Phenolic antioxidants – radical‐scavenging and chain‐breaking activity: A comparative study

Fifty phenolic antioxidants (AH) (42 individual compounds and 8 binary mixtures of two antioxidants) were chosen for a comparative analysis of their radical‐scavenging (H‐donating) and chain‐breaking (antioxidant) activity. Correlations between experimental (antiradical and antioxidant) and predictable (theoretical) activities of 15 flavonoids, 15 hydroxy cinnamic acid derivatives, 5 hydroxy chalcones, 4 dihydroxy coumarins and 3 standard antioxidants (butylated hydroxytoluene, hydroquinone, DL ‐α‐tocopherol) were summarized and discussed. The following models were applied to explain the structure‐activity relationships of phenolic antioxidants of natural origin: (a) model 1, a DPPH assay used for the determination of the radical‐scavenging capacity (AH + DPPH? → A? + DPPH‐H); (b) model 2, chemiluminescence of a model substrate RH (cumene or diphenylmethane) used for the determination of the rate constant of a reaction with model peroxyl radicals (AH + RO₂? → ROOH + A?); (c) model 3, lipid autoxidation used for the determination of the chain‐breaking antioxidant efficiency and reactivity (AH + LO₂? → LOOH + A?; A? + LH (+O₂) → AH + LO₂?); and (d) model 4, theoretical methods used for predicting the activity (predictable activity). The highest lipid oxidation stability was found for antioxidants with a catecholic structure and for their binary mixtures with DL ‐α‐tocopherol, as a result of synergism between them.     

Kinetic study of the prooxidant effect of tocopherol. Hydrogen abstraction from lipid hydroperoxides by tocopheroxyls in solution

A kinetic study of the prooxidant effect of vitamin E (tocopherol, TocH) has been carried out. The rates of hydrogen abstraction (k₋₁) from methyl linoleate hydroperoxide (ML-OOH) by α-tocopheroxyl (α-Toc^.) (1) and eight types of alkyl substituted Toc^. radicals, (2–9) in benzene solution have been determined spectrophotometrically. The results show that the rate constants decrease as the total electron-donating capacity of the alkyl substituents on the aromatic ring of Toc^. increases. The k₋₁ value (5.0×10⁻¹M⁻¹s⁻¹) obtained for α-Toc^. (1) was found to be about seven orders of magnitude lower than the k₁ value (3.2×10⁶M⁻¹s⁻¹) for the reaction of α-TocH with peroxyl radical, which is well known as the usual radical-scavenging reaction of α-TocH. The above reaction rates (k₋₁) obtained were compared with those (k₃) of methyl linoleate with Toc^. (1–9) in benzene solution. The rates (k₋₁) were found to be about six times larger than those (k₃) of the corresponding Toc^.. The results suggest that both reactions may relate, to the prooxidant effect of α-TocH at high concentrations in foods and oils. The effect of the phytyl side chain on the reaction rate, of Toc^. in micellar dispersions has also been studied. We have measured the rate constant, k₋₁, for the reaction of phosphatidylcholine hydroperoxide with a Toc^. radical in benzene,tert-butanol and in Triton X-100 micellar dispersions, and compared the observed k₋₁ values with the corresponding values for ML-OOH.     

Kinetic study of methanol oxidation on carbon-supported PtRu electrocatalyst

Methanol electrooxidation was investigated on the carbon-supported PtRu electrocatalyst (1:1 atomic ratio) in acid media. X-ray diffraction measurement indicated alloying of Pt and Ru. Cyclic voltammetry of the sample reflects the amount of Ru in the catalyst and its ability to adsorb OH radicals. Tafel plots for the oxidation of 0.02-3 M methanol in the solutions containing 0.05-1 M HClO₄ and in the temperature range 27-40 °C showed reasonably well-defined linear region with the slope of about 115 mV dec⁻¹ at the low currents, irrespective of the experimental conditions employed. Reaction order with respect to methanol was found to be 0.5. A correlation between methanol oxidation rate and pseudocapacitive current of OH adsorption on Ru sites was established. It was proposed that bifunctional mechanism is operative with the reaction between methanol residues adsorbed on Pt sites and OH radicals adsorbed on Ru sites as the rate-determining step.     

Evaluation of the antioxidant capacity of synthesized coumarins

Coumarins are secondary metabolites that are widely distributed within the plant kingdom, some of which have been extensively studied for their antioxidant properties. The antioxidant activity of coumarins assayed in the present study was measured by different methods, namely the 1,1-diphenyl-2-picryl-hydrazyl (DPPH^•) method, cyclic voltammetry and the antioxidant capacity against peroxyl radicals (ACAP) method. The 7,8-dihydroxy-4-methylcoumarin (LaSOM 78), 5-carboxy-7,8-dihydroxy-4-methylcoumarin (LaSOM 79), and 6,7-dihydroxycoumarin (Esculetin) compounds proved to be the most active, showing the highest capacity to deplete the DPPH radicals, the highest antioxidant capacity against peroxyl radicals, and the lowest values of potential oxidation.     

The antioxidant capacity of complex mixtures by kinetic analysis of crocin bleaching inhibition

The capability of a compound or of a mixture of compounds to quench peroxyl radicals was measured by analyzing the kinetics of the competition of a parallel reaction where peroxyl radicals bleach the carotenoid crocin. This kinetic approach, originally described for the analysis of antioxidants reacting with hydroxyl radicals in water, was modified by both decreasing the polarity of the solvent, thus allowing the analysis of lipophilic compounds, and by substituting a source of peroxyl radicals for the hydroxyl radical generating system. Single compounds as well as complex mixtures were analyzed by kinetic data processing. Overall antioxidant capacity, relative to that of α-tocopherol or of its soluble analog Trolox C, was calculated. As examples of the use of this test, the antioxidant capacities of a crude rosemary extract, Maillard reaction products, and virgin olive oils were measured.     

Comparative evaluation of the efficiency of a series of commercial antioxidants studied by kinetic modeling in a liquid phase and during the melt processing of different polyethylenes

An antioxidant response in condensed polymeric environments is often ambiguous and may vary strongly depending on the nature of the polymer and the conditions of polymer storage, processing, and use. The impact of polymeric environments during melt processing on the intrinsic efficiency of a set of commercial antioxidants was studied. The antioxidative activity of primary antioxidants Lowinox CPL, Lowinox 22IB46, Naugard 445, hydroxylamine Genox EP, and secondary phosphite Weston TNPP were determined by using two versions of the model reaction of cumene initiated (2,2′‐azobisisobutyronitrile, AIBN, and cumyl hydroperoxide, ROOH) oxidation. The melt stabilizing efficiency of the antioxidants was also studied during multipass extrusion testing in HDPE (Phillipstype), metallocene LLDPE, and (Ziegler‐Natta) LLDPE. The kinetic measurements showed that each of the three functional hydroxyl groups of Lowinox CPL is consumed in the model reaction (version 1) with the same high inhibition rate constant (k₇), whereas the two functional groups of Lowinox 22IB46 have different activity stipulated by hydrogen bonding between the hydroxyls. All the primary stabilizers involved afforded transformation products with additional antioxidative activity. For phenolic Lowinox CPL and amine Naugard 445, these products exhibited lower inhibition rate constants than that of the main functionality, but for Lowinox 22IB46, the discrepancy was not observed. Genox EP revealed three inhibition centers with different rate constants which, however, have low values of the inhibition coefficients (f). This effect is presumably due to the versatility of the inhibition pathways for the antioxidant and its intermediates, including the path of active interception of cumylalkyl (R^?) radicals. The secondary stabilizer Weston TNPP, tested by means of the second version of the model system, along with the expected decomposition of hydroperoxide appeared to be an effective radical scavenger. Kinetic parameters of the antioxidizing activity of the stabilizers – inhibition rate constants, coefficients of the oxidation chain termination, and total antioxidative activity {A = ∑[k_7(i) (fn[InH])_(i)]} — were determined for each functional group and for the whole antioxidant molecule. The phenolic stabilizers manifested powerful antioxidative activity. Their strongest functional groups have very high inhibition rate constant values: (log k₇) = 5.4 ± 0.15 (Lowinox 22IB46) and 5.2 ± 0.1 M^?1s^?1 (Lowinox CPL). In terms of the total inhibiting activity in the liquid system the antioxidants can be ordered as: Lowinox CPL > Lowinox 22IB46 > Naugard 445 > Genox EP > Weston TNPP. The effect of stabilizers during multipass extrusion experiments was assessed via melt flow rate and yellowness index measurements conducted as a function of the number of passes. Phenolic antioxidants and Genox EP significantly improved the melt stability of the polyethylenes in terms of melt viscosity retention and in partial compliance with the data from kinetic modeling measurements. According to the melt stabilizing efficiency data, the antioxidants can be arranged as: Lowinox 22IB46 > Lowinox CPL > Genox EP > Naugard 445 > Weston TNPP. The Lowinox 22IB46 with relatively lower molecular weight exhibited the best results among the primary stabilizers because of the unrestricted molecular dynamics in the viscous‐flow state of the polymer. Yellowness index measurements made after multiple extruder passes indicated that Weston TNPP effectively decreased the color development caused by the phenolic antioxidants. Genox EP displayed high efficiency as an antioxidant and melt‐processing stabilizer and additionally provided good color protection. Generally, we received a good correlation between the activity of the antioxidants in the model system and their melt stabilization performance in HDPE, metallocene LLDPE, and LLDPE. The model reaction of cumene‐initiated oxidation has demonstrated excellent applicability as an effective tool for preliminary quantitative assessment of antioxidant radical‐scavenging efficiency. J. VINYL ADDIT. TECHNOL., 2010. © 2009 Society of Plastics Engineers     

Water density effects on methanol oxidation in supercritical water at high pressure up to 100 MPa

Reaction kinetics of methanol oxidation in supercritical water at high pressure condition (420 °C; 34-100 MPa; ρ = 300-660 kg/m³) was investigated. Pseudo-first order rate constant for methanol decomposition increased with increasing water density. Effects of supercritical water on the reaction kinetics were investigated using a detailed chemical kinetics model. Incorporating the effect of diffusion in a reduced model revealed that overall kinetics for SCWO of methanol is not diffusion-limited. Roles of water as a reactant were also investigated. The dependence of sensitivity coefficient for methanol concentration and rate of production of OH radical on water density indicated that a reaction, HO₂ + H₂O = OH + H₂O₂, enhanced the OH radical production and thereby facilitated the decomposition of methanol. It is presumed that concentration of key radicals could be controlled by varying pressure intensively.     

Synthesis of Ni(OH)<Subscript>2</Subscript> nanostructures by the hydrothermal treatment in the presence of guanidine carbonate and their electrochemical properties

Nanostructure control of β-Ni(OH)₂ was attempted by adopting hydrothermal treatment for the high specific surface area (high-SSA) β-Ni(OH)₂ in the presence of guanidine carbonate. β-Ni(OH)₂ nanosheets-linked structures could be synthesized from high-SSA β-Ni(OH)₂ owing to the strong effects of guanidine carbonate for keeping or enlarging SSA during hydrothermal treatment. The discharging capacities of the high-SSA sample showed the highest capacity above 80 % at 0.2C. However, the capacities were decreased with decreasing SSA of the β-Ni(OH)₂ prepared. It was also found that, at lower SSA, charging/discharging rates below 1C did not affect the discharging capacity, i.e., the almost same capacities for 0.2C and 1C. Cyclic voltammetry revealed that the electrochemical reactions for the charging/discharging were considered to be reversible and smooth at various scanning rates. The rate-determining step of the electrochemical reaction is considered to be regulated by the diffusion of the active species involved for the lower SSA samples. However, for the higher SSA sample (280 m² g⁻¹) obtained at 200 °C in the presence of 1.0 × 10⁻¹ mol L⁻¹ guanidine carbonate strongly indicates the effects of the rate of surface reaction or electronic conductivity on rate-determining step of charging/discharging of the material.     

Covalently and ionically crosslinked sulfonated poly(arylene ether ketone)s as proton exchange membranes

A series of covalently and ionically crosslinked sulfonated poly(arylene ether ketone)s (SPAEKs) were prepared via the cyclocondensation reaction of crosslinkable SPAEKs with 3,3′-diaminobenzidine to form quinoxaline groups, where crosslinkable SPAEKs were synthesized by copolymerization of 4,4′-biphenol with 2,6-difluorobenzil, 4,4′-difluorobenzophenone, and 5,5′-carbonyl-bis(2-fluorobenzene sulfonate). The SPAEK membranes had high mechanical properties and the isotropic membrane swelling. The covalent and ionical crosslinking significantly improved the membrane performance, i.e., the crosslinked membranes showed the lower membrane dimensional change, lower methanol permeability, and higher oxidative stability than the corresponding uncrosslinked membranes, with keeping the reasonably high proton conductivity. The crosslinked membrane (CK3) with measured ion exchange capacity of 1.62 mequiv g⁻¹ displayed a reasonably high proton conductivity of 110 mS/cm with water uptake of 33 wt% at 80 °C, and exhibited a low methanol permeability of 1.7 × 10⁻⁷ cm² s⁻¹ for 32 wt% methanol solution at 25 °C. The covalently and ionically crosslinked SPAEK membranes have potential for polymer electrolyte membrane fuel cells and direct methanol fuel cells.     

Reaction of dipyridamole with the hydroxyl radical

Dipyridamole [2,6-bis-diethanolamino-4,8-dipiperidinopyrimido-(5,4-d) pyrimidine], a well known platelet aggregation inhibitor, shows powerful hydroxyl radical scavenging activity by inhibiting OH-dependent salicylate and deoxyribose degradation. Steady-state competition kinetics experiments with deoxyribose were carried out to evaluate the second-order rateconstant for the reaction between hydroxyl radical and dipyridamole. OH· radicals were generated either by a Fenton-type reaction or by X-ray irradiation of water solutions. A second-order rate constant k_{(Dipyridamole+OH·)} of 1.72±0.11×10¹⁰M⁻¹ s⁻¹ and of 1.54±0.15×10¹⁰ M⁻¹ s⁻¹ was measured by Fenton chemistry and by radiation chemistry, respectively. Mannitol was used as an internal standard for hydroxyl radicals in steady-state competition experiments with deoxyribose. A rate constant k_{(Mannitol+OH·)} of 1.58±0.13×10⁹ M⁻¹ s⁻¹ and 1.88±0.14×10⁹ M⁻¹ s⁻¹ was measured in the Fenton model and in the water radiolysis system, respectively. Both these rate constants are in good agreement with the published data obtained by the “deoxyribose assay” and by pulse radiolysis.     

Quenching mechanism and kinetics of ascorbyl palmitate for the reduction of the gamma irradiation-induced oxidation of oils

The effects of 0, 250, 500, and 1000 ppm (wt/vol) ascorbyl palmitate (AP) on the gamma irradiation-induced oxidation of soybean oil, cottonseed oil, corn oil, tallow, lard, or linoleic acid either in a solvent mixture (benzene/methanol, 4:1 vol/vol) or in methanol, was studied immediately after gamma irradiation with a dose of 1–5 kGy. Steady-state kinetic approximation was used to determine a quenching mechanism and quenching rate constant of AP on the gamma irradiation-induced oxidation of purified soybean oil in a solvent mixture (benzene/methanol, 4:1 vol/vol). Irradiation greatly increased oxidation of all oils, as was expected. AP was extremely effective at minimizing oxidation in all oils, and its effectiveness was concentration dependent. AP showed significantly greater antioxidative activity than α-tocopherol for the reduction of oxidation in all oils (P<0.05). The steady-state kinetic studies indicated that AP quenched oxygen only to minimize the oxidation of oils. The calculated total quenching rate of AP was 7.51×10⁷ M⁻¹s⁻¹. The present results clearly show the effective oxygen quenching ability of AP for the reduction of gamma irradiation-induced oxidation of oils.     

Kinetic Studies on Oxirane Cleavage of Epoxidized Soybean Oil by Methanol and Characterization of Polyols

The kinetics of the oxirane cleavage of epoxidized soybean oil (ESO) by methanol (Me) without a catalyst was studied at 50, 60, 65, 70 °C. The rate of oxirane ring opening is given by k[Ep][Me]², where [Ep] and [Me] are the concentrations of oxiranes in ESO and methanol, respectively and k is a rate constant. From the temperature dependence of the kinetics thermodynamic parameters such as enthalpy (ΔH), entropy (ΔS), free energy of activation (ΔF) and activation energy (ΔE _a) were found to be 76.08 (±1.06) kJ mol⁻¹, −118.42 (±3.12) J mol⁻¹ k⁻¹, 111.39 (±2.86) kJ mol⁻¹, and 78.56 (±1.63) kJ mol⁻¹, respectively. The methoxylated polyols formed from the oxirane cleavage reaction , were liquid at room temperature and had three low temperature melting peaks. The results of chemical analysis via titration for residual oxiranes in the reaction system showed good agreement with IR spectroscopy especially the disappearance of epoxy groups at 825, 843 cm⁻¹ and the emergence of hydroxy groups at the OH characteristic absorption peak from 3,100 to 3,800 cm⁻¹.     

Radical scavenging activity of canola hull phenolics

Possible use of canola hulls as a source of natural anti-oxidants was explored. Cyclone canola hulls were extracted with methanol (30 to 80%, vol/vol) and acetone (30 to 80%, vol/vol). The free radical-scavenging activity of phenolic extracts so prepared was evaluated using the 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonate) (ABTS) radical ion (ABTS^o−), 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical, and chemiluminescence assays. The total content of phenolics in prepared extracts from canola hulls ranged from 15 to 136 mg sinapic acid equivalents per gram of extract. Higher levels of condensed tannins were detected in the acetone extracts than in the corresponding methanolic counterparts. Seventy and 80% (vol/vol) acetone extracts displayed markedly stronger antioxidant activity than any of the other extracts investigated. Statistically significant linear correlations were found between TEAC (Trolox equivalent antioxidant capacity) values (expressed in mM of Trolox equivalents per gram of extract) and total pehnolics, TEAC and total condensed tannins (i.e., determined using the modified vanillin and pronthocyanidin assays), as well as TEAC and protein precipitation activity of phenolic extracts (i.e., measured using the dye-labeled assay). The antioxidant activities of extracts as determined by the ABTS^o− radical ion assay correlated highly with those of the chemiluminescence and DPPH radical assays.     

Steady-state isotopic transient kinetic analysis of steam reforming of methanol over Cu-based catalysts

Mechanistic aspects of steam reforming of methanol were studied via steady-state isotopic transient kinetic analysis over three copper-based catalysts, namely combustion-synthesized Cu-Ce-O and Cu-Mn-O, and commercial Cu-ZnO-Al₂O₃. The “C-path” and “O-path” for the production of CO₂ via steam reforming of methanol was analysed with the following step changes in the feed: ¹²CH₃OH/H₂O/Ar/He → ¹³CH₃OH/H₂O/He and CH₃OH/H₂¹⁶O/Ar/He → CH₃OH/H₂¹⁶O/H₂¹⁸O/He. The presence of CH₃¹⁸OH in the products after the switch to ¹⁸O-labeled water indicates that a major path of the reaction is the one involving a methyl formate intermediate. This appears to be the main path over the Cu-Mn-O catalyst, while parallel paths via dioxomethylene and methyl formate intermediates appear to be operative over Cu-Ce-O and Cu-ZnO-Al₂O₃ catalysts.     

Reaction Kinetics of Methanol Carbonylation to Methyl Formate Catalyzed by CH3O− Exchange Resin

The carbonylation of methanol with CO using CH₃O⁻ exchange resin as a heterogeneous catalyst at temperatures near 350 K is examined systematically in an attempt to derive kinetic rate expressions for the reaction. The activation energies for the carbonylation and decarbonylation reactions are found to be 68 kJ/mol and 105 kJ/mol, respectively. The CH₃O⁻ exchange resin is also shown to suffer no degradation of catalytic activity upon repeated separation and re-use at 353 K.     

Electrochemical pretreatment of distillery wastewater using aluminum electrode

Electrochemical (EC) oxidation of distillery wastewater with low (BOD₅/COD) ratio was investigated using aluminum plates as electrodes. The effects of operating parameters such as pH, electrolysis duration, and current density on COD removal were studied. At a current density of 0.03 A cm⁻² and at pH 3, the COD removal was found to be 72.3%. The BOD₅/COD ratio increased from 0.15 to 0.68 for an optimum of 120-min electrolysis duration indicating improvement of biodegradability of wastewater. The maximum anodic efficiency observed was 21.58 kg COD h⁻¹ A⁻¹ m⁻², and the minimum energy consumption observed was 0.084 kWh kg⁻¹ COD. The kinetic study results revealed that reaction rate (k) decreased from 0.011 to 0.0063 min⁻¹ with increase in pH from 3 to 9 while the k value increased from 0.0035 to 0.0102 min⁻¹ with increase in current density from 0.01 to 0.03 A cm⁻². This study showed that the COD reduction is more influenced by the current density. The linear and the nonlinear regression models reveal that the COD reduction is influenced by the applied current density.     

Kinetic modeling of H2O2-enhanced oxidation of flue gas elemental mercury

Mercury emissions from coal-fired power plants account for 40% of the anthropogenic mercury emissions in the U.S. The speciation of mercury largely determines the amount of mercury capture in control equipments. Conversion of insoluble Hg⁰ into more soluble Hg²⁺ facilitates its removal in scrubbers. Past studies suggest that an added supply of OH radicals possibly enhance the mercury oxidation process. This study demonstrates that the application of H₂O₂, as source of OH radicals, accelerates the oxidation of Hg⁰ into Hg²⁺. A detailed kinetic reaction mechanism was compiled and the reaction pathways were established to analyze the effect of H₂O₂ addition. The optimum temperature range for the oxidation was 480–490 °C. The sensitivity analysis of the reaction mechanism indicates that the supply OH radicals increase the formation of atomic Cl, which accelerates the formation of HgCl₂ enhancing the oxidation process. Also, the pathway through HOCl radical, generated by the interactions between chlorine and H₂O₂ was prominent in the oxidation of Hg⁰. The flue gas NO was found to be inhibiting the Hg⁰ oxidation, since it competed for the supplied H₂O₂. Studying the interactions with the other flue gas components and the surface chemistry with particles in the flue gas could be important and may improve the insight into the post combustion transformation of mercury in a comprehensive way.     

An efficient strategy for the partial oxidation of methane into methanol over POM-immobilized MOF catalysts under ambient conditions

One of the scientific obstacles in the direct conversion of methane to methanol lies in how to improve product selectivity. A dark photocatalysis mode that decouples solar energy in the dark may effectively alleviate this challenge. Herein, a range of composites had been prepared in which Fe-BTC was employed as the gas enrichment unit, while phosphotungstic acid (PTA) acted mainly as an electron storage within the framework. The modified reaction system allowed for the achievement of methanol yields up to 968 μmol·g_cat⁻¹·h⁻¹ under ambient conditions. Besides extrinsic factors, such as a gentle reaction environment and avoidance of the oxidation reactions between photogenerated holes and alcohols, we also demonstrated through in situ EPR measurements that the introduced PTA effectively regulated •OH radicals and suppressed their further reaction with methanol. Moreover, the strategy demonstrated its universality by effectively enhancing the performance of other POM-immobilized MOF composites. This work opens a new path for the efficient and targeted conversion of inert methane to methanol.     

Peroxyl and hydroxyl radical scavenging activity of some natural phenolic antioxidants

The autoxidation of linoleic acid dispersed in an aqueous media and the antioxidant effect of hydroxytyrosol, oleuropein, caffeic acid and tyrosol were studied. Linoleic acid autoxidation rate was estimated by the increase of conjugated diene level and by the decrease of linoleic acid content in the samples. The phenolic compounds exhibited an antioxidant activity which increased in the order: tyrosol < caffeic acid < oleuropein < hydroxytyrosol. The analysis of the hydroperoxide isomers pointed out that hydroxytyrosol, oleuropein and caffeic acid at a concentration of 10⁻⁴M inhibited the formation oftrans- trans isomers in the increasing order: caffeic acid < oleuropein < hydroxytyrosol. This inhibition could be related to the ability of phenolic compounds to scavenge peroxyl radical. Tyrosol did not inhibit the formation oftranstrans isomers. Phenolic compounds were degraded as a consequence of their antioxidant activity and their degradation rate was positively correlated to their antioxidant efficacy. These phenolic compounds, at a concentration of 6 × 10⁻³M, also scavenged hydroxyl radical, with an efficiency which increased in the order: tyrosol < hydroxytyrosol < oleuropein < caffeic acid. Polar substituents at the para position, such as in caffeic acid and oleuropein, were correlated with higher hydroxyl radical quenching ability.     

Thermal decomposition of additive polymerization initiators. I. Azobisisobutyronitrile

We measured the rate of thermal decomposition of azobisisobutyronitrile (AIBN) from the rate of loss of 2,2-diphenylpicrylhydrazyl (DPPH) on reaction of the free radicals formed by thermal decomposition of azobisisobutyronitrile (AIBN) in organic solvents with DPPH. There has been some doubt about the quantitative relation between AIBN and DPPH required to get the rate constant of thermal decomposition of AIBN. In the past, AIBN has been used in excess, and the rate constants measured by using DPPH are smaller than they ought to be. In our experiments we used as little of AIBN relative to DPPH as possible and obtained a value of 3.8 × 10^?4 min.^?1 at 50°C. as the rate constant of the first-order reaction, which was shown satisfactorily by the linearity of the graph. We also obtained 32 kcal./mole as activation energy. We were also able to make clear the meaning of the result when a large quantity of AIBN was used.