Oxygen, determination time constants

The fluorescence lifetime was determined to be 1124ps for 35a, 785 ps for 35b, and 831 ps for 43 in dichloromethane, whereas in the corresponding amorphous films a nonexponential decay with shorter time constants was observed [118, 119]. These lifetimes are similar to the parent oligophenyls but different from fluorene (10 ns) [120, 121]. When applying oligophenyls as luminescent films, however, we must consider that photooxidation may occur if molecular oxygen is present [122, 123], The proposed pathway for the decomposition is... 

The experimental curve in Figure 3 demonstrates overshoot in the tissue oxygen response. It was determined previously (22) that a term representing pure delay along with the steady-state blood flow vs. arterial oxygen tension data would cause overshoot. In this investigation it was found that a first-order time constant delay would also produce overshoot. Therefore, since exact controller mechanisms are not being postulated, the flow controlled dynamics used in this study include pure delay and time constant lag. To consider the problem of sensor location, feedback and feedforward control loops were superimposed on the capillary-tissue model. 

Although the one-layer model is an oversimplification of actual conditions, its application to the case where the oxygen partial pressure is allowed to change with time illustrates how electrode properties affect transient dissolved oxygen measurements. Pick s second law is needed to describe the unsteady-state diffusion in the membrane, and shows that the diffusion coefficient of the membrane directly determines how fast an electrode will respond to a step change in the oxygen partial pressure (Aiba et al., 1968 Lee and Tsao, 1979 Sobotka et al., 1982). Lee and Tsao (1979) showed mathematically that the electrode response time, for the one-layer model, depends on the electrode time constant defined as... 

The time constants of membrane covered polarographic oxygen electrodes are determined by the shape and diameter of the electrode and by the diffusive fluxes through the diffusion layers (polarization layer and electrolyte, membrane and front layer (Fig. 14-lb)). 

In this expression, C and V represent capacitance and electrode potential, with dl and bulk denoting double layer and bulk solution, respectively. The Faradaic current (e.g., due to oxygen reduction) ordinarily decays much more slowly than the charging current (cells with no supporting electrolyte are notable exceptions). Typically the cell time constant r=R Ci, where is the uncompensated resistance and Cd is the double-layer capacitance, is very useful to determine the timescale of... 

The decomposition of nitrous oxide (NjO) to nitrogen and oxygen is preformed in a 5.0 1 batch reactor at a constant temperature of 1,015 K, beginning with pure NjO at several initial pressures. The reactor pressure P(t) is monitored, and the times (tj/2) required to achieve 50% conversion of N2O are noted in Table 3-19. Use these results to verify that the N2O decomposition reaction is second order and determine the value of k at T = 1,015 K. 

For simplicity the residence times, r, of 160 and of 180- are assumed to be equal here. With Equations 12 and 13 it is now possible to determine the rate constants kx and k2 at different Vr. The results are shown in Table II and agree fairly well with the rate constants obtained using ordinary oxygen. The ratios of the rate constants for the two mechanisms are constant in the range of repeller potentials from 7 to 12 volts. 

---