Measurement of Oxygen Absorption

The first two determinations by radiation absorption require accurate measurements of the extinction coefficients of ozone (a measurement of the absorption efficiency of the incoming radiation at a maximal absorption wavelength) in the ultraviolet and the infrared. Three different principles have been used over the last 20 years to measure the extinction coefficient of ozone in the ultraviolet at 254 nm manometric, decomposition stoichiometry, and gas-phase titration. The manometric method, which is based on pressure measurements of gaseous ozone, requires (in at least one case ) a substantial and somewhat uncertain correction for decomposition and the method of decomposition stoichiometry depends on the pressure change that accompanies the decomposition of ozone to oxygen, 20, 30,. Clyne and Coxon determined ozone... 

Several researchers have measured the absorption rate at the presence of dispersed organic phase [1,17-18,37,39,49,51-53]. Bruining et al. [37] measured the oxygen absorption in stirred vessels with plane interface in the presence of small amounts of decane, hexadecane, c = 0.01 - 0.1 while van Ede et al. [49] applied octene as a dispersed phase. Littel et al.[39] used carbon dioxide for absorption in dispersion of toluene droplets with c = 0 - 0.4. The theoretical data in the literature were mostly verified by the experimental results of the above... 

Fluorene and 9,9-dideuteriofluorene oxidized at the same rate in DMSO and in tert-butyl alcohol solution. This observation is consistent with a rapid, reversible ionization step. In tert-butyl alcohol the exchange of alpha deuterium atoms of dideuteriofluorene was measured (see experimental section) and a second-order rate constant for ionization calculated to be 0.12 = = 0.01 mole"1 per second. Under the conditions of this experiment the rate of oxygen absorption of undeuterated fluorene was approximately 1/50 the rate of deuterium exchange from the 9,9-dideuteriofluorene. 

The rate of oxidation of fluorene was measured in the presence of various concentrations of nitrobenzene and the better electron acceptor, ra-trifluoromethylnitrobenzene. It was felt that the rate of Reaction 12 should be given by the rate of oxygen absorption (Reaction 13). 

Sulfite Oxidation Method The sulfite oxidation method is a classical, but still useful, technique for measuring /cgfl (or [4]. The method is based on the air oxidation of an aqueous solution of sodium sulfite (Na SOg) to sodium sulfate (Na.,SO ) with a cupric ion (Cu " ") or cobaltous ion (Co ) catalyst. With appropriate concentrations of sodium sulfite (about 1 N) or cupric ions (>10 inolH ), the value of k for the rate of oxygen absorption into sulfite solution, which can be determined by chemical analysis, is practically equal to Zr, for the physical oxygen absorption into sulfate solution in other words, the enhancement factor E, as defined by Equation 6.20, is essentially equal to unity. 

Fermentation broths are suspensions of microbial cells in a culture media. Although we need not consider the enhancement factor E for respiration reactions (as noted above), the physical presence per se of microbial cells in the broth will affect the k a values in bubbling-type fermentors. The rates of oxygen absorption into aqueous suspensions of sterilized yeast cells were measured in (i) an unaerated stirred tank with a known free gas-liquid interfacial area (ii) a bubble column and (iii) an aerated stirred tank [6]. Data acquired with scheme (i) showed that the A l values were only minimally affected by the presence of cells, whereas for schemes (ii) and (iii), the gas holdup and k a values were decreased somewhat with increasing cell concentrations, because of smaller a due to increased bubble sizes. 

So far, only transitions involving unperturbed molecules have been considered. If collisions between molecules are allowed, then electric dipole transitions of otherwise forbidden systems become more probable. From the measurements of the absorption in oxygen, Badger et al.13 predict that the effective first-order transition probability, A, for the O-st Aj, -> 3S5") system, will be given by... 

The most recent measurements of the absorption coefficients of oxygen have been made by Watanabe et al. (101). For the transition 32u- -32 absorption intensities in the Schumann-Runge bands are shown in Figure 2 and in the Schumann-Runge continuum in Figure 3. 

The reactivity of each of the phenols in homopolymerization was determined by following the rate of oxygen absorption in a closed system. In each case, a plot of oxygen absorption against time was linear over at least 80 of the total reaction. Measurements were made at 25°C with a cuprous chloride-pyridine catalyst ai d at 60°C with a more active catalyst, cuprous bromide-tetramethylethylenediamine (TMEDA). Relative rates, from the slope of the linear portion of the oxygen absorption curves, are summarized in Table I. DMP is about 30 times more reactive than DDP at 25° C and five times more reactive at 60° C. MPP is intermediate in reactivity (as expected from its structure) at both temperatures but is comparable at the lower temperature with DMP and at 60°C with DPP (about a third slower than DMP at 25°C and 50 faster than DPP at 60°C). 

To evaluate the activity of antioxidants, we measured the time needed for the absorption of 0.2, 0.5, 1.0, and 1.5 ml. of oxygen in 20 mg. of stabilized polypropylene at constant conditions. The values were denoted r0.2, r0.5, ri.0, and ri.5, respectively. The scattering of the values of r0.2 and especially of the values of r measured at oxygen absorption lower than 0.2 ml. 02 per 20 mg. polypropylene was considerably greater than that of to.5. Figure 1 shows that evaluating antioxidants at to.jt-ti.s values leads to the same conclusions about structural influences. It is advantageous to use more exact data. Therefore, the value of t0. 5 is used in the tables because it illustrates the initial phase of the reaction and it is less influenced by the apparatus error than the value of t02. 

Holroyd and Kenney argued that if the overall reaction could be described by a mechanism of this type, it should be possible to study some of the individual steps by adopting the techniques for investigating gas absorption with the chemical reactions developed for aqueous systems. The rate of absorption of sulphur dioxide was found to be proportional to (S02)(V20s) the rate of oxygen absorption into a partially reduced melt, assuming the vanadium is present only as and is proportional to (02)° (V ) . The implications of these types of measurements are discussed below. 

Direct methods of analysis such as ultraviolet (UV) absorption, infrared spectroscopy (IR), fluorescence, phosphorescence [13], X-ray fluorescence [14-16] and thermal analysis [17] have been reported. However, these methods generally lack specificity [18]. In Fourier transform IR (FTIR), overlapping bands of other species may interfere with the absorbance bands of the analyte, and in UV analysis the absorbance bands of different antioxidants can be very similar. UV and FTIR analysis are especially useful techniques when an antioxidant system is already known. X-ray fluorescence and elemental analysis are fast and useful techniques for the determination of antioxidants containing phosphorus or sulfur. The measurement of oxygen consumption... 

Factors Determining the Effectiveness of Antioxidants. It was for many years a source of puzzlement to polymer chemists that the rating of antioxidants in polymers and in model hydrocarbons appeared to be very different. The widely used antioxidant BHT (I) is one of the most "efficient" antioxidants known for liquid hydrocarbons as measured by oxygen absorption but is virtually ineffective in rubbers... 

Cheung, A.S.-C., K. Yoshino, W.H. Parkinson, S.L. Guberman, and D.E. Freeman, Absorption cross section measurements of oxygen in the wavelength region 195-241 nm of the Herzberg continuum. Planet Space Sci 34, 1007, 1986. 

Jenouvrier, A., B. Coquart, and M.F. Merienne-Lafore, New measurements of the absorption cross sections in the Herzberg continuum of molecular oxygen in the region between 205 and 240 nm. Planet Space Sci 34, 253, 1986. 

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