Destruction Of H2O2 Research Articles

Physical and chemical effects on crystalline H2O2 induced by 20 keV protons

We present laboratory studies on radiation chemistry, sputtering, and amorphization of crystalline H(2)O(2) induced by 20 keV protons at 80 K. We used infrared spectroscopy to identify H(2)O, O(3), and possibly HO(3), measure the fluence dependence of the fraction of crystalline and amorphous H(2)O(2) and of the production of H(2)O and destruction of H(2)O(2). Furthermore, using complementary techniques, we observe that the sputtering yield depends on fluence due to the buildup of O(2) radiation products in the sample. In addition, we find that the effective cross sections for the destruction of hydrogen peroxide and the production of water are very high compared to radiation chemical processes in water even though the fluence dependence of amorphization is nearly the same for the two materials. This result is consistent with a model of fast cooling of a liquid track produced by each projectile ion rather than with the disorder produced by the formation of radiolytic products.

Direct H2-to-H2O2 Oxidation in Aqueous Acidic Medium Containing Br Promoter Over Pd/Al2O3 and Pd/C Catalysts Thermally Pretreated Under Different Conditions

Influence of thermal pretreatments (under N2, air or H2 gas atmosphere at 500 °C or 700 °C) has been investigated for the Pd/Al2O3 and Pd/Carbon catalysts in terms of its effect on their Pd particle size and performance in the H2-to-H2O2 oxidation and H2O2 destruction (by decomposition and/or hydrogenation) reactions in aqueous acidic medium containing Br promoter. The influence on the net H2O2 formation is found to depend strongly upon the catalyst support due to support–Pd cluster interactions. For both the catalysts, the thermal treatments (except in air) caused a large increase in their Pd particle size. The increase in Pd particle size caused an increase in the H2O2 formation activity of Pd/Al2O3 but a decrease in the H2O2 formation activity of Pd/C.

Decomposition and/or hydrogenation of hydrogen peroxide over Pd/Al2O3 catalyst in aqueous medium: Factors affecting the rate of H2O2 destruction in presence of hydrogen

Hydrogen peroxide destruction by its decomposition and/or hydrogenation to water in the presence of H2 over Pd(5 wt.%)/Al2O3 catalyst in an aqueous reaction medium, similar to that used in the H2-to-H2O2 oxidation, has been thoroughly investigated at different reaction conditions. The H2O2 destruction is strongly influenced by the oxidation state of Pd in the catalyst and also by the presence of different halide anions (viz. F−, Cl−, Br− and I−) in the acidic aqueous medium or in the catalyst, depending upon the concentration of halide anions. The cations associated with halide anions have, however, a little or no influence on the H2O2 destruction. The iodide anions strongly poisoned the catalyst, even at their very low concentration. The fluoride anions have only a small effect on the H2O2 destruction. The chloride or bromide anions drastically inhibit the rapid H2O2 decomposition, but promote the slower H2O2 hydrogenation. The H2O2 destruction reactions are strongly influenced by the halide anion and acid concentrations. The presence of acid (protons) plays a very important role in drastically reducing the H2O2 destruction, particularly in the presence of chloride anions. Both in the presence and absence of Cl− or Br− anions, the H2O2 destruction activity of the catalyst is markedly reduced because of the oxidation of Pd but it is drastically increased due to the presence of H2. A plausible reaction mechanism for the rapid H2O2 decomposition and slower H2O2 hydrogenation reactions, prevailing under the different reaction conditions, has also been discussed.

Direct formation of H2O2 from H2 and O2 and decomposition/hydrogenation of H2O2 in aqueous acidic reaction medium over halide-containing Pd/SiO2 catalytic system

Formation of H2O2 from H2 and O2 and decomposition/hydrogenation of H2O2 have been studied in aqueous acidic medium over Pd/SiO2 catalyst in presence of different halide ions (viz. F−, Cl− and Br−). The halide ions were introduced in the catalytic system via incorporating them in the catalyst or by adding into the reaction medium. The nature of the halide ions present in the catalytic system showed profound influence on the H2O2 formation selectivity in the H2 to H2O2 oxidation over the catalyst. The H2O2 destruction via catalytic decomposition and by hydrogenation (in presence of hydrogen) was also found to be strongly dependent upon the nature of the halide ions present in the catalytic system. Among the different halides, Br− was found to selectivity promote the conversion of H2 to H2O2 by significantly reducing the H2O2 decomposition and hydrogenation over the catalyst. The other halides, on the other hand, showed a negative influence on the H2O2 formation by promoting the H2 combustion to water and/or by increasing the rate of decomposition/hydrogenation of H2O2 over the catalyst. An optimum concentration of Br− ions in the reaction medium or in the catalyst was found to be crucial for obtaining the higher H2O2 yield in the direct synthesis.

Formation from direct oxidation of H2 and destruction by decomposition/hydrogenation of H2O2 over Pd/C catalyst in aqueous medium containing different acids and halide anions

Abstract Direct oxidation of H2 by O2 to H2O2 and decomposition/hydrogenation of H2O2 (at 27 °C and atmospheric pressure) over Pd/C catalyst in an aqueous acidic reaction medium have been thoroughly investigated using different mineral acids (viz. 0.1N H2SO4, H3PO4, H3BO3, HNO3, HCl, HBr or HI) and/or halide anions (viz. F−, Cl−, Br− or I−), covering their wide concentration range (0–24 mmol/dm3), in the reaction medium. In the absence of any externally added halide, appreciable H2O2 formation in the H2 oxidation occurred only when the medium contained HCl. However, the rates of H2O2 decomposition and hydrogenation are greatly reduced because of the presence of any acid; the influence on the H2O2 decomposition was, however, much larger for the halo acids. In the presence of acid (0.1N H3PO4 or H2SO4), the H2-to-H2O2 formation reaction is greatly enhanced by the externally added halide anions up to their optimum concentrations. However, above the optimum halide anion concentration, it is inhibited more than the H2-to-water formation (parallel) reaction, causing a decrease in both the H2O2 yield and selectivity. Whereas, the H2 conversion and H2O2 destruction activities of the catalyst are decreased continuously with increasing the halide anion (except F−) concentration. Among the halides, chloride is the best halide promoter for Pd/C catalyst in the H2-to-H2O2 oxidation. At the optimum Cl− concentration (5.4 mmol/dm3), both the H2 conversion and H2O2 yield are passed through a maximum and the H2O2 decomposition is greatly inhibited with increasing the phosphoric acid concentration. In the absence of either the chloride anions or the acid (or both) in the reaction medium, only a little or no H2O2 is formed in the H2 oxidation and also the rate of H2O2 destruction is very fast, particularly in the presence of H2; the rapid destruction of H2O2 is mainly due to its decomposition rather than its hydrogenation. The best H2-to-H2O2 oxidation results are obtained at the optimum concentrations of both the acid and halide anions. In the presence of acid and chloride (or bromide) promoter, the H2O2 hydrogenation dominates the H2O2 destruction and hence the net H2O2 formation is mainly controlled by the H2O2 hydrogenation.

Paired Bacillus anthracis Dps (Mini-ferritin) Have Different Reactivities with Peroxide

Dps (DNA protection during starvation) proteins, mini-ferritins in the ferritin superfamily, catalyze Fe(2+)/H(2)O(2)/O(2) reactions and make minerals inside protein nanocages to minimize radical oxygen-chemistry (metal/osmotic/temperature/nutrient/oxidant) and sometimes to confer virulence. Paired Dps proteins in Bacillus, rare in other bacteria, have 60% sequence identity. To explore functional differences in paired Bacilli Dps protein, we measured ferroxidase activity and DNA protection (hydroxyl radical) for Dps protein dodecamers from Bacillus anthracis (Ba) since crystal structures and iron mineralization (iron-stain) were known. The self-assembled (200 kDa) Ba Dps1 (Dlp-1) and Ba Dps2 (Dlp-2) proteins had similar Fe(2+)/O(2) kinetics, with space for minerals of 500 iron atoms/protein, and protected DNA. The reactions with Fe(2+) were novel in several ways: 1) Ba Dps2 reactions (Fe(2+)/H(2)O(2)) proceeded via an A(650 nm) intermediate, with similar rates to maxi-ferritins (Fe(2+)/O(2)), indicating a new Dps protein reaction pathway, 2) Ba Dps2 reactions (Fe(2+)/O(2) versus Fe(2+)/O(2) + H(2)O(2)) differed 3-fold contrasting with Escherichia coli Dps reactions, with 100-fold differences, and 3) Ba Dps1, inert in Fe(2+)/H(2)O(2) catalysis, inhibited protein-independent Fe(2+)/H(2)O(2) reactions. Sequence similarities between Ba Dps1 and Bacillus subtilis DpsA (Dps1), which is regulated by general stress factor (SigmaB) and Fur, and between Ba Dps2 and B. subtilis MrgA, which is regulated by H(2)O(2) (PerR), suggest the function of Ba Dps1 is iron sequestration and the function of Ba Dps2 is H(2)O(2) destruction, important in host/pathogen interactions. Destruction of H(2)O(2) by Ba Dps2 proceeds via an unknown mechanism with an intermediate similar spectrally (A(650 nm)) and kinetically to the maxi-ferritin diferric peroxo complex.

Examination of Lactobacillus plantarum lactate metabolism side effects in relation to the modulation of aeration parameters

The characterization of global aerobic metabolism of Lactobacillus plantarum LP652 under different aeration levels, in order to optimize acetate production kinetics and to suppress H2O2 toxicity. Cultures of L. plantarum were grown on different aeration conditions. After sugar exhaustion and in the presence of oxygen, lactate was converted to acetate, H2O2 and carbon dioxide with concomitant ATP production. Physiological assays were performed at selected intervals in order to assess enzyme activity and vitality of the strain during lactic acid conversion. The maximal aerated condition led to fast lactate-to-acetate conversion kinetics between 8 and 12 h, but H2O2 immediately accumulated, thus affecting cell metabolism. Pyruvate oxidase activity was highly enhanced by oxygen tension and was responsible for H2O2 production after 12 h of culture, whereas lactate oxidase and NADH-dependent lactate dehydrogenase activities were not correlated to metabolite production. Limited NADH oxidase (NOX) and NADH peroxidase (NPR) activities were probably responsible for toxic H2O2 levels in over-aerated cultures. Modulating initial airflow led to the maximal specific activity of NOX and NPR observed after 24 h of culture, thus promoting H2O2 destruction and strain vitality at the end of the process. Optimal aeration conditions were determined to minimize H2O2 concentration level during growth on lactate.

Infrared Spectra and Radiation Stability of H2O2 Ices Relevant to Europa

In this paper we present spectra of H2O2-containing ices in the near- and mid-infrared (IR) regions. Spectral changes on warming are shown, as is a comparison of near-IR bands of H2O and H2O2-containing ices. An estimate of the A-value (absolute intensity) for the largest near- IR feature of H2O2 is given. Radiation-decay half-lives are reported for 19 K and 80 K, and are related to the surface radiation doses on Europa. The radiation data show that H2O2 destruction is slower at 80 K than 19 K, and are consistent with the claim that icy material in the outermost micrometer of Europa's surface has been heavily processed by radiation.

Pulsed excitation source multiplexed fluorometry for the simultaneous measurement of multiple analytes. Continuous measurement of atmospheric hydrogen peroxide and methyl hydroperoxide.

Presently, solid-state sources such as light-emitting diodes (LEDs) provide for intense, nearly monochromatic light. They are available over a broad range of emission wavelengths. Unlike incandescent and discharge lamps, LEDs can be turned on and off at high speeds. The resulting light pulses are highly reproducible. This allows the use of a single photomultiplier tube (PMT), often the most expensive component in a high-sensitivity measurement system, as a multiplexed detector with multiple, fiber-optic-coupled, fluorescence-detection cells excited by solid-state sources. A time resolution of 1 min is adequate in many continuous detection schemes. This enables multiple-channel single-detector multiplexed measurement without any loss of S/N. On the basis of this principle, we describe a new automated continuous instrument for the simultaneous measurement of atmospheric hydrogen peroxide and methyl hydroperoxide (MHP). A Nafion membrane diffusion scrubber (DS) is used with hematin-catalyzed oxidation of thiamine to thiochrome for the measurement of H2O2, and an expanded poly(tetrafluoroethylene) (ePTFE) DS is used with a H2O2 destruction catalyst and horseradish peroxidase-catalyzed oxidation of thiamine to thiochrome for the measurement of MHP. The respective limits of detection are 25 pptv and 15 pptv. Design, performance details, and illustrative results from a field campaign (Philadelphia NEO3PS study, 2001) are presented.

Removal of Hydrogen Peroxide by Thiol-specific Antioxidant Enzyme (TSA) Is Involved with Its Antioxidant Properties: TSA POSSESSES THIOL PEROXIDASE ACTIVITY

The thiol-specific antioxidant protein (TSA) protects glutamine synthetase from inactivation by a metal-catalyzed oxidation (MCO) system comprised of dithiothreitol (DTT)/Fe3+/O2 but not by the ascorbate/Fe3+/O2 MCO system. The removal of sulfur-centered radicals or H2O2 has been proposed as the protective mechanism of TSA. Like catalase, TSA prevents the initiation of the rapid O2 uptake phase during MCO of DTT but causes only partial inhibition when added after the reaction is well into the propagation phase. Stoichiometric studies showed that the antioxidant property of TSA is, at least in part, due to its ability to catalyze the destruction of H2O2 by the overall reaction 2 RSH + H2O2 --> RSSR + H2O. Results of kinetic studies demonstrate that the removal of H2O2 by TSA correlates with its ability to protect glutamine synthetase from inactivation. In the presence of thioredoxin, TSA is more active, whereas C170S (an active mutant of TSA in which cysteine 170 was replaced by a serine) and open reading frame 6 (a human antioxidant protein homologous to TSA with only one conserved cysteine residue) are only slightly affected. The thiol specificity of the protective activity of TSA derives from the fact that the oxidized form of TSA can be converted back to its sulfhydryl form by treatment with thiols but not by ascorbate.

Catalase and glutathione peroxidase are equally active in detoxification of hydrogen peroxide in human erythrocytes

Genetic deficiencies of glucose-6-phosphate dehydrogenase (G6PD) and NADPH predispose affected erythrocytes to destruction from peroxides. Conversely, genetic deficiencies of catalase do not predispose affected erythrocytes to peroxide-induced destruction. These observations have served to strengthen the assumption that the NADPH/glutathione/glutathione peroxidase pathway is the principal means for disposal of H2O2 in human erythrocytes. Recently, however, mammalian catalase was found to have tightly bound NADPH and to require NADPH for the prevention and reversal of inactivation by its toxic substrate (H2O2). Since both catalase and the glutathione pathway are dependent on NADPH for function, this finding raises the possibility that both mechanisms destroy H2O2 in human erythrocytes. A comparison of normal and acatalasemic erythrocytes in the present study indicated that catalase accounts for more than half of the destruction of H2O2 when H2O2 is generated at a rate comparable to that which leads to hemolysis in G6PD- deficient erythrocytes.

Catalase and Glutathione Peroxidase Are Equally Active in Detoxification of Hydrogen Peroxide in Human Erythrocytes

Genetic deficiencies of glucose-6-phosphate dehydrogenase (G6PD) and NADPH predispose affected erythrocytes to destruction from peroxides. Conversely, genetic deficiencies of catalase do not predispose affected erythrocytes to peroxide-induced destruction. These observations have served to strengthen the assumption that the NADPH/glutathione/glutathione peroxidase pathway is the principal means for disposal of H2O2 in human erythrocytes. Recently, however, mammalian catalase was found to have tightly bound NADPH and to require NADPH for the prevention and reversal of inactivation by its toxic substrate (H2O2). Since both catalase and the glutathione pathway are dependent on NADPH for function, this finding raises the possibility that both mechanisms destroy H2O2 in human erythrocytes. A comparison of normal and acatalasemic erythrocytes in the present study indicated that catalase accounts for more than half of the destruction of H2O2 when H2O2 is generated at a rate comparable to that which leads to hemolysis in G6PD- deficient erythrocytes.

Field evidence for the oxidation of SO2 by H2O2 in cap clouds

The reaction of SO2, dissolved in cloud droplets, with atmospheric oxidants such as H2O2 and ozone is thought to be an important pathway for the production of atmospheric sulphate, and may be significant in determining the pattern of acidic deposition over a region. Direct observation of in-cloud oxidation of SO2 by these species, however, has proved extremely difficult. This is especially true for H2O2 because the reaction is so rapid. We have used a new experimental design to investigate the dominant oxidation pathways of SO2 in the conditions that occur in natural clouds. This involved modifying SO2 levels in capping cloud on a peak in the northern Pennine hills of England by releasing SO2 locally upwind and then measuring changes in cloud and air composition. Here we present initial results from an experiment performed on 12 December 1985. Under the conditions encountered it was found that sulphate production by both ozone and H2O2 was negligible. The destruction of H2O2 by reaction with SO2, however, was clearly demonstrated. By comparing H2O2 in the presence and absence of released SO2, the rate of destruction was estimated and found to be comparable with that expected from laboratory measurements of the reaction rate constant, and our knowledge of the cloud physics.

Antioxidant enzymes of larvae of the cabbage looper moth, Trichoplusia ni: subcellular distribution and activities of superoxide dismutase, catalase and glutathione reductase.

In the mid-fifth instar larvae of the cabbage looper moth, Trichoplusia ni, the subcellular distribution of total superoxide dismutase was as follows: 3.05 units (70.0%), 0.97 units (22.3%), and 0.33 units (7.6%) mg-1 protein in the mitochondrial, cytosolic and nuclear fractions, respectively. No superoxide dismutase activity was detected in the microsomal fraction. Catalase activity was unusually high and as follows: 283.4 units (47.3%), 150.1 units (25.1%), 142.3 units (23.8%), and 22.9 units (3.8%) mg-1 protein in the mitochondrial, cytosolic, microsomal (containing peroxisomes), and nuclear fractions. No glutathione peroxidase activity was found, but appreciable glutathione reductase activity was detected with broad subcellular distribution as follows: 3.86 units (36.1%), 3.68 units (34.0%), 2.46 units (23.0%), and 0.70 units (6.5%) mg-1 protein in the nuclear, mitochondrial, and cytosolic fractions, respectively. The unusually wide intracellular distribution of catalase in this phytophagous insect is apparently an evolutionary adaptation to the absence of glutathione peroxidase; hence, lack of a glutathione peroxidase-glutathione reductase role in alleviating stress from lipid peroxidation. Catalase working sequentially to superoxide dismutase, may nearly completely prevent the formation of the lipid peroxidizing .OH radical from all intracellular compartments by the destruction of H2O2 which together with O2- is a precursor of .OH.

Measurements of peroxides in cloudwater and rain

Measurements of total peroxide content are reported for 190 cloud water samples from nonsupercooled stratiform clouds, and for 54 rain samples, collected at several nonurban sites in the eastern United States. Observed aqueous peroxide concentrations ranged from less than 0.1 μM to over 100 μM, with rain samples, precipitating cloudwater samples, and nonprecipitating cloudwater samples all exhibiting median values of less than 10 μM and similar concentration distributions. Specific analyses for H2O2 on 48 of the samples showed that in the majority of samples tested, H2O2 was the sole contributor to the total peroxide concentration, but in some samples, substantial signal was observed after selective destruction of H2O2, possibly indicating the presence of organic peroxides. Calculation of the total potential partial pressure of H2O2 for the samples collected in nonprecipitating clouds, using the measured aqueous concentrations and cloud liquid water content values, indicates a median potential partial pressure of H2O2 of 0.1 ppbv, and a maximum value of approximately 1 ppbv, Gaseous SO2 and aqueous peroxide were rarely simultaneously present in substantial concentrations; generally one reactant was present in great excess over the other, consistent with the occurrence of the aqueous phase reaction of peroxide with S(IV) in which one or the other species is the limiting reagent. Evaluation of the rates of S(IV) oxidation by peroxide and ozone, under conditions representative of the clouds sampled in this study, indicates that the former reaction, but not the latter, can produce substantial acidification in such clouds.

The influence of catalyst-supporting methods on electrochemical activity and the resultant stability of air electrodes activated with iron phthalocyanine

To improve the performance of air electrodes, the dependence of iron phthalocyanine (FePc) catalytic effects on preparation methods was examined. The methods used were mixture (Electrode 1), impregnation (Electrode 2) and direct synthesis (Electrode 3). Electrodes 2 and 3 showed higher potentials during cathodic polarization up to 10 mA cm−2 than Electrode 1. The rate of chemical destruction of H2O2 decreased in the order Electrode 3 > Electrode 2 > Electrode 1. Electrode 3 showed the smallest potential drop for a discharge at 10 mA cm−2, 0.09 V after 50 h. However, the potential of Electrode 2 decreased with discharge, becoming 0.09 V lower than that of Electrode 3 after a 50 h discharge at 10mA cm−2. Once the potential drop occurred, the potential was not recovered by resting or by drying the electrode. The potential drop may be caused by deactivation of FePc. One possible reason for such deactivation is the presence of H2SO4, which remained on the electrode after impregnation of the FePc-H2SO4 solution.

The effect of selenium-deficiency on rat fat-cell glucose oxidation

When rats are fed a selenium-deficient diet, the glutathione peroxidase activity of epididymal fat-cells decreases to 5-9% of that of control rats fed the same diet supplemented with 0.5 p.p.m. of selenium as sodium selenite. [1-14C]Glucose oxidation in fat-cells from rats fed a selenium-deficient diet is unresponsive to the action of t-butyl hydroperoxide, which stimulates 14CO2 formation from [1-14C]glucose 4-fold in control rats. Insulin enhances [1-14C]glucose oxidation and incorporation into lipids in fat-cells from both groups of rats; however, the response elicited is reduced in fat-cells prepared from selenium-deficient animals. The 'C-1/C-6 ratio' (ratio of glucose C-1 to glucose C-6 oxidized) is enhanced by insulin to a similar degree in fat-cells from both groups of animals. The stimulatory action of Zn2+ and dithiothreitol on [1-14C]glucose oxidation observed in fat-cells from selenium-supplemented rats is greatly reduced in fat-cells from selenium-deficient rats. [1-14C]Glucose oxidation in fat-cells from both groups of animals is highly sensitive to the stimulatory action of adenosine. It is concluded that the enhanced formation and glutathione-linked destruction of H2O2 plays, at the most, only a minor role in the stimulation of the flux of glucose through the pentose phosphate pathway elicited by insulin, although elimination of glutathione peroxidase activity may influence the action of insulin on glucose oxidation. Production and subsequent destruction of H2O2 may play an important role in the stimulatory action of Zn2+ and dithiothreitol on fat-cell [1-14C]glucose oxidation.

Interaction of ozone and hydrogen peroxide in water: Implications for analysis of H2O2 in air

We have attempted to measure gaseous H2O2 in air using an aqueous trapping method. With continuous bubbling, H2O2 levels in the traps reached a plateau, indicating that a state of dynamic equilibrium involving H2O2 destruction was established. We attribute this behavior to the interaction of ozone and its decomposition products (OH, O3−) with H2O2 in aqueous solution. This hypothesis was investigated by replacing the air stream with a mixture of N2, O2 and O3. The results of this experiment show that H O was both produced and destroyed in the traps. These results have led us to question the validity of techniques which employ aqueous traps to measure H2O2 in air.

ChemInform Abstract: OXIDATION OF FORMALDEHYDE IN KCL‐COATED VESSELS

In the temperature range 440–540°C, the oxidation of HCHO in KCl-coated vessels is reproducible and of short chain length for HCHO concentrations in the range 0.05–4 Torr. Interpretation of the [CO] against time profile permits evaluation of the unknown parameters k1, k7/k1/24 and k8. HCHO + O2→ HO2+ HCO (1), HO2+ HO2→ H2O2+ O2(4), HO2 [graphic ommitted] ½ H2O + O2(7), H2O2 [graphic ommitted] H2O +½ O2. (8). Comparison of the experimental values for k7/k1/24 and k8 with those calculated from diffusion theory indicates that the destruction of HO2 at the surface is fully diffusion controlled, whereas the destruction of H2O2, though moderately efficient, is not fully diffusion controlled. From measurements at HCHO concentrations below 0.5 Torr, where the chain length is close to unity, effectively direct measurements of k1 have been obtained, the values being 7.65 × 10–1, 2.08 × 10–1, 8.50 × 10–2 and 2.20 × 10–2 dm3 mol–1 s–1 at 542.5, 500, 471 and 440.5°C, respectively. These values give the Arrhenius parameters A1= 2.04 × 1010 dm3 mol–1 s–1, E1= 38.9 ± 1.5 kcal mol–1. The absence of any diameter effect on the value of k1 confirms that the parameters refer to the homogeneous process.

Oxidation of formaldehyde in KCl-coated vessels

In the temperature range 440–540°C, the oxidation of HCHO in KCl-coated vessels is reproducible and of short chain length for HCHO concentrations in the range 0.05–4 Torr. Interpretation of the [CO] against time profile permits evaluation of the unknown parameters k1, k7/k1/24 and k8. HCHO + O2→ HO2+ HCO (1), HO2+ HO2→ H2O2+ O2(4), HO2 [graphic ommitted] ½ H2O + O2(7), H2O2 [graphic ommitted] H2O +½ O2. (8). Comparison of the experimental values for k7/k1/24 and k8 with those calculated from diffusion theory indicates that the destruction of HO2 at the surface is fully diffusion controlled, whereas the destruction of H2O2, though moderately efficient, is not fully diffusion controlled. From measurements at HCHO concentrations below 0.5 Torr, where the chain length is close to unity, effectively direct measurements of k1 have been obtained, the values being 7.65 × 10–1, 2.08 × 10–1, 8.50 × 10–2 and 2.20 × 10–2 dm3 mol–1 s–1 at 542.5, 500, 471 and 440.5°C, respectively. These values give the Arrhenius parameters A1= 2.04 × 1010 dm3 mol–1 s–1, E1= 38.9 ± 1.5 kcal mol–1. The absence of any diameter effect on the value of k1 confirms that the parameters refer to the homogeneous process.