What are the distinguishing features of the gravimetric analysis method? Let us consider in more detail its essence and varieties.
Specifics
The gravimetric method of analysis is based on the law of conservation of the 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 method of analysis is divided into three main groups: distillation, isolation, precipitation.
About selection method
It is based on the extraction of the desired component from the analyzed chemical substance in 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 ash in solid fuels. For calculations, the crucible is weighed, a sample of fuel is burned in it, and the resulting ash is weighed. Having the mass of the residue, according to the formula for the mass fraction of a substance in a mixture, a quantitative indicator is calculated.
Distillation
This method of analysis is gravimetricby content, since it involves the complete removal of the calculated component as a gaseous compound and subsequent weighing of the solid residue. This technique 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 considered sample of the selected material is first determined. Then the component to be determined is completely removed from it. The difference between the mass before and after calcination or drying is the mass of the detected chemical component. According to the mass fraction formula, quantitative calculations are carried out.
Deposition method
What is this method of analysis? The gravimetric precipitation method is based on the quantitative precipitation of the desired ion as a poorly soluble substance with a certain chemical composition. The precipitate formed is filtered, washed, dried and then calcined. After complete removal of water from it, weigh. Knowing the mass of the precipitate, it is possible to calculate the quantitative content of molecules or ions of the desired component in the test sample.
Precipitation requirements for gravimetric analysis
And yet - what is the gravimetric method of analysis? The main operations in the precipitation method are related to the process of precipitation. The accuracy of the result obtained during the analysis directly depends on the chemical compositionsubstances, sediment structure, 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 precipitate obtained during its calcination. The precipitated form is the chemical composition of the precipitate obtained.
Basic methods of gravimetric analysis require an accurate result. That is why certain requirements are imposed on the gravimetric and sedimentable form of sediment.
- It should have minimal solubility, ideally being an insoluble chemical compound.
- Should form large crystals. In this case, there will be no problems during the filtration process, since the pores are not clogged. Large crystals have a small surface, they adsorb from the available solution at a minimum rate, and they are easy to wash. Amorphous precipitates of iron hydroxide (3) adsorb impurities without problems, they are difficult to wash from the latter, the filtration of this compound is slow.
- Completely and within a short period of time, move into gravity form.
Gravity Shape Requirements
Let's analyze the gravimetric method of analysis. The essence of the method is that accuracy is important in it. The gravimetric form must be with a specific chemical formula used to calculate the content of specific components in the sample. The calcined sediment during the cooling and weighing procedure must not absorb water vapor from the air,recover or oxidize. If the precipitate has similar physical characteristics, it is initially converted into a stable form using special chemicals. For example, if it is required 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. To do this, the calcined precipitate is treated with sulfuric acid, observing the temperature regime (500 ° C).
Dishes for research
What is needed to carry out such an analysis method? The gravimetric option involves the use of special chemical glassware of large sizes. Here, thin-walled glasses of various sizes, funnels, glass rods, watch glasses, porcelain crucibles, glass boxes are used. Gravimetric and titrimetric methods of analysis involve the use of only clean containers to avoid errors in the calculations. Dry spots or drops indicate the presence of fatty components on the glass surface. Precipitation will stick to such a layer, as a result, their full transfer to the filter will become more difficult. The gravimetric method of analysis involves thorough washing of dishes with detergents. To clean porcelain crucibles, dilute hot hydrochloric acid is used, then a solution of a chromium mixture. It is advisable to ignite clean dishes before starting work.
Research equipment
What is the difference between the gravimetric method of analysis? The essence of the method is in quantitativedetermining the components in a substance. The equipment that will be required for such studies is similar to that used in qualitative analysis. For the practical part, you will need water baths, porcelain triangles, ovens, crucible tongs, muffle furnaces, gas burners. For calcining porcelain crucibles on gas burners, triangles are used, made of porcelain tubes mounted on a metal base. Choose a triangle of such size that the crucible protrudes from it by a third of its height. The crucibles are brought into the oven using long tongs with flat, upward curved tips. They must not be immersed in sediment. Before use, the ends of the tongs 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 bottom of the desiccator is filled with a hygroscopic substance:
- pieces of calcium oxide;
- phosphorus oxide (5);
- concentrated sulfuric acid.
Sulfuric acid absorbs moisture intensely. When working with a desiccator, it is important to ensure that there is a layer of lubricant on the ground parts.
Sampling rules for experiment
The considered classification of gravimetric analysis methods involves working with substances. An average is considered to be such a sample, which contains a small amount of the analyzed material, which has chemical and physical properties characteristic of the main batch. The correctness of sampling affects the accuracy of setting the chemical and physical characteristics and the chemical composition of the analyzed material. The selection of the average sample is carried out with special care, otherwise there is a high probability of error, obtaining an inaccurate result of the study. It must be remembered that large pieces of chemical composition may differ significantly from dust. Therefore, there are three options:
- primary sample - needed for the first stage of the experiment;
- passport or laboratory sample - obtained by reducing the initial sample to the mass that is needed for chemical and physical-mechanical analysis;
- analytical - taken 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 bottles 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 the analyzed sample, which is weighed for chemical analysis. An important role in the quantitative determination is played by the sample size. The greater the amount of test sample 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, and washing becomes more complicated. For these reasons, the analysis time is significantly extended. In small sample sizes, the determination accuracy is significantly reduced. Small watch glasses are used to weigh weighings of solid components. Volatile, hygroscopic substances must be weighed in a closed bottle.
Deposition conditions
A presentation would be good for covering this material. The gravimetric method of analysis at this stage involves the quantitative translation of the desired component into a specific chemical substance. Knowing the mass of the sediment, it is possible to calculate the percentage of the component to be determined. The accuracy of the analysis performed directly depends on the completeness of the precipitation. Among the reasons due to which not the entire calculated component will precipitate, one can mention the incompleteness of precipitation. It is practically impossible to achieve absolute settling, 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 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;
- Heated solutions are precipitated with hot precipitators.
For the experiment, a high-quality reagent is selected for the ion to be determined. It is difficult to choose a specific precipitant for each ion to be determined. Concerningmasking of those particles that can interfere with full precipitation is carried out, or they are removed from the test solution before performing a quantitative analysis.
It is practically impossible to select specific precipitants for all ions being determined. Then it is necessary either to mask the ions that interfere with precipitation or to separate them from the solution before precipitation. Knowing about the features of crystalline precipitation, one can use conditions that favor 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 the 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 poured 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. Stirring stimulates crystal growth as it reduces the number of crystal centers.
- Withstand sediment for several hours. Amorphous precipitates are deposited under special conditions, as they are prone to the process of adsorption of various impurities and the appearance of colloidal solutions.
Problems of gravimetric analysis
The quality of the sludge affects the accuracy of quantitative calculations. When it is contaminated, the measurement accuracy is significantly reduced, and the error increases. The cause of pollution is co-precipitation, that is, precipitation insediment of foreign matter. There are two types of codeposition:
- surface adsorption;
- occlusion.
To check the completeness of the precipitation of the separated ion, add a few drops of the reagent 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 test material. The main tasks of qualitative analysis are the detection and identification of certain components in the selected sample: ions or chemical elements, a specific substance or functional group. The fractional method of analysis is suitable for the study of simple mixtures, when searching for a small number of components. Such gravimetric analysis requires separate samples and a negligible number 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 discovered using specific reactions. Systematic qualitative analysis makes it possible to increase the reliability of the obtained analytical information. Before proceeding with quantitative analysis, it is important to have an idea of the qualitative composition of the test sample in order to select the optimal method.
