Arsenic impurity

Also present are 0.3 wt % mixed metal oxides (R2O2) and 5 ppm of arsenic. Impurities in fluorspar may affect yield, plant operabiUty, or product quaUty. 

Arsine is formed when any inorganic arsenic-bearing material is brought in contact with zinc and sulfuric acid. The arsenides of the electropositive metals are decomposed with the formation of arsine by water or acid. Calcium arsenide [12255-53-7] Ca2As2, treated with water gives a 14% yield of arsine. Better yields (60—90%) are obtained by decomposing a solution of sodium arsenide [12044-25-6] Na As, in Hquid ammonia with ammonium bromide (14,15). Arsine may be accidentally formed by the reaction of arsenic impurities in commercial acids stored in metal tanks, so that a test should be made for... 

The sulfiu can be piped long distances in liquid form or transported molten in ships, barges or rail cars. Alternatively it can be prilled or bandied as nuggets or chunks. Despite the vast bulk of liquid sulfur mined by the Frasch process it is obtained in very pure form. There is virtually no selenium, tellurium or arsenic impurity, and the product is usually 99.5-99.9% pure. ... 

Clearly pollution hazards exist when inorganic arsenic compounds are introduced into an environment where anaerobic bacteria are growing. Arsenic impurities in the phosphate used in detergents and for agricultural practices may pose serious problems because of the continuing synthesis of deadly poisonous methylarsenic compounds. 

Arsenic recoveries from the zinc column in the range 0.1-5pg ml-1 arsenic exceeded 97%. The concentrations at which certain elements interfere are shown in Table 12.16. Various other elements [A1 m, B m, Ca II, Cd II, Co II, Cr VI, Fe III, K I, Li I, Mg II, Mn H, Na I, Ni II, Pb II, S VI, Sn II and Zn II] showed no significant interference at the 500pg level. Only low senium concentrations in extracts can be tolerated. However, few environmental samples contain appreciable amounts of selenium. As selenium is not reduced to hydrogen selenide on the column, selenium will not interfere in the final determination step, but probably suppresses either arsenic reduction or arsine formation. Selenium appears to suppress arsine generation at high arsenic concentrations but causes a slight enhancement at low arsenic concentrations (around O.lpg), which could not be traced to arsenic impurities in the selenium standard used. 

Arsine is used as a reducing agent and to synthesize many organoarsine derivatives. It is also used as a doping agent for solid state electronic components. Earlier, it was used as a military poison gas. It does not occur freely in nature but is susceptible to form upon contact of arsenic compounds with acid in presence of a metal. Thus commercial acids stored in metal tanks and contaminated with arsenic impurities may produce arsine. 

If tungsten is recovered from the wolframite group mineral, the wolframite concentrate is boiled or pressure-digested with 50% caustic soda solution. Alternatively, they may be fused or sintered with caustic soda, caustic potash or sodium carbonate and the fused mass then leached with water. The solution is filtered to separate sodium tungstate solution. The fdtrate is subjected to various treatments to remove molybdenum, phosphorus, and arsenic impurities. The filtrate at this point is essentially a solution of sodium tungstate and is treated in the same way as that obtained from the scheehte concentrate discussed above. 

Pyrite is the most common sulfide mineral. It is a major contributor to the formation of mine drainage and sulfate-rich natural runoff. The oxidation of pyrite and other Fe(II) sulfides (e.g. marcasite and pyrrhotite) involves both iron and sulfur, as well as any arsenic impurities. Activation energies suggest that surface reactions dominate the oxidation of pyrite (Lengke and Tempel, 2005). Furthermore, evidence from pyrites in coal and ore deposits suggests that arsenian pyrite is more susceptible to oxidation from weathering than low-arsenic pyrite (Savage et al., 2000, 1239). 

As discussed earlier in Section 3.17, the excessive application of arsenic-bearing pesticides and phosphate fertilizers on agricultural lands, golf courses, and lawns may locally contaminate surface waters and ground-waters (Welch et al., 2000), (Lewis et al., 2002), 590. Phosphates desorb arsenic from mineral surfaces and readily interfere with the sorption and coprecipitation of arsenic onto iron (oxy)(hydr)oxides (Campos, 2002). Commercial phosphate fertilizers also frequently contain >13 mg kg-1 of arsenic impurities (Campos, 2002), which may further contribute to groundwater contamination. 

Arsenic impurities may be removed from synthetic gas with various sorbents, such as zinc ferrite (ZnFe204) or, under carefully controlled temperatures, mixtures of copper(II) oxide and carbon (Quinn et al., 2006 Diaz-Somoano, L6pez-Ant6n and Martfnez-Tarazona, 2004). Specifically, zinc ferrite may capture AS4 vapors through the following reaction (Diaz-Somoano, Lopez-Anton and Martfnez-Tarazona, 2004) ... 

This always holds when the semiconductor is clean, without any added impurities. Such semiconductors are called intrinsic. The balance (4.126) can be changed by adding impurities that can selectively ionize to release electrons into the conduction band or holes into the valence band. Consider, for example, an arsenic impurity (with five valence electrons) in gennanium (four valence electrons). The arsenic impurity acts as an electron donor and tends to release an electron into the system conduction band. Similarly, a gallium impurity (three valence electrons) acts as an acceptor, and tends to take an electron out of the valence band. The overall system remains neutral, however now n p and the difference is balanced by the immobile ionized impurity centers that are randomly distributed in the system. We refer to the resulting systems as doped or extrinsic semiconductors and to the added impurities as dopants. Extrinsic semiconductors with excess electrons are called n-type. In these systems the negatively charged electrons constitute the majority carrier. Semiconductors in which holes are the majority carriers are calledp-type. 

Generation of Volatile Hydrides (11). The use of commercial sodium borohydride as a reducing agent for the generation of volatile arsine (AsH3) in trace arsenic analysis is often complicated by trace (ppb) arsenic impurities in the borohydride. The following procedure using polymer-bound borohydride has eliminated these problems ... 

The characterization of arsenobetaine was performed using element analysis, H NMR, mass spectrometry, thermogravimetric/thermodifferential analysis and separative methods such as HPLC, GC and CZE. From all the results obtained it was concluded that the maximum amount of water present in the solid (if stored and handled under dry atmosphere) is 1% (w/w). In addition, arsenic impurities represent less than 0.15% (w/w) other impurities can be neglected. Therefore, the arsenobetaine purity of the calibrant obtained is >98.9%. 

Generally, the aim of doping a semiconductor is to control the electronic properties exclusively by shifting the Fermi energy. In the study of a-Si H, the question arose early as to whether the incorporation of dopants causes side effects as well. The formation of a phosphorus impurity band 0.13 eV below the conduction-band mobility edge E. has been proposed by LeComber et al (1977) from their results of Hall effect experiments. An arsenic impurity level 0.35 eV below E,. and a boron impurity level 0.42 eV above the valence-band mobility edge Ey have been inferred by Jan et al (1979,... 

Alkali Salts of Sulfurous Acid The ease with which they could be used either in the dry form or as solutions has long made the alkali sulfites and bisulfites particularly attractive in sulfiting a wide variety of products. Potassium metabisulfite has long been used in wine making for disinfection, control of fermentation, and preservation. Stability, freedom from heavy metal and arsenic impurities, and cost are the chief factors that determine selection of salts. In addition their possible effect in reducing... 

At the same time, the low melting point of copper chloride meant that the catalyst operated as a liquid in the pores of the baked clay support. The process could not be used successfully on a large scale until the sulfur and arsenic impurities in the hydrochloric acid gas were removed by scmbbing with hot sulfuric acid, which is an early example of gas purification to remove catalyst poisons. ... 

---