Hydration, Dehydration, Stability

The thermal dehydration of Th(N03)4 5H20 and Th(N03)4 4H20 has been the subject of several investigations [9 to 13,31,41,60,61,64 to 66] since publication of Thorium 1955 (see pp. 243/4). As indicated earlier (p. 75) there is more recent evidence for the formation of a trihydrate [9 to 12, 60, 61], but no other lower hydrate, and the subsequent formation of an oxide nitrate [9, 61] or oxide nitrate hydrate [41] has been suggested (Section 4.6.6, p. 99) but such phases are not well characterized. The ultimate product of thermal dehydration/denitration is thorium dioxide and there have been numerous studies into the preparation of it by this route (see, for example, references [14,15, 67 to 77] and Thorium Suppl. Vol. C 1,1978, pp. 61/7). 

Solubility data reported by Phillips, Huber [56] for the Th(N03)4-Al(N03)3-HN03 system at 25 and 50°C are listed in Table 26. 

Some data for the solubility of Th(N03)4 5H20 and Th(N03)4-4H20 in non-aqueous media are listed in Table 27. More extensive results reported by Katzin et al. [81] on isobutyl alcohol, hexone, diethyl ether, ethylene glycol diethyl ether, ethylene glycol dibutyl ether, diethylene 

Thorium Nitrate Solubility in Nitric Acid-Aluminium Nitrate at 25 and 50°C [56]. 

Composition of Organic Layers in Equilibrium with Saturated Thorium Nitrate [81]. 

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The most common agents to stabilize an emulsion are surfactants. Different effects contribute to the stabilization of emulsions. Steric repulsion between those parts of the surfactant, which are in the continuous phase, is an important effect. For a water-in-oil emulsion the hydrocarbon chains are hindered in their thermal movements if two water drops approach each other too closely. For an oil-in-water emulsion there is an additional effect the hydrophilic head groups have to be dehydrated to come into close contact. The resulting hydration repulsion stabilizes the emulsion. 

If the equilibrium constant of the chemical reaction (such as complex stability constant, hydration-dehydration equilibrium constant, or the piCa of the investigated acid-base reaction) is known, limiting currents can be used to calculate the rate constant of the chemical reaction, generating the electroactive species. Such rate constants are of the order from 104 to 1010 Lmols-1. The use of kinetic currents for the determination of rate constants of fast chemical reactions preceded even the use of relaxation methods. In numerous instances a good agreement was found for data obtained by these two independent techniques. 

The equilibrium thermodynamics of stoichiometric hydrates has been described by several authors. The overview presented here is intended both to review the basic thermodynamics of crystalline hydrate formation/stability and to highlight the intrinsic differences between polymorphic systems and hydrate systems (a discussion of the kinetics of dehydration/hydration will be given in Section IV). The following description is a hybrid based on the work of Grant and Higuchi [7] and that of Carstensen [8]. 

The presence of such a free Ser hydroxymethyl group as a component giving rise to rapidly relaxing species however, seems to be very important as a site for hydration in stabilizing the silk 1 form. In other words, a solvent-induced conformational change could be associated with the destabilization of the silk I or silk 1 type form as a result of dehydration. 

That hydration of DNA affects its conformation is also evident from the fact that dehydration stabilizes the A phase. Rather remarkably, quantum effects might be largely responsible for this. The zero-point motions of protons are entirely responsible for the binding of water to A-DNA, and in more hydrated conditions a change in the zero-point kinetic energy is sufficient in itself to motivate the transition to B-DNA [104]. Whether the protons concerned are those of water molecules in the hydration shell or those in the DNA s H-bonds (or perhaps a bit of both) is not yet clear. 

CO3 species was formed and the X-ray structure solved. It is thought that the carbonate species forms on reaction with water, which was problematic in the selected strategy, as water was produced in the formation of the dialkyl carbonates. Other problems included compound solubility and the stability of the monoalkyl carbonate complex. Van Eldik and co-workers also carried out a detailed kinetic study of the hydration of carbon dioxide and the dehydration of bicarbonate both in the presence and absence of the zinc complex of 1,5,9-triazacyclododecane (12[ane]N3). The zinc hydroxo form is shown to catalyze the hydration reaction and only the aquo complex catalyzes the dehydration of bicarbonate. Kinetic data including second order rate constants were discussed in reference to other model systems and the enzyme carbonic anhy-drase.459 The zinc complex of the tetraamine 1,4,7,10-tetraazacyclododecane (cyclen) was also studied as a catalyst for these reactions in aqueous solution and comparison of activity suggests formation of a bidentate bicarbonate intermediate inhibits the catalytic activity. Van Eldik concludes that a unidentate bicarbonate intermediate is most likely to the active species in the enzyme carbonic anhydrase.460... 

In view of the ready commercial availability and apparent stability of the hexahy-drate, it is probable that the earlier report of explosion on impact, and deflagration on rapid heating [1] referred to the material produced by partial dehydration at 100°C, rather than the hexahydrate [2], The caked crystalline hydrated salt, prepared from aqueous perchloric acid and excess cobalt carbonate with subsequent heated evaporation, exploded violently when placed in a mortar and tapped gently to break up the crystalline mass, when a nearby dish of the salt also exploded [3]. Subsequent investigation revealed the probable cause as heating the solid stable hexahydrate to a temperature ( 150°C) at which partial loss of water produced a lower and endothermic hydrate (possibly a trihydrate) capable of explosive decomposition. This hazard may also exist for other hydrated metal perchlorates, and general caution is urged [4,5],... 

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