Scavenger concentration

Clearly, further investigation is desirable before these observed variations in polarization with scavenger concentration can be unequivocally interpreted. However, these studies hold out high promise not only of... 

Here cs is scavenger concentration, and fe and fes are the specific rates of the recombination and scavenging reactions, respectively. Integrating (7.12) over... 

Mahlman and Sworski (1967) found that curves for scavenging of H2 by NOj- have nearly the same shape for neutral and 0.1 M acid solutions, over the concentration range 1 mM to 0.4 M of the solute, despite the fact that nitrate reacts much faster with eh than with H. Schwarz points out, however, that when two solutes (H+ and N03-) compete for the same intermediate (eh), the extrapolation to zero scavenger concentration is not a valid procedure. The calculation of the diffusion model agrees with experiment over the entire N03- concentration range. Further, the model predicts a lower H2 yield at very low scavenger concentrations. 

Rzad et al. (1970) obtained the relative lifetime distribution of electron-ion recombination in cyclohexane by 1LT of Eq. (7.26). Denoting the probability that the lifetime would be between t and t + dt as f(t) dt, the thusly defined scavenging function at scavenger concentration c% is given by... 

A special situation arises in the limit of small scavenger concentration. Mozumder (1971) collected evidence from diverse experiments, ranging from thermal to photochemical to radiation-chemical, to show that in all these cases the scavenging probability varied as cs1/2 in the limit of small scavenger concentration. Thus, importantly, the square root law has nothing to do with the specificity of the reaction, but is a general property of diffusion-dominated reaction. For the case of an isolated e-ion pair, comparing the t—°° limit of Eq. (7.28) followed by Laplace transformation with the cs 0 limit of the WAS Eq. (7.26), Mozumder derived... 

A generalized flowsheet for treatment of sulphide-dominated PMG ores is presented in Figure 18.9. There can be some variation in this flowsheet, such as (a) retreatment of the cleaner tailings, (b) regrinding the scavenger concentrate and (c) the number of cleaning stages. 

In spite of the number of attempts to selectively float Ta/Nb and Zr, it was not possible to either float Ta/Nb and depress the zircon or float the zircon and depress the Ta/Nb. In fact, it was found that the recovery of Ta/Nb in the bulk rougher-scavenger concentrate was strongly related to the recovery of zircon. Figure 23.13 shows this relationship. 

Figure 23.13 Relationship between Ta/Nb and Zr recovery in the bulk rougher-scavenger concentrate.

The concentration of the spin trap is usually not critical, although care must be exercised in quantitative studies (next Section). When reactive radicals are being trapped in competition with attack on substrate, the scavenger concentration may have to be adjusted in order to detect substrate-derived radicals. In these experiments the variation of the scavenger concentrations can give useful information, as in the example of alcohol oxidation discussed earlier. 

One important advantage of the method is the small sample size required. The volume of solution need be no more than 0.1-0.2 ml. and a few milligrams of reagents and of scavenger are normally sufficient for a number of experiments using different scavenger concentrations. 

Fig. 7 Esr spectra obtained the polymerization of styrene by t-butoxyl radicals in the presence of MNP (Chalfont et al., 1968). Traces (a) and (b) were recorded on different scales using high scavenger concentration (c) represents an intermediate, and (d) a low scavenger concentration. The shoulder arrowed in (b) reveals a trace of growing-chain radical [29], whilst the lines marked x in (a) are due to [27]...

When a strong electron acceptor (electron scavenger) is added in the irradiated system, some of the electrons will react with the scavenger before they recombine with their parent cations. Therefore the measurement of the yield of the scavenging products as a function of the scavenger concentration can be used to monitor the geminate recombination, and the electron scavenging technique has proved to be an important tool in experimental studies. 

Here denotes the rate constant for the scavenging reaction and Cs is the scavenger concentration. The probability density for the geminate pairs, Ws(r,t,Cs), may be related to the corresponding probability density in the absence of scavenger, through... 

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