Scavenger reaction

Most oxygen scavengers perform best within an alkaline pH range, usually around 9.5, as a result of the catalyst or scavenger deprotonation step before the actual oxygen scavenging reaction. Typically, the minimum reaction pH is 8.5 (hence, the general minimum recommendation for FW pH). 

Where larger, semibulk quantities of sulfite-based oxygen scavengers are required, a 20 to 25% bisulfite solution is recommended. It is sufficiently concentrated to provide reasonable freeze protection. The overall oxygen scavenging reaction is ... 

Where dry, catalyzed sodium sulfite is used as the scavenger source, the provision of 2 to 3% metabisulfite into the day-tank batch provides sufficient pH level reduction to ensure the cobalt catalyst does not precipitate. The overall oxygen scavenging reaction is as follows ... 

The hydrazine-oxygen scavenging reaction is pH-dependent (like sulfite), and an increase in pH from 8 to 9 produces a threefold increase in reaction rate and a further three fold increase from pH 9 to 10. 

Homogeneous oxygen scavenging reaction, occurring in FW solution ... 

Following the formation of dehydroascorbic acid, the reaction proceeds irreversibly the degradation process produces oxalates, formates, and carbon dioxide, depending on temperature and pH. As usual, the oxygen-scavenging reaction rate is increased by raising the pH and the temperature. 

Because HQ has limited volatility at lower pressures, when acting as a catalyst with other more volatile oxygen scavengers, it will not always follow the primary scavenger around the boiler cycle system. This creates operational difficulties because scavenging reaction rates are slowed and, in lower pH condensate systems, the post-boiler oxygen scavenging reaction may cease entirely. 

The results showed that the presence of oxygen caused a decrease in the yield of As 03 and — As0j> and an increase (not equal to the sum of the other decreases) in >As0. The decrease in clearly due to thermal scavenging effects, while the increase in As0 was ascribed to scavenging reactions such as... 

The efficiency of the antioxidant will depend on the ratio of the rates of Reaaion 2.10 to those for Reactions 2.11 and 2.12. A compound that is capable of reducing the antioxidant radical (A ) back to the parent compound (AH) will compete with Reactions 2.11 and 2.12, and so increase the efficiency of peroxyl radical scavenging (Reaction 2.10). In addition, the steady-state concentration of the antioxidant wiU be maintained at its initial concentration for a longer period and this should also result in more efficient suppression of the peroxidation reaction. The net result of these effects will be a synergistic enhancement of antioxidant activity. 

Matsuo, M. Matsumoto, S. Iitaka, Y. Niki, E. Radical-scavenging reactions of vitamin E and its model compound 2 2,5,7,8-pentamethylchroman-6-ol, in a tert-butylperoxyl radical generating system. J. Am. Chem. Soc. 1989, 111, 7179-7185. 

This suggests the possible deleterious effects of carotenoids, for example, on membrane proteins, if, following a radical scavenging reaction, the radical cations so formed are not efficiently repaired. 

Mortensen, A. and Skibsted, L.H. 1997a. Importance of carotenoid structure in radical scavenging reactions. J. Agric. Food. Chem. 45 2970-2977. 

Packer, J.E., Mahood, J.S., Mora-Arellano, V.O., Slater, T.F., Willson, R.L., and Wolfenden, B.S. 1981. Free radicals and singlet oxygen scavengers Reaction of a peroxy-radical with P-carotene, diphenyl furan and 1,4-diazobicyclo (2,2,2)-octane. Biochem. Biophys. Res. Commun. 98 901-906. 

Magee and Chatterjee (1980) give a different criterion for the size of a chemical core depending on a scavenger reaction in competition with radical recombination. Its nature, however, is extraneous to the physical track structure. 

Operationally, a procedure may be based on measuring the yield of a reaction traceable to ionization, usually giving a lower limit to the ionization yield. Thus, in the radiation chemistry of hydrocarbon liquids, the product of an electron scavenging reaction (for example, C2H3- radical from the scavenger C2H5Br)... 

Samuel and Magee (1953) employed a 1-radical model to find the relative forward yield in water radiolysis as a function of radiation quality. In such models, no distinction is made between reactive radicals or molecular products. The products of radiolysis are called forward (F) to denote observable molecular yield or radical (R), denoting yield of scavenger reaction at small concentration. The aim of the theory is to calculate the relative forward yield G(F)/[G(F) + G(R)], where the G values refer to the respective yields for 100 eV energy absorbed in... 

Equation (7.9) implies that nothing survives radical-radical recombination for an isolated cylindrical track—that is, at zero dose. At a finite dose rate, tracks will overlap at a large, but finite time and, for even a very small solute concentration, a scavenger reaction may precede intertrack overlap. Thus, the relevant question is the determination of the maximum time to be used in Eq. (7.9) consistent with a given dose rate. 

The Ganguly-Magee model is a 1-radical-1-scavenger model where competition between recombination and scavenging reaction is represented as follows ... 

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

In the infinite time limit, no radical survives. They either combine to give molecular products or undergo scavenging reactions. The probability of the latter is given by... 

In addition, there are scavenging reactions for the primary species as appropriate to the solution. 

Equation (7.30) shows that the fundamental information on recombination kinetics is contained in the solution of the scavenger-free case, from which the recombination kinetics with a scavenger may be obtained via an exponential transformation. The scavenger reaction probability is now given by... 

After obtaining from the measured value of kl by this procedure, one can determine the attachment efficiency in the quasi-free state, rj = fe1f/fed.ff, by the same procedure as for scavenging reactions (see Eq. 10.11 et seq.). Mozumder (1996) classifies the attachment reactions somewhat arbitrarily as nearly diffusion-controlled, partially diffusion-controlled, and not diffusion-controlled depending on whether the efficiency p > 0.5, 0.5 > r > 0.2, or r < 0.2, respectively. By this criterion, the attachment reaction efficiency generally falls with electron mobility. Nearly diffusion-controlled reactions can only be seen in the liquids of the lowest mobility. Typical values of r] are (1) 0.65 and 0.72 respectively for styrene and p-C6H4F2 in n-hexane (2) 0.14 and 0.053 respectively for a-methylstyrene and naphthalene in isooctane (3) 1.8 X 10-3 for C02 in neopentane and (4) 0.043 and 0.024 respectively for triphenylene and naphthalene in TMS. 

Sanchez-Moreno and others (1998 1999a,b) proposed a new methodology for the evaluation of the antiradical efficiency toward DPPH. Their procedure takes into account not only the concentration of the antioxidant but also the reaction time to reach the plateau of the scavenging reaction, a modification that could be an advantage... 

Scavenger Reaction time Pd(AcO)2 Pd2(C3H5)2Cl2 Pd(PPh3)4 Pd2(dba)3... 

---