Oxygen buffer

Figure 4.1 Time-course of free-radical production during aerobic (a) or anoxic (b) reperfusion of the isolated rat heart. Radical production was assessed using e.s.r. and quantified as the formation of a Af-tert-butyl-a-phenylnitrone (PBN) spin adduct. After a 35 min stabilization period of aerobic perfusion, hearts were made globally ischaemic for 15 min. Hearts were then reperfused, either with oxygenated buffer (a) (n = 6), or with anoxic buffer, switching to an oxygenated buffer after 10 min (b) (n = 5). The bars represent the standard errors of the means. Redrawn with permission from Garlick et af. (1987).

Another advantage to the use of a thiol additive is that the abundance of free thiol groups in the reaction environment will prevent the oxidation of the cysteine thiol at the N-terminal of the other peptide. Without added thiol transesterification catalysts, disulfide formation resulting in dimerization of the Cys-peptide would be a dominant side reaction in aqueous, oxygenated buffer conditions. 

In the gut sac technique, animal intestinal segments are divided into 2-cm sacs by tying off each end. The sacs, with or without musculature, can be everted or not to measure directional drug transport into or out of the sacs. The sacs are placed in a vessel containing an oxygenated buffer and transport is measured with normalization for the sac size. The intestinal gut sac method is a relatively fast and inexpensive technique for measuring permeability that includes all intestinal cell types and mucus layer. 

One pecuhar oxygen buffer reaction is the well-known thermobarometric equation of Buddington and Lindsley (1964), which furnishes information on both T and the activity of molecular oxygen. The reaction uses the equilibrium among titanomagnetite (or ulvospinel ), magnetite, hematite, and ilmenite components in the hemo-ilmenite and spinel phases ... 

Hemingway, B.S. (1990) Thermodynamic properties for bunsenite, NiO, magnetite, Fe304, and hematite, Fe203, with comments on selected oxygen buffer reactions. Am. Min. 75 781-790... 

In order to clarify the meaning of D in the case of incomplete (local) defect equilibration, let us consider a linear diffusion geometry and assume that the equilibration of the defects with the external oxygen buffer occurs only at one end of the sample. The fluxes of the components can then be expressed as... 

Place 2.5 ml oxygenated buffer in the electrode cell, replace the adjustable plug, and set its depth to ensure that no air bubbles are trapped beneath the plug. Switch on the magnetic stirrer. 

Add 1.5 ml oxygenated buffer followed by 0.5 ml water and 0.5 ml enzyme extract to electrode chamber. 

A basalt plus groundwater oxygen buffering experiment was conducted at Arizona State University using Umtanum basalt plus synthetic Grande Ronde groundwater at 200°C in an autoclave equipped with the Teflon membrane previously described. The gas diffusing across the membrane initially rose to a maximum pressure of 10.62 bars after 242 hr. This was followed by a pressure decrease to a minimum of 4.4 bars after 358 hr. Thereafter the pressure steadily increased at a rate of 4.5 x 10 3 bars/hr. A possible explanation of the decrease in pressure from 10.62 to 4.4 bars is the reduction of sulfate ... 

Toward bioapplications Since cerium occurs in both trivalent (+ 3) state and tetravalent (+4) state, it may switch between these two states in a redox reaction in ceria to make excellent oxygen buffers. Therefore, ceria NPs with increased surface area and oxygen vacancies exhibit excellent catalytic activities. In biological systems, reports indicated that ceria NPs... 

Fig. 84. Isobaric sections of diagram of mineral equilibria in metamorphosed silicate-carbonate iron-formations under reducing conditions (graphite-oxygen buffer).

The concept of relative /o was proposed by Sato (1978). Because most solid-solid oxygen buffers are parallel to equilibria involving mineral solid solutions in T- fo space, reporting oxygen... 

Figure 2 Effect of oxygen fugacity on Z)(Ni), D(Ge), and D(P). Note that the partition coefficients for nickel decrease, for phosphorus increase and for germanium are constant as T increases parallel to a buffer. IW-1 refers to an oxygen fugacity 1 log/o unit below the IW oxygen buffer (sources Holzheid et aL, 1997 Schmitt etal., 1989 Hillgren, 1993 Newsom and Drake, 1983).

Dobson D. and Brodholt J. P. (1999) The pressure medium as a solid-state oxygen buffer. Geophys. Res. Lett. 26, 259—262. 

Holzheid A., O Neill H. St. C. (1999) The Cr-Cr203 oxygen buffer and the free energy of formation of Cr203 from high temperature electrochemical measurements. Geochim. Cosmochim. Acta 59, 475—479. 

Myers J. and Eugster H. (1983) The system Ee-Si-O oxygen buffer calibrations to 1,500 K. Contrib. Mineral. Petrol. 82, 75-85. 

Pownceby M. 1. and O Neill H. St. C. (1994) Thermodynamic data from redox reactions at high temperatures IV. Calibration of the Re-Re02 oxygen buffer from EMF and NiO -I- Ni — Pd redox sensor measurements. Contrib. Mineral. Petrol. 118, 130-137. 

Before and during an experiment buffer must be aerated continuously with a gas mixture (95% oxygen and 5% carbon dioxide) to oxygenate buffer and to stabilize the pH. 

Nojeim et al (utilized an electrolytic cell model system, free of oxygen, buffered with phthalate to pH k.2 and designed to provide a redox potential between +300 and +650 mv to evaluate the effect of redox potential on the ionization and valence of four iron compounds El, FS, FOP, and SFEDTA, described previously (1+3.). Data obtained were used to predict ionization and valence trends in actual food systems of different redox potentials. Redox potential was found to have no significant effect on the ionization of any of the four compounds evaluated. However, in the case of El and FS lower potentials in the environment favored the reduced form of iron. This is to be expected since a greater difference between the +770 mv potential of the ferric to ferrous couple and the potential of its chemical environment would cause the reduction to be more spontaneous. 

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