Zinc mark

In animal studies, decreased zinc status also contributes to lead and cadmium toxicity. In studies with rats, Cerklewski and Forbes (1976) demonstrated that an increase of zinc in the diet decreased the tissue lead levels and reduced other indicators of lead toxicity. Cerklewski (1979) also demonstrated that high levels of zinc fed to pregnant rats resulted in significantly lower levels of lead in the blood and liver of the rat pups. Using Japanese quail, Jacobs et al. (1977) reported that supplemental zinc markedly decreased concentrations of cadmium in the liver, kidney and small intestine, while Fox et al. (1979) showed that marginally adequate levels of dietary zinc markedly increased retention of cadmium in the duodenum, jejunum, ileum and liver as compared with zinc-supplemented birds. The association between increased lead burdens and lower serum zinc levels in children was reported by Markowitz and Rosen (1981). However, the mean levels of serum zinc in the children with elevated blood lead levels were not considered to be outside the lower limits of normal for plasma cited by Hambidge (1977). 

These elements formed Group IIB of Mendeleef s original periodic table. As we have seen in Chapter 13, zinc does not show very marked transition-metaf characteristics. The other two elements in this group, cadmium and mercury, lie at the ends of the second and third transition series (Y-Cd, La-Hg) and, although they resemble zinc in some respects in showing a predominantly - - 2 oxidation state, they also show rather more transition-metal characteristics. Additionally, mercury has characteristics, some of which relate it quite closely to its immediate predecessors in the third transition series, platinum and gold, and some of which are decidedly peculiar to mercury. 

The copper-zinc system (which includes brasses) has one eutectoid reaction. Mark the eutectoid point on the phase diagram (Fig. A 1.38). 

Rubin and Blossey standardized the experimental conditions for the Serini reaction, using freshly activated zinc in refluxing xylene for 20-24 hr with vigorous stirring yields of68-100 % could then be obtained. They noted, however, that the presence of a 16a- or -methyl group markedly decreased the yield, and that a primary-secondary glycol monoacetate failed to react. 

With special techniques for the activation of the metal—e.g. for removal of the oxide layer, and the preparation of finely dispersed metal—the scope of the Refor-matsky reaction has been broadened, and yields have been markedly improved." The attempted activation of zinc by treatment with iodine or dibromomethane, or washing with dilute hydrochloric acid prior to use, often is only moderately successful. Much more effective is the use of special alloys—e.g. zinc-copper couple, or the reduction of zinc halides using potassium (the so-called Rieke procedure ) or potassium graphite. The application of ultrasound has also been reported. ... 

Handels-verein, m. commercial association, commercial union, -ware, /. commercial article, article of commerce, merchandise, -wert, m. commercial value, -zeichen, n. trade-mark. -zink, n. commercial zinc. 

The rate (or kinetics) and form of a corrosion reaction will be affected by a variety of factors associated with the metal and the metal surface (which can range from a planar outer surface to the surface within pits or fine cracks), and the environment. Thus heterogeneities in a metal (see Section 1.3) may have a marked effect on the kinetics of a reaction without affecting the thermodynamics of the system there is no reason to believe that a perfect single crystal of pure zinc completely free from lattic defects (a hypothetical concept) would not corrode when immersed in hydrochloric acid, but it would probably corrode at a significantly slower rate than polycrystalline pure zinc, although there is no thermodynamic difference between these two forms of zinc. Furthermore, although heavy metal impurities in zinc will affect the rate of reaction they cannot alter the final position of equilibrium. 

Alloys containing only a few per cent of zinc may fail if the stresses are high and the environment sufficiently corrosive. Most types of brass, besides the plain copper/zinc alloys, appear to be susceptible to stress corrosion. An extensive investigation of the effect of additions to 70/30 brass was carried out by Wilson, Edmunds, Anderson and Peirce , who found that about 1% Si was markedly beneficial. Other additions were beneficial under some circumstances and none of the 36 additions tested accelerated stress-corrosion cracking. Further results are given in later papers ... 

The effect of temperature on the corrosion of zinc in water It is found that the temperature has a marked effect on the rate at which zinc corrodes in water. The corrosion rate in distilled water reaches a maximum in the temperature range 65-75°C. This variation in the corrosion rate with temperature is attributed to changes in the nature of the protective film. At lower temperatures the film is found to be very adherent and gelatinous, while at temperatures around 70°C it becomes distinctly granular in character and much less adherent. Above 75°C it again tends to become more adherent and assumes a very compact and dense form. It is believed that the granular coating formed at temperatures around 70°C is more porous... 

Reagents. Standard zinc solution. Dissolve about 4.0 g, accurately weighed zinc shot in 35 mL concentrated hydrochloric acid, and dilute with distilled water to 1 L in a graduated flask. Pipette 10.0 mL of this solution into a 1 litre graduated flask and dilute to the mark with distilled water. 

The widespread use of many metals such as silver, cadmium, copper, mercury, nickel, lead, and zinc has resulted in their accumulation in the environment. Sediments are often the repositories of toxic metals (e.g.. Table 15-2). For example, copper is used as an anti-biofouling agent in marine paints and many harbor sediments contain markedly elevated levels of copper. 

The zinc polycarboxylate cement sets within a few minutes of mixing and hardens rapidly. Strength is substantially developed within an hour. However, even when fully hardened the cement exhibits marked plastic behaviour. Its most important property is its ability to bond permanently to untreated dentine and enamel. 

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