Stoichiometry gravimetric analysis

Now that we have discussed the concentration and dilution of solutions, we can examine the quantitative aspects of reactions in aqueous solution, or solution stoichiometry. Sections 4.6. 8 focus on two techniques for studying solution stoichiometry gravimetric analysis and titration. These techniques are important tools of quantitative analysis, which is the determination of the amount or concentration of a substance in a sample. 

Quantitative Calculations When needed, the relationship between the analyte and the analytical signal is given by the stoichiometry of any relevant reactions. Calculations are simplified, however, by applying the principle of conservation of mass. The most frequently encountered example of a direct volatilization gravimetric analysis is the determination of a compound s elemental composition. 

In a gravimetric analysis a measurement of mass or change in mass provides quantitative information about the amount of analyte in a sample. The most common form of gravimetry uses a precipitation reaction to generate a product whose mass is proportional to the analyte. In many cases the precipitate includes the analyte however, an indirect analysis in which the analyte causes the precipitation of another compound also is possible. Precipitation gravimetric procedures must be carefully controlled to produce precipitates that are easily filterable, free from impurities, and of known stoichiometry. 

The ultimate goal of any titrimetric analysis is to determine the amount of the analyte in a sample. This involves the stoichiometry calculation mentioned in the Work the Data section of the analytical strategy flow chart in Figure 4.1. This amount of analyte is often expressed as a percentage, as it was for the gravimetric analysis examples in Chapter 3. This percentage is calculated via the basic equation for percent used previously for the gravimetric analysis examples ... 

As with gravimetric analysis, the weight of the sample (the denominator in Equation (4.33)) is determined by direct measurement in the laboratory or by weighing by difference. The weight of the analyte in the sample is determined from the titration data via a stoichiometry calculation. As discussed previously, we calculate moles of substance titrated (in this case, the analyte) as in Equation (4.21) ... 

Gravimetric analysis utilizes primarily weight measurements and may or may not involve chemical reactions. Titrimetric analysis utilizes both weight and volume measurements and always involves solution chemistry and stoichiometry. 

Stoichiometry and Gravimetric Analysis.. Gravimetric Analysis-Hard Water Testing... 

When chemists are faced with problems that require them to determine the quantity of a substance by mass, they often use a technique called gravimetric analysis. In this technique, a small sample of the material undergoes a reaction with an excess of another reactant. The chosen reaction is one that almost always provides a yield near 100%. If the mass of the product is carefully measured, you can use stoichiometry calculations to determine how much of the reactant of unknown amount was involved in the reaction. Then by comparing the size of the analysis sample with the size of the original material, you can determine exactly how much of the substance is present. 

Solution Stoichiometry Quantitative studies of reactions in solution require that we know the concentration of the solution, which is usually represented by the molarity unit. These studies include gravimetric analysis, which involves the measurement of mass, and titrations in which the unknown concentration of a solution is determined by reaction with a solution of known concentration. 

In Chapter 3 we studied stoiehiometric calculations in terms of the mole method, which treats the eoeffieients in a balanced equation as the number of moles of reactants and products. In working with solutions of known molarity, we have to use the relationship MV = moles of solute. We will examine two types of common solution stoichiometry here gravimetric analysis and acid-base titration. 

Certain aqueous reactions are useful for determining how much of a particular substance is present in a sample. For example, if we want to know the concentration of lead in a sample of water, or if we need to know the concentration of an acid, knowledge of precipitation reactions, acid-base reactions, and solution stoichiometry will be useful. Two common types of such quantitative analyses are gravimetric analysis and acid-base titration. 

The product is characterized by X-ray powder diffraction and thermo-gravimetric analysis. All diffraction lines could be indexed in orthorhombic symmetry, space group Pbam, a = 5.5491(1) k, b = 10.4782(2) A, c = 3.8796(1) A. TGA is consistent with the stoichiometry LaCu02.so-... 

Precipitation reactions have several applications in analysis in gravimetric methods, in precipitation titrations, and in separations. Gravimetry, which used to be a major l>art of analytical chemistry, has expanded less rapidly than other aspects of analysis and does not now occupy a prominent place. Precipitation titrimetry always has been restricted in application because most precipitation reactions fail to meet the requirements of rapid reaction rate and adequate stoichiometry. In separations, precipitation reactions are used in two ways in one the precipitate involved is of direct concern, and in the other it acts as a carrier for another substance of interest. The application of precipitation reactions to separations is described in Chapter 22. 

When the product is purified, its identity is established by H NMR, IR, FAB-MS, etc. An elemental analysis associated with a fusion point determination by thermo-gravimetric methods helps to establish the identity of the substance and its stoichiometry. Then the product is analysed on various GC and HPLC phases with various detectors. Impurities are often present in amounts that do not allow H NMR identification. 

From chemical, gravimetric and X-ray structural analysis a complete solid solution Lai jSm B5 was found, usually deviating from stoichiometry towards boron defects. Comparison of pycnometric (measured in toluene) and X-ray densities revealed the existence of metal vacancies. Due to the change in the proportion of the Sm " " and Sm + ions the magnetic susceptibility depends nonlinearly on the La concentration (30% Sm + in SmB to 50% Sm + for 80% LaB ). 

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