The Effect of Atmospheric Organic Compounds on the Fe-Catalyzed S(IV) Autoxidation in Aqueous Solution

Laboratory experiments were conducted with real atmospheric aerosol particles as well as with synthetic solutions under dark conditions, to simulate some of the chemical features of aerosols. In solutions obtained by the leaching of aerosols (size range >D _ae: 0.4–1.6 m) that contained sufficient amounts of transition metal ions (e.g. Fe) and organic species (e.g. oxalate), S(IV) oxidation rates were significantly lower than those expected from the Fe-catalyzed S(IV) autoxidation in Milli-Q water. The results suggest that oxalate is responsible for much of the observed inhibition. Acetate and formate also inhibit the reaction, but to a much lesser extent. Oxalate has a strong inhibiting effect on the Fe-catalyzed S(IV) autoxidation at all investigated pH values (2.8, 3.7 and 4.5). It was established that Fe(III)-oxalato complexes affect the redox cycling of Fe(II)/Fe(III) and that the observed decrease of the reaction rate is caused by the reduced amount of catalytically active Fe(III) due to the complexation with oxalate. For the system Fe-S(IV)-O₂-oxalate at initial pH 3.7 the reaction rate was calculated using exponential simplification to account for oxalate influence on the amount of free Fe(III) by the following equation:–r_S(IV) = k · [S(IV)] · [Fe(III))] · e ^-b·[Ox]     

Mn(II)-catalysed S(IV) oxidation and its inhibition by acetic acid in acidic aqueous solutions

The kinetics of the S(IV) oxidation by oxygen in the presence of Mn(II) ions and acetic acid has been studied. Experiments were carried out at 25°C, 3.5?≤?pH?≤?5.0, [S(IV)]≈1?×?10^?3 mol/dm³, 1?×?10^?6 mol/dm³?≤?[Mn(II)]?≤?1?×?10^?5 mol/dm³, 1?×?10^?6 mol/dm³?≤?[CH₃COOH]?≤?1?×?10^?4 mol/dm³. Based on the experimental results, rate constants and orders of the reactions were determined. Depending on the reaction conditions, the observed rate constants for the Mn(II)-catalysed S(IV) oxidation ranged between 3.91?×?10^?8 and 8.89?×?10^?7 (mol/dm³) s^?1, and in the presence of acetic acid they ranged between 2.95?×?10^?8 and 7.45?×?10^?7 (mol/dm³) s^?1. The reaction order in S(IV) was zero for both reactions. The effect of Mn(II) ion and acetic acid concentrations as well as an initial pH of the solution on the S(IV) oxidation rate was discussed. It was found that the rate of the S(IV) oxidation depends on the initial pH of the solution but it is independent of the pH change during the reaction. Acetic acid has a weak inhibiting effect on the Mn(II)-catalysed S(IV) oxidation. Under the experimental conditions the S(IV) oxidation rate decreased no more than twice.     

S(IV) Autoxidation in Atmospheric Liquid Water: The Role of Fe(II) and the Effect of Oxalate

The reactivity of dissolved iron compounds towards different pollutants and photooxidants in atmospheric liquid water depends upon the oxidation state and speciation of iron. Our measurements of the oxidation state of dissolved iron eluted from aerosol particles (Dae: 0.4–1.6 m) collected in the urban atmosphere of Ljubljana showed that a large fraction of the iron content is present as Fe(II). The concentration ratio [Fe(II)]/[Fe(III)] varied between 0.9 and 3.1. The kinetics of S(IV) autoxidation catalyzed by Fe(II) under the conditions representative for acidified atmospheric liquid water and the influence of oxalate on this reaction under dark conditions was investigated. The reaction rate is the same if Fe(II) or Fe(III) is used as a catalyst under the condition that Fe(II) can be oxidized in Fe(III), which is the catalytically active species. Oxalate has a strong inhibiting effect on the S(IV) autoxidation in the presence of Fe(II). The reaction is autocatalytic with an induction period, that increases with higher concentrations of oxalate. The inhibiting effect of oxalate differs according to whether iron is initially in the Fe(II) or Fe(III) state. However, in both cases the inhibition by oxalate is a result of the formation of complexes with the catalyst.     

Aqueous Oxidation of Sulfur(IV) Catalyzed by Manganese(II): A Generalized Simple Kinetic Model

The reaction kinetics of S(IV) autoxidation catalyzed by Mn(II) in the pH range 3–5 typical for atmospheric liquid water, was investigated. For reactions with pH maintained constant during the reaction course, the predictions obtained by a simple integral approach cover kinetic results only for concentrations of HSO ₃ ⁻ up to 0.2 mM at pH 4.5. Thus, a generalized simple kinetic model, which can be used for predicting the reaction kinetics in wider concentration, pH and temperature ranges, was derived. This model is based on the assumption that the reaction rate is proportional to the concentration of a transient manganese-sulfito complex formed in the initial step of a radical chain mechanism. In the proposed power law rate equation

Characterization of iron and manganese species in atmospheric aerosols from anthropogenic sources

Electron paramagnetic resonance (EPR) was used to study the chemical form of iron, manganese and other paramagnetic species in airborne particles collected on southern coastal part of the North Sea, located in France. In parallel, chemical analysis was performed to obtain the metal concentrations in samples whereas an individual analysis of particles was provided by scanning electron microscopy coupled to an energy-dispersive X-ray spectrometer (SEM-EDX). EPR spectra have evidenced Fe³⁺ and Mn²⁺ ions in the form of isolated or agglomerated species, as well as carbonaceous products with variable signal intensities according to the wind direction. The monitoring of a signal of isolated Mn²⁺ ions on a distance close to 90 km was proposed as tracer of particles from a Mn local emission source. Fe³⁺ signals are relative to agglomerated species and differences in the type of interaction between these species were evidenced following the wind direction. Fe³⁺ EPR signals parameters revealing antiferromagnetic contribution in Fe-rich particles were found for an industrial origin at Dunkerque.     

Characterization of iron and manganese species in atmospheric aerosols from anthropogenic sources

Electron paramagnetic resonance (EPR) was used to study the chemical form of iron, manganese and other paramagnetic species in airborne particles collected on southern coastal part of the North Sea, located in France. In parallel, chemical analysis was performed to obtain the metal concentrations in samples whereas an individual analysis of particles was provided by scanning electron microscopy coupled to an energy-dispersive X-ray spectrometer (SEM-EDX). EPR spectra have evidenced Fe³⁺ and Mn²⁺ ions in the form of isolated or agglomerated species, as well as carbonaceous products with variable signal intensities according to the wind direction. The monitoring of a signal of isolated Mn²⁺ ions on a distance close to 90 km was proposed as tracer of particles from a Mn local emission source. Fe³⁺ signals are relative to agglomerated species and differences in the type of interaction between these species were evidenced following the wind direction. Fe³⁺ EPR signals parameters revealing antiferromagnetic contribution in Fe-rich particles were found for an industrial origin at Dunkerque.     

Inhibition of the S(IV) Autoxidation in the Atmosphere by Secondary Terpenic Compounds

To better understand the role of terpenic compounds in atmospheric chemistry the influence of sobrerol, a water-soluble product of photochemical autoxidation or ozonolysis of -pinene, on the rate of S(IV) autoxidation was studied. Laboratory experiments were performed under heterogeneous conditions, at constant supply with S(IV) by dissolution of CaSO₃.1/2 H₂O fine particles and with air oxygen by diffusion through a planar gas-liquid interface. The progress of S(IV) autoxidation, uncatalysed or catalysed by Fe₂(SO₄)₃ or CoSO₄, was followed conductometrically. The inhibiting effect of sobrerol was discussed in terms of the chain reaction terminated in a step linear with respect to sulphoxy radicals. A method was proposed for quantification of the inhibiting effect and the rate constant (2.0–3.8)10⁸ M^–1s^–1 was given for the step eliminating sulphate radicals. Further consequences of the inhibition: shifting the reaction locus from the gas-liquid (droplet) interface to the liquid (droplet) bulk and strong oscillations in the rate of S(IV) autoxidation resulting of the activity of sobrerol-derived radicals were also shown.     

Iron Speciation in Precipitation in the North-Eastern Mediterranean and Its Relationship with Sahara Dust

Precipitation samples collected at Erdemli, Turkey, during February 1996–June 1997 were analysed to determine iron content and speciation. The purpose of the measurements was to examine the atmospheric abundance of iron and to quantify its solubility in the region. Spectrophotometric analyses of Fe(II) and reducible Fe(III) in precipitation samples, along with measurements of pH, conductivity, filterable iron (Fe_filt), particulate aluminium (Al_par) and particulateiron (Fe_par) were performed to determine iron solubility, which principally affects its bioavailability. Backward trajectories corresponding to the sampling dates were analysed to determine the sources of atmospheric constituents arriving at the site. Among these, the mineral dust transported from the Great Sahara to the region is considered to be a rich source of iron. The concentration of Fe(II) varied from below detection limit (0.02 M) up to 0.42 M,while the maximum concentration of total reactive Fe (referred as Fe(II) + reducible Fe(III) = Fe_reac) was found to be 1.0 M in precipitation. A strong correlation was found between particulate Fe and Al fractions, both of crustal origin. No correlation was observed between the soluble and insoluble fractions of iron. The soluble iron fraction, Fe(II) concentration varied independently from the concentrations of reducible Fe(III), Fe_filt, Fe_par, and from the pH of the precipitation. The Fe_filt fraction (size < 0.45 m), measuredby Atomic Absorption Spectrophotometer, and frequently interpreted to be the soluble iron fraction in the literature, was found to be significantly higher than the corresponding Fe_reac fraction inprecipitation samples, most likely due to the colloidal iron content of the Fe_filt fraction passing through the 0.45 m pore size filter. The volume weighted mean Fe_filt concentration of the precipitation samples collected during the episodic `red rain' events was found to be relatively higher. The geometric mean ratios of soluble Fe(II) and of Fe_reac to Total Fe (Fe_filt + Fe_par), werefound to be 1.6% and 2.1%, respectively, while the mean ratioof Fe_filt to Total Fe was 9.6%. The flux of bioavailable iron (Fe_reac)fraction in most atmospheric wet deposition events was found to be sufficient for supporting the maximum primary production rates that are typical for the Eastern Mediterranean Sea.     

Manganese in coastal rainwater: speciation,photochemistry and deposition to seawater

Concentrations of manganese in 56 rain events in Wilmington, NC, USA rainwater from April 1, 2005 to March 31, 2006 were 11 ± 3 nM for dissolved Mn and 1.2 ± 0.4 nM for particulate Mn. Concentrations of both forms of Mn were higher in terrestrial storms relative to marine events. This observation along with the positive correlation of Mn with pollutant indicators suggests anthropogenic inputs to rain at this location, as has been observed at other locations. The ratio of Mn_part/Mn_diss was threefold larger in summer relative to winter rain, which matched the increase of particulate to dissolved Fe in rainwater suggesting influence of Saharan dust during the summer. Like Fe in rain, Mn undergoes photoreduction in rainwater, which has also been shown to be important in Mn cycling in seawater. The flux of Mn removed from the atmosphere via wet deposition is 1.5 × 10⁻⁵ moles m⁻² yr⁻¹ at this location, which is approximately twice the flux reported from two rainwater studies conducted in the early 1980s on Bermuda. Atmospheric input of Mn to the oceans is important because Mn like Fe is an essential and potentially limiting nutrient. Experiments mixing authentic rainwater and seawater demonstrate that rainwater dissolved Mn does not rapidly precipitate in seawater suggesting wet deposition is an important source of soluble, stable Mn to surface seawater.     

On the pH-dependent formation constants of iron(III)-sulfur(IV) transient complexes

An experimental study is described of Fe(III)-S(IV) formation constants measured as a function of pH (1–3), ionic strength (0.2–0.5 M) and [Fe(III)] _T (2.5–5.0×10^–4 M) using a continuous-flow spectrophotometric technique to make observations 160 ms after mixing. Preliminary experiments using pulse-accelerated-flow (PAF) spectrophotometry to measure rate constants on a microsecond timescale are also described. The conditional formation constant at 25 °C can be modeled with the following equation: {ie307-1} where {ie307-2}K ₇ andK ₈ can be interpreted as intrinsic constants for the coordination of HSO ₃ ^– by FeOH²⁺ and Fe³⁺, respectively, but until further evidence is obtained they should be regarded as fitting constants. PAF spectrophotometry showed that the initial reaction of Fe(III) with S(IV) (pH 2.0) is characterized by a second-order rate constant of 4×10⁶ M^–1 s^–1 which is comparable to rate of reaction of FeOH²⁺ with SO ₄ ^2– . However, the PAF results should be regarded as preliminary since unexpected features in the initial data indicate that the reaction may be more complex than expected.     

Kinetics of ammonia and ammonium ion inhibition of the atmospheric oxidation of aqueous sulfur dioxide by oxygen

This is the first study, which shows both NH₃ and NH₄⁺ to inhibit the autoxidation of aqueous SO₂ in the pH range 7.0–8.5. The rate of the autoxidation, R _aut , in both buffered and unbuffered media at a fixed pH is in conformity with the rate law:

Inorganic ions and trace metals bulk deposition at an Atlantic Coastal European region

The inorganic chemical composition (major ions and trace metals) of bulk deposition samples collected monthly with bulk collectors at seven Atlantic Coastal European cities (Galicia, Northwest of Spain) during wet season (September 2011 to March 2012) has been assessed and compared. Trace metals (Al, As, Ba, Co, Cr, Cu, Fe, Mn, Ni, Pb, Sr, V and Zn) were analysed in soluble fraction and non-soluble fraction (after acid extraction) of the bulk deposition by inductively coupled plasma-mass spectrometry. Major inorganic ions (Cl^?, NO₃ ^?, SO₄ ^2?, Na⁺, K⁺, Ca²⁺, Mg²⁺ and NH₄ ⁺) were analysed in the soluble fraction of the bulk deposition by capillary zone electrophoresis. Univariate analysis (ANOVA and Multiple Range Test) according to the location of each sampling site was performed. Results also suggest a great influence of cleaner Atlantic air masses. After partition coefficients and enrichment factor estimation, similar sources could be assigned for the ionic and metal composition of bulk deposition at seven urban sites.     

Degradation of Isoprene in the Presence of Sulphoxy Radical Anions

Autoxidation of S(IV) initiated by manganese sulphate or potassium peroxydisulphate in alkaline aqueous solutions was significantly slowed down by dissolved isoprene, which decayed in the process. The laboratory experiments were carried out in a batch, perfectly mixed reactor, which had no gas space. The concentration–time profiles of oxygen were measured with a Clark-type electrode. The profiles of sulphite species and of isoprene were evaluated from the UV spectra of solutions. The kinetic analysis indicated that isoprene reacted directly with sulphate radical anions produced during the S(IV) autoxidation. A relative second-order rate constant of (2.12 ± 0.37) × 10⁹ M^–1 s^–1 was determined for this reaction at 25 °C, pH (8.0–8.5) and ionic strength of (1.7–4.9) × 10^–3 M (the reference rate constant of the reaction of sulphate radical anions with sulphite ions equalled 3.4 × 10⁸ M^–1 s^–1). A tentative mechanism of isoprene oxidation during S(IV) autoxidation, which included formation of isoprene – SO^– ₄ adduct, was based on the analogy to the gas-phase reactions of isoprene and to the liquid-phase reactions of sulphate radical anions with other compounds. Atmospheric significance of the aqueous-phase reaction of isoprene with sulphate radicals was discussed. Approximate analysis showed the reaction is a potential sink for isoprene in the aqueous phase and in the gas–liquid systems of high liquid water content (LWC > 10^–5 m³ m^–3). The aqueous-phase oxidation of isoprene can produce secondary pollutants, and influence transformation and the long-range transport of SO₂ in the atmosphere.     

Temporal Variability of Iron Speciation in Coastal Rainwater

Iron occurs in rain as particulateand dissolved Fe and includes both Fe(II) and Fe(III)species. Model calculations and correlation analysisindicate Fe(II)(aq) occurs almost exclusively as thefree ion whereas Fe(III)(aq) occurs as both ironoxalate and Fe(OH)₂ ⁺(aq) with largevariations over the pH range from 4.0 to 5.0. Complexation with humic-like compounds may also beimportant for Fe(III)(aq); however, the concentrationand structural characteristics of these compounds haveyet to be determined. 112 rain samples were collectedfor iron analysis in Wilmington, North Carolina,between 1 July 1997, and 30 June 1999. Total iron,particulate iron and Fe(III)(aq) were higher inconcentration in summer and spring rain relative towinter and autumn rain. Fe(II)(aq) concentrations, incontrast, did not vary seasonally. Particulate iron,which was approximately half the total rainwater iron,was highest between noon and 6 p.m. (EST), probably dueto more intense regional convection including land-seabreezes during that time. The ratio ofFe(II)(aq)/Fe(III)(aq) was also highest in rainreceived between noon and 6 p.m., which most likelyreflects photochemical reduction of Fe(III)(aq)complexes to form Fe(II)(aq). A conceptual modeldepicting the interplay between iron species, lightintensity and organic ligands in rainwater ispresented.     

Effects of Monochromatic UV-Visible Light and Sunlight on Fe(III)-Catalyzed Oxidation of Dissolved Sulfur Dioxide

The effect of UV-visible light and natural sunlight on the Fe(III)-catalyzed oxidation of dissolved sulfur dioxide has been studied under the conditions representative for those of acidified atmospheric liquids. The experimental results have shown that both sunlight and UV-visible light enhance the rate of Fe(III)-catalyzed oxidation of aqueous sulfite with wavelength ranging from 300 to 575 nm. The light enhanced oxidation is mainly due to photochemical formation of OH radicals from Fe(OH)²⁺ complexes in the wavelength region below 420 nm and SO₃^•− free radicals from Fe(III) sulfite complexes above 420 nm in the absence of organic ligands. Like the Fe(III)-catalyzed thermal chemical oxidation, the Fe(III)-catalyzed photochemical oxidation is also first order with respect to sulfite ion concentration. The sunlight irradiation can increase the Fe(III)-catalyzed oxidation of S(IV) over 45%. The presence of organic complex ligands, such as oxalate, can completely inhibit the Fe-catalyzed oxidation of S(IV) in the dark. However, the photolysis of Fe(III)-oxalato complexes generates oxalate free radicals, leading to the formation of H₂O₂ and OH radicals and the oxidation of S(IV). The rate of Fe(III)-catalyzed oxidation of S(IV) species is found to increase with increasing light intensity. The effects of sunlight on the Fe(III)-catalyzed oxidation of S(IV) should be taken into account when predicting the daytime rates of sulfuric acid formation in atmospheric water droplets.     

Determination of inorganic ions and trace elements in total suspended particles at three urban zones in the Mexico City Metropolitan Area and one rural site

Inorganic ions and trace metals in total suspended particles were measured during the period 2006–2007 at four sites; three urban sites in the Mexico City Metropolitan Area (MCMA) and one nearby rural site in the state of Morelos. SO₄²⁻, NO₃⁻, Cl⁻ and NH₄⁺ ions were analyzed by ion chromatography; Na⁺, K⁺, Ca²⁺ and Mg²⁺ by flame atomic absorption spectroscopy, and Al, Cd, Cr, Mn, Pb and V by an atomic absorption spectrometer with a graphite furnace attachment. The results indicated that SO₄²⁻ was the most abundant ion. All trace elements except Mn and V showed statistically significant differences between sampling sites. Pearson's correlation applied to all data showed a high correlation among SO₄²⁻, NO₃⁻ and NH₄⁺, indicating a common anthropogenic origin. In addition, the correlation observed between Ca²⁺ and Al indicated a crustal origin, as supported by the enrichment factors. Over the total sampling period, significant differences in particles and trace metals were found between sites and meteorological seasons. To gain a better insight into the origin of trace metals and major inorganic ions, a Principal Component Analysis was applied to the results for six trace metal and eight inorganic ions.     

The Influence of Oxalate on Fe-Catalyzed S(IV) Oxidation by Oxygen in Aqueous Solution

This study demonstrates that oxalate has a strong inhibiting effect onFe-catalyzed S(IV) oxidation by oxygen in aqueous solution. While thepseudo-first order rate constant of S(IV) oxidation was determined to be1.6 × 10³ M^-1 s^-1 in experimentswithout oxalate, the oxidation of S(IV) was totally inhibited at a molarconcentration ratio of iron:oxalate = 1:5 at an oxalate concentration of 4M. Under these conditions, the Fe(II)/Fe(III) ratio remained nearlyconstant during the observed reaction time. The determined rate constants wereindependent of the initial oxidation state of iron. However, with increasingconcentrations of oxalate, a longer induction period is observed forexperiments with iron initially in the Fe(II) oxidation state.     

The Effect of Temperature and Humidity on the Reaction of Ozone with Combustion Soot: Implications for Reactivity near the Tropopause

The effect of temperature (296–238 K) on the reaction of combustion soot (n-hexane) with ozone at low concentration (6–8 ppm) has been measured. Long optical path FTIR spectroscopy has revealed the rate law for ozone loss beyond initial stages, second order in O₃, to be the same over this range of conditions. The reaction rate is 3.5 times lower at 238 K than at 296 K, and reveals an activation energy of 12.9 ± 0.5 kJ mol^–1. The effect of humidity on the reaction has been estimated using its recently determined rate law dependence (p^0.2). These data, differing from O₃ reaction kinetics obtained from other types of carbonaceous particles used as surrogates for atmospheric soot, have implications for the role of combustion soot in atmospheric chemistry. Any involvement of aircraft soot in ozone depletion near the tropopause, for example, should be estimated using these temperature and humidity dependences.     

Fast Oxidation Reaction of Nitrite by Dissolved Oxygen in the Freezing Process in the Tropospheric Aqueous Phase

Nitrite oxidation in the tropospheric aqueous phase by freezing was evaluated by freezing a field sample. Nitrite oxidation by dissolved oxygen in the freezing process is much faster than by other oxidation processes, such as reactions with ozone, hydrogen peroxide or dissolved oxygen in an aqueous solution at pHs 3 to –6. At pH 4.5 and 25°C, the lifetime of nitrite in the aqueous phase is ca. 1 hr in oxidation by ozone (6×10^-10 mol dm^-3), ca. 10 hr in oxidation by H₂O₂ (2×10^-4 mol dm^-3), and 7.5 hr (Fischer and Warneck, 1996) in photodissociation at midday in summer. Under the same conditions at a temperature below 0°C, the lifetime of nitrite in the freezing process is estimated as ca. 2 sec when the droplets are frozen within a second. The reaction by freezing is affected by the presence of salts, such as NaCl or KCl, or orgnaic compounds, such as methanol or acetone. The results of freezing a field rain or fog sample showed that nitrite oxidation proceeds below pH 6, and the conversion ratio of nitrate from nitrite increases with decreasing pH. The oxidation of nitrite by freezing was also observed in freezing fog particles generated by an ultrasonic humidifier. The ratios of the concentrations of ions in the winter sample to those in the summer sample (or those in the fog sample) were almost the same values. However, the concentration of nitrite in the winter sample was lower than that estimated by the ratios of other ions. From the present study, it seems that the freezing process plays an important role in the nitrite sink process in the tropospheric aqueous phase.     

The Reactivity of Biogenic Monoterpenes towards OH· and SO4-· Radicals in De-Oxygenated Acidic Solution

The reactivity of some selected biogenic monoterpenecompounds towards important aqueous phase free-radicaloxidants, namely OH· and SO₄ ^-·, have beeninvestigated using the complementary experimentaltechniques of pulse radiolysis and laser flashphotolysis ( = 248 nm). Rate constants forthe reactions of the OH· radical with cis-verbenol andmethacrolein have been determined to be (6.8 ± 0.5) ×10⁹ dm³ mol^-1 s^-1 and (8.0± 0.7) × 10⁹ dm³ mol^-1s^-1,respectively (T = 20 °C, pH 4.0, Ionic strength 0 mol dm^-3). Rate constants and activationenergies for the reactions of the SO₄ ^-·radical have been measured for the following compounds(T = 20 °C, pH 4.0, Ionic strength = 0.03 moldm^-3): -pinene (k = (3.1 ± 0.1) ×10⁹ dm³ mol^-1 s^-1;E _act. =(8.9 ± 1.3) kJ mol^-1), -terpineol(k = (4.1 ± 0.1) × 10⁹ dm³mol^-1s^-1; E _act. = (13.4 ± 0.6) kJmol^-1), cis-verbenol (k = (3.2 ± 0.2) ×10⁹ dm³ mol^-1 s^-1;E _act. =(10.0 ± 0.7) kJ mol^-1), verbenone (k = (1.6± 0.1) × 10⁹ dm³ mol^-1s^-1;E _act. = (6.1 ± 0.7) kJ mol^-1), myrtenal(k = (1.85 ± 0.1) × 10⁹ dm³mol^-1s^-1; E _act. = (7.5 ± 0.7) kJmol^-1)and methacrolein (k = (1.18 ± 0.1) × 10⁹dm³ mol^-1 s^-1). In most instances theabsorption spectra of the intermediate products formedby these reactions have been measured which, inconjunction with strategic conductiometric studies,have been used to suggest plausible mechanisms for theoxidation in acidic de-oxygenated solution.