Scavenged behaviour

Elements that are highly particle-reactive, characterized by large z/r ratios (see Section 5.2), often have vertical profiles with surface maxima and decreasing concentrations with depth aluminium (Al) 

Component North Atlantic North Pacific Estimated oceanic residence time (years) 

Scavenged species are all metals and their residence times in seawater are estimated to be a few hundred years, short in comparison with nutrient and conservative elements (Table 6.9). These rapid removal rates imply that river inputs are 

Element Hydrothermal plume removal flux River input flux % removal of river input flux 

---

As shown on Figure 9.1 when the circuit is opened (I = 0) the catalyst potential starts increasing but the reaction rate stays constant. This is different from the behaviour observed with O2 conducting solid electrolytes and is due to the fact that the spillover oxygen anions can react with the fuel (e.g. C2H4, CO), albeit at a slow rate, whereas Na(Pt) can be scavenged from the surface only by electrochemical means.1 Thus, as shown on Fig. 9.1, when the potentiostat is used to impose the initial catalyst potential, U r =-430 mV, then the catalytic rate is restored within 100-150 s to its initial value, since Na(Pt) is now pumped electrochemically as Na+ back into the P"-A1203 lattice. 

In practice, all inhibitors show some deviations from ideal behaviour for example when diphenylpicrylhydrazyl is used as a radical scavenger it is converted to products which are reactive towards radicals as shown by the fact that polymerizations occurring after the induction periods are obviously retarded. The use of 14C-hydrazyl in the radical polymerization of styrene has shown that substantial quantities of the material are combined in the polymer 36). 

The regiochemistry of the bromination (Scheme 20) of a,/l-unsaturated ketones101 (in methanol) by bromine is affected by the presence of A-bromosuccinimide, which also depresses the overall rate of bromination. A possible explanation of the observed behaviour is the presence of a complex (50) involving the C=C double bond of olefin, TV-bromosuccinimide and bromine, as shown in Scheme 21. Probably, NBS acts as a hydrogen bromide scavenger, thereby causing a change in the bromination mechanism from an acid-catalysed pathway to a bromonium ion mechanism. 

Clearly, the removal mechanisms have an appreciable effect on dissolved elemental abundances. The two major processes in operation are uptake by biota and scavenging by suspended particulate material. In the first instance, the constituent mimics the behaviour of nutrients. This is evident in the metal/nutrient correlation for Cd/P and Zn/Si (Figure 12). 

The study of kinetics and mechanism of pyrolysis is of considerable scientific interest, with regard to the thermal behaviour of organic molecules, the precise constitution of molecules or copolymers, including irregularities, such as structural defects and incorporation of initiator molecnles and radical scavengers, used to control the MW. 

The results of spin-trapping experiments 18) also support this view no radicals were trapped on photolysis of benzil in benzene solutions of compounds I and II (see Section II. A. 1) but in methanol solutions of II, CH OH radicals were scavenged, indicating that hydrr en abstraction by photoexcited benzil had occurred 18). Thus the behaviour of benzil in these experiments contrasted markedly with that of benzoin and benzoin methyl ether, described previously. 

The chemistry of dissolved metals in seawater can be grouped into three classes, which describe the behaviour of the metal during chemical cycling. These classes—conservative, nutrient-like and scavenged—have been recognized by the shapes of concentration profiles when plotted against depth in the oceans. 

Removal of the latter in the cyclic regeneration mechanism of piperidine stabilisers by reaction with hindered phenol radical intermediates is also believed to be important in antagonism. During thermal oxidation the interactions were more favourable and synergism prevailed due here to the complementary behaviour of nitroxyl free radicals and antioxidant radicals in free radical scavenging processes. 

The redox reaction also extends to the participation of hydroperoxides, but their efficient decomposition depends on the formation of a non-radical product such as an alcohol. Another example of a redox couple is found in the behaviour of the nitroxyl radical (R NO )- Depending on the structure of R, these are efficient radical scavengers and a redox couple between the mdical and the hydroxyl amine (R NO /R NOH) is formed (which is analogous to the galvinoxyl radical G-/GH). It is noted that the hindered amine stabilizers (e.g. Tinuvin 770 and the monomeric and polymeric analogues) are ineffective as melt antioxidants, possibly because of reaction with hydroperoxides or their sensitivity to acid. 

---