Notes on qualitative analysis

For a suggested theory to explain this colour change in the presence of benzidine, see (Fenton s) Note on Qualitative Analysis, B. C. Saunders (Cambridge University Press, 1953),... 

Figure 4.23. The results of a qualitative analysis of a multiple phase sample. Three crystalline phases are clearly identifiable lithium silicate - Li2Si03, silicon oxide - SiOj (quartz), and a different pol)imorph of silicon oxide - tridymite. A low quality diffraction pattern collected during a fast experiment was employed in this example. The data shown on top were smoothed, the background was subtracted, and the Ktt2 components were stripped before the digitized pattern (shown below the smoothed profile) was obtained using an automatic peak search. Note, that many weak Bragg reflections were missed in the peak search,...

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. 

To choose a method for the determination of one or more species in a sample, it is necessary to know what other elements or compounds are present. If you are lacking such information, a qualitative analysis must be undertaken to identify components that are likely to interfere in the various methods under consideration. As we have noted repeatedly, most analytical methods are based on reactions and physical properties that are shared by several elements or compounds. Thus, measurement of the concentration of a given element by a method that is simple and straightforward in the presence of one group of elements or compounds may require many tediou.s and time-consuming separations in the presence of others. A solvent suitable for one combination of compounds may be totally unsatisfactory when applied to another. It is very important to know the approximate chemical composition of the sample before selecting a method for the quantitative determination of one or more of its components. 

As expected, the lattice enthalpy for LiF is higher than for CsF. LiF is insoluble, because the hydration enthalpy for Li" (956 kJ mol" ) is insufficient to compensate for the lattice enthalpy of LiF. On the other hand, hydration enthalpy for Cs (710 kJ mol" ) is about the lattice enthalpy for CsF, and it is expected that CsF should be more soluble than LiF. Note that both LiF and CsF contain the same anion, F", and that the hydration of this anion should also be taken into account. However, the hydration enthalpy for F" is insignificant (particularly when compared to the error produced by the Kapustinskii equation) and constant for both compounds, thus it can be ignored for a qualitative analysis like this one. 

Note that the turbulent viscosity parameter has an empirical origin. In connection with a qualitative analysis of pressure drop measurements Boussinesq [19] introduced some apparent internal friction forces, which were assumed to be proportional to the strain rate ([20], p 8), to fit the data. To explain these observations Boussinesq proceeded to derive the same basic equations of motion as had others before him, but he specifically considered the molecular viscosity coefficient to be a function of the state of flow and not only on the system properties [135]. It follows that the turbulent viscosity concept (frequently referred to as the Boussinesq hypothesis in the CFD literature) represents an empirical first attempt to account for turbulence effects by increasing the viscosity coefficient in an empirical manner fitting experimental data. Moreover, at the time Boussinesq [19] [20] was apparently not aware of the Reynolds averaging procedure that was published 18 years after the first report by Boussinesq [19] on the apparent viscosity parameter. 

Qualitative analysis of the spectra clearly outline the experimental dependencies on the extent of sulfur adsorption. The sulfation of ceria-only model catalysts is heavily dependent on total pressure and insensitive to temperature. The normalized sulfur transitions for exposures at 1 Torr total pressure yielded an average amplitude of 223 a.u., while exposures at 1000 Torr resulted in an average sulfur amplitude of 951 a.u. This represents an increase of 4.3 times the surface sulfur concentration for a corresponding increase of pressure by 1000 Torr. However, it should also be noted that the increase in overall pressure also represents an increase of sulfur dioxide partial pressure from 5x10 to 5x10 Torr. 

Wet reactions are carried out with dissolved substances. Such reactions lead to (a) the formation of a precipitate (b) the evolution of a gas or (c) a change of colour. The majority of reactions in qualitative analysis are carried out in solution, and details of these are given in later chapters. The following general notes on basic laboratory operations should be carefully studied. 

However, often a mere qualitative analysis (i.e., noting all significant changes in the spectrum) suffices to map the ligand s binding site on the protein structure, as described in the next Section. 

Most interestingly, this article contributed to the appreciation of the usefulness of computer graphics visualization of molecular structures and dynamics for complex molecular systems [4]. Rahman and Stiliinger saved ten intermediate structures and then viewed them in stereo. Equally important as the quantitative analysis noted above were the qualitative insights into the water structure they found this way, as noted on pages 3348-3349 ... 

---