Precipitation element

With tellurium, precipitation of the element with sulphur dioxide is slow in dilute hydrochloric acid solution and does not take place at all in the presence of excess of acid moreover, the precipitated element is so finely divided that it oxidises readily in the subsequent washing process. Satisfactory results are obtained by the use of a mixture of sulphur dioxide and hydrazinium chloride... 

Losi M.E., Frankenberger W.T. Microbial oxidation and solubilization of precipitated elemental selenium in soil. J Environ Qual 1998 27 836-843. 

As with selenium analysis, HGAAS also finds a place in Te determination. Basnayake et al. amended cultures of P. fluorescens K27 with 0.1 mM tellurite and, after 92-h growth, determined that approximately 34% of added Te was present as either precipitated, elemental Te in/or on centrifuged cells the balance of added Te remained in solution.190 In this same study, GC/MS was used for determination of DMTe in the same facultative anaerobe amended with tellurate.190 Earlier, GC/MS was used to analyze the headspace of a tellurium-resistant fungus amended with tellurite.215 This last is one of the few reports of the detection of dimethyl ditelluride in microbial headspace (see below). 

Alternatively, raw anode slimes are aerated with hot dilute sulfuric acid to remove copper. Slimes are then mixed thoroughly with sodium carbonate and roasted in the presence of sufficient air. Sodium selenate formed is leached with water. Hydrochloric acid is added to this selenate solution. Treatment with sulfur dioxide precipitates elemental selenium. Alternatively, the selenate solution is evaporated to dryness. Sodium selenate is reduced to sodium selenide by heating with carbon at high temperatures. Sodium selenide is leached with water. Air is blown over the solution. Selenide is oxidized to elemental selenium which precipitates. 

H2S was measured by slowly adding 25.00 mL of aqueous H2S to 25.00 mL of acidified standard 0.010 44 M I3 to precipitate elemental sulfur. (If [H2S1 > 0.01 M, then precipitated sulfur traps some I3 solution, which is not subsequently titrated.) The remaining If was titrated with 14.44 mL of 0.009 336 M Na2S203. Find the molarity of the H2S solution. Should starch indicator be added to this titration at the beginning or just before the end point ... 

The bisulfite leaving the scrubber is then reduced with gaseous hydrogen sulfide, which precipitates elemental sulfur by a modified Qaus reaction ... 

Many fungi can precipitate reduced forms of metals and metalloids (e.g. elemental silver, selenium, tellurium) within and around fungal cells (Gadd, 2004). The reductive ability of fungi is manifest by the appearance of black coloration of fungal colonies precipitating elemental silver or tellurium, and a red coloration for precipitation of elemental selenium (Gharieb et al., 1999). 

The efficiency of wet removal of gases and particles is due to the fact that the falling speed of precipitation elements greatly exceeds the dry deposition velocity of trace constituents. In discussion of removal caused by clouds and precipitation it is reasonable to differentiate processes taking place in the clouds (rain-out) and beneath the cloud base (wash-out). 

The other obvious possibility is that materials absorbed are carried by precipitation to the surface of the Earth, that is they are definitively removed from the air. There is no intention here to discuss the formation of precipitation. We only mention that it is believed (Fletcher, 1962) that, in winter layer clouds with small liquid water content, ice crystals play an important role in precipitation formation, while in summer convective clouds the coalescence of large drops with smaller ones is the dominant process. At the same time we have to emphasize that the wet removal of trace constituents is continued by falling precipitation elements (snow crystals, raindrops) below the cloud base. This removal mechanism is called washout. 

Aerosol particles below the cloud base are captured by precipitation elements due to gravitational coagulation. This type of coagulation is caused by the difference between falling speeds of the aerosol particles and the raindrops or snow crystals. In other words, this means that precipitation elements overtake the particles. The air molecules go around the falling drops (or crystals) while large particles are impacted against the drops due to their inertia. For this reason precipitation elements are considered to be small impactors (see Subsection 4.1.2). 

The composition of cloud and precipitation water was investigated by Petrenchuk and Drozdova (1966), among others they developed a special cloud water collector that worked at positive as well as at negative temperatures. Their results, obtained over the European parts of the U.S.S.R., are given in Table 28. It can be seen that over clean northern regions the difference between the sum of ions in cloud and precipitation elements is not great. In these areas the concentration of sulfate and nitrate ions is relatively small while the chloride content is great. This situation can be explained by maritime influences. In comparison, sulfate is the... 

On the basis of our foregoing discussion, the pattern represented by Fig. 44 may be explained as follows. Let us consider a cloud with precipitation which moves over our sampling site. At the beginning, precipitation elements fall from the frontal part of the cloud. In this part, near the edge of the cloud, the liquid water content is low and consequently the water is more concentrated. Since there is a direct relation between liquid water content in the cloud and precipitation intensity at the surface the rainfall rate is also low at this time. The light rain falls in an unwashed polluted atmosphere which is frequently unsaturated with water vapour. Both circumstances... 

It follows implicitly from this discussion that smaller precipitation elements are generally more concentrated than larger ones since rain of low intensity is composed of smaller drops (Best, 1950). This relation was experimentally proved by Georgii and Wotzel (1967) who constructed a special rainwater collector that classified rain drops according to their size. In this way they demonstrated that the concentration increased with decreasing drop size. 

The presence of carbide and nitride precipitates in alloy steels can have a beneficial effect on the mechanical properties of the steels concerned. How-ever, the amounts, morphology and distribution of the precipitated phases must be carefully controlled in order to achieve the properties required. Because the presence of hard precipitates in a steel during hot-rolling operations can result in damage both to the rollers and to the steel, it is important that information be available on the ranges of temperature and composition in which precipitated phases are stable. For this reason, and also to achieve desired precipitation characteristics using the minimum amounts of expensive precipitating elements such as niobium, titanium, vanadium, etc., it is helpful to cany out prior calculations of the stability of precipitates in steels of different compositions. 

The condensation of water vapor and its precipitation from the atmosphere in the form of rain, snow, sleet, or hail are important not only for the water cycle, but also because they bring to the earth surface other atmospheric constituents, primarily those substances that have a pronounced affinity toward water in the condensed state. Cloud and precipitation elements may incorporate both aerosol particles and gases. The uptake mechanisms are discussed in this chapter, together with the inorganic composition of cloud and rain water that they determine. These processes are, in principle, well understood. Another subject requiring discussion is the occurrence of chemical reactions in the liquid phase of clouds. The oxidation of S02 dissolved in cloud water is considered especially important. As a result of laboratory studies, the conversion of S02 to sulfate is now known to proceed by several reaction pathways in aqueous solution. 

Falling precipitation elements incorporate particles by collisional capture. For water drops, the usual assumption that particles and drops merge upon contact has been experimentally confirmed by Weber (1969). Consider then a single rain drop of radius r, moving with velocity v. It would sweep out a cylinder with cross section irr and collect the particles contained therein,... 

Although preliminary separations may be required, in other instances the precipitation step in gravimetric analysis is sufficiently selective that other separations are not required. The pH is important because it often influences both the solubility of the analytical precipitate and the possibility of interferences from other substances. For example, calcium oxalate is insoluble in basic medium, but at low pH the oxalate ion combines with the hydrogen ions to form a weak acid. 8-Hydroxy-quinoline (oxine) can be used to precipitate a large number of elements, but by controlling pH, we can precipitate elements selectively. Aluminum ion can be precipitated at pH 4, but the concentration of the anion form of oxine is too low at this pH to precipitate magnesium ion. 

The second type of cell is a mercury pool type. A mercury cathode is particularly useful for separating easily reduced elements as a preliminary step in an analysis. l or example, copper, nickel, cobalt, silver, and cadmium are readily separated from ions such as aluminum, titanium, the alkali metals, and phosphates. The precipitated elements dissolve in the mercury little hydrogen evolution occurs even at high applied potentials because of large overvoltage effects. A coulomet-ric cell such as that shown in Figure 24-5b is also useful for coulometric determination of metal ions and certain types of organic compounds as well. 

A number of the observed minerals (formulae given in Table 16.4) do not form in the simulation. Wurtzite is metastable with respect to sphalerite, so it cannot be expected to appear in the calculation results. Similarly, the formation of pyrite in the simulation probably precludes the possibility of pyrrhotite precipitating. In the laboratory, and presumably in nature, pyrite forms slowly, allowing less stable iron sulfides to precipitate. Elemental sulfur at the site probably results from incomplete oxidation of H2S(aq), a process not accounted... 

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