Metal ions as impurities

Most modem therapeutic drugs are organic compounds. Such pharmaceuticals may contain metallic ions as impurities, arising from starting... 

Deionized water contains ppb levels of heavy metals, mostly zinc, copper, and lead. Which method would you suggest for determining these levels All common chemicals contain metal ions at the same concentration level or greater. How would you choose your supporting electrolyte (Suggestion Gases usually do not contain metal ions as impurities.)... 

Commercial phosphoric acid contains fluorides, chlorides, sulfates, and heavy metal ions as impurities, however, which significantly increase its corrosivity and makes its corrosion characteristics unpredictable. Chloride contamination significantly increases acid corrosion of austenitic stainless steels and requires the use of nickel-based alloys. Very good corrosion behavior is reported for the superferrite XlCrNiMoNb28-4-2, Fig. 1-44 (Thyssen Edelstahl, 1979). 

It has been indicated by several investigators that strong acids rather than weak acids, Lewis acids rather than Bronsted acids favor coke formation, and that the presence of transition metal ions as impurities, e.g., Fe and Ni ions, accelerate the formation of coke. It was reported that coke formation on a nonacidic silica was less than one-twentieth that on acidic silica —alumina. Coke deposition became less serious as the strong acid sites of silica —alumina were weakened by NaOH treatments. Coke formation from hydrocarbons is usually less serious in the case of solid bases. It is reported that the deactivation of MgO and Li/MgO for methane coupling was due to sintering and loss of alkali. ... 

Corundum, when pure, is also known as the gemstone white sapphire. Certain other gemstones consist of corundum with small amounts of transition metal ions as impurities Cr in ruby and Fe and Ti in blue sapphire, for example. Artificial gemstones are made by fusing corundum with carefully controlled amounts of other oxides. 

The HP is converted to insoluble Na2Sip6, and the gypsum is filtered off along with other insoluble impurities. The phosphoric acid is concentrated by evaporation. Phosphoric acid obtained by this "wet process" contains a variety of metal ions as impurities and is dark green or brown. Nevertheless, it is satisfactory for the manufacture of fertilizers and for metallurgical operations. 

The material is impact-sensitive when dry and is supplied and stored damp with ethanol. It is used as a saturated solution and it is important to prevent total evaporation, or the slow growth of large crystals which may become dried and shock-sensitive. Lead drains must not be used, to avoid formation of the detonator, lead azide. Exposure to acid conditions may generate explosive hydrazoic acid [1], It has been stated that barium azide is relatively insensitive to impact but highly sensitive to friction [2], Strontium, and particularly calcium azides show much more marked explosive properties than barium azide. The explosive properties appear to be closely associated with the method of formation of the azide [3], Factors which affect the sensitivity of the azide include surface area, solvent used and ageing. Presence of barium metal, sodium or iron ions as impurities increases the sensitivity [4], Though not an endothermic compound (AH°f —22.17 kJ/mol, 0.1 kj/g), it may thermally decompose to barium nitride, rather than to the elements, when a considerable exotherm is produced (98.74 kJ/mol, 0.45 kJ/g of azide) [5]. 

Our results clearly show that modification of the electronic state of titanium oxide by metal ion implantation is closely associated with the strong and longdistance interaction which arises between the titanium oxide and the metal ions implanted, as shown in Fig. 13, and not by the formation of impurity energy levels within the band gap of the titanium oxides resulting from the formation of impurity oxide clusters which are often observed in the chemical doping of metal ions, as shown in Figs. 6 and 13. 

Polarographic reductions have been studied for many kinds of metal ions and in a variety of lion-aqueous solvents. Large amounts of data on half-wave potentials are available and have been compiled in some books and reviews [18]. However, many of the old data were obtained using different reference electrodes or aqueous SCE, for which the problem of the liquid junction potential exists (Section 6.1.2). Gritzner [19] compiled half-wave potentials for metal ions as values referred to the BCr+/BCr system, which was recommended by IUPAC. Some of these are listed in Table 8.2. The potential of the BCr+/BCr system is not seriously affected either by the presence of water and other impurities or by differences in experimental conditions. Thus, although the determination of the half-wave poten-... 

When the support contains cations that are not easily reducible (i.e. those of Al, Si, Mg, Zr), hydrogen spillover occurs above 573 K without observable chemical reaction (Class A). However if it contains ferric ions as impurities, as is often the case with alumina, reduction to ferrous ion is detectable by EPR and if it contains sulfate ion, as may be the case with titania, reduction of the precursor with hydrogen automatically generates hydrogen sulfide which poisons the metal (Class B). If deuterium is used in place of hydrogen, support hydroxyls... 

Other impurities can enter the slurry by being leached from the refractories themselves. Calcium and Magnesium are impurities that can have very deleterious effects on the stability of colloidal silica. Some refractories like Zircon are acidic in their reaction with colloidal silica and tend to reduce the pH over time. The stability of alkali stabilized colloidal silicas depend both on minimizing the presence of impurities (especially polyvalent metal ions) as... 

Effect of Purityt Trace Elements. There are cases where aluminum has been established as suitable for use with a specihc product, but the metal has corroded because of contamination of that product with trace amounts of heavy metal ions. These impurities can have virtually no effect on the product but can cause... 

Viable glass fibers for optical communication are made from glass of an extremely high purity as well as a precise refractive index stmcture. The first fibers produced for this purpose in the 1960s attempted to improve on the quahty of traditional optical glasses, which at that time exhibited losses on the order of 1000 dB/km. To achieve optical transmission over sufficient distance to be competitive with existing systems, the optical losses had to be reduced to below 20 dB/km. It was realized that impurities such as transition-metal ion contamination in this glass must be reduced to unprecedented levels (see Fig. 

Solid-State Lasers. Sohd-state lasers (37) use glassy or crystalline host materials containing some active species. The term soHd-state as used in connection with lasers does not imply semiconductors rather it appHes to soHd materials containing impurity ions. The impurity ions are typically ions of the transition metals, such as chromium, or ions of the rare-earth series, such as neodymium (see Lanthanides). Most often, the soHd material is in the form of a cylindrical rod with the ends poHshed flat and parallel, but a variety of other forms have been used, including slabs and cylindrical rods with the ends cut at Brewster s angle. 

In natural water, the half-hves fall between these extremes. For example, the half-life of Lake Zbrich water (pH 8, 1.5 meq/LHCO ) is 10 min (27). The decomposition in natural water also can be initiated by trace metal ions, eg, Fe , promoted by impurities such as organic matter, and inhibited by HO radical scavengers, eg, HCO3, COg , HPO (25,28). 

Calcium Pyrophosphates. As is typical of the pyrophosphate salts of multiple-charged or heavy-metal ions, the calcium pyrophosphates are extremely insoluble ia water. Calcium pyrophosphate exists ia three polymorphic modifications, each of which is metastable at room temperature. These are formed progressively upon thermal dehydration of calcium hydrogen phosphate dihydrate as shown below. Conversion temperatures indicated are those obtained from thermal analyses (22,23). The presence of impurities and actual processing conditions can change these values considerably, as is tme of commercial manufacture. 

Most of the heavy-metal impurities present in 2inc salt solutions must be removed before the precipitation reaction, or these form insoluble colored sulfides that reduce the whiteness of the 2inc sulfide pigment. This end is usually achieved by the addition of 2inc metal which reduces most heavy-metal ions to their metallic form. The brightness of 2inc sulfide can be improved by the addition of a small amount of cobalt salts (ca 0.04% on a Co/Zn basis) (20). Barium sulfate [7727-43-7] formed in the first step is isolated and can be used as an extender. 

The ultraviolet cutoff or the absorption edge for pure vitreous siUca is 8.1 eV or 153 nm (171). This uv cutoff is influenced by the impurity level and stoichiometry of the material. Several impurities, such as the transition metals (Fe, Cu, Ti, etc) and alkaU metal ions (Na, Li, K), degrade the ultraviolet performance, shifting the uv cutoff to longer wavelengths. Ferric ions (Fe " ) cause absorption or result in network defects under reducing conditions. This contaminant at only a few ppm can be detected as an absorption at 230 nm and below (176). 

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