Oxygen diffusion rate

The ODR method relies on amperometric or polarographic techniques to measure the oxygen concentration of the soil pores. The amperometric techniques imply the measurement of current. The polarographic techniques imply that the electrode at which reduction of reactants occurs is in a polarized condition. Under polarized conditions, the concentration of reactants at the electrode surface is low, whereas the concentration of products is high. Under these conditions, the amount of current that the electrode will pass is directly proportional to the flux of reactant to the electrode surface. These principles are used in the ODR method. 

The ODR method involves the chemical reduction of oxygen at the surface of a thin Pt electrode maintained at constant potential and negatively polarized with respect to a saturated calomel or 

the current increases rapidly with each increment in voltage. This is because electrons on the Pt electrode surface acquire a potential sufficient to cause them to react with some of the dissolved oxygen at the surface of the electrode. As the potential increases, the reaction rate increases until a point is reached when the oxygen concentration at the electrode surface is zero. At voltages between 

2 and 0.8 V, electrons on the Pt surface have sufficient activation energy to reduce the dissolved oxygen in the solution. The plateau region between 0.2 and 0.8 V indicates a point at which the current becomes independent of the applied voltage and is limited by the diffusion of oxygen to electrode surface. 

The current intensity is proportional to oxygen flux at the electrode surface and is described as 

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As mentioned earlier, there is an inverse relationship between water volumes and oxygen concentration in soil. As soils dry, conditions become more aerobic and oxygen diffusion rates become higher. The wet-dry or anaerobic-aerobic alternation, either temporal or spatial, leads to higher corrosion rates than would be obtained within a constant environment. Oxygen-concentration-cell formation is enhanced. This same fluctuation in water and air relations also leads to greater variation in biological activity within the soil. 

SRB, a diverse group of anaerobic bacteria isolated from a variety of environments, use sulfate in the absence of oxygen as the terminal electron acceptor in respiration. During biofilm formation, if the aerobic respiration rate within a biofilm is greater than the oxygen diffusion rate, the metal/biofilm interface can become anaerobic and provide a niche for sulfide production by SRB. The critical thickness of the biofilm required to produce anaerobie conditions depends on the availability of oxygen and the rate of respiration. The corrosion rate of iron and copper alloys in the presence of hydrogen sulfide is accelerated by the formation of iron sulfide minerals that stimulate the cathodic reaction. 

Peck W.H., Valley J.W., and Graham C.M. (2003) Slow oxygen diffusion rates in igneous zircons from metamorphic rocks. Am. Mineral. 88, 1003-1014. 

Sharp Z.D. (1991) Determination of oxygen diffusion rates in magnetite from natural isotopic variations. Geology 19, 653-656. 

Sharp Z.D., Giletti B. J., and Yoder H. S. (1991) Oxygen diffusion rates in quartz exchanged with CO2. Earth Planet. Sci. Lett. 107, 339-348. 

Minimum rate, may be limited by oxygen diffusion rate. 

Instead of assuming an oxygen diffusion rate as in this example, it is also possible to define a pyrite oxidation rate R that is a function of e.g. 02, pH, temperature, the amount of microorganisms and the nutrient supply. Examples using direct reaction rates in PHREEQC follow in the next exercises. 

A third situation that is possible is intermediate between the two extreme cases shown in Figs. 8.7a and b. In this case, the boundary between the reacted and unreacted composite is not sharp, and a region of semi-reacted SiC particles separates the two (Fig. 8.7c). This case can occur in two circumstances. When the oxygen diffusion rate through the oxidation product is comparable to, but slightly slower than, through the matrix, a semi-reacted zone will appear, the thickness of which depends on the SiC particle size. A similar situation will arise when the initial oxidation production reacts with the... 

Poly(vinyl halides) - Photodehydrochlorination occurs in PVC with a foam backing as determined by Raman spectroscopy. For PVC stabilised with Zn/Ca stearates phooxidation indicates that dehydrochlorination dominates over oxidation at the inner layers beyond 100 microns " . The effect of polyene formation also appears to be a function of light intensity as well as oxygen diffusion rate. 

Upon application of a certain potential across the platinum electrode and a reference electrode inserted in the soil, oxygen is reduced at the platinum surface. The electric current flowing between the electrodes is proportional to the rate of oxygen reduction which in turn is related to the rate of oxygen diffusion to the electrode. The oxygen diffusion rate (ODR) is calculated from the measured electric current according to the following equation. 

Fig. 2.14. Schematic diagram of oxygen diffusion rate (ODR) measurement.

Shaikh, A.U., Hawk, R.M., Sims, R.A. Scott, H.D. (1985) Graphite electrode for the measurement of redox potential and oxygen diffusion rate in soils. Nuclear and Chemical Waste Management,... 

The solution oxidation technique allows the study of polyolefin autoxi-dation under conditions where the temperature, concentration of reactants, and rates of radical initiation can be controlled. The results should be considered as a useful prelude to any fundamental understanding of the autoxidation processes which occur in neat polymers where the effects of very high viscosity, partial crystallinity, and oxygen diffusion rates are included. The objective of our work was to determine the kinetics and stoichiometry of the inhibited autoxidation of polypropylene in solution. A relatively detailed study of the oxidation of polypropylene inhibited by 2,6-di-terf-butyl-4-methylphenol [butylated hydroxytoluene (BHT)] has been made for comparison with data obtained in polypropylene oxidations inhibited by a variety of other stabilizers which include commercial polyfunctional antioxidants. Singly hindered phenols appeared to be superior in the inhibited-solution oxidation of polypropylene, and the application of this finding to stabilization technology was investigated briefly. 

A key point here is that the correlations based on either ionic or anion porosity, however well they work, are strictly empirical and cannot have much of a microscopic basis. This is because of the fact that the porosities of nominally anhydrous minerals do not change at all due to the presence of trace amounts of water, but oxygen diffusion rates do, typically by orders of magnitude. One can speculate that this behavior is linked. 

Figure 33. Pressure dependence of oxygen diffusion rates in silicate melts, modified after Tinker and Lesher (2001). Data for the sodium-silicate melts are taken from Poe et al. (1997), dacite from Tinker and Lesher (2001), basalt from Lesher et al. (1996) and diopside from Reid et al. (2001). Inset shows results from computer simulations (Angell et al. 1982 — NaAlSi20e, solid Nevins and Spera 1998 — CaAl2Si20s, open) at mnch higher temperatnres of 4000 to 5000°C which show qnalitatively similar behavior. Note how the maxima in pressure dependence shifts to lower pressures for more polymerized, Al-bearing melts whereas in diopside there is actually a minima in the data set see text for details.

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