Half-live

All elements of atomic number greater than 83 exhibit radioactive decay K, Rb, Ir and a few other light elements emit p particles. The heavy elements decay through various isotopes until a stable nucleus is reached. Known half-lives range from seconds to 10 years. 

The question is now Which reaction pathways arc Followed, and to what extent This asks for a detailed modeling of the kinetics of the individual reaction steps of this network. This can be achieved on the basis of the half-lives of four s-triazinc herbicides in soil [17]. Figure 10.3-13 shows the four compounds For which data were Found in the literature. 

The half-lives for these four compounds taken from the literature allowed the estimation of the Four reaction rates necessai to model their degradation [18], As a first approximation, the rate of hydrolysis of the C-Cl bond of all Four, -triazine compounds was assumed to be the same and to be 5.0 x 10 s on the basis of literature precedence. This approximation seems reasonable as the four structures differ only in the alkyl groups at a site quite remote from the C-CI bond. Furthermore, among the Four reaction steps hydrolysis is the slowest anyway. 

Twenty one isotopes of samarium exist. Natural samarium is a mixture of several isotopes, three of which are unstable with long half-lives. 

Olah and Overchuk also attempted to discover evidence of slow mixing by carrying out reactions in high-speed flow systems. Evidence, including the isolation of dinitro compounds (> i %), was indeed found, but held to show that the effect of imperfect mixing was only minor. The reactions were, unfortunately, too fast to permit determinations of absolute rates (half-lives of about io s). 

Since the half-life is independent of the number of radioactive atoms, it remains constant throughout the decay process. Thus, 50% of the radioactive atoms disintegrate in one half-life, 75% in two half-lives, and 87.5% in three half-lives. 

Table 13.1 provides a list of several isotopes commonly used as tracers. The half-lives for these isotopes also are listed. What is the rate constant for the radioactive decay of each isotope ... 

Thus, the ratios of lead isotopes 204,206,207 and 208 can vary markedly depending on the source of the lead. One use of these ratios lies in determination of the ages of rocks from the abundances of the various isotopes and the half-lives of their precursor radioactive isotopes. 

Many artificially made isotopes are known, and most have very short half-lives. For example, Au has a half-life of 53 sec. 

Halex reaction Halfan [36167-63-2] Half-life data Half-lives Halftones Halide glasses Halide ions Halides... 

Half-Lives and Decay Constants. Each nuclear state, whether an unstable ground state or an excited level, has a characteristic probabiUty... 

Therefore, in these cases it is only necessary that the gaseous members be retained for periods of several of their half-lives for the measured ages to be unaffected by loss of the gases. In the chain, the Rn has a half-life of 3.8 d, so that diffusion out of the region of the may occur, but in the Th chain the Rn has a half-life of only 55 s, so that diffusion would be less likely. 

Procainamide may be adininistered by iv, intramuscular (im), or po routes. After po dosing, 75—90% of the dmg is absorbed from the GI tract. About 25% of the amount absorbed undergoes first-pass metaboHsm in the fiver. The primary metabolite is A/-acetylprocainamide (NAPA) which has almost the same antiarrhythmic activity as procainamide. This is significant because the plasma concentration of NAPA relative to that of procainamide is 0.5—2.5. In terms of dmg metabolism there are two groups of patients those that rapidly acetylate and those that slowly acetylate procainamide. About 15—20% of the dmg is bound to plasma proteins. Peak plasma concentrations are achieved in 60—90 min. Therapeutic plasma concentrations are 4—10 lg/mL. Plasma half-lives of procainamide and NAPA, which are excreted mainly by the kidneys, are 2.5—4.5 and 6 h, respectively. About 50—60% is excreted as unchanged procainamide (1,2). 

In terms of cost and versatility, the stirred batch reactor is the unit of choice for homogeneous or slurry reactions and even gas/liquid reactions when provision is made for recirculation of the gas. They are especially suited to reactions with half-lives in excess of 10 min. Sam-... 

Cosmogenic radionuclides are formed in the upper atmosphere by the interaction of cosmic rays, primarily from the sun, with elements present in the atmosphere (e.g. 0, and " Ar). Their half-lives range from months to... 

The abundance of a trace element is often too small to be accurately quantihed using conventional analytical methods such as ion chromatography or mass spectrometry. It is possible, however, to precisely determine very low concentrations of a constituent by measuring its radioactive decay properties. In order to understand how U-Th series radionuclides can provide such low-level tracer information, a brief review of the basic principles of radioactive decay and the application of these radionuclides as geochronological tools is useful. " The U-Th decay series together consist of 36 radionuclides that are isotopes (same atomic number, Z, different atomic mass, M) of 10 distinct elements (Figure 1). Some of these are very short-lived (tj j 1 -nd are thus not directly useful as marine tracers. It is the other radioisotopes with half-lives greater than 1 day that are most useful and are the focus of this chapter. 

Figure 2 Exponential decay of a hypothetical radionuclide (N) to a stable daughter (D) as measured in half-lives (ti/,)-Note that as t approaches CO, N approaches 0 and D approaches Nq (Adapted from Faure" )...

The two remaining thorium isotopes have much shorter half-lives. Thorium-228 has a half life of 1.91 years and is produced from Ra, with as a very... 

PROTECTION FOR THE HYDROXYL GROUP, INCLUDING 1.2- AND 1,3-DIOLS Half-lives for cleavage of 5 -protected thymidine in 80% AcOH at 15 ... 

This group was introduced and removed using the same conditions as for the MTMB group. The half-lives for ammonolysis of acetate, MTMB, and MTMT are 5 min, 15 min, and 6 h, respectively. ... 

The Fmoc group is cleaved under mild conditions with an amine base to afford the free amine and dibenzofulvene. The approximate half-lives for the. deprotection of Fmoc-ValOH by a variety of amine bases in DMF are as follows ... 

The half-lives shown in the table ". ill vary depending on the structure of the Fmoc-amine derivative. 

If the decomposition reaction follows the general rate law, the activation energy, heat of decomposition, rate constant and half-life for any given temperature can be obtained on a few milligrams using the ASTM method. Hazard indicators include heats of decomposition in excess of 0.3 kcal/g, short half-lives, low activation energies and low exotherm onset temperatures, especially if heat of decomposition is considerable. 

Multiple pathways are a major concern since depostion of PIC would have occurred. Specific soil conditions determine attenuation rates of penta PIC leachate. Once penta reaches the water table, other transport and fate processes become important. Penta exists in two forms ionized and non-ionized. The ionized form is soluble in water, while the non-ionized form is not. The ratio of the two forms in water is dependent on the pH of the aquifer. In alkaline environments penta PIC tend to be more soluble and more susceptible to advective transport and biological decay. Half-lives of penta leachate in groundwater have been estimated ranging from 27 days to 58 years. 

Creosote is a complex mixture of toxic chemicals, which can have both immediate and chronic effects on exposed organisms. PIC of creosote are of particular concern due to long half-lives of some chemicals, and because of multiple pathways to the environment from ash and soot. 

Equation 3-39 shows that in the first order reaetions, the half-life is independent of the eoneentration of the reaetant. This basis ean be used to test whether a reaetion obeys first order kineties by measuring half-lives of the reaetion at various initial eoneentrations of the reaetant. 

Plots of in tj/2 versus in from a series of half-life experiments are shown in Figure 3-9. Table 3-3 gives some expressions for reaetion half-lives. 

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