Oxygen migration rate

After the 20K step increase in feed temperature, not much change could be observed for two minutes. Then the last thermocouple started to increase from 560 to 1200 K level, and the hot zone widened. The forward migration rate of the hot zone was about 5 cm/min. After about six minutes, the oxygen content of the cycle gas became very low and temperature slowly started to decline. With this the experiment terminated. 

Racemization (1,2-oxygen migration) occurred between 2a and 2b (97T12203). The racemization took place slowly in solution at room temperature and obeyed reversible first-order kinetics krac = 4.3 x 10 6 s-1 (25°C, CH2C12). Tlie AH and AS, calculated from rate constants at temperatures between 20° and 40°C, were 24.3 kcal/mol and -2.0 eu, respectively. The rate of the racemization was hardly dependent on either the concentration of the solution or the polarity of the solvent examined. The low activation energy and the analogy of the mechanism to that for the dispro-... 

Volatility and Migration Rate. A study of the half-lives (T ) of the antioxidants in the polymer (see Table 1) at the same temperature suggests a reason for the lack of correlation between the two sets of results. In an oxygen absorption test, volatilization cannot occur, and the result is a true measure of the intrinsic activity of the antioxidant molecule. In an air oven test, on the other hand, physical loss of the antioxidant by migration and volatilization from the surface must dominantly influence the test results. Billingham and his coworkers ( 2) have shown that these two physical parameters determine the rate of loss of antioxidants from polymers. Increase in molecular mass generally decreases molecular mobility as well as volatility, and which factor dominates depends on the thickness of the sample ( 2). 

These values are similar to the coefficient of diffusion for A0, measured at equivalent metal loading, which indicates that the rate of oxygen migration is virtually independent of the nature of the alumina used. 

The results, recorded in Table 6, demonstrate that rhodium remains the most active metal in the promotion of oxygen migration on the support. Platinum is approximately four times less active than rhodium, and palladium cannot promote, at a measurable rate, the reaction of exchange at 400°C. 

The 17.8-ns lifetime of the protoporphyrin IX emission is sufficient to permit oxygen situated in any part of the protein at the instant of fluorophore excitation to reach the porphyrin during the persistence of the excited state therefore the calculated migration rate k for oxygen through the protein represents an average value for all domains of the protein. 

Table 9.9. Oxygen entry rate (k ), exit rate (k ) and migration rate (x) determined on - ...

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