-800 nm and infrared (IR) with a wavelength of 800-40000 nm. Such areas of radiation are otherwise called the "optical range".
- Luminescent (fluorescent) method consists in observing the emission of light by the substance under study due toexposure to ultraviolet rays. The test sample can be an organic or mineral compound, as well as some medications. When exposed to UV radiation, the atoms of this substance pass into an excited state, characterized by an impressive energy reserve. During the transition to the normal state, the substance luminesces due to the residual amount of energy.
- X-ray diffraction analysis is carried out, as a rule, using x-rays. They are used to determine the size of atoms and how they are located relative to other sample molecules. Thus, the crystal lattice, the composition of the sample, and the presence of impurities in some cases are found. This method uses a small amount of analyte without the use of chemical reactions.
- Mass-spectrometric method. Sometimes it happens that the electromagnetic field does not allow certain ionized particles to pass through it due to too large a difference in the ratio of mass and charge. In order to determine them, this physical method of analysis is needed.
Thus, these methods are in high demand, compared to conventional chemical ones, because they have a number of advantages. However, the combination of chemical and physical methods of analysis in analytical chemistry gives a much better and more accurate result of the study.
Physico-chemical (instrumental) methods of qualitative analysis
These categories include:
- Electrochemical methods that consist in measuringelectromotive forces of galvanic cells (potentiometry) and electrical conductivity of solutions (conductometry), as well as in the study of the motion and rest of chemical processes (polarography).
- Emission spectral analysis, the essence of which is to determine the intensity of electromagnetic radiation on a frequency scale.
- Photometric method.
- X-ray spectral analysis, which examines the spectra of x-rays that have passed through the sample.
- Method for measuring radioactivity.
- The chromatographic method is based on the repeated interaction of sorption and desorption of a substance when it moves along an immobile sorbent.
You should know that basically physical-chemical and physical methods of analysis in chemistry are combined into one group, so when they are considered separately, they have a lot in common.
Physico-chemical methods of separation of substances
Very often in laboratories there are situations when it is impossible to extract the required substance without separating it from another. In such cases, methods of separation of substances are used, which include:
- Extraction - a method by which the necessary substance is extracted from a solution or mixture by means of an extractant (corresponding solvent).
- Chromatography. This method is used not only for analysis, but also for the separation of components that are in the mobile and stationary phases.
- Separation by ion exchange. As a resultthe desired substance may precipitate, insoluble in water, and can then be separated by centrifugation or filtration.
- Cryogenic separation is used to extract gaseous substances from the air.
- Electrophoresis is the separation of substances with the participation of an electric field, under the influence of which particles that do not mix with each other move in liquid or gaseous media.
Thus, the laboratory assistant will always be able to get the required substance.
