Acid system used

In the extraction and separation of zirconium from hafnium in a nitric acid system, using TBP, the system operates best if run at about 10% less than saturation [56]. As saturation of the solvent is approached, a zirconium compound precipitates in the presence of the solvent, causing cruds and emulsions. This problem is also encountered in rare earth circuits using DEHPA. 

Carbohydrate-based adhesives, in which the formulation begins with the carbohydrate, have been reported (130), but the acid system used during formulation readily degrades the original carbohydrate to furan intermediates that... 

World nickel metal production in 2002 was 678000 tons [39]. Hydrometallurgy has typically been applied to the treatment of nickel-copper mattes, anode nickel, and reduced laterite ore. The sulfide concentrates are usually oxidized by roasting and then smelted to copper-iron-nickel sulfide matte (75-80% Cu-Ni), which is refined or used directly to make M onel metal. Cathode nickel can be produced from a variety of electrolytes, including chloride, sulfate, or a mixed chloride-sulfate. The former two are acid systems used in leaching and electrowinning. Mixed chloride-sulfate electrolytes are used for electrorefining the nickel sulfide matte from the traditional matte-smelting operations. 

The VLE phase behaviour of the C02/palmitic acid system using the data of Schwarz and Knoetze [24] is shown in Figure 4. Schwarz and Knoetze found their data for C02/hexadecanoic acid to be in agreement with that of Kramer and Thodos [41] and Bharath et al. [33]. However, the data of Penedo et al. [44] appeared to be at too low pressures, most probably due to the fact that they used an extraction type set-up instead of a phase equilibrium... 

Molecular Dynamics Simulations on Nucleic Acid Systems Using the CorneU et al. Force Field and Particle Mesh Ewald Electrostatics... 

The former two are acid systems used in leaching and electrowinning. Mixed chloride-sulfate electrolytes are used for electrorefining the nickel sulfide matte from the traditional matte-smelting operations. 

Although this transformation worked well enough in small reactions, it proved inefficient upon scale-up. A superior, scalable one-pot process (Scheme 3) from diazocinedione to HNFX was developed more recently [37]. Here, efficient protolytic N-denosylation is effected by trifiuoromethanesul-fonic acid added to the crude 3,3,7,7-tetrakis(difluoramino)octahydro-l,5-bis(4-nitrobenzenesulfonyl)-l,5-diazocine intermediate (4), still contained in the acid system used for difluoramination after expulsion of volatiles such as excess difluoramine and trichlorofluoromethane. The denosylated intermediate, a 3,3,7,7-tetrakis(difluoramino)octahydro-l,5-diazocinium salt (5), is straightforwardly nitrated to HNFX by added nitric acid. 

While the reprecipitation reactions in HF acidizing are various and complex, they do not preclude success, by any means. Reprecipitation reactions are not completely avoidable, but their effect on stimulation response can be minimized or eliminated with proper fluid selection, modern acid system use, and treatment design, as discussed in chapter 6. Thus, there is hope and reason to be encouraged. 

The formation is readily soluble in the acid system used, but the fracture face dissolves uniformly so that differential etching does not take place. When the fracture closes, conductivity is lost. 

Over a decade of research, we were able to show that practically all conceivable carbocations could be prepared under what became known as stable ion conditions using various very strong acid systems (see discussion of superacids) and low nucleophilicity solvents (SO2, SO2CIF, SO2F2, etc.). A variety of precursors could be used under appropriate conditions, as shown, for example, in the preparation of the methylcyclopentyl cation. 

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

Acids are not limited to liquid (or gaseous) systems. Solid acids also play a significant role. Acidic oxides such as silica, silica-alumina, etc. are used extensively as solid acid catalysts. New solid acid systems that are stronger than those used conventionally are frequently called solid superacids. 

We found a way to overcome charge-charge repulsion when activating the nitronium ion when Tewis acids were used instead of strong Bronsted acids. The Friedel-Crafts nitration of deactivated aromatics and some aliphatic hydrocarbons was efficiently carried out with the NO2CI/3AICI3 system. In this case, the nitronium ion is coordinated to AICI3. 

Assisted model building with energy refinement (AMBER) is the name of both a force field and a molecular mechanics program. It was parameterized specifically for proteins and nucleic acids. AMBER uses only five bonding and nonbonding terms along with a sophisticated electrostatic treatment. No cross terms are included. Results are very good for proteins and nucleic acids, but can be somewhat erratic for other systems. 

Even less dangerous in this respeet are the nitrating systems using alkyl nitrates and sodium ethoxide. Noteworthy examples of the use of these less acidic or basic nitrating systems are found in the pyrrole series. 

A mixture of methanesulfonic acid and P Oj used either neat or diluted with sulfolane or CH2CI2 is a strongly acidic system. It has been used to control the rcgiosclcctivity in cydization of unsymmetrical ketones. Use of the neal reagent favours reaction into the less substituted branch whereas diluted solutions favour the more substituted branch[3]. 

Howardt describes a model system used to test the molecular weight distribution of a condensation polymer The polymer sample was an acetic acid-stabilized equilibrium nylon-6,6. Analysis showed it to have the following end group composition (in equivalents per 10 g) acetyl = 28.9,... 

Nitric acid acidulation of phosphate rock produces phosphoric acid, together with dissolved calcium nitrate. Separation of the phosphoric acid for use as an intermediate in other fertilizer processes has not been developed commercially. Solvent extraction is less effective in the phosphoric—nitric system than in the phosphoric—hydrochloric system. Instead, the nitric acid acidulate is processed to produce nitrophosphate fertilizers. 

The second ceUulosic fiber process to be commercialized was invented by L. H. Despeissis (4) in 1890 and involved the direct dissolution of cotton fiber in ammoniacal copper oxide Uquor. This solvent had been developed by M. E. Schweizer in 1857 (5). The cuprammonium solution of ceUulose was spun into water, with dilute sulfuric acid being used to neutralize the ammonia and precipitate the ceUulose fibers. H. Pauly and co-workers (6) improved on the Despeissis patent, and a German company, Vereinigte Glanstoff Eabriken, was formed to exploit the technology. In 1901, Dr. Thiele at J. P. Bemberg developed an improved stretch-spinning system, the descendants of which survive today. 

Combination Flame Retardant—Durable Press Performance. Systems using THPC, urea, and TMM can be formulated to give fabrics which combine both flame-retardant performance and increased wrinkle recovery values (80). Another system employs dimethylol cyanoguanidine with THPC under acidic conditions (115). Both of these systems lead to substantial losses in fabric tensile and tearing strength. 

Medicated Dandruff Shampoos. Dandmff is a scalp condition characterized by the production of excessive cellular material (18). A number of shampoos have been marketed which are designed to control and alleviate this condition, and many additives have been included in shampoo compositions to classify them as treatment products for dandmff. These additives include antimicrobial additives, eg, quaternary ammonium salts keratolytic agents, eg, saUcychc acid and sulfur heavy metals, eg, cadmium sulfide coal tar resorcinol and many others. More recent (ca 1993) systems use selenium sulfide [7488-56-4] or zinc pyrithione [13463-41 -7] as active antidandmff shampoo additives. Both of these additives are classified as dmgs, but can be found in over-the-counter products. A stronger version, incorporating the use of higher levels of selenium sulfide in a shampoo, is available but requires a prescription for purchase. 

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