Jerzy Holcman

REACTIONS OF THE FERRYL ION WITH SOME COMPOUNDS FOUND IN CLOUD WATER

The lifetime of the ferryl ion FeO2+, (minutes at pH below 1) is reduced by a factor of 50 at pH above 3. This is rationalized in terms of an acid-base equilibrium between two different hydrolytic forms of this species (Fe(OH)++2 and Fe(OH)+3) with a pKa=2.0. The rate constants for reactions between FeO2+ and selected compounds found in cloud water were measured in acid solutions by stopped-flow technique. For inorganic reactants: kHNO2=1.1×104 M−1 s−1, kNO2−≤105 M−1 s−1, kCl−=1.0×102 M−1 s− 1, kHSO3−=2.5×105 M−1 s−1, kSO2=4.5×105 M−1 s−1, and kMn(II)=1.0×104 M−1 s−1 were obtained and for the organic reactants: kHCOOH=160 M−1 s−1, kHCOO−=3×105 M−1, kCH3COOH=3.1 M−1 s−1, kCH2(OH)2=400 M−1 s−1, kCH3COCH3=16 M−1 s−1, kCH3CH20H=2.5×103 M−1 s−1, kC6H5OH=4.0×103 M−1 s−1, and kC6H5COOH=80 M−1 s−1 were obtained. A good correlation between log(k) and the standard one electron reduction potential (E°) indicates that the reactions of inorganic compounds proceed by electron transfer. The reaction mechanisms in case of organic compounds are very similar to the reactions of the OH radical, i.e., H-abstraction, and a fairly good correlation between log(k) and the bond dissociation energy BDE was obtained. Activation parameters were measured for the reaction of FeO2+ with HNO2 (Ea=34.5 kJ/mol); Mn2+ (Ea=21.3 kJ/mol); HCOOH (Ea=22.3 kJ/mol); CH2 (OH)2 (Ea=44.5 kJ/mol); and C6H5 OH (Ea=28.1 kJ/mol).

OXIDATION OF MANGANESE(II) BY OZONE AND REDUCTION OF MANGANESE(III) BY HYDROGEN PEROXIDE IN ACIDIC SOLUTION

Manganese(II) is oxidized by ozone in acid solution, k=(1.5±0.2)×103 M−1 s−1 in HClO4 and k=(1.8±0.2)×103M−1 s−1 in H2SO4. The plausible mechanism is an oxygen atom transfer from O3 to Mn2+ producing the manganyl ion MnO2+, which subsequently reacts rapidly with Mn2+ to form Mn(III). No free OH radicals are involved in the mechanism. The spectrum of Mn(III) was obtained in the wave length range 200–310 nm. The activation energy for the initial reaction is 39.5 kJ/mol. Manganese(III) is reduced by hydrogen peroxide to Mn(II) with k(Mn(III)+H2O2)=2.8×103M−1 s−1 at pH 0–2. The mechanism of the reaction involving formation of the manganese(II)-superoxide complex and reaction of H2O2 with Mn(IV) species formed due to reversible disproportionation of Mn(III), is suggested.

A pulse radiolysis study of the OH radical induced autoxidation of methanesulfinic acid

Abstract Methanesulfinic acid, CH 3 SO 2 H, reacts with OH radicals at pH 7 forming CH 3 SO 2 radicals with a rate constant k =(6.0±1.0)×10 9 M −1 s −1 . The CH 3 SO 2 radicals absorbs at 325 nm with an extinction coefficient of 900±100 M −1 cm −1 and disappears in a second order self-reaction with k =(1.0±0.2)×10 9 M −1 s −1 . This radical reacts with oxygen, k =(1.2±0.3)×10 9 M −1 s −1 , forming a peroxy radical which absorbs in the UV below 300 nm. The peroxy radical reacts in turn with methanesulfinic acid reforming the CH 3 SO 2 radical whereby a chain oxidation of sulfinic acid takes place. During the course of the chain oxidation a peroxyacid, presumably methaneperoxymonosulfonic acid, is formed and accumulated. This acid absorbs in the UV and eventually decays by reaction with excess methanesulfinic acid k =5×10 3 M −1 s −1 . The final product of the chain autoxidation is methanesulfonic acid. The chain is very efficient and proceeds until either oxygen or methanesulfinic acid is exhausted. The mechanism is compared to the chain autoxidation of sulfite.

Pulse radiolysis of pyridine and methylpyridines in aqueous solutions

Abstract The radicals formed from pyridine, 3-methylpyridine, 3,5-dimethylpyridine, 2,6-dimethylpyridine and 2,4,6-trimethylpyridine by attack of H, e-aq, OH and O•- in acqueous solutions were investigated by pulse radiolysisin the pH-range 1–13.8. The UV-vis. absorption spectra as well as the formation and decay kinetics for the protonated and unprotonated forms of the methylpyridine radicals studied are presented. The pKa-values for the OH-adducts were determined.

Pulse radiolysis of methyl viologen in aqueous solutions

Pulse radiolysis of air-free aqueous methyl viologen (MV2+) solutions was carried out at various pH. The attack of e–aq on MV2+, with k(e–aq+ MV2+)= 7.5 × 1010 dm3 mol–1 s–1, leads to the formation of the long-lived radical cation (MV˙+), which possesses two absorption maxima at 392.5 nm (Iµ392.5= 4200 m2 mol–1) and 600 nm (Iµ600= 1450 m2 mol–1). The H-atoms react with MV2+ at pH 1 forming two species, which have superimposed absorption bands. By means of a computer simulation they are resolved in the absorptions belonging to: (1) a protonated form of the radical cation (MV˙+H+), which is produced with k(H + MV2+)=(3.5 ± 0.2)× 108 dm3 mol–1 s–1, has 2 absorption maxima at 390 nm (Iµ390= 1700 m2 mol–1) and 595 nm (Iµ595= 760 m2 mol–1) and decays by second-order kinetics with k= 3.5 × 109 dm3 mol–1 s–1; (2) an H-adduct (MV˙2+H) on the ring carbon, which is formed with k(H + MV2+)= 2.5 × 108 dm3 mol–1 s–1, absorbs at 310 nm (Iµ310= 900 m2 mol–1) and 470 nm (Iµ470= 630 m2 mol–1) and decays by conversion into MV˙+H+ in a first-order process with k= 6 × 103 s–1. For the equilibrium MV˙+H+⇌ MV˙++ H+ pK= 2.9 ± 0.1 was determined. The presented data explain, at least partly, the instability of MV2+ when used as an electron acceptor in various devices for the utilization of solar energy.

Activation parameters of ferryl ion reactions in aqueous acid solutions

The temperature dependence of the oxidation kinetics of Fe2+ by O3 at pH 0–3 was studied by stopped-flow technique in the temperature range 5–40°C. Activation parameters of the reactions involved in formation and decay of the ferryl ion (iron(IV)), FeO2+ are determined. The reaction of Fe2+ + FeO2+ was found to branch into two channels forming iron(III)-dimer, Fe(OH)2Fe4+, and Fe3+. The yield of the dimer, Fe(OH)2Fe4+, increases with temperature on the expense of the Fe3+ yield. On the basis of the overall rate constant and relative yield of Fe(OH)2Fe4+ the activation energy is determined for both channels. The activation parameters of the hydrolysis of the ferryl ion and its reaction with H2O2 were also determined.

Rate constants of the equilibrium reactions SO⨪4 + HNO3 ⇄ HSO-4 + NO.̇3 and SO⨪4 + NO-3 ⇄ SO2-4 + NO.̇3

Abstract Rate constants of the following equilibrium reactions were determined by pulse radiolysis at high solute concentrations: SO⨪ + HNO3 ⇄ HSO-4 + NO . 3 [kf = (2.7 ± 0.5) × 106 M-1 s-1, kr = (5.6 ± 1.0) × 103 M-1 s-1] and SO⨪4 + NO-3 ⇄ SO2-4 + NO . 3 [kf = (5.0 ± 2.0) ×104 M-1 s-1, kr = (1.0 ± 0.2) × 105 M-1 s-1]. By extrapolation of the latter to zero ionic strength an approximate reduction potential of NO3/NO-3, E0 = 2.45 ± 0.05 V can be estimated.

Reactivity of OH and O– with aqueous methyl viologen studied by pulse radiolysis

The behaviour of aqueous MV2+ towards oxidizing radicals (OH and O–) has been investigated in the pH range from 6 to 14 by means of pulse radiolysis. A semi-linear optimization method was applied for resolving the complex reaction mechanism. In the pH range from 6 to 8 the rate constant for attack by OH is k=(2.5±0.2)× 108 dm3 mol–1 s–1. The resulting transient absorbs at λmax= 470 nm (Iµ470= 1600±70 m2 mol–1) and decays with 2k=(1.3±0.2)× 108 dm3 mol–1 s–1.In strongly alkaline solutions (pH 13.8) the O– radical anion reacts preferentially by hydrogen abstraction from the methyl group, k=(1.4±0.2)× 109 dm3 mol–1 s–1, forming a radical which then decays by reaction with OH–(k= 2.8 × 106 dm3 mol–1 s–1) to produce a modified radical cation; this has absorption maxima at 392 and 605 nm (Iµ392= 4300 m2 mol–1, Iµ605= 1500 m2 mol–1) and is relatively long lived.The remaining part (< 10%) of O– attacks MV2+ at the ring carbon atom, k=(1.0±0.4)× 108 dm3 mol–1 s–1, resulting in an O– adduct, which has λmax= 470 nm (Iµ470= 2200±100 m2 mol–1).

Gamma-ray initiated decomposition of aqueous ozone solutions

Abstract The free-radical initiated chain decomposition of aqueous O 3 -O 2 solutions has been studied as a function of dose rate and O 3 concentration in the pH range, 2.0–7.0. At dose rates of 10 rad s − or less, extensive chain decomposition of O 3 occurs. Under these conditions impurities arising from the water, glassware, acids and buffers suppress decomposition. In the absence of impurities, the decomposition is first order in O 3 concentration in the range 50–200 μM and achieves chain lengths of 200–300. A typical decomposition curve is autocatalytic, with a low initial rate, a rapid middle rate and, as the O 3 concentration approaches zero, a slow final rate. These complex decomposition curves have been analyzed by measuring the rates at 50 and 30% of the initial O 3 concentration. The yields per 100 μM O 3 increase with increasing pH to a broad maximum at pH 4.0 and thereafter decline slightly. The propagation reactions in this chain mechanism are: O 3 + O − 2 = O − 3 + O 2 ; O − 3 + H + = O 2 + OH; OH + O 3 = HO 2 + O 2 ; HO 2 = H + + O - 2 . The termination reactions are: HO 2 + OH = H 2 O + O 2 (dominant at low pH); OH + OH = H 2 O 2 (dominant at pH > 4.0), OH + X = X OH (dominant with X as an impurity). The decay curves may be simulated by the water radiolysis mechanism including the above reactions. The overall behavior of the system conforms to an impurity level with kc in the range of 100–500 s −1 . For example, at an impurity level of 1.0 μM( X ) and a rate constant, k , of 10 8 M −1 s −1 , kc = 100 s −1 .

Reactivity of H, OH and e-aq with nicotinic acid: A pulse radiolysis study

Abstract The reactivity of aqueous nicotinic acid (NA) towards OH, e-aq and H-atoms has been investigated in the pH-range 0.3–13.8. The OH attack on NA [k = (2.5 ± 0.2) × 109 M-1 s-1] and its N-protonated forms [k = (2.2 ± 0.2) × 107 M-1 s-1] gives OH-adducts with pH-dependent optical spectra [NĊ5H4(OH)COO-:λmax = 310 nm, ϵ310 = 1800 ± 200 M-1 cm-1; +HNĊ5H4(OH)COO-:λmax = 325 nm, ϵ325 = 2000 ± 200 M-1 cm-1]. pKa = 4.5 ± 0.2 was determined for these two transients. The reaction of H-atoms with NA in the pH-range 6–12 [k = (6.0 ± 0.5) × 108 M-1 s-1] results in the formation of one type of transient, the H-adducts on ring carbons (λmax = 315 nm, ϵ315 = 4500 ± 200 M-1 s-1; pK = 6.7 ± 0.2). With the N-protonated forms of NA, however, two kinds of radicals are produced: pyridinyl (λmax = 285 nm, ϵ285 = 7800 ± 200 M-1 cm-1; λmax = 410 nm, ϵ410 = 3100 ± 100 M-1 cm-1) and H-adduct on ring carbons (λmax = 340 nm, ϵ340 = 4600 ± 200 M-1 cm-1). The reaction of e-aq with NA was reinvestigated. The kinetic and spectroscopic data are in good agreement with those previously reported.