Oxides precipitation

In neutral solution, the indicator is potassium chromate(VI). In acid solution the CrOj" ion changes to CrjO (p. 378). and since silver dichromatefVI) is soluble, chromate(VI) is not a suitable indicator other methods can be used under these conditions. (In alkaline solution, silverfl) oxide precipitates, so silver(I) nitrate cannot be used under these conditions.)... 

The brine clean-up consists of skimming and settling steps to free the solution from oil, clays, and other impurities. Sulfuric acid is then added until a pH of <2.5 is reached ensuring iodine Hberation by oxidation, precipitation of the soluble barium contained in the brine, and recovery of the remaining iodine. 

When equal amounts of solutions of poly(ethylene oxide) and poly(acryhc acid) ate mixed, a precipitate, which appears to be an association product of the two polymers, forms immediately. This association reaction is influenced by hydrogen-ion concentration. Below ca pH 4, the complex precipitates from solution. Above ca pH 12, precipitation also occurs, but probably only poly(ethylene oxide) precipitates. If solution viscosity is used as an indication of the degree of association, it appears that association becomes mote pronounced as the pH is reduced toward a lower limit of about four. The highest yield of insoluble complex usually occurs at an equimolar ratio of ether and carboxyl groups. Studies of the poly(ethylene oxide)—poly(methacryhc acid) complexes indicate a stoichiometric ratio of three monomeric units of ethylene oxide for each methacrylic acid unit. 

Nickel plating solutions may contain excess iron and unknown organic contaminants. Iron is removed by peroxide oxidation, precipitation at a pH of about 5, then filtered out. The more complex, less water-soluble organic contaminants along with some trace metals are removed with activated carbon treatments in separate treatment tanks. About 5 g/L of plating-grade activated carbon is mixed in the plating solution for at least 1—2 hours, usually at warmer temperatures. 

Reaction.—. dA Fehling s solution to benzoin dissolved in alcohol. Benzil is formed and cuprous oxide precipitated. Benzil is also formed on oxidation with nitric acid. 

The acetic acid mother liquor, containing the rest of the reaction product, was concentrated in vacuo. The residue was dissolved in methylene chloride and washed with ice cold sodium carbonate solution. The organic solution was dried, concentrated in vacuo to a small volume and diluted with ether and petroleum ether. Fine yellow needles of 2-chloro-methyl-4-phenyl-6-chloroquinazoline 3-oxide precipitated. The pure base was recrystallized from a mixture of methylene chloride, ether and petroleum ether, MP 133° to 134°C. 

Depending on electrolyte composition, the metal will either dissolve in the anodic reaction, that is, form solution ions [reaction (1.24)], or will form insoluble or poorly soluble salts or oxides precipitating as a new solid phase next to the electrode surface [reaction (1.28)]. Reacting metal electrodes forming soluble products are also known as electrodes of the first kind, and those forming solid products are known as electrodes of the second kind. 

Strong adsorption on Fe and Mn oxides and hydrous oxides Precipitation... 

Chrome green is produced by mechanically mixing chrome yellow and iron blue pigments in water. An iron blue, [Fe(NH4) (FeCN6)], is an oxidized precipitate product of the reaction between an aqueous solution of iron sulfate and ammonium sulfate with sodium hexacyanoferrate. 

Compared to the importance of Fe and Mn oxides as metal adsorbing surfaces, the primary role of layer silicates is as a substrate on which Fe and Mn oxides precipitate and coat. This is especially true in arid... 

An appropriate amount of hydrated iron (III) or bismuth oxide was added the oxide precipitates were prepared separately and washed thoroughly with distilled water before use [43]. After about 24 h, the samples were filtered on 0.4 jtm Nuclepore filters. The separated precipitates were dissolved with hydrochloric acid and the solutions obtained were used for /-activity measurements. In the examination of solvent extraction, chromium was measured by using 51Cr, while iron and bismuth were measured by electrothermal AAS (EAAS). The decomposition of organic complexes and other procedures were also examined by EAAS. 

Fe(III)(hydr)oxides introduced into the lake and formed within the lake - Strong affinity (surface complex formation) for heavy metals, phosphates, silicates and oxyanions of As, Se Fe(III) oxides even if present in small proportions can exert significant removal of trace elements. - At the oxic-anoxic boundary of a lake (see Chapter 9.6) Fe(III) oxides may represent a large part of settling particles. Internal cycling of Fe by reductive dissolution and by oxidation-precipitation is coupled to the cycling of metal ions as discussed in Chapter 9. 

Aluminum is produced according to the Hall-Heroult process [42-44]. At the cathode, AlxFy species are reduced and lead to liquid aluminum. As the electrolysis proceeds, the metal from the aluminum oxide precipitates at the bottom of the cell. At the anode, oxygen evolution takes place producing carbon dioxide/monoxide and hence resulting in current and performance losses [42-44]. 

Figure 15. Illustration of possible variations in isotopic fractionation between Fe(III),q and ferric oxide/ hydroxide precipitate (Aje(,n),q.Fenicppt) and precipitation rate. Skulan et al. (2002) noted that the kinetic AF (ni)aq-Feiricppt fractionation produced during precipitation of hematite from Fe(III), was linearly related to precipitation rate, which is shown in the dashed curve (precipitation rate plotted on log scale). The most rapid precipitation rate measured by Skulan et al. (2002) is shown in the black circle. The equilibrium Fe(III),-hematite fractionation is near zero at 98°C, and this is plotted (black square) to the left of the break in scale for precipitation rate. Also shown for comparison is the calculated Fe(III),q-ferrihydrite fractionation from the experiments of Bullen et al. (2001) (grey diamond), as discussed in the previous chapter (Chapter lOA Beard and Johnson 2004). The average oxidation-precipitation rates for the APIO experiments of Croal et al. (2004) are also noted, where the overall process is limited by the rate constant ki. As discussed in the text, if the proportion of Fe(III),q is small relative to total aqueous Fe, the rate constant for the precipitation of ferrihydrite from Fe(III), (Ai) will be higher, assuming first-order rate laws, although its value is unknown.

Although the majority of attention in discussions on the origins of BIFs has been on the oxide facies, siderite facies rocks are equally important in many BIF sequences. Reaction of Fe(II)aq and dissolved carbonate with hematite to form siderite and magnetite has been hypothesized to be an important diagenetic process in marine basins during formation of some BIFs if sulfate contents were low (e.g., Klein and Beukes 1989 Beukes et al. 1990 Kaufman 1996 Sumner 1997). In Figure 18 we assume that Fe(II)aq was derived either from MOR sources or DIR, or a combination of the two, which reacted with ferric oxide precipitates to form magnetite or dissolved carbonate to produce siderite. 

Figure 10,20 Major element concentrations in effusive products of Boina series plotted as functions of degree of fractional crystallization, based on equation 10.132. I = first discontinuity (transition from olivine-dominated to plagioclase-dominated fractionation) II = second discontinuity (appearance of Fe-Ti oxides) III = third discontinuity (field of Si02 oversaturated trachytes apatite and Mn-oxides precipitate) IV = beginning of peralkalinity field. From Barberi et al. (1975), Journal of Petrology, 16, 22-56. Reproduced with modifications by permission of Oxford University Press.

If the analyte is subject to change (bacterial, air oxidation, precipitation, adsorption on container, etc.) provisions should be made for its preservation. 

The reaction produces hydroxyl ions which react directly with the Fe ions to produce an oxide precipitate. The combined anodic and cathodic reactions form the corrosion cell, the electrochemical potential of which lies between the single potential of the two half reactions. This mixed potential is termed the corrosion potential, corr> and for corrosion to proceed beyond the equilibrium state, the corrosion potential must be more positive than the equilibrium single potential of iron. For iron in water at pH 7 and with [Fe j = 10" M, for example, the potential of the anodic reaction is. 

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