Pulse radiolysis of 3-pyridine methanol and 3-pyridine carboxaldehyde in aqueous solutions

Reactions of e_aq^-, H-atom and OH radicals with 3-pyridine methanol (3-PM) and 3-pyridine carboxaldehyde (3-PCA) have been studied at various pHs using pulse radiolysis technique. e_aq^- was found to be highly reactive with both 3-PM and 3-PCA (k approx. 10¹⁰ dm³ mol¹ s^-1). Semi-reduced species formed in both cases were strongly reducing in nature. In the case of 3-PM, electron addition leads to the formation of pyridinyl radicals whereas in the case of 3-PCA, PyCHOH type radicals are formed. At pH 6.8, H-atom reaction with 3-PCA also gives semi-reduced species (PyCHOH), whereas at pH 1, H-atoms add to the ring. (CH₃)₂˙COH radicals were found to transfer electron to 3-PCA at all the pH values tested and by making use of changes in the absorption spectra, pK_a values of the semi-reduced species were determined to be 4.5 and 10.6. OH radicals were found to undergo addition reaction with 3-PCA, whereas in the case of 3-PM they reacted by H-abstraction as well as addition reaction. By following the yield of methylviologen radical cation formed by electron transfer reaction, it was estimated that approx. 50% of OH radicals react with 3-PM by H-atom abstraction at pH 6.8, giving reducing radicals, whereas at pH 3.2, where 3-PM is in the protonated form, the same is only about 10%. At pH 13, O^-˙ radical anions were found to react exclusively by H-atom abstraction. Reaction of SO₄^-˙ radicals with 3-PCA was found to give a species identical to the one formed by one electron reduction of nicotinic acid at acidic pH values.     

Pulse radiolysis of 3-pyridine methanol and 3-pyridine carboxaldehyde in aqueous solutions

Reactions of e_aq ^-, H-atom and OH radicals with 3-pyridine methanol (3-PM) and 3-pyridine carboxaldehyde (3-PCA) have been studied at various pHs using pulse radiolysis technique. e_aq ^- was found to be highly reactive with both 3-PM and 3-PCA (k approx. 10¹⁰ dm³ mol 1 s^-1). Semi-reduced species formed in both cases were strongly reducing in nature. In the case of 3-PM, electron addition leads to the formation of pyridinyl radicals whereas in the case of 3-PCA, PyCHOH type radicals are formed. At pH 6.8, H-atom reaction with 3-PCA also gives semi-reduced species (PyCHOH), whereas at pH 1, H-atoms add to the ring. (CH₃)₂ ^·COH radicals were found to transfer electron to 3-PCA at all the pH values tested and by making use of changes in the absorption spectra, pK _a values of the semi-reduced species were determined to be 4.5 and 10.6. OH radicals were found to undergo addition reaction with 3-PCA, whereas in the case of 3-PM they reacted by H-abstraction as well as addition reaction. By following the yield of methylviologen radical cation formed by electron transfer reaction, it was estimated that approx. 50% of OH radicals react with 3-PM by H-atom abstraction at pH 6.8, giving reducing radicals, whereas at pH 3.2, where 3-PM is in the protonated form, the same is only about 10%. At pH 13, O^-· radical anions were found to react exclusively by H-atom abstraction. Reaction of SO₄ ^-· radicals with 3-PCA was found to give a species identical to the one formed by one electron reduction of nicotinic acid at acidic pH values.     

Nature of transient species formed during pulse radiolysis of 4-mercaptopyridine in aqueous solutions: Formation of a dimer radical species by one-electron reduction reaction

Reactions of one-electron reducing as well as oxidizing radicals with 4-mercaptopyridine (4-MPy) were studied in aqueous solutions at different pH values. One-electron oxidizing radicals such as N₃ and Br₂ ^–, react with 4-MPy by electron transfer reaction at pH 11 to give 4-pyridylthiyl radical. The reduction potential for the couple 4-PyS /4-PyS^– was estimated to be 0.93V vs. NHE by equilibrium reaction with I₂ ^– /2I^– couple. At pH 6.8, where the compound is predominantly present in the thione form, the transient species formed is a cation radical. OH radicals react with 4-MPy by addition to the pyridine ring at pH 6.8 and 11. At pH 0, OH radicals as well as one-electron oxidants like Cl₂ ^– and Br₂ ^– radicals react with 4-MPy to produce the protonated form of 4-pyridylthiyl radical. At pH 6.8 and 11, e_aq ^– reaction with 4-MPy gave an initial adducts which reacted with the parent molecule to give dimer radicals. Acetone ketyl radicals were unable to reduce 4-MPy at neutral pH. Reducing radicals like H-atoms and acetone ketyl radicals reacted with 4-MPy at acidic pH by H-abstraction reaction to give the same species as produced by oxidizing radicals.     

Pulse radiolysis study of dithio-oxamide in aqueous solutions

The reactions of e⁻ _aq, H-atoms, OH radicals and some one electron oxidants and reductants were studied with dithio-oxamide (DTO) in aqueous solutions using pulse radiolysis technique. The transient species formed by the reaction of e⁻ _aq with DTO at pH 6.8 has an absorption band with λ _max at 380 nm and is reducing in nature. H-atom reaction with DTO at pH 6.8 also produced the same transient species. The semi-reduced species was found to be neutral indicating that the electron adduct gets protonated quickly. However at pH 1, the species produced by H-atom reaction had a different spectrum with λ _max at 360 and 520 nm. Reaction of acetone ketyl radicals and CO₂ ⁻ radicals with DTO at pH 6.8 gave transient spectra which were identical to that obtained by e⁻ _aq reaction. However at pH 1, the spectrum obtained by the reaction of acetone ketyl radicals with DTO was similar to that obtained by H-atom reaction at that pH. The transient species formed by OH radical reaction with DTO in the pH range 1–9.2 also has two absorption maxima at 360 and 520 nm. This spectrum was identical with the spectrum obtained by H-atom reaction at pH 1. This means that all these radicals viz. OH, H-atom and (CH₃)₂COH radicals react with DTO at pH 1 by H-abstraction mechanism. The transient species produced was found to be sensitive to the presence of oxygen. One-electron oxidizing radicals such as Br₂ ^−· and SO₄ ^−· radicals reacted with DTO at neutral pH to give the same species as produced by OH radical reaction having absorption maxima at 360 to 520 nm. At acidic pHs, only Br₂ ^−· and Cl₂ ^−· radicals were able to oxidize DTO to give the same species as produced by OH radical reaction. The semioxidized species is a resonance stabilized species with the electron delocalized over the-N-C-S bond. This species was found to be neutral and non-oxidizing in nature.     

Nature of the transient species formed in pulse radiolysis of n-allylthiourea in aqueous solutions

Reactions of e⁻_aq, OH radicals and H atoms were studied with n-allylthiourea (NATU) using pulse radiolysis. Hydrated electrons reacted with NATU (k = 2.8×10⁹ dm³ mol⁻¹ s⁻¹) giving a transient species which did not have any significant absorption above 300 nm. It was found to transfer electrons to methyl viologen. At pH 6.8, the reduction potential of NATU has been determined to be −0.527 V versus NHE. At pH 6.8, OH radicals were found to react with NATU, giving a transient species having absorption maxima at 400–410 nm and continuously increasing absorption below 290 nm. Absorption at 400–410 nm was found to increase with parent concentration, from which the equilibrium constant for dimer radical cation formation has been estimated to be 4.9×10³ dm³ mol⁻¹. H atoms were found to react with NATU with a rate constant of 5 × 10⁹ dm³ mol⁻¹ s⁻¹, giving a transient species having an absorption maximum at 310 nm, which has been assigned to H-atom addition to the double bond in the allyl group. Acetoneketyl radicals reacted with NATU at acidic pH values and the species formed underwent reaction with parent NATU molecule. Reaction of Cl^.−₂ radicals (k = 4.6 × 10⁹ dm³ mol⁻¹ s⁻¹) at pH 1 was found to give a transient species with λ_max at 400 nm. At the same pH, reaction of OH radicals also gave transient species, having a similar spectrum, but the yield was lower. This showed that OH radicals react with NATU by two mechanisms, viz., one-electron oxidation, as well as addition to the allylic double bond. From the absorbance values at 410 nm, it has been estimated that around 38% of the OH radicals abstract H atoms and the remaining 62% of the OH radicals add to the allylic double bond.     

Nature of the transient species formed during pulse radiolysis of thioacetamide in aqueous solutions

Thioacetamide (TA) is an organic compound having thioamide group similar to that in thiourea derivatives. Its reactions with e_aq⁻, H-atom and OH radicals were studied using the pulse radiolysis technique at various pHs and the kinetic and spectral characteristics of the transient species were determined. The initial adduct formed by the reaction of TA with OH radicals at pH 7 does not absorb light in the 300–600 nm region but reacts with the parent compound to give a transient species with an absorption maximum around 400 nm. At pH 0, the reaction of OH radicals with TA directly gives a similar transient species with absorption maximum at 400 nm. Transient species formed by H-atom reaction with TA and pH 0 has no absorption in the 300–600 nm region but at higher acidity a new transient species is formed which has absorption maximum at 400 nm. This transient absorption observed in the case of both OH and H atom reaction with TA is ascribed to the formation of a resonance stabilized radical similar to that obtained in the case of thiourea derivatives. The species produced by electron reaction viz. electron adduct was found to be a strong reductant and could reduce MV²⁺ with a high rate constant. H₂S was produced as a stable product in the reaction of e_aq⁻ and its G-value was determined to be about 0.8.     

Radiation chemistry of xylenol orange in aqueous solutions at different acidities

The radiolytic decolourization and peroxide formation have been studied in aqueous solutions of xylenol orange (XO) at different acidities. The G(-XO) increases from 0.78 at pH 11, to 3.70 at pH 3. The peroxide yield also increases from 1.19 at pH 11, to 3.34 in 0.025 mol dm^-3 H₂SO₄. In alkaline solutions only the OH^. decolourizes XO whereas in acidic solutions both the H and OH^. decolourize XO though G(-XO) due to H-atoms is less. The ionization of the phenolic group in XO influences the e^-_aq reaction with it. In alkaline solution, the oxidized and reduced XO formed by OH^. and e^-_aq reactions, respectively, react together regenerating original XO. Near 0.025 mol dm^-3 H₂SO₄, there is an abstraction of H-atom from XO by HO₂ whereas at other acidities, H₂O₂ is formed by disproportionation of peroxides. Reaction schemes have been given to explain the various radiolytic yields.     

Pulse radiolysis investigations on the reactions of primary radiolytic species of water with atropine

On pulse radiolysis of N₂O saturated aqueous solutions of atropine, an optical absorption band (_max at 320 nm,e=2.81·10³ dm³·mol^–1·cm^–1) was observed, which is assigned to the product of reaction of OH radicals with the solute. This absorption decayed following second order kinetics with a rate constant of 4.5·10⁸ dm³·mol^–1·s^–1. The rate constant for the reaction of OH radicals with atropine as estimated by following the build-up kinetics is 2.7·10⁹ dm³·mol^–1·s^–1. The H atoms also reacted with this compound to produce a transient absorption band behaving similarly to the one observed in the case of reaction with OH radicals. The transient species formed in both cases is assigned to a radical derived by H atom abstraction by H/OH radicals from the parent compound. This radical was unreactive towards 2-mercaptoethanol. e _aq ^– was found to react with atropine forming a transient band with _max at 310 nm (=3.55·10³ dm³·mol^–1). Its decay was also second order with a rate constant of 1.64·10⁹ dm³·mol^–1·s^–1. The bimolecular rate constant for the reaction of e _aq ^– with atropine as estimated from the decay of e _aq ^– absorption at 720 nm is 3.9·10⁹ dm³·mol^–1·s^–1. Specific one-electron oxidizing and reducing agents (such as Cl ₂ ^– , Tl²⁺, SO ₄ ^– and (CH₃)₂COH, CO ₂ ^– , respectively) failed to oxidize or reduce this compound in aqoues solutions. The radical anion of atropine formed by its reaction with e _aq ^– was found to reduce thionine and methyl viologen with bimolecular rate constant of 3.8·10⁹ and 3.2·10⁹ dm³·mol^–1·s^–1, respectively.     

Pulse radiolysis of ethyl propionate in aqueous solution

Hydrated electrons (e _aq ^– ) formed in water radiolysis react with ethyl propionate with a rate parameter of 7.5×10⁷ mol^–1 dm³ s^–1. The electron adduct in acidic solutions immediately (<100 ns) dissociates, yielding CH₃CH₂C=0 radical. This process in alkaline solutions is slower, k=1.4×10⁵ s^–1. The hydroxyl radicals abstract H atoms in about 50% from the -position of propionate.     

Redox reactions of transient species formed during pulse radiolysis of 2-pyridine carboxaldehyde and 2-pyridine methanol in aqueous solutions

Rate constants for the reactions of e _aq ^? , H and OH radicals with 2-pyridine carboxaldehyde and 2-pyridine methanol have been determined by pulse radiolysis technique. Reactions of reducing radicals such as acetone ketyl radicals and CO₂ ^? with these compounds were also evaluated at various pHs. The species produced by the reaction of reducing radicals with these solutes was a strong reductant itself. While pyridinyl were produced in the case of 2-pyridine methanol, one-electron reduction of 2-pyridine carboxaldehyde led to the formation of PyCHOH radical. The one-electron reduction potential of PyCHOH radicals was estimated by establishing an equilibrium with MV⁺ radical cations to be ?0.6V vs NHE. OH radical reaction with 2-pyridine carboxaldehyde gave an OH adduct, while in the case of 2-pyridine methanol, OH radicals reacted partly by H-abstraction from the ?CH₂OH group. SO₄ ^? radical reaction with 2-pyridine carboxaldehyde produced a species which was reducing in nature. The rate constants for the reaction of e _aq ^? and OH radicals are compared with similar values obtained in the case of other 2-pyridine derivatives to see if there is any electron-inductive effect.     

Nature of sulfur centered radicals formed during pulse radiolysis of 2-mercaptopyridine in aqueous solutions

Spectral, redox and kinetic properties of the transient species formed by the reaction of 2- mercaptopyridine (2-MPy) with oxidants such as OH, Br¯₂ ^. ; N ^. ₃ and Cl¯₂ ^. radicals and reductants such as e¯_aq, H-atoms and (CH₃)₂ ^. COH radicals have been studied by pulse radiolysis technique. Reaction of one-electron oxidants with 2-MPy at pH 11.5 led to the formation of 2-pyridyl thiyl radical. The reduction potential for the couple C₅H₄NS ^. /C₅H₄NS¯ was estimated to be 0.84 V vs NHE from the equilibrium studies with I¯₂ ^. /2I¯ couple. At pH 6.8, the reaction of N ^. ₃ radical with 2-MPy gave a cation radical derived from 2-MPy. At pH 6.8 and 11.5, OH radicals react with 2-MPy by addition pathway. Reaction of e¯_aq with 2-MPy was found to give a reducing radical capable of transferring electron to methyl viologen. At acidic pH, the reaction of (CH₃)₂ ^. COH radicals and H-atoms with 2-MPy gave transient species identical to those produced by the reaction of oxidising radicals, namely, OH radicals, Cl¯₂ ^. and Br¯₂ ^. radicals.     

Raman Spectroscopic Study of Aluminum Silicate Complexes at 20°C in Basic Solutions

Raman spectroscopic measurements were performed at ambient temperature onaqueous silica-bearing solutions (0.005 < m _Si < 0.02; 0 < pH < 14). The spectraare consistent with the formation of monomeric Si(OH)^o ₄, SiO(OH)^– ₃ andSiO₂(OH)^2– ₂ species at acid to neutral, basic, and strongly basic pH, respectively.Raman spectra of aqueous Al-bearing solutions at basic pH confirm thepredominance of the Al(OH)^– ₄ species in a wide concentration range (0.01 < m _Al < 0.1).Raman spectra of basic solutions (12.4 < pH < 14.3), containing both Al andSi, exhibit a strong decrease in intensities of SiO(OH)^– ₃, SiO₂(OH)^2– ₂, andAl(OH)^– ₄ bands in comparison with Al-free Si-bearing and Si-free Al-bearingsolutions of the same metal concentration and pH, suggesting the formation ofsoluble Al—Si complexes. The amounts of complexed Al and Si derived fromthe measurements of the Al and Si band intensities in strongly basic solutions(pH 14) are consistent with the formation, between Al(OH)^– ₄ andSiO₂(OH)^2– ₂, of the single Al—Si dimer SiAlO₃(OH)^3– ₄ according to the reactionSiO₂(OH)^2– ₂ + Al(OH)^– ₄ SiAlO₃(OH)^3– ₄ + H₂OAt lower pH ( 12.5) the changes in band intensities are consistent with theformation of several, likely more polymerized, Al—Si complexes.     

Chromium(III) Hydroxide Solubility in The Aqueous Na+-OH?-H2PO?4-HPO2?4-PO3?4-H2O System: A Thermodynamic Model

Chromium(III)-phosphate reactions are expected to be important in managing high-level radioactive wastes stored in tanks at many DOE sites. Extensive studies on the solubility of amorphous Cr(III) solids in a wide range of pH (2.8–14) and phosphate concentrations (10^–4 to 1.0 m) at room temperature (22±2)°C were carried out to obtain reliable thermodynamic data for important Cr(III)-phosphate reactions. A combination of techniques (XRD, XANES, EXAFS, Raman spectroscopy, total chemical composition, and thermodynamic analyses of solubility data) was used to characterize solid and aqueous species. Contrary to the data recently reported in the literature,⁽¹⁾ only a limited number of aqueous species [Cr(OH)₃H₂PO^–₄, Cr(OH)₃(H₂PO₄)^2–₂), and Cr(OH)₃HPO^2–₄] with up to about four orders of magnitude lower values for the formation constants of these species are required to explain Cr(III)-phosphate reactions in a wide range of pH and phosphate concentrations. The log K^o values of reactions involving these species [Cr(OH)₃(aq)+H₂PO^–₄&#x21CC;Cr(OH)₃H₂PO^–₄; Cr(OH)₃(aq)+2H₂PO^–₄&#x21CC;Cr(OH)₃(H₂PO₄)^2–₂; Cr(OH)₃(aq)+HPO^2–₄&#x21CC;Cr(OH)₃HPO^2–₄] were found to be 2.78±0.3, 3.48±0.3, and 1.97±0.3, respectively.     

Gamma radiolysis of anthraquinone dye aqueous solution

The decoloration of Acid Blue 62 in aqueous solution was mainly attributed to the attack of the e _aq ^– and OH radicals on the dye molecule. The mechanisms of these reactions were investigated in detail including the influence of pH, dose rate and oxygen.     

Redox reactions of basic fuchsin in homogeneous aqueous media

The pulse-radiolysis technique has been employed to understand the reaction mechanism and to characterize the transient species involved in the redox processes taking place in the radiation chemistry of basic fuchsin (BF⁺). One-electron reduction and oxidation reactions of BF⁺ have been carried out in homogeneous aqueous solutions employing various reducing (e _aq ^– , (CH₃)₂COH, CO ₂ ^– ) and oxidizing (N ₃, Br, Cl ₂ ^– Br ₂ ^– ) radicals. The absorption spectra of the transients formed in the above reactions have been attributed to semi-reduced and semi-oxidized species of BF⁺, respectively. The kinetic and spectroscopic properties of these transients have been evaluated. The reaction with H and OH radicals have also been performed and compared with those of specific one-electron reducing and oxidizing radicals. These reactions have been inferred predominantly by addition to BF⁺. Protolytic equilibria involving semi-reduced species of basic fuchsin have been studied over the pH range from 2 to 10 and the pK _a has been determined to be 3.9.     

Pulse radiolysis of thionicotinamide in aqueous solutions: formation of resonance stabilized species on one electron oxidation

Pulse radiolysis studies on thionicotinamide (TNA) have been carried out in aqueous solutions at pH 6.8 and 1. The initial species formed by OH radical reaction with TNA at pH 6.8 was found to react with the parent molecule to give a dimer radical species (_max = 420-440 nm). Reaction of Br₂ ^- radicals with TNA was found to give a transient species having _max at 380 nm. This spectrum has been assigned to a resonance stabilized species with free electron delocalised over the -N-C-S bond. Reactions of OH and Cl2₂ ^- radicals with TNA at pH 1 were found to give identical transient species with _max at 420 nm, which decayed by first-order kinetics at a rate of about 8.0 × 10³ s^-1. This species is suggested to be the protonated form of the resonance-stabilized species formed at pH 6.8 in the reaction of Br₂ ^- with TNA. The rate constant for the reaction was 4 × 10⁹ dm³ mol^-1 s^-1. Semi-reduced species formed by the reaction of e-_aq with TNA (k = 1.6 × 10¹⁰ dm³ mol^-1 s^-1) was found to be a good reductant which could transfer electron to methyl viologen. CO₂ ^- radicals also reacted with TNA to give a reducing species. Although, the absorption peaks in the two cases were at the same wavelengths viz. 380 and 480 nm, the ratios of the peak heights were different suggesting the formation of different species. Hydrogen atoms and (CH₃)₂COH radicals were found to transfer an electron to TNA at pH 1, as seen by quantitative electron transfer to methyl viologen from the transient species.     

The volume change for the dissociation of telluric acid

The hydrolysis constants of telluric acid were determined by potentiometric titrations at 25°C andI=1.0 mol kg^–1 NaClO₄. Using these results the partial molar volume change according to the dissociation reaction Te(OH)_6(aq) TeO(OH) _5(aq) ^– +H _(aq) ⁺ was measured densitymetrically.

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Das Dissoziationsvolumen der Tellursäure (Kurze Mitteilung)

Zusammenfassung Die Hydrolysekonstanten der Tellursäure wurden bei 25°C undI=1.0 mol kg^–1 NaClO₄ durch potentiometrische Titrationen bestimmt. Diese Ergebnisse wurden verwendet, um die Volumsänderung zufolge der Dissoziationsreaktion Te(OH)_6(aq) TeO(OH) _5(aq) ^– +H _(aq) ⁺ durch Dichtemessungen zu ermitteln.

     

Reaction of Plutonium(VI) with Orthosilicic Acid Si(OH)4

Reaction of Pu(VI) with Si(OH)₄ (at concentration 0.004–0.025 mol l^–1) in a 0.2 M NaClO₄ solution at pH 3–8 is studied by spectrophotometric method. In the range of pH 4.5–5.5, PuO₂(H₂O)₄OSi(OH)₃ ⁺ complex is formed, while at pH > 6, PuO₂(H₂O)₃O₂Si(OH)₂ or hydroxosilicate complex PuO₂(H₂O)₃(OH)OSi(OH)₃ is recorded. The equilibrium constants are calculated for the reactions of formation of PuO₂(H₂O)₄OSi(OH)₃ ⁺ and PuO₂(H₂O)₃O₂Si(OH)₂ and their concentration stability constants: log K ₁ = –3.91 ± 0.17 and log K ₂ –10.5; log ₁= 5.90 ± 0.17 and log ₂ 12.6. The PuO₂(H₂O)₄OSi(OH)₃ ⁺ complex is significantly less stable than analogous complex of U(VI). Calculations of the forms of Pu(VI) occurrence at the Si(OH)₄ concentration equal to 0.002 mol l^–1 showed that the maximum fraction of the PuO₂(H₂O)₄OSi(OH)₃ ⁺ complex is 10% (pH 6.5), while the fraction of PuO₂(H₂O)₃O₂Si(OH)₂ is almost 40% (pH 8).     

Interaction of Aluminium (III) with the f-Family Ions Np(VI), Pu(VI), Np(V), Np(IV), Pu(IV), Nd(III), and Am(III) in the Course of Simultaneous Hydrolysis

The interaction of Np(VI), Pu(VI), Np(V), Np(IV), Pu(IV), Nd(III), and Am(III) with Al(III) in solutions at pH 0–4 was studied by the spectrophotometric method. It was shown that, in the range of pH 3–4, the hydrolyzed forms of neptunyl and plutonyl react with the hydrolyzed forms of aluminium. In the case of Pu(VI), the mixed hydroxoaqua complexes (H₂O)₃PuO₂(-OH)₂Al(OH)(H₂O)₃ ²⁺ or (H₂O)₄PuO₂OAl(OH)(H₂O)₄ ²⁺ are formed at the first stage of hydrolysis. Np(VI) also forms similar hydroxoaqua complexes with Al(III). The formation of the mixed hydroxoaqua complexes was also observed when Np(IV) or Pu(IV) was simultaneously hydrolyzed with Al(III) at pH 1.5–2.5. The Np(IV) complex with Al(III) has, most likely, the formula (H₂O) _n (OH)Np(-OH)₂Al(OH)(H₂O)₃ ³⁺. At pH from 2 to 4.1 (when aluminium hydroxide precipitates), the Np(V) or Nd(III) ions exist in solutions with or without Al(III) in similar forms. When pH is increased to 5–5.5, these ions are almost not captured by the aluminium hydroxide precipitate.     

Pulse radiolysis of pyridinecarboxylic acids in aqueous solution

The reactivity of OH, e^-_aq and H radicals towards aqueous carboxypyridines: picolinic acid (2-pyridinecarboxylic acid), PA; isonicotinic acid (4-pyridinecarboxylic acid), i-NA; 2,6-pyridinedicarboxylic acid, 2,6-PDCA; and 3,5-pyridinedicarboxylic acid, 3,5-PDCA was investigated in the pH-range 1–13.8. The absorption spectra of the OH-adducts, H-adducts and pyridinyl radicals are given as well as the formation and decay kinetics. In acid (but not in alkaline) solution, the reaction of H-atoms leads to the formation of two distinct products, namely H-adduct and pyridinyl radicals. The yields of pyridinyl radical are: 20% for PA, 75% for i-NA, 60% for 2,6-PDCA and 25% for 3,5-PDCA (a yield of 50% has been found earlier for nicotinic acid, NA).