Fractionation at low temperatures

Sharpie A process for purifying paraffin wax by crystallization from a petroleum fraction at low temperature. 

Mechanical treatment of samples or different synthetic procedures have been shown to influence strongly SCO behaviour. The first observation of the effect of grinding a sample was reported by Hendrickson et al. for an iron(III) SCO complex [154]. This resulted in the flattening of the ST curve with an increase of the residual HS fraction at low temperatures. Similar effects were later observed in other systems. The SCO characteristics may also be influenced by the synthetic procedure, as illustrated for [Fe(phen)2(NCS)2]. This can be prepared in two principal ways by precipitation from methanol or by extraction with acetone of a phen molecule from [Fe(phen)3](NCS)2-H20... 

The experimentally measured temperature dependence of the Fe(III)aq-Fe(II)aq fractionation at low temperatures is ... 

There is a further fascinating chemistry associated with isotopic fractionation . At low temperatures, the difference in zero-point energies of the reactants and products in the reaction... 

Other than the suppression of isotope fractionation at low temperatures for mesophilic sulfate reducers, as discussed above, temperature has played a prominent role in the discussion of the factors influencing fractionation. Thus, for Desulfovibrio desulfuricans, specific rates of sulfate reduction increase with increasing temperature, and reduced fractionations down to 16-17 %o have been observed between 40 to 45°C (Kaplan and Rittenberg 1964). These results have been extrapolated to suggest higher specific rates should accompany fractionations reduced to near 0 %o at even higher... 

On the other hand, the extreme depletion in 0 of the water in isotopic equihbrium with the black calcite at 0°C is consistent with the 5 0 values of the ice exposed in the Elephant Moraine which range from -42.6%c to 9.7%c (Faure et al. 1993). Similar calculations for aU of the calcites in Table 17.2 yield 8 0 values of the water between -47.3%o and -54.9%o. The evident depletion of the water in 0 is characteristic of meteoric precipitation in the polar regions and is the result of isotope fractionation at low temperature during mnltiple cycles of evaporation of water from the surface of the Earth and condensation of water vapor in the atmosphere. This process can be modeled by the Rayleigh equations (Hoefs 1997, pp. 9-10). 

Schomacker etal. (1988) labelled human blood serum with Yb in vitro and showed, by alcohol fractionation at low temperature, that 95% of the radioactivity was associated with the albumin. Equilibrium dialysis studies in vitro using HSA-ianthan-ide solutions or whole human serum yielded values for the association constant for the lanthanide-albumin complex ranging, with increasing atomic number and decreasing ionic radius, from log Kp, = 4.7 for Ce to log = 9.5 for Yb, table 9. 

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