What are the hallmarks of a gravimetric analysis method? Let us consider in more detail its essence and varieties.
Specificity
The gravimetric analysis method is based on the law of conservation of mass of substances and the constancy of composition. 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 analysis method is divided into three main groups: distillation, separation, deposition.
About the allocation method
It is based on the extraction of the desired component in the free form from the analyzed chemical substance and its subsequent accurate weighing. For example, such a gravimetric method of quantitative analysis allows us to determine the mass content of ash residue in solid fuel. For calculations, the crucible is weighed, a portion of fuel is burned in it, the resulting ash is weighed. Having a mass of residue, according to the formula of the mass fraction of the substance in the mixture, a quantitative indicator is calculated.
Stripping
This method of analysis is gravimetric in content, since it involves the complete removal of the calculated component as a gaseous compound and the subsequent weighing of the solid residue. Using this technique, you can determine the moisture content of various materials, calculate the quantitative content of crystallization water in crystalline hydrates. To perform such a calculation, the mass of the sample in question of the selected material is initially determined. Then the determined component is completely removed from it. The difference between the mass before calcination or drying and after them is the mass of the detected chemical component. According to the mass fraction formula, quantitative calculations are carried out.
Deposition technique
What is this analysis method? The gravimetric deposition method is based on the quantitative deposition of the desired ion as a sparingly soluble substance with a specific chemical composition. The precipitate formed is filtered, washed, dried, and then calcined. After complete removal from it, the water is weighed. Knowing the mass of the precipitate, it is possible to calculate the quantitative content in the sample of molecules or ions of the desired component.
Precipitation requirements for gravimetric analysis
And yet - what is the gravimetric method of analysis? The main operations in the deposition method are associated with the process of precipitation. The accuracy of the result obtained during the analysis directly depends on the chemical composition of the substance, the structure of the precipitate, and the degree of purity. In addition, the calculations are related to the behavior of the precipitate during drying and calcination. Quite often, the chemical composition of the resulting precipitate changes during its calcination. The precipitated form is called the chemical composition of the obtained precipitate.
The main methods of gravimetric analysis involve obtaining an accurate result. That is why certain requirements are imposed on the gravimetric and precipitated sediment form.
- It should have minimal solubility, ideally be an insoluble chemical compound.
- Must form large crystals. In this case, there will be no problems in the filtering process, since the pores are not clogged. Large crystals have a small surface, they adsorb from the existing solution with minimal speed, they are easy to wash. Amorphous precipitation of iron hydroxide (3) adsorbs impurities without problems, it is difficult to wash them off, the filtration of this compound is slow.
- Completely and in a short period of time go into gravitational form.
Gravity form requirements
We analyze the gravimetric analysis method. The essence of the method is that accuracy is important in it. The gravimetric form should be with a specific chemical formula used to calculate the content of specific components in the sample. The calcined precipitate during cooling and weighing procedures must not absorb water vapor from the air, or be reduced or oxidized. If the precipitate has similar physical characteristics, it is initially converted into a stable form using special chemicals. For example, if you want to calculate the mass fraction of calcium carbonate in 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).
Research glassware
What is needed to carry out such an analysis method? The gravimetric version involves the use of special chemical glassware of large sizes. Thin-walled glasses of different volumes, funnels, glass sticks, watch glasses, porcelain crucibles, glass boxes are used here. Gravimetric and titrimetric methods of analysis involve the use of only clean containers so that there are no errors in the calculations. Dry spots or drops indicate the presence of fatty components on the glass surface. Precipitation will adhere to such a layer; as a result, their full transfer to the filter will become more complicated. The gravimetric method of analysis involves thorough washing of dishes with detergents. To clean porcelain crucibles, diluted hot hydrochloric acid is used, then a solution of the chromium mixture. It is advisable to bake clean dishes before starting work.
Research Equipment
What is the difference between the gravimetric analysis method? The essence of the method is the quantitative determination of the components in a substance. The equipment required for such studies is similar to that used in qualitative analysis. The practical part will require water baths, porcelain triangles, ovens, crucible tongs, muffle furnaces, gas burners. For calcination on gas burners of porcelain crucibles, triangles are used, made of porcelain tubes mounted on a metal base. Choose a triangle of such a size that the crucible protrudes from it to a third of the height. The crucibles are introduced into the furnace using long tongs having flat, upwardly bent tips. They should not be immersed in sediment. Prior to 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 closed with a polished lid. The lower part of the desiccator is filled with a hygroscopic substance:
- pieces of calcium oxide;
- phosphorus oxide (5);
- concentrated sulfuric acid.
Sulfuric acid intensively absorbs moisture. When working with a desiccator, it is important to ensure that there is a layer of lubricant on the ground parts.
Experiment Sampling Rules
The considered classification of gravimetric analysis methods involves working with substances. The average is considered such a sample, which contains a small amount of the analyzed material having chemical, physical properties characteristic of the main batch. Correct sampling affects the accuracy of the installation of the chemical and physical characteristics and chemical composition of the analyzed material. An average sample is taken with particular care, otherwise there is a high probability of error, of obtaining an inaccurate research result. It must be remembered that large pieces in chemical composition can differ significantly from dust. Therefore, there are three options:
- primary test - 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 - selected from a 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 humidity and chemical composition of the substance, materials for gravimetric analysis are stored in containers tightly closed with lids. Part of the sample is required for direct analysis, and part remains as a reserve.
Preparation of sample for research
A sample is considered a small mass of an analytical sample of an analyzed sample, which is weighed to carry out a chemical analysis. An important role in the quantification is played by the size of the sample. The larger the number of test samples taken for gravimetric analysis, the more accurate the result will be. But this complicates the process of filtering the resulting precipitate, its calcination, washing. For these reasons, the analysis time is significantly extended. In small samples, the accuracy of determination is significantly reduced. To carry out weighing of samples of solid components, small watch glasses are used. Volatile, hygroscopic substances must be weighed in a closed bottle.
Deposition conditions
A presentation would be good for covering this material. The gravimetric analysis method at this stage involves the quantitative translation of the desired component into a specific chemical substance. Knowing the mass of sediment, you can calculate the percentage of the determined component. The accuracy of the analysis directly depends on the completeness of the deposition. Among the reasons due to which not all of the calculated component will precipitate, we can mention the incompleteness of the deposition. It is practically impossible to achieve absolute precipitation, it is only possible to minimize possible losses. For analysis, a precipitant is chosen - an almost insoluble precipitate. It is taken in excess to avoid similar chemical reactions. There are certain conditions that must be met in order to obtain a crystalline precipitate:
- from dilute solutions, precipitation is carried out with weak precipitant solutions;
- heated solutions precipitated with hot precipitators.
For the experiment, a high-quality reagent for the determined ion is selected. It is difficult to choose a specific precipitant for each ion to be determined. In this regard, the masking of those particles that can interfere with the full deposition, or remove them from the test solution before performing a quantitative analysis.
It is practically impossible to select specific precipitators for all the ions to be determined. Then you have to apply or mask ions that interfere with the deposition, or separate them from the solution before deposition. Knowing the features of crystalline precipitation, you can use conditions that contribute to the formation of large crystals.
- Precipitation is carried out from dilute hot solutions with a precipitant taken in a small concentration. When heated, the solubility of small crystals increases, so the concentration of precipitant and ions in the solution increases. Due to this phenomenon, large crystals are formed that do not have time to dissolve when heated.
- The precipitant is added to the substance to be determined at a low speed. For mixing, a glass rod is used, which should not touch the bottom and walls of the glass. With stirring, crystal growth is stimulated, since the number of crystal centers decreases.
- Withstand the sediment for several hours. Amorphous precipitates precipitate under special conditions, as they are prone to the process of 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. With its contamination, the accuracy of measurements is significantly reduced, the error increases. The cause of contamination is the coprecipitation, that is, the precipitation of foreign substances. There are two types of coprecipitation:
- surface adsorption;
- occlusion.
To check the completeness of precipitation of the separated ion, add a few drops of the reagent to the solution formed above the precipitate. With the complete precipitation of the separated ion, the solution will remain transparent.
Conclusion
Qualitative analysis involves the quantitative determination of inorganic ions in the test material. The main tasks of a qualitative analysis are the detection in the selected sample and the identification of certain components: ions or chemical elements, a specific substance or functional group. The fractional analysis method is suitable for the study of simple mixtures when searching for a small number of components. Such gravimetric analysis requires separate samples and an insignificant amount of qualitative reactions. In order to fully determine the inorganic components in the test substance, the initial mixture is initially divided into separate “analytical groups”, then each desired ion is opened using specific reactions. Systematic qualitative analysis allows you to increase the reliability of the received analytical information. Before embarking on a quantitative analysis, it is important to have an idea of the qualitative composition of the test sample in order to select the optimal technique.