Sensing cadmium

Key words gas sensing, cadmium selenide (CdSe) quantum dots, photoluminescence, nanocomposites. 

Potentially more significant is the fact that a single ion is used to represent the dissolved form of the contaminant in question, an assumption that can lead to serious error. Cadmium in a model calculated at pH 12, for example, is present primarily as the species Cd(OH)2 almost no free ion Cd++ occurs. Employing the reaction Kd model in terms of Cd++ in this case would predict a contaminant distribution unlike that suggested by the distribution coefficient, applied in the traditional sense. We see the importance of applying a Kd model to systems similar to that for which it was originally determined. 

Mercury is directly below cadmium in the periodic table, but has a considerably more varied and interesting chemistry than cadmium or zinc. Elemental mercury is the only metal that is a liquid at room temperature, and its relatively high vapor pressure contributes to its toxicological hazard. Mercury metal is used in electric discharge tubes (mercury lamps), gauges, pressure-sensing devices, vacuum pumps, valves, and seals. It was formerly widely used as a cathode in the chlor-alkali process for the manufacture of NaOH and Cl2, a process that has been largely discontinued, in part because of the mercury pollution that resulted from it. 

What is a cumulative poison In what sense is cadmium a cumulative poison What might be a metabolic explanation for why a poison is cumulative ... 

These techniques have crossovers in the sense that, for example, LCVD can be done with metal-organic precursors. Cheon and Zink (1997) deposited thin films of ZnS, CdS, and Zn,Cdi S onto quartz substrates. The source of the metal and the sulfur was the organometallic compound diethyl dithiocarbamate zinc or cadmium M(S2CNEt2)2- In this case a single precursor was used to produce a binary compound as a thin film. 

Use and exposure Zinc is available as a silver or bluish-white foil or powder. It is incompatible with amines, cadmium, sulfur, chlorinated solvents, strong acids, and strong bases. The important use of zinc is to coat iron or steel in a process called galvanization to prevent rust. Zinc powder is very flammable. Zinc is another essential micronutrient that is important in immunity and antioxidation. Zinc is an essential mineral that is found in almost every cell function. It stimulates the activity of approximately 100 enzymes, which are substances that promote biochemical reactions in the body. Zinc supports a healthy immune system that the body requires for wound healing. It helps to maintain a sense of taste and smell and is needed for DNA synthesis. Zinc supports normal growth and development during pregnancy, childhood, and adolescence. ... 

Lea and Boyle (1989) showed that the barium content in foraminifera shells is, like cadmium, controlled by the barium content in bottom waters. Barium, in a broad sense, also cycles like a nutrient (depleted in surface waters, and higher in deep waters), but its regeneration occurs deeper than the organic matter. This results in a close correlation between barium and alkalinity in today s ocean (Lea, 1993). Like Cd/Ca, Ba/Ca in foraminifera has also been used, as a paleo-tracer of water masses (e.g.. Lea and Boyle (1990), but suffers the same carbonate-saturation-state-linked effect as Cd/Ca (McCorkle et al., 1995). 

There is a problem with the widespread use of arsenic, cadmium, and selenium in electronic and photovoltaic devices. Cadmium mercury telluride is used in infrared-sensing night goggles. Cadmium sulfide, cadmium selenide, gallium arsenide, and analogues, are used in solar cells. If their use becomes widespread, then an efficient system of collecting used cells for reprocessing will be needed. Some workers feel that it will be better to use nontoxic silicon cells wherever possible. (Solar cells are discussed in Chap. 15.)... 

A closely analogous state of affairs is seen in the systems NiTe2-NiTe and TiTe2-TiTe, in which the AB2 compound has the cadmium iodide structure and the AB compound that of nickel arsenide. In both of these structures the B atoms are arranged as in hexagonal close-packing, but in other respects the relationship between them is precisely the same as that between the cadmium chloride and sodium chloride structures, so that solid solution can take place by the same mechanism. Examples such as this, and many others which could be quoted, emphasize that solid solution is in no sense a satisfactory criterion for isomorphism, and that substances may form solid solution even if their structures are formally quite different. 

However, It Is well to note that Na is not an activator in the strictest sense but is a member of the lattice structure, placed there to avoid formation of a lattice vacancy. If a -vacancy is formed, it is likely that the resulting "phosphor" may be non-luminescent or will exhibit a low degree of luminescent efficiency. Although you will find that many phosphors like ZnS Ag, Al K are listed as requiring the K activator, in reality this cation is present for charge compensation in the lattice. This fact has not been recognized by many prior workers in the field. Furthermore, some zinc (cadmium) sulfides have several listed "activators", part of which are undoubtedly there for charge compensation. Elxamples Include ... 

Finally, there is the curious result that if the electrons that carry the current are in levels near the top of a band, then the field pushes the electrons in the wrong direction wrong in the sense that they are accelerated in the direction opposite to the usual one. These electrons behave as if they were positively charged. This happens with zinc and cadmium as well as a number of other metals. The effect is detected in the Hall experiment the Hall potential for these metals has the opposite sign when compared with a metal such as copper. 

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