Release rate constant

Some introductory comments on the conceptual basis of SPMD uptake (ku) and release (ke) rate constants and the associated sampling rates (i.e., Rs) are in order. The can be conceptualized as the volume of air or water cleared of chemical per unit sampler mass or volume per unit time (e.g., mL g d or mL mL d ) and Rs is the volume of air or water cleared per unit time (e.g., L d ). Thus, the only difference between ku and Rs is that Rs is not normalized to a unit mass or unit volume of sampler. In the context of organism exposure (see Section l.L), the SPMD is equivalent to the encounter volume times the fractional bioavailability of the chemical (which excludes dietary uptake). The release rate constant (d ) is equal to kuK J. 

In the systems of Mo and Co sulfides, TAA was assumed to release sulfide ions by hydrolysis accelerated by hydrazine. Since the concentration of S2 in equilibrium with TAA is extremely low despite the exceedingly high release rate constant of S2- in the reversible reaction of Eq. (1), this assumption is reasonable if the concentrations of the free metal ions are too low for the nucleation of these metal sulfides. However, if the role of hydrazine is different than an accelerator of hydrolysis of TAA, and if the deposition rate of the metal sulfide monomers or the release rate of metal ions from the metal ion complexes such as orthomolybdate or cobalt... 

The release rate constants for 15% polyvinyl chloride matrix tablets of 4 kp and 8kp hardness were 32.40 0.93 and 33.99+4.22 respectively. No significant differences in the release rate constants were observed (p>0.05). 

This study indicates that drug release from these formulations is by a matrix-diffusion-controlled process. The release rate constant shows an inverse relationship with the concentration of carbomer only. For all the formulations, release rate constants were independent of tablet hardness. 

Formulation 4kp hardness release rate constant (h 1/2) (meaniSD (CV)) 8 kp hardness release rate constant (h"1/2) (meaniSD (CV))... 

Since r is constant for a non-biodegradable carrier such as PMMA, the slopes a, P, etc., should reflect changes in the diffusion coefficient D, and therefore differences in porosity, between formulations. The first-order drug release rate constants can be calculated from the slopes of the a and P phases of the release profile. 

Soil Sorption-Desorption Rate Constants release rate constants kd for labile PCBs sorbed to utility substation soils are k = 1.90 d 1 from Conkelley subsurface soil, 0.4-m deep, consist of sand and silt with 0.13% OC, k = 0.88 d 1 from Tarehee surface soil consist of sand and silt with 0.02% OC and k = 0.37 d 1 from Conkelley surface soil consist of silt with 0.01% OC, rates derived from first day gas-purge experiments release rate constants kd for nonlabile PCBs sorbed to utility substation soils are k = 0.00413 d 1 from Conkelley subsurface soil, 0.4-m deep, consist of sand and silt with 0.13% OC, k = 0.00099 d 1 from Tarehee surface soil consist of sand and silt with 0.02% OC and k = 0.00052 d 1 from Conkelley surface soil consist of silt with 0.01% OC, rates derived from 120-195 d experiments (Girvin et al. 1997). 

When equilibrium is attained, the concentration of the compound in the SPMD is equal to the concentration in water multiplied by ratio of compound uptake and release rate constants. The ratio describes partition coefficient between SPMD and water, Ksw. Equilibrium for some chemicals with high fugacities may occur in less than one month. 

FIGURE 1.7 Relationship between release rate constant (Ar) and water activity of carrier matrices (a ). Symbols are the same as in Figure 1.6. (From Soottitantawat, A. et al., J. Agric. Food Chem., 52, 1269, 2004a. With permission.)... 

The release rate constants of AITC and D-limonene included in a-, P-, and y-CD can be estimated by means of Equation 1.2. Since the release rate of the flavor is very sensitive to the environmental humidity, it can be presumed that the included flavor was replaced by the water molecules. In Figure 1.25a and b, the release rate constant, k, is plotted against moisture concentration in humid air flowing through the sample bottle. The values are well correlated with the moisture concentration in the humid air, C. In both AITC and D-limonene cases,... 

FIGURE 1.25 Plot of release rate constant, k, against moisture concentration, C , in humid air flowing through the sample bottle for (a) AITC and (b) D-limonene. O a-CD, A (3-CD, y-CD. (From Shiga, H. et ah. Release characteristics of allyl isothiocyanate encapsulated in cyclodextrins, in Proceedings of the 10th International Cyclodextrin Symposium, Ann Arbor, MI, 2000, pp. 553-558.)... 

Comparison of Release Activation Energy, f r, and Release Rate Constant, k, of Various Flavors... 

The release profiles were also independently evaluated for the type of release kinetics observed i.e. zero order, SQRT of time and first order release. It appeared that for all the copolymers studied, the release was best described by first order release kinetics. The release profiles were thus fitted to the following equation to obtain the first order release rate constant, Kr ... 

Copolymer used to prepare the device Erosion rate cm/day Drug release rate due to erosion, B Co gm/cm2/day First order release rate constant, Kr. Day " ... 

Fig. 19. Correlation of first order release rate constant with erosion rate of the P(FAD-SA) device...

FIGURE 10.17 Dependence of vancomycin release rate constant from IPNs net-PP-g-PNIPAAm-inter-net-PAAc on the amount of PNIPAAm grafted to PP (percentage referred to the weight of PP substrate) and the cross-linking dose ( , 10 kGy , 40 kGy and , 70 kGy) (a) and vancomycin release profiles from the nef-PP-g-PNIPAAm-lnfer-nef-PAAc containing 80% or 125% PNIPAAm (b) and 145% PNIPAAm (c) in pH 7.4 phosphate buffer. (Reprinted from Munoz-Munoz, F. et al., Radiat. Phys. Chem., 81, 531, 2012. With permission.)... 

R is the retention (%) of the volatile, t is the releasing time (s), k is the releasing rate constant and n is a parameter representing the releasing mechanism. 

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