Qualitative analysis for the elements

It is usually advisable to carry out the ignition test first. This will provide useful information as to the general properties of the compound and, in particular, the residue may be employed for the detection of any inorganic elements which may be present. 

Place about 0-1 g. of the compound in a porcelain crucible or crucible cover. Heat it gently at first and finally to dull redness. Observe  

If an appreciable amount of residue remains, note its colour. Add a few drops of water and test the solution (or suspension) with litmus or with Universal indicator paper. Then add a little dilute hydrochloric acid and observe whether effervescence occurs and the residue dissolves. Apply a flame test with a platinum wire on the hydrochloric acid solution to determine the metal present. (In rare cases, it may be necessary to subject a solution of the residue to the methods of qualitative inorganic analysis to identify the metal or metals present.) If the flame test indicates sodium, repeat the ignition of the substance on platinum foil. 

Evidence of the organic nature of the substance may, be provided by the behaviour of the compound when heated on porcelain or platinum or other comparatively inert metal (e.g., nickel) the substance is inflammable, burns with a more or less smoky flame, chars and leaves a black residue consisting largely of carbon (compare Ignition Test above). 

If it is desired to test directly for the presence of carbon and hydrogen in a compound, mix 0-1 g. of the substance with 1-2 g. of ignited, fine 

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Qualitative analysis for the elements. This includes an examination of the effect of heat upon the substance—a test which inter alia will indicate the presence of inorganic elements—and quahtative analysis for nitrogen, halogens and sulphur and, if necessary, other inorganic elements. It is clear that the presence or absence of any or all of these elements would immediately exclude from consideration certain classes of organic compounds. 

Let the student be given an unknown organic compound, which may be any one from among thousands of known compounds. Obviously, it would be a waste of time to search through the literature in order to find constants and reactions of known compounds which check with the physical and chemical properties of the unknown. We shall seek first the class to which the unknown belongs. The determination of its melting- or boiling-point will exclude certain classes of compounds the qualitative analysis for the elements (C, H, N, S, X, etc.), will further limit the possible classes, and after the application... 

Muller, J. Amer. Ohem. Soc., 1911, 33, 1306 Noyes and Bray, Qualitative Analysis for the Bare Elements (Macmillan, London), 1927, 77 Dittrich and Freund, Zeitsch. anorg. Ohem., 1907, 56, 344, 346. 

Compare Noyes and Bray, Qualitative Analysis for the Bare Elements (Macmillan, London), 1927, 106. 

To separate and purify the radionuclide of interest in the sample, the analyst can depend on the similar behavior of the stable element and its radioisotopes. Chemical reactions involving the radionuclide will proceed with essentially the equilibrium and rate constants known for the stable element in the same chemical form. Slight differences result from small differences between the isotopic mass of the radionuclide and the atomic mass (i.e., the weighted average of the stable isotopic masses) of the stable element. Because of this similarity in chemical behavior, many ra-dioanalytical chemistry procedures were adapted from classical quantitative and qualitative analysis. For the same reason, new methods published for separating chemical substances by processes such as precipitation, ion-exchange, solvent extraction, or distillation are adapted for and applied to radionuclides. One exception occurs when the radionuclides to be separated are two or more isotopes of the same element. Here, effective separation can be accomplished by mass spectrometer (see Chapter 17). 

QUALITATIVE ANALYSIS FOR METALLIC ELEMENTS We explain how the principles of solubility and complexation eqiilibria can be used to identify ions in solution. 

A survey XPS scan on a material can provide a qualitative analysis of the elemental eomposition on the surface. The XPS survey scan spectra of Pt/MWCNT nanocomposites before and after reduction treatment are shown in Figure 10.15(e). Elements of Pt, Cl, C, and O existed in flie Pt/MWCNT nanocomposite before the reduetion treatment. However, the Cl 2p peak vanished after the reduction, indicating removal of chloride ions in the reduced sample. The removal of halide ion impurities sueh as Cf ean improve the activities of the catalyst for CO oxidation and also direct methanol oxidation reactions in fuel cells [136,137]. 

Noyes AA, Bray WC (1952) A system of qualitative analysis for the rare elements. Macmillan, New York... 

In connection with the identification of an organic compound, time will usually not permit a quantitative analysis for the elements (step three, above), since it is desired to identify a compound not in a few days time, but during a few hours. For the same reason, molecular weight determinations are applied only in exceptional instances. Step five, the assignment of structure, often involves years of investigational work. Fortunately, this work has already been accomplished for an enormous number of organic compounds, and the path has thus been cleared in the direction of qualitative identification when these compounds are again met. 

X-Ray Fluorescence (XRF) is a nondestructive method used for elemental analysis of materials. An X-ray source is used to irradiate the specimen and to cause the elements in the specimen to emit (or fluoresce) their characteristic X rays. A detector s)rstem is used to measure the positions of the fluorescent X-ray peaks for qualitative identiflcation of the elements present, and to measure the intensities of the peaks for quantitative determination of the composition. All elements but low-Z elements—H, He, and Li—can be routinely analyzed by XRF. 

In addition to qualitative identification of the elements present, XRF can be used to determine quantitative elemental compositions and layer thicknesses of thin films. In quantitative analysis the observed intensities must be corrected for various factors, including the spectral intensity distribution of the incident X rays, fluorescent yields, matrix enhancements and absorptions, etc. Two general methods used for making these corrections are the empirical parameters method and the fimdamen-tal parameters methods. 

The lines in the spectrum from any element always occur in the same positions relative to each other. When sufficient amounts of several elements are present in the source of radiation, each emits its characteristic spectrum this is the basis for qualitative analysis by the spectrochemical method. It is not necessary to examine and identify all the lines in the spectrum, because the strongest lines will be present in definite positions, and they serve to identify unequivocally the presence of the corresponding element. As the quantity of the element in the source is reduced, these lines are the last to disappear from the spectrum they have therefore been called the persistent lines or the rates ultimes (R.U. lines), and simplify greatly the qualitative examination of spectra. 

Analytical electron microscopy permits structural and chemical analyses of catalyst areas nearly 1000 times smaller than those studied by conventional bulk analysis techniques. Quantitative x-ray analyses of bismuth molybdates are shown from lOnm diameter regions to better than 5% relative accuracy for the elements 61 and Mo. Digital x-ray images show qualitative 2-dimensional distributions of elements with a lateral spatial resolution of lOnm in supported Pd catalysts and ZSM-5 zeolites. Fine structure in CuLj 2 edges from electron energy loss spectroscopy indicate d>ether the copper is in the form of Cu metal or Cu oxide. These techniques should prove to be of great utility for the analysis of active phases, promoters, and poisons. 

The ICP torch provides a rich source of free atoms and ions from the elements comprising the sample. In ICP-MS, part of the sample stream from a point close to the centre of the fireball is directed to a mass spectrometer. The resulting mass spectrum can be used to identify elements from the mass numbers of the ion peaks and the peak size for quantitative analysis. Moreover, the whole spectrum can be displayed at the same time providing qualitative analysis for a wide range of elements from one display... 

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