Zero-order degradation rate model

A zero-order degradation rate model can be represented as follows ... 

The speed of autoxidation was compared for different carotenoids in an aqueous model system in which the carotenoids were adsorbed onto a C-18 solid phase and exposed to a continnons flow of water saturated with oxygen at 30°C. Major products of P-carotene were identified as (Z)-isomers, 13-(Z), 9-(Z), and a di-(Z) isomer cleavage prodncts were P-apo-13-carotenone and p-apo-14 -carotenal, and also P-carotene 5,8-epoxide and P-carotene 5,8-endoperoxide. The degradation of all the carotenoids followed zero-order reaction kinetics with the following relative rates lycopene > P-cryptoxanthin > (E)-P-carotene > 9-(Z)-p-carotene. 

P is the number of polymer molecules of degree of polymerization n, R is the number of radicals found in a volume V, R is the number of polymer radicals with degree of polymerization n found in a volume, V. For other definitions, please use the nomenclature associated with Table 15.2. Noting equation 15.14, the kinetics of polymer degradation are very complex. Only the most simple mechanisms have been thoroughly researched. These simplified reactions presented in Table 15.2 are sometimes zero order, more frequently first order, and infiiequently second order in polymer mass. These simplified rate expressions are typically used to model binder burnout. 

To develop a PD model for cholinesterase-inhibiting compounds, the steady-state levels (p,mol) of B-esterase enzymes (AChE, BuChE. and CarbE) were determined for the variou.s tissues (c.g., brain, blood, liver, and diaphragm) based on the rates of enzyme synthesis (zero-order) and degradation (first-order) (Gearhart et al., 1990). The... 

Through a preliminary study they demonstrated that, starting from the L-H kinetic model, the rate of the photocatalytic degradation followed a pseudo-zero order kinetics (Equation [21.1a]). Moreover, performing dialysis experiments (without irradiation), they assumed that mass transport of solutes was due to diffusion only and no exchange between the two compartments occurred. The authors described also the variation of concentration in the feed tank and in the reactor by differential equations (Equation [21.1b]). 

Because the radiation in most cases will not penetrate the entire sample, the concentration of the reactant is unlikely to approach zero at infinite time. A plot of remaining concentration vs. time will therefore level off at a value greater than zero. This should be taken into account when selecting the kinetic model for studies of solid-state degradation (Sande, 1996). The solid-state degradation will in some cases appear to consist of a series of consecutive processes with different mechanisms and rates (Carstensen, 1974). Such a stepwise change in reaction rate is most likely caused by an alteration in sample surface and fading of subsequent layers. The concept of reaction order may not be useful for photodecomposition in the solid state (De VUliers et al 1992). 

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