Absorption tungsten

In a r-1. three-necked flask fitted with a mechanical stirrer, a reflux condenser, a thermometer, and a dropping funnel is placed LOO g. (0.58 mole) of -bromotoluene (Org. Syn. Coll. Vol. i, 131). The stem of the dropping funnel and the thermometer should reach nearly to the bottom of the flask. The upper end of the condenser is connected to a gas absorption trap (Org. Syn. 14, 2). The flask is heated with stirring in an oil bath until the temperature of the liquid reaches 105. The liquid is illuminated with an unfrosted 150-watt tungsten lamp, and 197 g. (61.8 cc., 1.23 moles) of bromine is added slowly from the separatory funnel (Note i). About one-half of the bromine is added during the first hour, during which time the temperature is kept at 105-110°. The rest is added during about two hours, while the temperature is raised to 135°. When all the bromine has been added the temperature is raised slowly to 150°. 

In these instruments the monochromated beam of radiation, from tungsten and deuterium lamp sources, is divided into two identical beams, one of which passes through the reference cell and the other through the sample cell. The signal for the absorption of the contents of the reference cell is automatically subtracted from that from the sample cell giving a net signal corresponding to the absorption for the components in the sample solution. 

Fig. 5-3. The spectrum, recorded photographically, from a tungsten tube. The silver and bromine absorption edges due to the selective absorption by the chemicals in the film are clearly shown.

Moxnes24 used the characteristic lines of one element to bracket the absorption edge of another. He showed, for example, that the addition of 2% ZnO to AI2O3 shifted the intensity ratio of tungsten L(3S to L/34 from 3 2 to about 1 1. 

The constancy of the quotient in the last line of Table 7-2 is greatly improved over that in the line above the last, proving that Equation 7-5 holds. So far as we know, this is the first case in which the absorption effects for a series of solutions have been obtained so precisely. Examples of this kind place on a firmer basis the calculation of semiquantitative analytical results from measured intensities when the composition of the matrix (all of S but E, the element sought) in a sample is approximately known. For example, tungsten contents could be estimated from measurements of L7I intensity for sodium tungstate solutions even when other salts are present in the absence of such salts, tungsten contents... 

Recent results from the authors laboratory69 on the x-ray emission spectrography of tungsten or molybdenum in solution illustrate some of the points made in Section 7.13. The also show the usefulness of internal standards (7.12). Finally, the work on tungsten is closely related to the experiments on the absorption effect in sodium tungstate solutions, the results of which are summarized in Table 7-2. 

Broadening, of monochromatic beams by crystals, 115, 116, 118 of pulse-size range, 47 Bromine, as internal standard in tungsten analysis, 194-196 determination, by absorption-edge method, 138-142... 

Lewis Bases. A variety of other ligands have been studied, but with only a few of the transition metals. There is still a lot of room for scoping work in this direction. Other reactant systems reported are ammoni a(2e), methanol (3h), and hydrogen sulfide(3b) with iron, and benzene with tungsten (Tf) and plati num(3a). In a qualitative sense all of these reactions appear to occur at, or near gas kinetic rates without distinct size selectivity. The ammonia chemisorbs on each collision with no size selective behavior. These complexes have lower ionization potential indicative of the donor type ligands. Saturation studies have indicated a variety of absorption sites on a single size cluster(51). 

Hydrogenolysis of 2-methylpentane, hexane, and methylcyclopentane has been also studied on tungsten carbide, WC, a highly absorptive catalyst, at 150-350 °C in a flow reactor [80], These reforming reactions were mainly cracking reactions leading to lower molar mass hydrocarbons. At the highest temperature (350 °C) all the carbon-carbon bonds were broken, and only methane was formed. At lower temperatures (150-200 °C) product molecules contained several carbon atoms. 

In the meantime temperature-dependent stopped-flow measurements were conducted on the latter complex in order to determine the activation parameters of the N-N cleavage reaction (24). Plots of the absorption intensity at 418 nm vs. time at T — —35 to +15°C indicate biphasic kinetics with two rate constants 0bs(p and obs(2)> in analogy to our measurements of the tungsten complex. This time, however, both rates depended upon the acid concentration. Interestingly much smaller rate constants 0bs(i) and 0bs(2)> were found for all acid concentrations than given by Henderson et al. for his (single) rate constant kobs (up to 1 order of magnitude). Furthermore plots of 0bs(i) and kohs(2) vs. the acid concentration showed no saturation behavior but linear dependencies with slopes k and k and intercepts k und k, respectively (s — acid dependent and i — acid independent), Eq. (2) ... 

Figure 5.3 K and L absorption edges of tungsten. The absorption of the solid decreases as the energy of the X-rays increases (i.e., as the wavelength decreases), but when the energy exceeds the threshold for a particular excitation process to occur (e.g., the eviction of an Lm electron at 10.2 KeV), the absorption jumps substantially.

Zhou, Y., Parsons, P. J., Aldous, K. M., Brockman, P., and Slavin, W. (2001). Atomization of lead from whole blood using novel tungsten filaments in electrothermal atomic absorption spectrometry. Journal of Analytical Atomic Spectrometry 16 82-89. 

---