Chemical divides

Specialty chemicals divide into fine and functional chemicals (Figure 1.37). Fine-chemicals manufacture today is undergoing a substantial change. Thus, before... 

If the fluid had been initially richer in calcium than carbonate (MCa++ > MhcOj )> 38 noted by Hardie and Eugster (1970), it would have followed a distinct reaction path. In such a case, calcite precipitation would deplete the fluid in carbonate, allowing the calcium concentration to increase until gypsum (CaS04 2H2O) saturates and forms. The point at which the calcium and carbonate are present at equal initial concentration (MCa++ = Mhco3 ) is known as a chemical divide. 

According to Hardie and Eugster s (1970) model and its later variants (see discussions in Eugster and Jones, 1979 Drever, 1988, pp. 232-250 and Jankowski and Jacobson, 1989), a natural water, as it evaporates, encounters a series of chemical divides that controls the sequence of minerals that precipitate. The reaction pathway specific to the evaporation of a water of any initial composition can be traced in detail using a reaction model like the one applied in this section to Sierra spring water. 

Halite forms in the calculation in far greater volume than any other mineral, reflecting the fact that seawater is dominantly an NaCl solution. The precipitation reaction represents a chemical divide, as discussed in Section 24.1. Since Na+ is less concentrated (on a molal basis) in seawater than Cl- (A/Na i < Mci ), it becomes depleted in solution. As a result (Fig. 24.8), seawater evolves with evaporation from a dominantly NaCl solution into an MgCl2 bittern. 

The CA concept uses the toxic unit (TU) or the toxicity equivalence factor (TEF), defined as the concentration of a chemical divided by a measure of its toxicity (e.g., EC50) to scale toxicities of different chemicals in a mixture. As a consequence, the CA concept assumes that each chemical in the mixture contributes to toxicity, even at concentrations below its no-effect concentrations. The IA or RA concept, on the other hand, follows a statistical concept of independent random events it sums the (probability of) effect caused by each chemical at its concentration in the mixture. In the case of IA, the only chemicals with concentrations above the no-effect concentration contribute to the toxicity of the mixture. The IA model requires an adequate model to describe the (full) dose-response curve, enabling a precise estimate of the effect expected at the concentration at which each individual chemical is present in the mixture. The concepts generally are used as the reference models when assessing mixture toxicity or investigating interactions of chemicals... 

To the use of powders, however finely mechanically divided, I think there are some objections particularly I doubt whether they could enter the minute vesicles of the lungs, but if such substances can be chemically divided and obtained in the state of solution in air of some congenial species, they might have their full effect.128... 

After the Ca-Mg carbonates, and again reflecting relative solubility, the next major chemical divide is created by the next most soluble phase, gypsum. At this stage, the carbonate and silica contents of the evaporating solution can be sufficient to drive the calcium and magnesium to very low concentrations, and preclude the... 

Zone dispersion (Z ) is the number of column sections containing the chemical divided by the concentration of the core containing the highest concentration... 

Figure 4. Chemical divide model for sulfate reduction of hydrocarbons, and their effect on diagenetic reactions. Numbers refer to reactions in Table II (based on 3, 25, 81).

Opium is the dried exudate of the unripe capsule of Papaver somniferum.4 Varieties of this plant have been cultivated and used for several millennia. Today, it is grown in Turkey, Iran, Afghanistan, Russia, China, Southeast Asia, and Mexico, as well as in other areas of the world, both licitly and illicitly. Crude opium is known to contain over 25 different alkaloids that can be chemically divided into two categories the phenanthrenes and the ben-zylisoquinolines (Fig. 5-7). Only morphine, codeine, papaverine, and nascopine are of clinical interest thebaine is used as a starting compound for some semisynthetic opioids.5... 

Mushrooms have inspired the cuisines of many cultures (notably Chinese, lapanese and European) for centuries and many species have been used in folk medicine for thousands of years, but approximately 70% of poisonings caused by natural substances can be attributed to mushrooms. About 5000 mushrooms species occur in nature, of which approximately 50-100 are known to be poisonous, only about ten are fatally poisonous and only 200-300 species have been clearly established to be eaten safely. However, many people are still hospitalised after eating mushrooms and several deaths are also reported annually after mushroom poisoning. Mushroom poisoning is more common in Europe than in America. The only way to be able to tell if a particular fungus is edible is to correctly identify its species. There are no short cuts. Toxic substances occurring in mushrooms can be chemically divided into ... 

The definition of the median dose is that half of the animals in the test population manifest the specific toxic endpoint (e.g., cancer, blindness, anemia, birth defect, death) at doses below the median, and the other half manifests the specific toxic effect at doses above the median. In other words, the median is the midpoint of the frequency distribution of the toxic effect of concern. It is important to note that the median dose is not the same as the mean, or average, dose. The average is calculated as the sum of all the doses of the test chemical divided by the number of dosage groups. It turns out that the mean dose coincides with the median dose only if the dose-effect histogram is a perfect bell-shaped curve. In practice, the frequency distribution is seldom perfectly bell-shaped, and the mean is rarely equal to the median. 

D is the diffusion constant of the chemical in the agar gel T is the half-life time of the chemical divided by In 2. 

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