Dispersive element dissociation

We have now discussed three types of intermolecular forces dispersion forces, dipole forces, and hydrogen bonds. You should bear in mind that all these forces are relatively weak compared with ordinary covalent bonds. Consider, for example, the situation in HzO. The total intermolecular attractive energy in ice is about 50 kj/mol. In contrast, to dissociate one mole of water vapor into atoms requires the absorption of928 kj of energy, that is, 2(OH bond energy). This explains why it is a lot easier to boil water than to decompose it into the elements. Even at a temperature of 1000°C and 1 atm, only about one H20 molecule in a billion decomposes to hydrogen and oxygen atoms. 

The general construction of an atomic absorption spectrometer, which need not be at all complicated, is shown schematically in Fig. 1. The most important components are the light source (A), which emits the characteristic narrow-line spectrum of the element of interest an absorption cell or atom reservoir in which the atoms of the sample to be analysed are formed by thermal molecular dissociation, most commonly by a flame (B) a monochromator (C) for the spectral dispersion of the light into its component wavelengths with an exit slit of variable width to permit selection and isolation of the analytical wavelength a photomultiplier detector (D) whose function it is to convert photons of light into an electrical signal which may be amplified (E) and eventually displayed to the operator on the instruments readout, (F). 

Figure 7.7 The line spectra of several elements. A, A sample of gaseous H2 is dissociated into atoms and excited by an electric discharge. The emitted light passes through a slit and a prism, which disperses the light into individual wavelengths. The line spectrum of atomic H is shown (top). B, The continuous spectrum of white light is compared with the line spectra of mercury and strontium. Note that each line spectrum is different from the others.

Figure 2 gives some characteristics of the size separation techniques that can be used to study the distribution of trace elements associated with various constituents of natural waters. It is obvious that the dimensions given in the figure are tentative as various factors influence the association/dissociation and aggregation/dispersion processes. However, preservation of real equilibria and labile species of elements, especially at concentrations of less than 10 g 1 prior to analysis is a much more serious problem encountered with methods that are not based on a direct physical separation. From this point of view, membrane filtration as well as some variants of field-flow fractionation (FFF) have advantages, although some uncertainties connected with equilibria shifts always exist. 

Nebulised samples enter the central channel of the plasma as a finely dispersed mist which is rapidly vaporised dissociation is virtually complete during passage through the plasma core with most elements fully ionised. 

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