Gravimetric analysis method: concept, types and features

What are the hallmarks of gravimetric analysis? Let us consider in more detail its essence and varieties.

Specificity

The gravimetric method of analysis based on the law of conservation of mass of substances and constancy of composition is based. In this regard, it is based on an accurate measurement of the mass of the desired component, which is obtained as a compound with a known chemical composition. The gravimetric method of analysis is divided into three main groups: distillation, isolation, precipitation.

About the selection method

It is based on the extraction of the desired component from the analyzed chemical substance in a free form and its subsequent accurate weighing. For example, such a gravimetric method of quantitative analysis makes it possible to determine the mass content of the ash residue in solid fuels. To carry out the calculations, weigh the crucible, burn the fuel sample in it, and the resulting ash is weighed. Having a mass of the residue, the quantitative index is calculated from the formula for the mass fraction of the substance in the mixture.

Distillation

This method of analysis is gravimetric in content, since it assumes a complete removal of the calculated component as a gaseous compound and subsequent weighing of the solid residue. By this technique, one can determine the moisture content of various materials, calculate the quantitative content in crystalline hydrates of crystallization water. To perform such a calculation, the mass of the sample weights of the selected material is initially determined. Then, the component to be determined is completely removed from it. The difference between the mass before calcining or drying and after them is the mass of the detected chemical component. By the mass fraction formula, quantitative calculations are carried out.

Method of precipitation

What is this method of analysis? The gravimetric method of precipitation is based on the quantitative precipitation of the desired ion as a sparingly soluble substance with a certain chemical composition. The resulting precipitate is filtered, washed, dried, then calcined. After completely removing water from it, weigh it. Knowing the mass of the sediment, it is possible to calculate the quantitative content of the molecules or ions of the desired component in the sample under study.

Requirements for precipitation in gravimetric analysis

And yet - what is the gravimetric method of analysis? The main operations in the deposition method are related to the precipitation process. The accuracy of the result obtained during the analysis directly depends on the chemical composition of the substance, the structure of the sediment, and the degree of purity. In addition, the calculations are related to the behavior of the precipitate during drying and calcination. Quite often there is a change in the chemical composition of the resulting precipitate during its calcination. A precipitated form is the chemical composition of the obtained precipitate.

The main methods of gravimetric analysis assume an accurate result. This is why certain requirements are imposed on the gravimetric and precipitated form of the sediment.

1. It should have minimal solubility, ideally an insoluble chemical compound.
2. Must form large crystals. In this case, there will be no problems in the filtering process, as the pores are not clogged. Large crystals have a small surface, they are adsorbed with minimal speed from the existing solution, they can easily be washed. Amorphous precipitates of iron hydroxide (3) adsorb impurities without problems, they are difficult to wash away from the impurities, the filtration of this compound is slow.
3. Completely and for a short time interval to move into a gravitational form.

Requirements for the gravitational form

Let's analyze the gravimetric method of analysis. The essence of the method is that accuracy is important in it. Gravimetric form should be with a certain chemical formula, used to calculate the content in a sample of specific components. The calcined sediment during cooling and the weighing procedure should not absorb water vapor from the air, recover or oxidize. If the sediment has similar physical characteristics, it is initially converted into a stable form using special chemical reagents. For example, if it is required to calculate the mass fraction of calcium carbonate in the materials, the gravimetric form of calcium oxide, capable of absorbing carbon dioxide and water, is converted to calcium sulfate. For this, the calcined precipitate is treated with sulfuric acid, observing the temperature regime (500 ° C).

Tableware for the study

What is needed to conduct such a method of analysis? Gravimetric version involves the use of special chemical dishes of large sizes. Here, thin-walled glasses of different volume, funnels, glass sticks, watch glass, porcelain crucibles, glass boxes are used. Gravimetric and titrimetric methods of analysis imply the use of pure containers only, so that there are no errors in the calculations. Dry spots or drops indicate the presence of fat components on the glass surface. Precipitation will adhere to such a layer, as a result, a complete transfer to the filter will become more complicated. The method of carrying out the gravimetric method of analysis involves careful washing of dishes with detergents. To clean porcelain crucibles, dilute hot hydrochloric acid is used, then a solution of chrome mixture. It is advisable to heat clean cookware before starting work.

Equipment for research

What is the difference between the gravimetric method of analysis? The essence of the method is the quantitative determination of the components in the substance. The equipment that will be required for such studies is similar to that used in qualitative analysis. For practical purposes, water baths, porcelain triangles, drying cabinets, crucibles, muffle furnaces, gas burners will be required. For calcination on gas burners of porcelain crucibles, triangles made of porcelain tubes, mounted on a metal base, are used. Choose a triangle of this size so that the crucible protrudes from it a third of the height. The crucibles are introduced into the furnace by means of long forceps having flat, upward curved tips. They should not be submerged. Before use, the ends of the forceps are cleaned, calcined on a gas burner or in an oven. Desiccators are used to cool calcined or heated substances to room temperature. It is a glass thick-walled vessel, which is covered with a ground cover. The lower part of the desiccator is filled with hygroscopic substance:

• Pieces of calcium oxide;
• Phosphorus oxide (5);
• Concentrated sulfuric acid.

Sulfuric acid absorbs moisture intensively. Working with the desiccator, it is important to ensure that the lubricated parts have a layer of lubricant.

Sampling rules for the experiment

The classification of methods of gravimetric analysis considered involves working with substances. The average is a sample that contains a small amount of the analyzed material, which has chemical, physical properties characteristic of the main lot. The correctness of sampling affects the accuracy of the chemical and physical characteristics and the chemical composition of the material being analyzed. The selection of the average sample is carried out with special care, otherwise the probability of error is inaccurate, obtaining an inaccurate result of the study. It must be remembered that large pieces of chemical composition can differ significantly from dust. Therefore, there are three options:

• Primary sample - needed for the first stage of the experiment;
• Passport, or laboratory test - obtained by reducing the initial sample to the mass that is needed for chemical and physical-mechanical analysis;
• Analytical - is selected from the laboratory sample for chemical analysis.

There is such a section as analytical chemistry. The gravimetric method of analysis is one of the ways to establish the quantitative composition of a substance. In order to avoid changes in the moisture content and chemical composition of the substance, materials for gravimetric analysis are stored in bags tightly closed with lids. Part of the sample is required for direct analysis, and some remain as a reserve.

Preparation of a sample for research

The sample is considered a small mass of the analytical sample of the analyzed sample, which is weighed for chemical analysis. The size of the sample plays an important role in the quantitative determination. The more the sample is taken for gravimetric analysis, the more accurate the result will be. But at the same time, the process of filtering the resulting precipitate, its calcination, washing is complicated. For these reasons, the analysis time is extended significantly. In small samples, the accuracy of the determination is significantly reduced. To carry out weighing of samples of solid components, use small watch glass. Volatile, hygroscopic substances must be weighed in a closed bag.

Precipitation conditions

For presentation of this material, a presentation would be a good idea. The gravimetric method of analysis at this stage involves the quantitative transfer of the desired component to a specific chemical substance. Knowing the mass of the sediment, you can calculate the percentage of the component being determined. The accuracy of the analysis depends on the completeness of the deposition. Among the reasons why not the entire calculated component will precipitate, one can mention the incompleteness of precipitation. It is practically impossible to achieve absolute precipitation, it is possible only to minimize possible losses. For analysis, a precipitant is chosen - an almost insoluble precipitate. It is taken in excess to avoid such chemical reactions. There are certain conditions that must be observed in order to obtain a crystalline precipitate:

• From dilute solutions, precipitation is carried out with weak solutions of the precipitant;
• The warmed solutions are precipitated by hot precipitators.

For the experiment, a qualitative reagent is selected for the detectable ion. It is difficult to choose a specific precipitant for each ion to be determined. In this connection, the particles that can interfere with complete precipitation can be masked, or they can be removed from the solution under investigation before quantitative analysis is performed.

It is practically impossible to select specific precipitators for all detectable ions. Then it is necessary to use either masking of ions interfering with precipitation, or to separate them from solution before precipitation. Knowing the peculiarities of crystalline precipitation, one can use conditions that promote the formation of large crystals.

1. The precipitation is carried out from the diluted hot solutions with a precipitator taken in a small concentration. When heated, the solubility of small crystals increases, so the concentration of the precipitant and ions in the solution increases. Due to this phenomenon, large crystals are formed, which do not have time to dissolve when heated.
2. The precipitant is precipitating to the substance being determined at a low rate. For mixing, use a glass rod, which should not touch the bottom and walls of the glass. With stirring, crystal growth is stimulated, as the number of crystal centers decreases.
3. Withstand the sediment for several hours. Amorphous sediments are precipitated under special conditions, since they are prone to adsorption of various impurities and to the appearance of colloidal solutions.

Problems of gravimetric analysis

The accuracy of quantitative calculations is affected by the quality of the sediment. When it becomes dirty, the accuracy of the measurements decreases significantly, the error increases. The reason for the contamination is in coprecipitation, that is precipitation of foreign substances. There are two types of coprecipitation:

• Surface adsorption;
• occlusion.

To check the completeness of the precipitation of the separated ion, several drops of the reagent are added to the solution formed above the precipitate. With complete precipitation of the separated ion, the solution will remain transparent.

Conclusion

Qualitative analysis involves the quantitative determination of inorganic ions in the material under study. The main tasks of qualitative analysis are the detection in the selected sample and identification of certain components: ions or chemical elements, a particular substance or a functional group. A fractional assay method is suitable for the investigation of simple mixtures, when searching for a small number of components. For such a gravimetric analysis, separate samples and an insignificant number of qualitative reactions are required. In order to fully determine the inorganic components in the investigated substance, the initial mixture is initially divided into separate "analytical groups," then each ion is discovered with the help of specific reactions. Systematic qualitative analysis makes it possible to increase the reliability of the resulting analytical information. Before proceeding with quantitative analysis, it is important to understand the qualitative composition of the sample in order to choose the optimal technique.