Iron hydroxides, dehydrations

A suite of both oxidized and reduced iron minerals has been found as efflorescences and precipitates in or near the acid mine water of Iron Mountain. The dominant minerals tend to be melan-terite (or one of its dehydration products), copiapite, jarosite and iron hydroxide. These minerals and their chemical formulae are listed in Table III from the most ferrous-rich at the top to the most ferric-rich at the bottom. These minerals were collected in air-tight containers and identified by X-ray diffractometry. It was also possible to check the mineral saturation indices (log Q(AP/K), where AP = activity product and K = solubility product constant)of the mine waters with the field occurrences of the same minerals. By continual checking of the saturation index (S.I.) with actual mineralogic occurrences, inaccuracies in chemical models such as WATEQ2 can be discovered, evaluated and corrected (19), provided that these occurrences can be assumed to be an approach towards equilibrium. 

Diagenesis in oxidizing conditions results mainly in dehydration, condensation, compaction, and crystallization of amorphous iron hydroxide and silica-gel sediments. The stable mineral forms—goethite and quartz—are formed in several stages via intermediate metastable phases. 

The sequence of dehydration reactions in the progressive metamorphism of silicate rocks depends on the presence of excess iron hydroxide in the original sediment, inasmuch as the lowest-temperature reaction is the transformation of the greenalite + hematite association into magnetite according to the reaction ... 

The conversion of goethite or iron hydroxides into hematite has been investigated experimentally in a great number of works, and the results obtained are rather diverse. Even at room temperature hematite often forms along with goethite, especially in acid and neutral environments. Apparently both these phases are characterized by similar thermodynamic properties and the AGj value of the reaction of dehydration of a-FeOOH at moderate temperatures is not large. 

Roasting raw umber q.v.) in an oxidising environment causes dehydration of die iron hydroxides, converting them into iron oxides and yields a pigment with a warm reddish brown hue (Weber, 1923) while retaining raw umber s abilities as a drier. 

Iron Oxide Yellows. From a chemical point of view, synthetic iron oxide yellows, also known as iron gelbs, are based on the iron(III) oxide—hydroxide, a-FeO(OH), known as goethite. Color varies from light yellows to dark buffs and is primarily determined by particle size, which is usually between 0.1 and 0.8 p.m. Because of their resistance to alkahes, these are used by the building industry to color cement. Thermally, iron oxide yellows are stable up to 177°C above this temperature they dehydrate to iron(III) oxide ... 

The ferrous hydroxide then combines with oxygen and water to produce ferric hydroxide, Fe(OH)2, which becomes common iron mst when dehydrated to Fe202. 

Dehydration to 2-vinylthiophene is better achieved from 2-(2-thienyl) ethanol with powdered potassium hydroxide in the presence of copper than from 1-(2-thienyl) ethanol. a-Chloro-2-thienylpro-pane undergoes a Wurtz reaction with active iron to give 3,4-di-(2-thienyl) hexane in low yield, which has also been obtained through coupling with n-butyllithium. ... 

Aluminum is the most abundant metallic element in the Earth s crust and, after oxygen and silicon, the third most abundant element (see Fig. 14.1). However, the aluminum content in most minerals is low, and the commercial source of aluminum, bauxite, is a hydrated, impure oxide, Al203-xH20, where x can range from 1 to 3. Bauxite ore, which is red from the iron oxides that it contains (Fig. 14.23), is processed to obtain alumina, A1203, in the Bayer process. In this process, the ore is first treated with aqueous sodium hydroxide, which dissolves the amphoteric alumina as the aluminate ion, Al(OH)4 (aq). Carbon dioxide is then bubbled through the solution to remove OH ions as HCO and to convert some of the aluminate ions into aluminum hydroxide, which precipitates. The aluminum hydroxide is removed and dehydrated to the oxide by heating to 1200°C. 

Figure 13.17 Role of clusters in substrate binding—in aconitase the cluster geometry shifts from 4- to 6-coordination on substrate binding. The coordinating iron atom abstracts the hydroxide anion during dehydration. (From Imlay, 2006. Reproduced with permission of Blackwell Publishing Ltd.)...

The oxide is prepared in industrial scale by first precipitating iron(II) hydroxide Fe(OH)2 by treating aqueous solutions of iron(II) sulfate and caustic soda. The Fe(OH)2 is then oxidized to iron(III) hydroxide by aeration. The latter is dehydrated by heating ... 

Ferric Oxide or Red Iron Trioxide (Hematite), FejjOj, mw 159.70, ted or blk trigonal crysts, sp gr 5.12, mp 1560°(dec) insol in w, sol in HCl other acids found in nature as hematite and is a by-product in some industries. Can be prepd by dehydrating ferric hydroxide or calcining ferrous oxalate or sulfate. Used in magnetic tapes in electronics and as an ingredient of some expl compns Refs l) Lange (1961), No 824 2) Cond-... 

Benzyl acetate was prepared by addition of benzyl chloride (containing 0.6% pyridine as stabiliser) to preformed sodium acetate at 70°, followed by heating at 115°, then finally up to 135°C to complete the reaction. On one occasion, gas began to be evolved at the end of the dehydration phase, and the reaction accelerated to a violent explosion, rupturing the 25 mm thick cast iron vessel. This was attributed to presence of insufficient pyridine to maintain basicity, dissolution of iron by the acidic mixture, and catalysis by ferric chloride of a Friedel-Craft type polycondensation reaction to polybenzyls, with evolution of hydrogen chloride, which at 130°C would produce an overpressure approaching 100 bar. Previously the chloride had been supplied in steel drums containing 10% sodium carbonate or 3% sodium hydroxide solutions as... 

Summary Copper-II-oxide is formed in a similar manner as for iron-II-oxide. It is prepared, first, by electrolyzing a solution of pickling salt using copper electrodes. During the electrolysis process, a messy precipitate of mixed hydrated copper hydroxides are formed. Thereafter, this precipitate is collected by filtration, and then dried. The dry mass is then roasted at high temperature for several hours to facilitate formation of copper-II-oxide, which is formed by the dehydration and oxidization of the hydrated copper... 

---