Aqueous radiolysis

The hydrated electron, if the major reducing species in water. A number of its properties are important either in understanding or measuring its kinetic behavior in radiolysis. Such properties are the molar extinction coefficient, the charge, the equilibrium constant for interconversion with H atoms, the hydration energy, the redox potential, the reaction radius, and the diffusion constant. Measured or estimated values for these quantities can be found in the literature. The rate constants for the reaction of Bag with other products of water radiolysis are in many cases diffusion controlled. These rate constants for reactions between the transient species in aqueous radiolysis are essential for testing the "diffusion from spurs" model of aqueous radiation chemistry. 

As can be seen, the agreement between the experiments and the calculations is excellent. It is also interesting to note that the surface reactions at the solid surface area to solution volume ratios used here do not influence the bulk concentrations of aqueous radiolysis products significantly. However, this is only valid when the dose is uniformly distributed in the whole volume. Under deep repository conditions, i.e. in systems dominated by a-radiolysis, the relative impact of H202has been shown to be >99.9% Consequently, H2O2 is the only radiolytical oxidant needed to be accounted for when... 

Table I lists the data obtained by our four methods for the reactivities of various organic compounds (relative to hexane) and also some of the previous data collected by radiolysis techniques. All of the data agree well with the exception of the reactivities of the alcohols as measured by aqueous radiolysis and the more reactive hydrocarbons as measured by radiolysis. The disagreement between our work and radiolysis for ethanol and 2-propanol cannot be attributed to experimental error since numerous laboratories have studied the radiolysis of these alcohols. The reason for this discrepancy is unknown at present. However, we feel that two arguments suggest that our data may be correct and that the radiolysis results may contain some subtle systematic error for ethanol and 2-propanol. The first is discussed below in connection with Table II. The second is the lack of internal consistency of the radiolysis data themselves. When compared with other compounds studied by radiolysis which contain RCH2OH or R2CHOH groups, ethanol and 2-propanol stand out as unusually reactive. This point is discussed by Pryor and Stanley (Table IV of Reference 4).

The data of Table II support the suggestion that there may be a systematic error in the values for 2-propanol and ethanol as determined by aqueous radiolysis. Om reactivity profile for the H atom is closer to what would be expected in comparison with other radicals than is the radiolysis data profile. 

The one-electron reduction of thiazole in aqueous solution has been studied by the technique of pulse radiolysis and kinetic absorption spectrophotometry (514). The acetone ketyl radical (CH ljCOH and the solvated electron e were used as one-electron reducing agents. The reaction rate constant of with thiazole determined at pH 8.0 is fe = 2.1 X 10 mole sec in agreement with 2.5 x 10 mole sec" , the value given by the National Bureau of Standards (513). It is considerably higher than that for thiophene (6.5 x 10" mole" sec" ) (513) and pyrrole (6.0 X10 mole sec ) (513). The reaction rate constant of acetone ketyl radical with thiazolium ion determined at pH 0.8 is lc = 6.2=10 mole sec" . Relatively strong transient absorption spectra are observed from these one-electron reactions they show (nm) and e... 

Miscellaneous Reactions. Radiolysis at room temperature of diluted aqueous solutions of succinic acid produces 1,2,3,4-butane tetracarboxyhc acid [1703-58-8] (122), which has numerous industrial and agricultural appHcations (eq. 12). 

In addition to the stable I2O5 and moderately stable I4O9 and I2O4, several short-lived radicals have been detected and characterized during y-radiolysis and flash photolysis of iodates in aqueous alkali ... 

An interesting example of the N(9)-C(8) prototropic tautomerism has been reported for the caffeine radical by pulse radiolysis studies in aqueous solution the transformation of the heteroatom-protonated electron adduct 25 into the carbon-protonated tautomer 26 occurred spontaneously in neutral media [95JCS(F)615]. 

Complexes of cations in unstable oxidation states in aqueous solutions as studied by pulse radiolysis. D. Meyerstein, Acc. Chem, Res., 1978,11,43-48 (95). 

Irradiation of dilute aqueous solutions results in the interaction ofthe ionizing radiation with water molecules. The radiolysis of water produces hydrated electrons (eaq ", G = 2.8), hydrogen atoms (G = 0.6) and hydroxyl radicals (G = 2.8) which react with the molecules of the solutes. The use of special scavengers can convert one species to another, e.g. 

Meissner and coworkers36 studied the pulse radiolysis of aqueous solutions of dimethyl sulfoxide. It was found that hydrated electrons react with DMSO with a rate constant of... 

Veltwisch and colleagues45 studied the reaction of OH with several sulfoxides by pulse radiolysis using electrical conductivity for the detection of formation or disappearance of ions. Pulse radio lysis of N20-saturated aqueous solution of DMSO (10-3m) leads to a decrease in conductivity at basic pH (pH = 9.0) and an increase in conductivity at acidic pH (pH = 4.4). This is explained by the reactions... 

TABLE 1. Yields of RS02 in the radiolysis of N20-saturated aqueous solutions of symmetrical sulfoxides (R2SO), the percent of OH forming RS02 [G(OH) = 6.0] and the rate constants for ion formation (equation 30)... 

In the case of diaryl sulfoxides the formation of both the aryl radical and the hydroxycyclohexadienyl radical was observed optically. Veltwisch and coworkers45 studied also the reaction of OH radicals from radiolysis of aqueous solutions of mixed (alkyl phenyl) sulfoxides (PhSOR). They found the formation of both alkylsulfinic and phenylsulfinic acids. 

They measured the formation of ionic species in the pulse radiolysis of a 10"3 m MTMSO aqueous solution saturated by N20 gas at pH 5.6 by conductivity detection and found an increase in conductivity. Assuming the molar conductivity of CH3SCH2S02 to be the same as that measured for CH3S02- 45, it is found that G(CH3SCH2S02") = 2.7. 

Nelson54 studied the products of radiolysis of aqueous solution by variable-field CIDNP pulse radiolysis. On the basis of the chemical shifts the following products were identified methyl methanesulfinate, methanol, l,2-bis(methylsulfinylethane) [CH3S(0)CH2CH2S(0)CH3], dimethyl sulfone, dimethyl sulfide, methane and ethane. The high field polarization was used to study the mechanism of formation of polarized products. 

Koulkes-Pujo and coworkers5 5 studied the formation of methane in the reaction of OH radicals and H atoms with aqueous DMSO in acidic media. In the radiolysis of deaerated acidic aqueous solution of DMSO they found that G(CH4) increases monotonously with CH4 concentration up to 0.8 m DMSO. Similar results were obtained for C2H6 but the yields of C2H6 are much lower than that of CH4. 

Nishimura and coworkers57-59 studied the y-radiolysis of aqueous solutions of sulfoxide amino acids. Sulfoxide amino acids are the precursors of the flavors of onions (S-propyl-L-cysteine sulfoxide, S-methyl-L-cysteine sulfoxide and S-(l-propenyl)-L-cysteine sulfoxide) and garlic (S-allyl-L-cysteine sulfoxide). In studies on sprout inhibition of onion by /-irradiation it was found that the characteristic flavor of onions became milder. In the y-radiolysis of an aqueous solution of S-propyl-L-cysteine sulfoxide (PCSO)57,58 they identified as the main products alanine, cysteic acid, dipropyl disulfide and dipropyl sulfide. In the radiolysis of S-allyl-L-cysteine sulfoxide (ACSO) they found that the main products are S-allyl-L-cysteine, cysteic acid, cystine, allyl alcohol, propyl allyl sulfide and diallyl sulfide. The mechanisms of formation of the products were partly elucidated by the study of the radiolysis in the presence of N20 and Br- as eaq - and OH radicals scavengers, respectively. 

In contrast with irradiation of ACSO and PCSO, where volatile products were formed (sulfides, disulfides and alcohols), no volatile products were formed in the radiolysis of aqueous solutions of S-(cis- l-propenyl)-L-cysteine. Here the authors found that reactions of OH" radicals are responsible for the formation of propyl-1-propenyl sulfides (cis and trans). 

The application of techniques of pulse radiolysis offers the potential to determine rates of primary radiolysis induced reaction processes. This knowledge can be of great value in the determination of redox processes of Pu ions occurring in a wide variety of aqueous solutions. As a matter of fact, such information is essential to a prediction of the Pu oxidation states to be expected in breached repository scenarios. For an... 

By pulse radiolysis of nitrous oxide-saturated aqueous solutions of ferricyanide (2 X 10 " M) and various alcohols (0.1 M), Adams and Willson " were able to obtain absolute rate coefficients for the ferricyanide oxidation of the radicals derived from the alcohols by attack of the solvent irradiation product, OH-. 

The standard reduction potentials determined in aqueous solution give hierarchies slightly different from the antioxidant hierarchy established in DMF. For the potential determined by pulse radiolysis the ordering according to tendency of regeneration is (Jovanovic et al, 1994) ... 

Sumiyoshi and coworkers studied the radiolysis of aqueous solution of methyl methylthiomethyl sulfoxide [ H3S(0) H2S H3 MTMSO] at various pH by pulse radiolysis. They found that the reaction of e with MTMSO (in the presence of 1M t-butyl alcohol to scavenge OH" radicals) leads to formation of a transient with a broad absorption band of = 375 nm. The absorbance at 375 nm as a function of pH are of S shape, indicating an equilibrium due to reaction with H. Similar to the finding in dimethyl sulfoxide they suggested the scheme. 

While aqueous solutions mean only dilute solutions, mixtures can be any proportion of HjO and DMSO. Cooper and coworkers found in the pulse radiolysis of H2O/DMSO mixtures two easily resolvable absorption bands at wavelengths >400nm. One band corresponds to the oxidizing species with a maximum at 600 nm and a relatively long half-life (1 to 4/is). The second band with higher wavelength (720-1500 nm) and shorter half-life is attributed to the solvated electrons. 

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