Qualitative analysis chemical identities

Precipitation reactions have many applications. One is to make compounds. The strategy is to choose starting solutions that form a precipitate of the desired insoluble compound when they are mixed. Then we can separate the insoluble compound from the reaction mixture by filtration. Another application is in chemical analysis. In qualitative analysis—the determination of the substances present in a sample—the formation of a precipitate is used to confirm the identity of certain ions. In quantitative analysis, the aim is to determine the amount of each substance or element present. In particular, in gravimetric analysis, the amount of substance present is determined by measurements of mass. In this application, an insoluble compound is precipitated, the precipitate is filtered off and weighed, and from its mass the amount of a substance in one of the original solutions is calculated (Fig. 1.6). Gravimetric analysis can be used in environmental monitoring to find out how much of a heavy metal ion, such as lead or mercury, is in a sample of water. 

Guideline 1. The most fundamental basis for mechanistic speculation is a complete analysis of the reaction products. It is important to obtain a complete quantitative and qualitative analysis for all products of the reaction. Inasmuch as many chemical reactions give a complex array of products, the relative proportions of which change as the time of reaction increases, it is very useful to carry out a complete analysis for the shortest possible reaction time and for successively longer periods. In this manner one can differentiate between primary products formed directly from the reactants and secondary products formed by subsequent reaction of the primary products. Such analyses can give valuable clues as to the identity of reaction intermediates. 

Analytical procedures can be classified in two ways first, in terms of the goal of the analysis, and second, in terms of the nature of the method used. In terms of the goal of the analysis, classification can be based on whether the analysis is qualitative or quantitative. Qualitative analysis is identification. In other words, it is an analysis carried out to determine only the identity of a pure analyte, the identity of an analyte in a matrix, or the identity of several or all components of a mixture. Stated another way, it is an analysis to determine what a material is or what the components of a mixture are. Such an analysis does not report the amount of the substance. If a chemical analysis is carried out and it is reported that there is mercury present in the water in a lake and the quantity of the mercury is not reported, then the analysis was a qualitative analysis. Quantitative analysis, on the other hand, is the analysis of a material for how much of one or more components is present. Such an analysis is undertaken when the identity of the components is already known and when it is important to also know the quantities of these components. It is the determination of the quantities of one or more components present per some quantity of the matrix. For example, the analysis of the soil in your garden that reports the potassium level as 342 parts per million (ppm) would be classified as a quantitative analysis. The major emphasis of this text is on quantitative analysis, although some qualitative applications will be discussed for some techniques. See Workplace Scene 1.1. 

The components of the sample that are to be determined are often referred to as analytes. While qualitative analysis reveals the chemical identity of the species in the sample, quantitative analysis establishes the relative amount of one or more of these species, the analytes, in numerical terms. Often separation is required as a necessary part of either qualitative or quantitative analysis. 

Analytical chemistry deals with methods for determining the chemical composition of samples. A compound can often be measured by several methods. The choice of analytical methodology is based on many considerations, such as chemical properties of the analyte and its concentration, sample matrix, the speed and cost of the analysis, type of measurements i.e., quantitative or qualitative and the number of samples. A qualitative method yields information of the chemical identity of the species in the sample. A quantitative method provides numerical information regarding the relative amounts of one or more of the species (the analytes) in the sample. Qualitative information is required before a quantitative analysis can be performed. A separation step is usually a necessary part of both a qualitative and a quantitative analysis. 

The Pathfinder example demonstrates that both qualitative information and quantitative information are required in an analysis. Qualitative analysis establishes the chemical identity of the species in the sample. Quantitative analysis determines the relative amounts of these species, or analytes, in numerical terms. The data from the APXS spectrometer on Sojourner contain both types of information. Note that chemical separation of the various elements contained in the rocks was unnecessary in the APXS experiment. More commonly, a separation step is a necessary part of the analytical process. As we shall see, qualitative analysis is often an integral part of the separation step, and determining the identity of the analytes is an essential adjunct to quantitative analysis. In this text, we shall explore quantitative methods of analysis, separation methods, and the principles behind their operation. 

For inorganic substances, chemical reactions may be carried out on a small scale on microscope slides, the crystallization of reaction products being watched. Tests for particular ions or atom groups have been devised, the criterion of identity being, not solubility or colour, as in macroscopic qualitative chemical analysis, but crystallographic properties. For information on such methods, see Handbook of Chemical Microscopy, by Chamot and Mason (1958). 

NIR spectroscopy is a popular method for qualitative and quantitative analysis. It is finding widespread use in many different industries for monitoring the identity and quality of raw materials and finished products in the food, agricultural, polymer, pharmaceutical, and organic chemical manufacturing industries. 

It is important to make the distinction between the determination of polymorphic identity and polymorphic purity. The former is essentially a qualitative determination, asking the question, Ts a particular polymorph present in a given sample The latter is a question of quantitative analysis, and it is generally (though not always) assumed that the sample is chemically pure, so the analytical problem to be addressed is the determination of the relative amounts of different polymorphs in the sample. Recalling that different polymorphs are for all intents and purposes different solids, the determination of polymorphic purity is then no different in principle from quantitative determination of the composition of a mixture of solids. Such quantitative determinations comprise one of the traditional activities of analytical chemistry, especially when the materials are different chemical entities. In those cases, a variety of different analytical methods may be employed. In the case of polymorphic mixtures, or the determination of polymorphic purity, the choice of analytical method is considerably more restricted, and X-ray diffraction is one of the most definitive techniques (see e.g. Stowell 2001). 

This paper reports further on the analysis of the Charles River system by a detailed qualitative examination of the organic compoimds which are present in the river sediment. This investigation of the sediment was undertaken from two points of view. First, the sediment is an obvious sink for pollutants which enter the river, and it was of interest to determine the identities of these compounds and, ultimately, their primary sources. Second, the sediment is a possible pollutant source in itself, i.e., organic compounds may be generated in the sediment by chemical or biological reactions and these would, in turn, pollute the water. In both of these cases, a comparison of the organic compounds found in the water and in the sediment would provide information on the natural processes which tend to modify the pollutant load of a body of water. 

Nuclei that are typically analyzed with this technique include those of 13C, 31P, 1SN, 2SMg, and 23Na. Different crystal structures of a compound can result in perturbation of the chemical environment of each nucleus, resulting in a unique spectrum for each form. Once resonances have been assigned to specific atoms of the molecule, information on the nature of the polymorphic variations can be obtained. This can be useful early in drug development, when the single-crystal structure may not be available. Long data acquisition times are common with solid-state NMR, so it is often not considered for routine analysis of samples. However, it is usually a very sensitive technique, and sample preparation is minimal. NMR spectroscopy can be used either qualitatively or quantitatively, and can provide structural data, such as the identity of solvents bound in a crystal. 

NIR analysts often use a statistical methodology called chemometrics to calibrate an NIR analysis. Chemometrics is a specialized branch of mathematical analysis that uses statistical algorithms to predict the identity and concentration of materials. Chemometrics is heavily used in NIR spectral analysis to provide quantitative and qualitative information about a variety of pure substances and mixtures. NIR spectra are often the result of complex, convoluted, and even unknown interactions of the different molecules and their environment. As a result, it is difficult to pick out a spectral peak or set of peaks that behave linearly with concentration or are definitive identifiable markers of particular chemical structures. Chemometrics uses statistical algorithms to pick out complex relationships between a set of spectra and the material s composition and then uses the relationship to predict the composition of new materials. Essentially, the NIR system, computer, and associated software are trained to relate spectral variation to identity and then apply that training to new examples of the material. 

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