Currently, there are many specialists who have devoted themselves to the physical or chemical sciences, and sometimes immediately to both. Indeed, most of the phenomena can be logically explained precisely through such experiments. Physical research methods will be considered 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 use requires modern laboratory equipment. It is divided into spectroscopic, nuclear-physical and radiochemical groups.
In addition, in chemistry there may be problems of various 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) of analysis.
Quantitative Analysis Methods
They allow you to determine the content of the starting substance in the sample. Total distinguish between chemical, physico-chemical and physical methods of quantitative analysis.
Chemical methods of quantitative analysis
They are divided into:
- Weight analysis, which allows to determine the substance content by weighing on an analytical balance and conducting further operations.
- Volume analysis, which involves the measurement of the volume of substances in different states of aggregation 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 desired substance is consumed, and then the spent volume is measured;
- The volumetric gas method is to analyze gas mixtures where the starting material is absorbed by another.
- volume sedimentation (from the Latin sedimentum - "subsidence") is based on layering by the disperse 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 very often it is necessary to separate the mixture in order to isolate the desired component. To perform such an operation without using chemical reactions, physical analysis methods are used. And to observe the change in the physical properties of the compound as a result of the reactions, physico-chemical.
Physical methods of quantitative analysis
They are used during many laboratory studies. Physical analysis methods include:
- Spectroscopic - based on the interaction of atoms, molecules, ions of the test compound with electromagnetic radiation, as a result of which photons are absorbed or emitted.
- 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 radiation. This works because the particle activity is directly proportional to the concentration of the element being studied.
- The radiochemical method consists in determining 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 analysis of matter, 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, apply methods of qualitative analysis, 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: by reactions in solutions and by dry reactions.
Wet reactions
The course of reactions in solutions has some conditions, one or more of which must be necessarily fulfilled:
- The formation of insoluble sediment.
- Change the color of the solution.
- Release of gaseous matter.
The formation of a precipitate can occur, for example, as a result of the interaction of barium chloride (BaCl2) and sulfuric acid (H2SO4). The reaction products are hydrochloric acid (HCl) and a white insoluble precipitate of white color - barium sulfate (BaSO4). Then the necessary condition for the occurrence of the chemical reaction will be satisfied. Sometimes the reaction products may be a pair of substances, which must be separated by filtration.
The color change of the solution as a result of chemical interaction is a very important sign of analysis. This is most often observed when working with redox processes or when using indicators in the process of acid-base titration. Substances that can stain the solution with the appropriate color include: potassium thiocyanate KSCN (its interaction with iron salts III is accompanied by a blood-red staining of the solution), ferric chloride (when it interacts with chlorine water, the slightly green color of the solution turns yellow), potassium dichromate (upon reduction and under the influence of sulfuric acid passes from orange to dark green) and others.
The reactions proceeding with evolution of gas are not basic and are used in rare cases. Most often, laboratories receive carbon dioxide CO2.
Dry reactions
Such interactions are carried out to determine the content of impurities in the analyte, in the study of minerals, and it consists of several stages:
- Fusibility test.
- Flame stain test.
- Volatility test.
- The ability to redox reactions.
Usually mineral substances are tested for their ability to melt by preheating a small sample of them over a gas burner and observing the curvature of its edges under a magnifying glass.
To check how much the sample is able to color the flame, it is introduced on a wire from platinum, first to the base of the flame, and then to the place heated most.
The volatility of the sample is checked in the assay cylinder, which is heated after the introduction of the test element.
Redox reactions 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 of carrying out this reaction: incandescent in a glass tube with alkali metals - Na, K, simple heating or heating on charcoal, and so on.
The use of chemical indicators
Sometimes in the methods of chemical analysis, various indicators are used that help determine the pH of the substance. The most commonly used are:
- Litmus. In an acidic environment, indicator litmus paper turns red, and in an alkaline one it turns blue.
- Methyl orange. When exposed to it, the acidic ion turns pink, alkaline - it turns yellow.
- Phenolphthalein. In an alkaline environment, a red color is characteristic of it, but in an acidic medium it has no color.
- Curcumin Used less frequently than other indicators. It turns brown under the influence of alkalis and yellow under the influence of acids.
Physical methods of qualitative analysis
Currently, they are often used in both industrial and laboratory research. Examples of physical analysis methods are:
- Spectral, which was already discussed above. It, in turn, is divided into emission and absorption methods. Depending on the analytical signal of the particles, atomic and molecular spectropy are distinguished. During emission, the sample emits quanta, and upon 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 also called the "optical range."
- The luminescent (fluorescent) method consists in observing the emission of light by the substance under study due to the action of ultraviolet rays on them. 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 go into an excited state, characterized by an impressive energy reserve. Upon transition to a 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 also 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 an electromagnetic field does not pass through these or those ionized particles due to too much difference in the ratio of mass and charge. In order to determine them, this physical analysis method is required.
Thus, these methods are in great demand, in comparison with conventional chemical methods, because they have a number of advantages. However, a combination of chemical and physical methods of analysis in analytical chemistry gives a much better and accurate research result.
Physicochemical (instrumental) methods of qualitative analysis
These include the following categories:
- Electrochemical methods, which consist in measuring the electromotive forces of galvanic cells (potentiometry) and the 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 as a result of which the spectra of x-rays transmitted through the sample are studied.
- Method for measuring radioactivity.
- The chromatographic method is based on the multiple interaction of sorption and desorption of a substance as it moves along a stationary sorbent.
You should know that basically the physicochemical and physical methods of analysis in chemistry are combined into one group, so when they are considered separately, they have much 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 is a method by which the necessary substance is extracted from a solution or mixture by means of an extractant (appropriate 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 result of this, the necessary substance may precipitate, insoluble in water, and then it will be possible to separate it by centrifugation or filtration.
- Cryogenic separation is used to produce gaseous substances from the air.
- Electrophoresis is the separation of substances with the participation of an electric field, under which particles immiscible with each other move in a liquid or gaseous medium.
Thus, the laboratory assistant will always be able to get the desired substance.