Autoxidation measurement

Figure 2. Apparatus for non-steady state autoxidation measurements...

The reactivity of a series of hydrocarbons toward oxygen measured under a standard set of conditions can give some indication of the susceptibility of various structural units to autoxidation. Table 12.10 gives the results for a series of hydrocarbons. These data indicate the activating effect of alkyl, vinyl, and phenyl substituents. 

Numerous autoxidation reactions of aliphatic and araliphatic hydrocarbons, ketones, and esters have been found to be accompanied by chemiluminescence (for reviews see D, p. 19 14>) generally of low intensity and quantum yield. This weak chemiluminescence can be measured by means of modern equipment, especially when fluorescers are used to transform the electronic excitation energy of the triplet carbonyl compounds formed as primary reaction products. It is therefore possible to use it for analytical purposes 35>, e.g. to measure the efficiency of inhibitors as well as initiators in autoxidation of polymer hydrocarbons 14), and in mechanistic studies of radical chain reactions. 

In a series of chemiluminescent autoxidation reactions the following quantum yields were measured 38) ... 

The peroxyl radical has been the most frequently used compound in assays to measure antioxidant capacity because it is a key component of autoxidation and can be easily produced by the decomposition of azo compounds. 

Rate Constants of Hydroperoxide Group Decomposition into Free Radicals Measured by Kinetics of Autoxidation and by Free Radical Acceptor Method... 

For initiated oxidation, the inhibitory criterion could be defined as the ratio v0/v or (v0/ v — v/v0), where v0 and v are the rates of initiated oxidation in the absence and presence of the fixed concentration of an inhibitor, respectively. Another criterion could be defined as the ratio of the inhibition coefficient of the combined action of a few antioxidants / to the sum of the inhibition coefficients of individual antioxidants when the conditions of oxidation are fixed (fx = IfiXi where f, and x, are the inhibition coefficient and molar fraction of z th antioxidant terminating the chain). It should, however, be noted that synergism during initiated oxidation seldom takes place and is typical of autoxidation, where the main source of radicals is formed hydroperoxide. It is virtually impossible to measure the initial rate in the presence of inhibitors in such experiments. Hence, inhibitory effects of individual inhibitors and their mixtures are usually evaluated from the duration of retardation (induction period), which equals the span of time elapsed from the onset of experiment to the moment of consumption of a certain amount of oxygen or attainment of a certain, well-measurable rate of oxidation. Then three aforementioned cases of autoxidation response to inhibitors can be described by the following inequalities (r is the induction period of a mixture of antioxidants). 

Fig. 1. Amperometric monitoring of the autoxidation of epigallocatechin gallete in the presence of (A) 0, (B) 2.0, (C) 5.0, (D) 10, (E) 20, and (F) 50 pm CuCl2. The measurements were performed in 0.1 M Tris buffer (pH 9.0) with a Clark type oxygen electrode at 28 °C. The epigallocatechin gallate concentration was fixed at 50 pm. The catechin stock solution was injected into the test solution at t = 0. The inset shows the (initial) steady-state autoxidation rate as a function of Cu2+ concentration. Reprinted from Biochimica et Biophysica Acta, vol. 1569, Mochizuki, M. Yamazaki, S. Kano, K. Ikeda,T., Kinetic analysis and mechanistic aspects of autoxidation of catechins, p. 35, Copyright (2002), with permission from Elsevier Science.

Although each of the metal electrodes has been polished immediately prior to the measurements that are illustrated in Figures 1-3, the initial negative scans often exhibit significant reduction peaks due to autoxidation by O2. The polishing process undoubtedly atomizes some of the M-M surface molecules, which probably initiates the process via a mechanism similar to that for the autoxidation of reduced iron porphyrins (17)... 

A widespread method for determining the induction period for autoxidation of oils and fats consists of passing a continuous stream of air through the heated sample and collecting the volatile acids evolved in a water trap, where they are determined on a real time basis. The time plot usually presents a flat appearance for a certain period and then takes off in an accelerated manner. This test is the basis of several national and international standards (e.g. AOCS Cd 12b-92—oil stability index" ISO 6886—accelerated oxidation test for oxidative stability of fats and oils ) and the design of the Rancimat equipment, where the end determination is based on conductivity measurements . In addition to oxidation stability as determined by the Rancimat method and POV, which negatively affects virgin olive oil stability, other nonstandard properties were proposed for better assessment of the quality of this oil, namely LC determination of Vitamin E (21), colorimetric determination of total polar phenols and UVD of total chlorophyll. ... 

Direct measurement of oxygen uptake can be performed with O2 sensors in the headspace of a closed system and organic deposition by measuring the weight changes with a quartz microbalance. This method was applied to investigate the dependence of jet fuel autoxidation on temperature and the presence of antioxidants . 

Tables I and II include data for the co-oxidations of styrene and butadiene in chlorobenzene and ferf-butylbenzene solutions, as well as with no added solvent. These solvents were chosen because the rate of oxidation of cyclohexene varies significantly in them at the the same rate of initiation (6). There is a variation in the over-all rate of oxidation under these solvent conditions, but there appears to be no significant difference in the measured ra and rb (Table II). If the solvent does affect the propagation reaction in autoxidation reactions, it affects the competing steps to the same degree.

Tphe original objectives of this work were to determine how much the relative reactivity of two hydrocarbons toward alkylperoxy radicals, R02, depends on the substituent R—, and whether there are any important abnormalities in co-oxidations of hydrocarbons other than the retardation effect first described by Russell (30). Two papers by Russell and Williamson (31, 32) have since answered the first objective qualitatively, but their work is unsatisfactory quantitatively. The several papers by Howard, Ingold, and co-workers (20, 21, 23, 24, 29) which appeared since this manuscript was first prepared have culminated (24) in a new and excellent method for a quantitative treatment of the first objective. The present paper has therefore been modified to compare, experimentally and theoretically, the different methods of measuring relative reactivities of hydrocarbons in autoxidations. It shows that large deviations from linear rate relations are unusual in oxidations of mixtures of hydrocarbons. 

Since the third objective of this paper is to compare, theoretically and experimentally, different methods of measuring reactivities of hydrocarbons in autoxidation, the most pertinent work is reviewed below. 

Let us now consider what we mean by the reactivity of a hydrocarbon in autoxidation. One measure is how fast it oxidizes by itself at unit concentration and unit rate of initiation. (Rates of thermal oxidation at unknown rates of initiation are not useful enough to be considered.) The first two columns of figures in Table VII give such comparisons in terms of kp/(2kt)l/l at 30° and 60°C. and determine the order in which hydrocarbons are listed in the table. The results of Ingold and Sajus agree fairly well the orders of reactivity are identical except for a trivial difference with the xylenes. The stated quotients at 60°C. are uniformly 2 to 3 times as large as at 30°C. for sec-butylbenzene and more reactive hydrocarbons but 3 to 6 times as large for less reactive hydrocarbons. 

Initiated Oxidation. The initial rates of oxidation of chloroprene, initiated with 2,2 -azobisisobutyronitrile, were measured in the range 20° to 40°C. at a total pressure of 700 mm. of Hg. The difficulty with these measurements was that chloroprene autoxidizes so readily that even when the initiator is used at the fairly massive concentration of 0.462M, the rate of oxidation is constant for only a few minutes before acceleration, resulting from a contribution to initiation from chloroprene peroxide. 

Peroxide Decomposition Mechanism. Since virtually no work has been reported which concerns only the mechanism by which zinc dialkyl di-thiophosphates act as peroxide decomposers, it is pertinent to discuss metal dialkyl dithiophosphates as a whole. The mechanism has been studied both by investigating the products and the decomposition rates of hydroperoxides in the presence of metal dithiophosphates and by measuring the efficiency of these compounds as antioxidants in hydrocarbon autoxidation systems in which hydroperoxide initiation is significant. 

Only ratios of rate constants of autoxidation can be determined by ordinary steady-state velocity measurements. Non-steady state methods, notably the sector method, have been used to evaluate the individual or absolute rate constants. We have devised a method for measuring the change in rate of autoxidation under non-stationary conditions directly, by means of sensitive manometric apparatus, and for deriving individual rate constants from these measurements. 

In our method reproducible non-stationary states are effected as follows the low stationary-state rate of an autoxidizing hydrocarbon is decreased by a factor of 2 to 5 by adding an appropriately small amount of inhibitor. Under the conditions outlined below, the time required to establish the new stationary state at the inhibited rate is not immeasurably small, as it would be in conventional measurements, but of the order of 100-300 sec. With sufficiently sensitive apparatus a number of determinations of the decreasing velocity can be made, which delineate the course of the non-steady state. Similarly a non-steady state with an increasing velocity can be realized by introducing a small amount of initiator. 

For experimental reasons (discussed below) the relative rate of decrease should be smaller than 0.01 sec."1. Since kA is of the order of 104 liter per mole per sec., [AH] must be of the order of 10 6 mole per liter. This low concentration of inhibitor will only have an appreciable effect on the rate of autoxidation if the rate of initiation—and consequently the rate of oxidation—is low. To measure the decrease in rate during the short-lived non-steady state, one must be able to determine these low velocities within short periods of time. From the usual inhibition formulas one can compute, for instance, that in order to obtain a ratio of original to inhibited rate of about 5 with [AH] = 10 6 mole per... 

Destruction of nitric oxide by superoxide in the buffers is more likely to account for the short half-life of nitric oxide in vitro. Superoxide dismutase (15-100 U/ml) substantially increased the apparent half-life of EDRF, strongly suggesting that superoxide contributes to the short biological half-life of nitric oxide. In the perfusion cascade bioassay system, the buffers are bubbled with 95% oxygen, contain 11 mM glucose as well as trace iron plus copper contamination and are incubated under the weak ultraviolet (UV) radiation of fluorescent lights. These are prime conditions for the autoxidation of glucose to form small amounts of superoxide in sufficient amounts to account for the short half-life of nitric oxide in nanomolar concentrations. The rate of reaction between superoxide and nitric oxide is 6.7 X 10 M sec L The shortest half-life of nitric oxide measured is approximately 6 sec. To achieve a half-life of 6 sec, the steady state concentration of superoxide would only need to be 17 pM, calculated as ln(2)/ (6 sec X 6.7 X 10 M" sec )-... 

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