Ferric salts, reduction

Reduction of Ferric Salts.—The reverse reactions, namely conversion of ferric salts into ferrous, are likewise easily effected by means of the usual reducing agents, such as nascent hydrogen, sulphur dioxide, etc. By the introduction of zinc into an acidified solution of a ferric salt, reduction is rapidly caused. Excess of acid slightly retards the reaction.4 This affords a convenient method of volumetrically determining the presence of ferric iron, the solution after reduction being titrated with permanganate. 

Glycohc acid also undergoes reduction or hydrogenation with certain metals to form acetic acid, and oxidation by hydrogen peroxide ia the presence of ferrous salts to form glyoxylic acid [298-12A], HCOCOOH, and ia the presence of ferric salts ia neutral solution to form oxaHc acid, HOOCCOOH formic acid, HCOOH and Hberate CO2 and H2O. These reduction and oxidation reactions are not commercially significant. 

Dithionite is a stronger reducing agent than sulfite. Many metal ions, eg, Cu", Ag", Pb ", Sb ", and Bi ", are reduced to the metal, whereas TiO " is reduced to (346). Dithionite readily reduces iodine, peroxides, ferric salts, and oxygen. Some of the decolorizing appHcations of dithionite, eg, in clay bleaching, are based on the reduction of ferric iron. 

Adsorption inhibitors act by forming a film on the metal surface. The action of traditional oil-based red lead paint formulations presumably involves the formation of soaps and the precipitation of complex ferric salts that reinforce the oxide film. There has been substantial interest in recent years in development of replacements for lead-based and chromate-based inhibitor systems. Adsorption inhibitors based on pol3rmers have been of particular interest. In this volume, Johnson et al. and Eng and Ishida discuss inhibitors for copper 2-undecylimidazole is shown to be effective in acid media, where it suppresses the oxygen reduction reaction almost completely. Polyvlnyllmidazoles are shown to be effective oxidation inhibitors for copper at elevated temperatures. Also in this volume, Chen discusses the use of N-(hydroxyalkyl)acrylamide copolymers in conjunction with phosphate-orthophosphate inhibitor systems for cooling systems. 

In another search for an alternative to Potier s modified Polonovski reaction of catharanthine A-oxide (45), it has now been found that anhy-drovinblastine (42) can be generated directly, in 77% yield, from a reaction of catharanthine and vindoline in 0.01 N acid, promoted by ionized ferric salts, followed by reduction with sodium borohydride (Scheme 30) (Wl). Remarkably, the cation radical 106 generated by Fe(III), in accord with other simple amine oxidations by Lindsay Smith and Mead (102), resulted in isoquinuclidine fragmentation and coupling to vindoline at 0°C, without the conformational inversion observed in the modified Polonovski reaction at that temperature (see Scheme 15). Other metal oxidants or ligand-bound Fe(lll) did not promote the coupling reaction. It will be of interest to see if the overwhelming competition of C-5-C-6 bond... 

For removing low levels of priority metal pollutants from wastewater, using ferric chloride has been shown to be an effective and economical method [41]. The ferric salt forms iron oxyhydroxide, an amorphous precipitate in the wastewater. Pollutants are adsorbed onto and trapped within this precipitate, which is then settled out, leaving a clear effluent. The equipment is identical to that for metal hydroxide precipitation. Trace elements such as arsenic, selenium, chromium, cadmium, and lead can be removed by this method at varying pH values. Alternative methods of metals removal include ion exchange, oxidation or reduction, reverse osmosis, and activated carbon. 

In the case of the reduction of ferric salts by sulphur dioxide it has been suggested 9 that the reduction most probably proceeds in stages, a red ferric ferrisulphite, Fe[Fe(S03)3], being first formed, which on warming yields the ferrous salt and the dithionate, thus ... 

This energetic reducing agent can be maintained at constant strength in aqueous hydrochloric acid solution for a reasonable period. It is advisable, however, to re-standardise it after 24 hours standing. It serves for the reduction of aromatic nitro compounds, some nitroso bodies, many azo dyes, and of nearly all the dyes which yield leuco-compounds. It is easily standardised against a ferric salt—say ferric alum—using potassium thiocyanate as indicator. From the equations —... 

Ferric salts and potassium ferricyanide are completely reduced by charged palladium foil or wire, and the reduction may be carried out quantitatively if required for analytical purposes. 

Ammonia is oxidised to oxides of nitrogen by means of oxygen in the presence of palladium foil heated to redness1 and, as has already been pointed out, palladium foil saturated with hydrogen effects the reduction of ferric salts, chlorine water, iodine water, etc., to ferrous salts, hydrochloric acid, and hydriodic acid respectively. Hydrocarbons are oxidised to carbon dioxide and water when passed with air over palladium wire heated to redness. In the absence of air they are decomposed, yielding a deposit of carbon. After a time the palladium becomes brittle, and its surface, seen through a lens, resembles coke.2... 

Ferric salts are reduced to ferrous,6 and ammonium molybdate likewise suffers reduction.7... 

Traces of ferric salts retard the dissolution of mercury in nitric acid, a phenomenon which is attributed5 to decomposition of the nitrous acid by the catalytic alternate reduction and oxidation of the iron-radicle. 

Reduction of ferric salts with potassium iodide is usually regarded as taking place as follows,2 in the case of ferric chloride —3... 

Photochemical Reduction.—Ferric salts are reduced by organic substances under the influence of light. Thus an alcoholic solution of ferric chloride, when exposed to sunlight, is converted into ferrous chloride, the alcohol being oxidised to aldehyde. For example, in the case of methyl 8 and ethyl alcohols —... 

Reduction of Iron Salts to Metallic Iron.—Both ferrous and ferric salts can be reduced to the metal in a variety of ways. In solution many of them are reduced by the introduction of more electro-positive metals such as magnesium, zinc, or aluminium, as also by electrolysis. In the dry way they are reduced by heating in a current of hydrogen... 

A new approach for the chemical removal of pyritic sulfur from coal is described. The process is based on the discovery that aqueous ferric salts selectively oxidize the pyritic sulfur in coal to chemical forms which can be removed by vaporiza-tion, steam, or solvent extraction. Data for removal of the pyritic sulfur from four major coals (Lower Kittanning, Illu nois No. 5, Herrin No. 6 and Pittsburgh) are presented together with a discussion of the process chemistry. The effect of variables, such as coal particle size, acid and iron concern tration, reaction time, and temperature are discussed. The results show that near complete removal of pyritic sulfur can be obtained under mild conditions, resulting in a reduction of the total sulfur content of the coals from 40 to 80%, depending on the original pyritic sulfur content. 

Properties Light buff, odorless crystals white when pure discolor on exposure to light and air. Mp 186-187C. Sparingly soluble in cold water soluble in hot water, glacial acetic acid, ethyl acetate. Unstable to ferric salts and oxidizing agents. Derivation Reduction of p-nitrobenzoic acid. Commercially available as the calcium, potassium, and sodium salts. Food source Widely distributed, especially in yeast. 

The general Haber-Weiss mechanism involves an intermediate reduction oxidation of the ferric-ferrous ion system. This has been accepted by a number of authors and Simon et al. (15) have shown that catalytic decomposition of hydrogen peroxide by ferric salts only takes place under conditions where the formation of ferrous ions can be demonstrated. There is, therefore, no experimental basis for a mechanism without the intermediate reduction of the ferric ions. 

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