Half-life The length of time it takes for

The maximum possible amount of digoxin present in the body at 12 hours, if it were completely absorbed and not lost at all, would be (0.5 -I- 0.5) = 1 mg. The plasma half-life (the length of time it takes for the plasma concentration to fall by 50%) of digoxin in normal individuals is around 36 hours, so at 12 hours after the first dose we would anticipate rather less than 1 mg to be retained - probably about 0.6 mg if digoxin in this case is 70% absorbed. So a very crude estimate of Vd would be... 

Half-life—The length of time it takes for the body to metabolize or eliminate half of a drug taken. 

Rotation around carbon-carbon bonds is one of the fastest processes m chemistry Among the ways that we can describe the rate of a process is by its half life which is the length of time it takes for one half of the molecules to react It takes less than 10 s for half of the molecules m a sample of ethane to go from one staggered conformation to another at 25°C... 

All radioactive decays occur with first-order kinetics, with the exception of electron capture, which is a two-particle collision. The differential rate law for radioactive decay is given by Equation (2.7). After integration, an alternative and more useful form of the rate law is shown by Equation (2.8). The half-life of radioactive decay is defined as the length of time it takes for the number of unstable nuclides to decrease to exactly one-half of their original value. The half-life, t can be calculated using Equation (2.9), where k is the first-order rate constant... 

The experimental rate law can be determined by monitoring the concentration of one of the reactants or products as a function of time using spectroscopic means. For instance, the Beer-Lambert law states that the absorbance of a colored compound is directly proportional to its concentration (for optically dilute solutions anyway), so that the absorbance can be measured as the course of the reaction proceeds. The data are then fit to a model, such as the function that results when integrating one of the differential rate law equations. The integrated rate laws for some commonly occurring kinetics are listed in Table 17.1. Half-life equations are also included for some of the reactions in this table, where the half-life ftyi) is defined as the length of time that it takes for half of the initial reactant concentration to disappear. 

The half-life of a radioactive nuclide is the length of time that it takes for half of a given number of atoms of the nuclide to decay. 

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