Chemical changes during concentration

ESR Studies of Surface Species. ESR has been applied widely in heterogeneous catalysis as a technique for examining the nature and concentration of possible catalytic sites when a material is activated either chemically or thermally (64, 65, 66). ESR studies on the Phillips polyethylene catalyst, Cr03/Si02, are a classical example of this application (67). The interpretation of such ESR studies is questionable since the chemical changes during activation are not well understood, and the nature of the surface species may have to be assumed. 

The work of Huraux, Soualmia, and Sheppard has demonstrated that it is possible to make a quantitative interpretation of the r values (within the limits of an admittedly difficult theory), and that the er value is directly linked to the chemical changes during cure. However, because of the complications introduced by dipole correlations, the relaxed permittivity is not a useful tool for routine quantitative determination of polar reactive group concentrations during cure. 

The cells considered thus far are ones in which there has been a net chemical change during the production of emf. It is also possible to set up a cel in which there is no net chemical change, the net effect being the transfer of a species from one solution to another, in which it is at a different concentration. There is a natural tendency for a species to pass from a solution in which it is at a high concentration to one in which it is at a lower concentration. It experiences a decrease in Gibbs energy... 

The most serious problem arises as a result of relatively fast reactions or, more precisely, relatively large values of the dimensionless rate constants K as defined in Eqs. 7.6 and 7.12. It has been pointed out (Nielsen et al 1987) that even moderate rates render the standard explicit simulation technique inaccurate. The actual limit depends upon ST a rough guide (Nielsen 1985) appears to be if the discrete term for the chemical change during the time interval ST exceeds a few % of the bulk concentration value, more sophisticated techniques are needed. Looking at Eq. 7.8, if KST > 0.01, EX will not work very well. So if, for example, we use ST = lO ", we are limited to K < 10. It is not... 

Table I. Chemical Changes during Processing of Tomato Juice (Concentrations in mmol/100 g dry matter)...

SIMS is, strictly speaking, a destructive teclmique, but not necessarily a damaging one. In the dynamic mode, used for making concentration depth profiles, several tens of monolayers are removed per minute. In static SIMS, however, the rate of removal corresponds to one monolayer per several hours, implying that the surface structure does not change during the measurement (between seconds and minutes). In this case one can be sure that the molecular ion fragments are truly indicative of the chemical structure on the surface. 

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