Prolonged depositions

In the case of prolonged deposition, due either to fallout from weapons detonations or operational releases from nuclear plant, Chamberlain (1970) developed a further function, the normalised specific activity (NSA) defined as ... 

It is obvious that there should be no overlap of the deposit upon prolonged deposition. Moreover, it should be noted that the profiles were calculated assuming a constant current density, which is not the case in a real system where the space in the vicinity of the comer vertex is increasingly screened as the deposit grows. This implies that the real distribution of the metal deposit in the comers is worse than that calculated and shown in Fig. 3.19a. 

EPCR is used in the charging of silver-zinc storage batteries, to prevent, or to delay, the formation of spongy and dendritic deposits of zinc [20, 21]. It is impossible to obtain smooth deposits of zinc from alkaline zincate solutions during prolonged deposition at a constant rate due to formation of spongy deposits at lower and dendritic deposits at higher overpotentials [21, 22]. 

Dinitrophenylhydra2ones usually separate in well-formed crystals. These can be filtered at the pump, washed with a diluted sample of the acid in the reagent used, then with water, and then (when the solubility allows) with a small quantity of ethanol the dried specimen is then usually pure. It should, however, be recrystallised from a suitable solvent, a process which can usually be carried out with the dinitrophenylhydrazones of the simpler aldehydes and ketones. Many other hydrazones have a very low solubility in most solvents, and a recrystallisation which involves prolonged boiling with a large volume of solvent may be accompanied by partial decomposition, and with the ultimate deposition of a sample less pure than the above washed, dried and unrecrystal-lised sample. 

Elaborate precautions must be taken to prevent the entrance of Pu iato the worker s body by ingestion, inhalation, or entry through the skin, because all common Pu isotopes except for Pu ate a-emitters. Pu is a P-emitter, but it decays to Am, which emits both (X- and y-rays. Acute intake of Pu, from ingestion or a wound, thus mandates prompt and aggressive medical intervention to remove as much Pu as possible before it deposits in the body. Subcutaneous deposition of plutonium from a puncture wound has been effectively controlled by prompt surgical excision followed by prolonged intravenous chelation therapy with diethylenetriaminepentaacetate (Ca " —DTPA) (171). 

Catalysts for dielectric surfaces are more complex than the simple salts used on metals. The original catalysts were separate solutions of acidic staimous chloride [7772-99-8J, used to wet the surface and deposit an adherent reducing agent, and acidic palladium chloride [7647-10-17, which was reduced to metallic palladium by the tin. This two-step catalyst system is now essentially obsolete. One-step catalysts consist of a stabilized, pre-reacted solution of the palladium and staimous chlorides. The one-step catalyst is more stable, more active, and more economical than the two-step catalyst (21,23). A separate acceleration or activation solution removes loose palladium and excess tin before the catalyzed part is placed in the electroless bath, prolonging bath life and stability. 

Although the corrosivity may not be high provided the condensed moisture remains uncontaminated, this rarely happens in practice, and in marine environments sea salts are naturally present not only from direct spray but also as wind-borne particles. Moreover, many marine environments are also contaminated by industrial pollution owing to the proximity of factories, port installations, refineries, power stations and densely populated areas, and in the case of ships or offshore installation superstructures by the discharge from funnels, exhausts or flares. In these circumstances any moisture will also contain S, C and N compounds. In addition, solid pollutants such as soot and dust are likely to be deposited and these can cause increased attack either directly because of their corrosive nature, or by forming a layer on the surface of the metal which can absorb and retain moisture. The hygroscopic nature of the various dissolved salts and solid pollutants can also prolong the time that the surface remains moist. 

Substantially all the zinc is absorbed by the surface of the components, and the resulting coating has a matt-grey appearance and consists essentially of a zinc-iron alloy averaging 90-95% Zn. If excess zinc is made available and the treatment is prolonged, pure zinc is deposited at the surface. 

When undiluted 2-vinylfuran was added to metallic sodium (mirror or particles) an orange colour developed and some resinous material was deposited on the metal surface. On prolonged contact much of the monomer was converted into a partly-insoluble reddish resin with spectra unrelated to those of standard poly(2-vinyl-furan). Reaction of diluted monomer with sodium gave a milder interaction, but no evidence of living anionic polymerization. 

As far as hydride decomposition is concerned, the relations are reversed. The larger the metal crystals are the slower their hydride decomposes (62). Moreover some deposits situated on the exit points of dislocations, for example on the surface of a nickel hydride crystal, inhibit hydrogen desorption and result in prolonging the hydride existence in the crystal (87). 

In those with gout, the serum uric acid level is usually elevated. Sulfinpyrazone increases the excretion of uric acid by the kidneys, which lowers serum uric acid levels and consequently retards the deposit of urate crystals in the joints. Probenecid (Benemid) works in the same manner and may be given alone or with colchicine as combination therapy when there are frequent, recurrent attacks of gout. Probenecid also has been used to prolong the plasma levels of the penicillins and cephalosporins. 

An interesting way to retard catalyst deactivation is to expose the reaction mixture to ultrasound. Ultrasound treatment of the mixture creates local hot spots, which lead to the formation of cavitation bubbles. These cavitation bubbles bombard the solid, dirty surface leading to the removal of carbonaceous deposits [38]. The ultrasound source can be inside the reactor vessel (ultrasound stick) or ultrasound generators can be placed in contact with the wall of the reactor. Both designs work in practice, and the catalyst lifetime can be essentially prolonged, leading to process intensification. The effects of ultrasound are discussed in detail in a review article [39]. 

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