Isotopic mixture

Solutions of Helium Isotopes 410. 7. Influence of Pressure on the Excess 

Recently Sommers, Keller and Dash [1953] have measured directly the heat of mixing of He and He. Their value (extrapolated to xa = xb = 0.5 using a parabolic law = 4 ALv xaXb) is 

A typical vapour pressure diagram for the mixture He —He is shown on Fig. 19.1.1 

Hoge and Arnold [1951] studied the thennodynamic properties of liquid solutions of and Hg—HD. They again found positive 

Finally we may mention that Kerr [1952] has measured the densities of the S3 stem —Tg and foimd the dilatation 

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The possibility that an even larger impact caused the P T extinction received support when Becker and Poreda found that helium and argon atoms were present in the inner cores of some of the fullerenes from the P T boundary sediments (The cover of this book shows a helium atom inside a mol ecule of Ceo) What is special about the fullerene trapped atoms is that the mixtures of both helium and argon isotopes resemble extraterrestrial isotopic mixtures more than earthly ones The He/ He ratio in the P T boundary fullerenes for example is 50 times larger than the natural abundance ratio... 

Each of the elements has a number of isotopes (2,4), all radioactive and some of which can be obtained in isotopicaHy pure form. More than 200 in number and mosdy synthetic in origin, they are produced by neutron or charged-particle induced transmutations (2,4). The known radioactive isotopes are distributed among the 15 elements approximately as follows actinium and thorium, 25 each protactinium, 20 uranium, neptunium, plutonium, americium, curium, californium, einsteinium, and fermium, 15 each herkelium, mendelevium, nobehum, and lawrencium, 10 each. There is frequently a need for values to be assigned for the atomic weights of the actinide elements. Any precise experimental work would require a value for the isotope or isotopic mixture being used, but where there is a purely formal demand for atomic weights, mass numbers that are chosen on the basis of half-life and availabiUty have customarily been used. A Hst of these is provided in Table 1. 

In addition to H2, D2, and molecular tritium [100028-17-8] the following isotopic mixtures exist HD [13983-20-5] HT [14885-60-0] and DT [14885-61-1]. Table 5 Hsts the vapor pressures of normal H2, D2, and T2 at the respective boiling points and triple points. As the molecular weight of the isotope increases, the triple point and boiling point temperatures also increase. Other physical constants also differ for the heavy isotopes. A 98% ortho—25/q deuterium mixture (the low temperature form) has the following critical properties = 1.650 MPa(16.28 atm), = 38.26 K, 17 = 60.3 cm/mol3... 

A few elements, among them fluorine and phosphoras, occur naturally with just one isotope, but most elements are isotopic mixtures. For example, element number 22 is titanium (Ti), a light and strong metal used in Jet engines and in artificial human Joints. There are five naturally occurring isotopes of Ti. Each one has 22 protons in its nuclei, but the number of neutrons varies from 24 to 28. In a chemical reaction, all isotopes of an element behave nearly identically. This means that the isotopic composition of an element remains essentially constant. The isotopic composition of Ti (number percentages) is... 

The following isotopic labeling experiment was performed in order to quantify the contribution of the direct and indirect reaction routes to CO formation After steady-state reaction with CH4/02/He was achieved, an abrupt switch of the feed from CH4/02/He to an isotopic mixture of CH4/1 02/ C 02/He was made, in which the partial pressures of CH4 and 62 were kept exactly the same as in the ordinary CH4/02/He mixture, so as not to disturb the steady-state condition. However, C 02 was added to the isotopic mixture in an amount corresponding to approximately 10-15% of the CO2 produced during reaction of the mixture. The purpose was to measure the production of C 0 due to reforming of CH4 with C 02 only (indirect reaction scheme) under steady-state conditions of the working catalyst surface. Figure 3 shows the transient responses of and C O... 

After approximately 3 min on stream in the isotopic mixture of CH4/02/C 02/He, a steady-state value in the rate of formation is obtained (Fig. 3). This value is used to estimate the relative contribution of the CO2-reforming route to the overall production of CO (direct + indirect routes). Proper analysis of this result, taking into consideration scrambling of isotopic oxygen Figure 3. Transient isotopic labelling experiment atoms between the CO2 and O2 molecules... 

Taylor PDF, De Bievre P, Walder AJ, Entwistle A (1995) Vahdation of the analytical linearity and mass discrimination correction model exhibited by a Multiple Collector Inductively Coupled Plasma Mass Spectrometer by means of a set of synthetic uranium isotope mixtures. J Anal At Spectrom 10 395-398... 

For the study of mixed oxides, one should characterize the various sites. In this case, the first step is to characterize the CO adsorption at various equilibrium pressures at low temperature, followed by evacuation at increasing temperatures to obtain information about the stabilities of the various species. Although the C—O stretching frequency is the most informative parameter, the data determining the stabilities of the various species can be decisive for the assignment of the bands. Multiple carbonyls adsorbed on the same metal cation are possible, and in order to identify them isotopic mixtures should be used. Sometimes the polycarbonyls are very stable and in this case, if 12CO is adsorbed first and then 13CO introduced, mixed species may not form at ambient temperature. 

Isotopic labeling studies, 21 137-139 Isotopic mixtures, hydrogen, 13 765 Isotrate ER, molecular formula and structure, 5 110t Isotretinoin, 25 789... 

Mulliken, R.S. and Harkins, W.D. (1922). The separation of isotopes. Theory of resolution of isotopic mixtures by diffusion and similar processes. Experimental separation of mercury by evaporation in a vacuum. Journal of the American Chemical Society 44 37-65. 

As described above, it is probably adequately clear that the vibrational spectroscopy of water is complicated indeed One can simplify the situation considerably by considering dilute isotopic mixtures. Thus one common system is dilute HOD in D2O. The large frequency mismatch between OH and OD stretches now effectively decouples the OH stretch from all other vibrations in the problem, meaning that the OH stretch functions as an isolated chromophore. Of course the liquid is now primarily D2O instead of H2O, which has slightly different structural and dynamical properties, but that is a small price to pay for the substantial simplification this modification brings to the problem. 

Infrared derivative spectra for different isotopic mixtures of C 0/ C 0 in the c(2 x 2) structure at lOOK on Cu(lOO). Reproduced by permission from Persson and Ryberg. )... 

SOL.37.1. Prigogine and A. Bellemans, On isotope mixtures. Proceedings, Symposium on Isotope... 

Another method, based on an old idea about radiation pressure, uses the local separation of different isotopes in atomic or molecular beams. If the laser beam which crosses the molecular beam at right angles is tuned to an absorption line of a defined isotope in a molecular beam containing an isotopic mixture, the recoil from the absorption of the laser photons results in a small additional transverse velocity component. This leads to a beam deflection for the absorbing molecules which enables the desired isotope to be collected in a separate collector 154g)... 

As logical as this diagnostic method is, one needs to realize its lack of absolute applicability. The observed magnitude of the kinetic isotopic effect is not great, and the aforementioned statement of independence of the electron affinity from the increase in molecular mass of the substrate is not obvious. This postulate should be proved in each case. Benzophenone, taken as an isotopic mixture of C=0 and C=0 gives a mixture of anion-radicals with a decreased proportion of C=0 isotomer when reduced with potassium in HMPA (Stevenson et al. 1987b). In effect, this means that for the heavier isotopomer of benzophenone, the electron affinity is smaller. 

Behavior of Isotope Mixtures in Electron-Transfer Reactions... 

The future will supposedly bring a more precise description of the trends considered here, and the reasons for the exceptions will be clearer. It is quite possible that in this field, too, the exceptions will only conhrm the rule. For now, it is worth concluding that all these regularities have very real practical applications. It is a fact that the equilibrium constants of the previously described reactions differ from unity. This provides an opportunity to separate and enrich isotopic mixtures. 

Figure 3c. Comparison of observed resonance Raman spectra for isotope mixture with predicted spectra for two possible coordination geometries.

For di- and triethylene lithium the use of isotopic mixtures enabled the authors to show that the Li(C2H4)(C2D4) and Li(C2H4)2(C2D4) species have only one isotopomer and therefore two and three equivalent ligand molecules (Scheme 4). 

Cations can also be obtained by the formal addition of a hydron (hydron is the recommended name for the normal isotopic mixture of protons, deuterons and tritons, see p. 7) to a binary hydride. In such cases, a formalism of substitutive nomenclature is used the suffix -ium is added to the name, slightly modified, of the parent hydride. The selection of permitted hydride names and their usage are discussed in Section 4.5 on substitutive nomenclature. 

Busca (18b) has evaluated the literature values for infrared bands attributed to coordinated and adsorbed dioxygen species. He concludes that it is very difficult to deduce the nature of the dioxygen coordination from measurements of the frequency shift, Av00, with respect to the stretching frequency of the free molecule. It needs to be stressed that it is also difficult to distinguish between mononuclear and molecular species from measurements of v00, and this can only be achieved by careful interpretation of experiments using 160/180 isotopic mixtures. The absence of such experiments very often accounts for the conflicting attributions in the literature which are discussed in later sections. 

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