Pseudo-first-order kinetics loss

Fig. 5 shows the time dependence of the solid-state ion exchange process. The process has pseudo first order kinetics in the investigated conversion range for both (i) the distribution and crystallinity loss of the CdCl2 salt and (ii) the formation of new Cd,H-Y phase. The rate constant obtained for the decay of... 

These prodrugs underwent spontaneous hydrolysis in aqueous solution. The mechanism of reaction (Fig. 11.10) was postulated to involve nucleophilic hydration of the C=N bond to yield an intermediate carbinolamine. The latter breaks down with loss of the dialkylamine to form an Af4-formyl intermediate, which, in turn, hydrolyzes to the active agent (11.68, Fig. 11.10). The hydrolysis of these prodrugs followed pseudo-first-order kinetics with tm values at pH 7.4 and 37° that were mostly influenced by the steric properties of the dialkylamino group. Thus, tm values were ca. 4, 9 - 10, 14 -15, and 48 - 52 h for R = Me, Et, Pr, and i-Pr, respectively. Interestingly, the rates of hydrolysis were decreased in human serum, indicating protective... 

A kinetic analyses of the data was performed by noting the pseudo-first order loss of substrate together with selectivity. This enabled a pseudo-first order kinetic description of the two pathways to be obtained. Table 1 lists the lifetimes of 2-butanone and 2-butanol production for the various experiments. Here the lifetimes refers to the inverse of the pseudo-first order reaction rate coefficients. 

A more fundamental difference between isotope and e-beam sources is dose rate. Whereas the high dose rates of radiation provided by e-beams are necessary for cost-effective water treatment, they also introduce complications resulting from the very high radical concentrations produced. High radical concentrations favor radical/radical recombination, resulting in a loss of reactive species. Gehringer [52] has shown a departure from pseudo-first-order kinetics in such situations, due at least in part to dose rate. 

According to Eq. 58, when 1/2(1) i/2(2), the overall half-life for loss of A is approximately equal to 1/2(2). In other words, the competitive reaction with the shortest half-life has the strongest influence on the half-life of the common substrate. It is often possible to apply pseudo-first-order kinetic methods to characterize time scales of complex reactions. 

This leads to a decrease in the measured rate constant as the reaction progresses (and the [(CF3)2NO ]/[(CF3)2NOHj ratio decreases. The loss of (CF3)2NO could be seen not to follow (pseudo-)first-order kinetics if the decay was monitored for 3 or more half-lives. 

In vitro studies with human term placental microsomes showed that PED displayed the typical properties of a mechanism-based inhibitor of cytochrome P450, namely (a) NADPH-dependent inhibition (b) time-dependent loss of enzymatic activity, followed by pseudo-first-order kinetics and (c) natural substrates delayed the irreversible inhibition. The irreversibility of inhibition was further substantiated by dialysis experiments ... 

Several experiments have shown that picloram will photochemically decompose in aqueous solutions. The loss follows pseudo first-order kinetics over a wide range of concentrations and depths. The half-lives of picloram in water are affected by depth, amounts of sunlight, and the presence of interferring substances. 

In a SIFT or SIFT-MS flow tube for an analyte A, even though the analyte A is at trace levels, its concentration [A] [H30T and therefore pseudo first order kinetics can be applied. The reaction time t can be defined as the reaction length /, divided by the ion velocity v. In addition to reactive ion loss, there is loss of H3O+ by diffusion. If > 3 + is the diffusion coefficient and A the diffusion length (which depends on the flow mbe diameter and length) then the kinetic equation expressing the loss of the reagent ion H3O is [97]... 

The initial anhydride concentration was about 3 x 10 M, and the amine concentration was much larger than this. The reaction was followed spectrophoto-metrically, and good first-order kinetics were observed hence, the reaction is first-order with respect to cinnamic anhydride. It was not convenient analytically to use the isolation technique to determine the order with respect to allylamine, because it is easier to observe the cinnamoyl group spectrophotometrically than to follow the loss of amine. Therefore, the preceding experiment was repeated at several amine concentrations, and from the first-order plots the pseudo-first-order rate constants were determined. These data are shown in Table 2-1. Letting A represent... 

Toth et al. then used laser flash photolysis as a means to determine the value of k x independently of the above study (8). They used 355 nm laser light to photolyze mixtures of C102 and Br2/Br s. Absorption of this light by Br3 led to the prompt formation of Br2, and the subsequent loss of Br2 was monitored by its absorbance at 360 nm. The loss of Br2 occurred with mixed 2nd- and lst-order kinetics due to the parallel 2nd-order self reaction of Br2 and its pseudo-first-order reaction with C102. These experiments led to a value of 3.6 x 109 M 1 s 1 for kh which is in good agreement with the approximate value (1.1 x 109 M 1 s ) originally obtained. 

Comparison of the observed pseudo-first-order decay of biological activity with a half-life of 30 sec at normal oxygen tensions versus decomposition via nitrogen dioxide by pseudo-second-order kinetics predicted by Reaction 4. The loss of nitric oxide through formation of nitrogen oxide is twice as fast as calculated by Reaction 4 because each nitrogen dioxide formed rapidly attacks a second nitric oxide to form nitrite. 

A variety of pulsed techniques are particularly useful for kinetic experiments (Mclver and Dunbar, 1971 McMahon and Beauchamp, 1972 Mclver, 1978). In these experiments, ions are initially produced by pulsing the electron beam for a few milliseconds. A suitable combination of magnetic and electric fields is then used to store the ions for a variable period of time, after which the detection system is switched on to resonance to measure the abundance of a given ionic species. These techniques allow the monitoring of ion concentration as a function of reaction time. Since the neutrals are in large excess with respect to the ions, a pseudo first-order rate constant can be obtained in a straightforward fashion from these data. The calculation of the rate constant must nevertheless make proper allowance for the fact that ion losses in the icr cell are not negligible. 

Polysaccharide pyrolysis at 375-520°C is accompanied by a higher rate of weight loss and evolution of a complex mixture of vapor-phase compounds preponderantly of HsO, CO, C02, levoglucosan, furans, lactones, and phenols (Shafizadeh, 1968). The volatile and involatile phase compositions are conditional on the rate of removal of the vapor phase from the heated chamber (Irwin, 1979), inasmuch as the primary decomposition products are themselves secondary reactants. The reaction kinetics is described as pseudo zero order (Tang and Neill, 1964) and zero order initially, followed by pseudo first order and first order (Lipska and Parker, 1966), suggesting an... 

Fourthly, the starting point for lifetime estimations is often laboratorygenerated kinetic data for reaction of the compound of interest with OH radicals. The bimolecular rate constants measured in laboratory kinetic experiments need to be converted into a pseudo first order rate constant for loss of the compound, k . In principal this conversion is simple, i.e., the bimolecular rate constant merely has to be multiplied by the OH concentration ([OH]). In practice there are difficulties associated with the choice of an appropriate value of [OH], At present we cannot measure the global OH concentration field directly. The OH radical concentration varies widely with location, season, and meteorological conditions. To account for such variations requires use of sophisticated 3D computer models of the atmosphere. 

Stopped-flow spectroscopy was used to relate the rate constants, activation parameters, and mechanisms of the oxidation of CIO2 and Br02 by O3. Ozone reactions with XO2 (X = Cl or Br) are studied under pseudo-first-order conditions with excess X02. The kinetic traces were obtained observing the loss of O3 at 260 nm and fitted to an exponential equation to give the pseudo-first-order rate constants. The 03/X02 reactions are first order in [O3] and... 

Golova and Krylova pyrolyzed cotton cellulose and measured the decrease in the D-glucose residues, instead of the weight-loss, as a function of time, and found that the reaction follows a zero order. In contrast, Tang and Neill, on the basis of thermogravimetric data to be discussed later (see p. 446), suggested that the initial state of pyrolysis of cellulose is controlled by pseudo-zero-order kinetics and the final state is of pseudo-first order. 

If the destruction of a component is presumed to be zero-order, a plot of C vs. t should give a straight line. Certain losses of vitamin C, particularly in frozen foods, are presumed to follow first-order kinetics (89). Labuza (76) observed that zero-order reaction rates for quahty losses may be assumed in some fluctuating temperature studies, but this may lead to a miscalculation of predicted changes. Therefore, from a theoretical standpoint, it is important that the proper order be used for predictions. In general, ascorbic acid destruction is assumed to be first-order, or pseudo first-order (17,27) except under specific conditions of heat and moisture (79). 

Reaction (15) (with R = H and R = C6H5, 4-MeC6H4, 4-CF3C6H4 and Me and with R = Me and R = 4-CF3C6H4) was investigated kinetically in dibutyl ether" under pseudo-first-order conditions ([22] [21] > 10). The reversible loss of CO led to complex kinetics that was difficult to evaluate. However, when the reaction was conducted either in the presence of a saturated solution of CO or under conditions that led to the continuous removal of CO so that [CO] 0, first-order kinetics was observed. The concentration dependence of the rate was shown to be consistent with the mechanism of Scheme 1 where 26 is a steady-state intermediate and hence the... 

In the estimation of rate constants through the fitting of degradation data to a kinetic model, the validity of the model and the reliability of the estimated rate constant should be evaluated, taking into account experimental errors. Additional data are sometimes required to obtain accurate estimates. For example, in the case of consecutive reactions, the time courses for both the parent drug and the intermediate are required to estimate the pseudo-first-order rate constant for the formation and loss of the intermediate (see Section 2.2.3.7.f), especially when k/k, is larger than O.5.297... 

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