Water concentration, time evolution measurements

The physical model of the reactor is a 350 mm high cylindrical vessel, with a diameter of 200 mm and an elliptical bottom. The operation volume is V = 12 10 m. The entrance of the reactants is placed near the middle of the reactor, more exactly at 130 mm from the bottom. The reactor s exit is positioned on the top of the vessel but below the liquid level. At the vessel centre is placed a mixer with three helicoidal paddles with d/D = 0.33. It operates with a rotation speed of 150 mirnf In order to establish the reactor flow model, this is filled with pure water which continuously flushes through the reactor at a flow rate of 6.6 10 5 m /s (similar to the reactants flow rate). At time t = 0, a unitary impulse of an NaCl solution with a Cq = 3.6 kg/m is introduced into the reactor input. The time evolution of the NaCl concentration at the exit flow of the reactor is measured by the conductivity. Table 3.5 gives the data that show the evolution of this concentration at the reactor exit. 

Besides fluorescence spectroscopy, time-resolved spectroscopy can rely on the measurement of excited (singlet or triplet) state absorption. Similarly to ground-state absorption, the spectral and absorbance properties may be altered by CyD complexation and yield information about the behavior of the complex in the excited state in addition, the time dependence (formation and decay) of the excited state absorption yields information about the kinetics and dynamics of the system. This is illustrated by the behavior of the lowest triplet state of naphthalene as measured by nanosecond spectroscopy using a Q-switched Nd YAG laser at 266 nm for excitation [21]. The triplet-triplet absorption spectra were measured in neat solvents (water and ethanol) and in the presence of a- and -CyD (Fig. 10.3.3). The spectra in ethanol and H2O had the same absorption maximum, but the transition was considerably weaker and broadened in H2O. Both CyDs induced a red shift, and a-CyD additionally narrowed the main band considerably. Fig. 10.3.4 shows the effect of a-CD concentration on the time evolution of the triplet-triplet absorption at 416 nm in the microsecond range. Triplet decay was caused by O2 quenching a detailed kinetic analysis of the time dependence yielded two main components which could be assigned to the free guest and the 1 2 complex, in full... 

The classic idea of a cosmic-ray exposure (CRE) age for a meteorite is based on a simple but useful picture of meteorite evolution, the one-stage irradiation model. The precursor rock starts out on a parent body, buried under a mantle of material many meters thick that screens out cosmic rays. At a time fj, a collision excavates a precursor rock—a meteoroid. The newly liberated meteoroid, now fully exposed to cosmic rays, orbits the Sun until a time ff, when it strikes the Earth, where the overlying blanket of air (and possibly of water or ice) again shuts out almost all cosmic rays (cf. Masarik and Reedy, 1995). The quantity ff — h is called the CRE age, f. To obtain the CRE age of a meteorite, we measure the concentrations in it of one or more cosmogenic nuclides (Table 1), which are nuclides that cosmic rays produce by inducing nuclear reactions. Many shorter-lived radionuclides excluded from Table 1 such as Na (ff/2 = 2.6 yr) and °Co ty = 5.27 yr) can also furnish valuable information, but can be measured only in meteorites that feu within the last few half-Uves of those nucUdes (see, e.g., Leya et al. (2001) and references therein). 

Figure 2.4.4 shows the evolution of nitrate concentration at the same sampling point (three times) during a period of 12 days. Even if the raw values of the nitrate concentration level measured by PASTEL UV are slightly overestimated in comparison with those obtained by laboratory method, this device can be used to assess trends in water quality evolution and then could help decision making in water management resources. The results obtained for ammonium concentration by Merck test kit lead to more or less to the same conclusions compare to reference method. 

In further temperature and concentration-dependent measurements of the helical pitch, only mixtures with formamide were chosen as the lengthy evolution time necessary for mixtures with water together with the ever present threat of solvent evaporation make such investigations of mixtures with water much more complicated. In Fig. 5.33 the helical pitch p is plotted versus the reduced temperature T - Tq for a sample with 18 wt% of formamide. The pitch shows the typical temperature dependence known from thermotropic SmC phases [30]. Right after the phase transition into the lyotropic SmC analog phase, the pitch increases rapidly to a value of about 5.5 pm and decreases more slowly towards a low temperature value of about 2.5 pm. However, by repeating the measurement with other concentrations of formamide, no significant difference in the value of p could be detected. 

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