Dissolution time constant

FIG. 32 Top Semilog plot of the time constant t for ionic motion as a function of RH for KF. Bottom Simultaneously measured contact potential. At a critical humidity A, there is a break or a change in slope in these two surface properties. Below A, water solvates preferentially cations at the step edges. Above A, the rates of dissolution (solvation) of anions and cations are similar and water uni-... 

Level B Correlation A predictive mathematical model for the relationship between summary parameters that characterize the in vitro and in vivo time courses, e.g., models that relate the mean in vitro dissolution time to the mean in vivo dissolution time, the mean in vitro dissolution time to the mean residence time in vivo, or the in vitro dissolution rate constant to the absorption rate constant. 

M = mass of solid remaining at a specific time k= cube root dissolution rate constant, which is represented as... 

In all previous dissolution models described in Sections 5.1 and 5.2, the variability of the particles (or media) is not directly taken into account. In all cases, a unique constant (cf. Sections 5.1, 5.1.1, and 5.1.2) or a certain type of time dependency in the dissolution rate constant (cf. Sections 5.1.3, 5.2.1, and 5.2.2) is determined at the commencement of the process and fixed throughout the entire course of dissolution. Thus, in essence, all these models are deterministic. However, one can also assume that the above variation in time of the rate or the rate coefficient can take place randomly due to unspecified fluctuations in the heterogeneous properties of drug particles or the structure/function of the dissolution medium. Lansky and Weiss have proposed [130] such a model assuming that the rate of dissolution k (t) is stochastic and is described by the following equation ... 

The first factor, kd, mimics the conventional dissolution rate constant it is inherent for every drug and takes values in the range 0 < kdi < 1. A value close to unity denotes a drug with rapid dissolution characteristics. Thus, a specific kdi value is conceived for a given drug under certain experimental conditions. As a probability value, kd corresponds to pd and it expresses the number of events occurring in a time unit. Consequently, kd has dimension of time-1. 

Figure 2 also demonstrates that the CTA is slowly degraded in the TFA-CH2CI2 mixture. As dissolution time increased, the viscosity peak shifted to higher concentrations. Polarized light microscopy indicated the concentration at which anisotropy occurs also increased slightly with dissolution time (Table DC) and correlates with the shift in the viscosity peaks. The concentration at which the viscosity minimum occurs remained essentially constant with time in solution as did the viscosity minimum ( 3.5 x 10 cps). 

FIGURE 5.58. Voltage versus time curve (solid line) for an n -type silicon electrode anodized with a constant current density in NH4F. The thickness of the anodic oxide was measured by ellipsometry (open circles, dashed line fitted as a guide to the eye). The OCP dissolution time of the anodic oxide in the electrolyte was measured (values above arrows) at different points of the oscillation. The bar graph visualizes the proposed oscillation mechanism. After Lehmann. (Reproduced by permission of The Electrochemical Society, Inc.)... 

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