Zero order kinetics, degradation rate

The rate of ethanol degradation in the liver is limited by alcohol dehydrogenase activity. The amount of NAD" available is the limiting factor. As the maximum degradation rate is already reached at low concentrations of ethanol, the ethanol level therefore declines at a constant rate (zero-order kinetics). The calorific value of ethanol is 29.4 kj g Alcoholic drinks—particularly in alcoholics—can therefore represent a substantial proportion of dietary energy intake. 

Example 8-1 (taken from Bolton et al, 2001a) 2000 L of wastewater containing 500 mg of TOC as phenol (CgHgO) are treated in a batch reactor for 10 h with a 30 kW AOT to yield an efQuent with TOC of 100 mg L . The rate of degradation followed zero order kinetics. Calculate the corresponding design parameter. 

The alternative kinetic situation that is often encountered experimentally in photochemical reactions is zero-order kinetics, corresponding to the situation where the rate of degradation of the drug is a constant, independent of concentration, as shown in Eq. 4... 

A perfect photosensitizer will not be transformed in the photosensitizing process, although in reality, some degradation of the photosensitizer will occur in time. Nonetheless, the rate of the photosensitized reaction will depend directly on the sensitizer concentration at low values where not all of the relevant incident radiation is being absorbed. Most studies of this type of photoreaction use a high concentration of the acceptor A, in which case the transformation of A will follow apparent zero-order kinetics. 

When k (0.01) k2 (0.0005) as shown in the middle simulation, the rate of degradation remains constant from about 400 sec. to 2000 sec. and represents zero order kinetics over most of the degradation. At relative values of k. and k2 of 1 to 20 and 20 to 1 the initial and final portions of these degradations are characteristic of acceleratory reactions. When ratios are 1 to 100 and 100 to 1, the acceleratory characteristics are not seen and the reactions are truly first order and zero order. The upper simulation shows a typical acceleratory reaction in which k- = k2 = 0.0008. An alpha-time plot for the data of the upper simulation gives the typical S-shaped curve of an acceleratory reaction. 

For reactions in which the catalytic effect of degradation products is negligible and the volume of the adsorbed moisture layer and the drug solubility can be regarded as constant, the rate should be described by Eq. (2.68) according to the adsorbed moisture layer, and the degradation should conform to apparent zero-order kinetics. 

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]). 

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. 

When microorganisms use an organic compound as a sole carbon source, their specific growth rate is a function of chemical concentration and can be described by the Monod kinetic equation. This equation includes a number of empirical constants that depend on the characteristics of the microbes, pH, temperature, and nutrients.54 Depending on the relationship between substrate concentration and rate of bacterial growth, the Monod equation can be reduced to forms in which the rate of degradation is zero order with substrate concentration and first order with cell concentration, or second order with concentration and cell concentration.144... 

Figure 9.9 Kinetic scheme for MBI.

Reservoir/matrix hybrid-type non-degradable polymeric implants are also available. Such systems are designed in an attempt to improve the M t1/2" release kinetics of a matrix system, so that release approximates the zero-order release rate of a reservoir device. Examples of these types of systems include ... 

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