Oxygen absorbance measurements

This method involves measurement of the oxidation rate of an aqueous sodium sulfite solution catalyzed by cupric or cobaltous ions. The oxygen absorbed reacts with the sulfite according to the equation ... 

FIGURE 1.13 Absorbance measurement of 2, 7 -dichlorodihydrofluorescein (DCDHF), a peroxynitrite-sensitive dye, as a function of nitrite (1 mM) and NADH (1 mM) introduction to a solution containing 100 pM DCDHF and 30mU/mL NR under ambient (a) and nitrogen saturated conditions (b). As can be seen, die absorbance of DCDHF increases upon nitrite and NADH introduction only under an oxygen atmosphere, indicative of peroxynitrite production. (Reprinted with permission from Elsevier Publishing [117].)... 

The absorber pump constantly recycled the fluid through the oxygen absorber so that the fluid was always essentially saturated with oxygen. A small side stream was removed from the gas absorber recycle by the reactor pump. The side stream could either flow through the reactor or bypass the reactor. The reactor bypass was used when it was desired to study the decomposition in solution of the humic acids. Down stream from the reactor was a filter to remove any coal fines produced, a sight glass to observe the fluid, a flow colorimeter to measure the rate of production of humic acids, and a flow rate indicator. 

There is no net change in gas flow rate in and out of the reactor, so that QG in = Qa out = QG (i. e. when nitrogen is used in kLa(02) measurements to produce oxygen-free water, the volume of nitrogen desorbed approximately equals the volume of oxygen absorbed.)... 

The semi-batch set-up uses a chemical reaction to immediately remove the absorbed gas from the liquid. In oxygen transfer measurements, sulfite (S032-) or hydrazine (N2H4) have been used to remove the oxygen transferred (Charpentier, pp. 42-49, 1981). For example, the addition rate of the reactant (S032- or N2H4) is adjusted until the system comes to a steady state dissolved oxygen concentration of about 2 mg/L. Then the sulfite addition rate equals the transfer rate. Mass transfer enhancement must be avoided, which is difficult with hydrazine, so that it is rarely used. 

The study of the kinetics of oxidation (photochemical oxidation in particular) of aldehydes in liquid phase by dissolved oxygen has generally been carried out by measuring, as a function of time, the volume of oxygen absorbed at constant pressure. In experiments of this kind, the rate of dissolution of oxygen is governed by the chemical phenomenon and not by a physical phenomenon of diffusion, provided the two following conditions are fulfilled simultaneously (cf. references 5a and 5b) ... 

The measurement is conducted by frequency modulating a diode laser by superimposing a radio frequency oscillation, Q, onto the diode injection current. The spectral output of a frequency modulated diode laser, shown at the top of Fig. 3, consists of a carrier frequency, a>c and side band frequencies, cOc H. When the laser is scanned through the wavelength of oxygen absorbance, the amount of light absorption, which is proportional to the gas concentration, is written into the side band frequencies by recording the difference in absorption between the two sidebands. 

The oxidation of thallium(I) with bromine occurs quantitatively in the cold. The excess of bromine is conveniently removed by reaction with phenol. The formation of a suspension of bromophenol (which interferes in the absorbance measurement) can be avoided, if the exeess of bromine is small. The aqueous glycerol solution of starch is stable and gives a good reproducibility of results. The excess of stareh does not affect the colour. The amount of the added KI should always be the same. Atmospheric oxygen slowly liberates iodine from the iodide, but the resulting increase in absorbance amounts to scarcely 7% in one hour. 

Analysis of the reaction rates is achieved in terms of oxygen uptake measured with an oxygen electrode, or by product separation and quantification. DPBF absorbs intensely at 415 nm and reacts rapidly with singlet oxygen to form a colorless intermediate endoperoxide. The DPBF reaction can be used as a benchmark against which the effect of an added quencher is compared. A note of caution must be applied the use of inhibitors and quenchers alone is not unambiguous in its outcome and should be strictly supplemented with flash photolysis experiments. Thus, if a photosensitized reaction is quenched by millimolar concentrations of azide ion, it should also be established that azide does not quench the triplet state of the sensitizer directly because that would also affect the reaction rate. 

Rates of Hs uptake were measured in Warburg respirometers at 1°. After gassing the vessels with H2 and before mixing with KCN and CoCl solutions, traces of O2 were removed by an oxygen absorber in the center well of each vessel. No buffer was employed in the Warburg experiments reported here. 

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