Hydrates metastable

Solutions of anhydrous stannous chloride are strongly reducing and thus are widely used as reducing agents. Dilute aqueous solutions tend to hydrolyze and oxidize in air, but addition of dilute hydrochloric acid prevents this hydrolysis concentrated solutions resist both hydrolysis and oxidation. Neutralization of tin(II) chloride solutions with caustic causes the precipitation of stannous oxide or its metastable hydrate. Excess addition of caustic causes the formation of stannites. Numerous complex salts of stannous chloride, known as chlorostannites, have been reported (3). They are generally prepared by the evaporation of a solution containing the complexing salts. 

Owing to this activation threshold, the first precipitation product from aqueous solutions of silicic acids will be an amorphous silica of some degree of hydration, while at room temperature the growth of vitreous and crystalline forms of silica from the precipitate (and thus the approach toward the absolute equilibrium) will proceed extremely slowly. With this understanding the data in Figure 1 are said to represent, an equilibrium—i.e., the reversible equilibrium between silicic acids in aqueous solution and metastable hydrated silica or polymeric silicic acid as precipitate. 

With the evolution of these new structures, the possibility of forming metastable hydrate phases (Section 3.2), and the fact that different structures form at different thermodynamic conditions (pressure, temperature, composition), it is clear that macroscopic methods cannot adequately predict the hydrate structure(s) present. 

A second example comes from the reaction of Xe with powdered THF hydrate . It is easy to see that a hydrate of sll forms quickly on the surface as it is not necessary to form to nucleate a new phase. Since the activity of the Xe guest is much greater than that of THF under the initial experimental conditions, Xe replaces THF in the surface layers of the hydrate. This metastable hydrate persists for several hundred seconds before it converts completely to si Xe hydrate (which does have to nucleate) plus a mixed THF-Xe hydrate. Such experiments reveal that hydrates are extremely labile at temperatures above 200 C, and that metastable hydrates may be important in the early stages of hydrate formation. 

It should be noted that it is possible to crystallize metastable hydrates from aqueous solution. A metastable hydrate is one that when saturated in water, where the water activity is maximal, there exists an anhydrous crystal form that is thermodynamically more stable. For a stable hydrate screen, solvent mediated conversion will occur in time, converting the metastable hydrate to the stable anhydrous form. Thus, a metastable hydrate should not be observed in a stable hydrate screen, provided the slurries have reached equilibrium. 

Other Systems.—In the case of sodium sulphate there is only one stable hydrate. Other salts are known which exhibit a similar behaviour and we shall therefore expect that the solubility relationships will be represented by a diagram similar to that for sodium sulphate. A considerable number of such cases have, indeed, been found, and in some cases there is more than one metastable hydrate. This is found, for example, in the case of nickel iodate, the solubility curves for which are given in Fig. 76. As can be seen from the figure, suspended transformation occurs, the solubility curves having in some cases been followed to a considerable distance beyond the transition point. One of the most brilliant examples, however, of suspended transformation in the case of salt hydrates, and the sluggish transition... 

Above 29 8° the stable hydrate is the a-tetrahydrate and its solubility curve extends to 45 3° (K), at which temperature it cuts the solubility curve of the dihydrate. The curve of the latter hydrate extends to 175 5 (L), and is then succeeded by the curve for the monohydrate. The solubility curve of the anhydrous salt does not begin until a temperature of about 260 . The whole diagram, therefore, shows a succession of stable hydrates, a metastable hydrate, and a metastable melting-point. 

The behaviour of the lower alcohols in aqueous solution merits a special comment. Despite their apparent complete miscibility with one another and with water, the alcohols display an extremely complex series of eutectic, peritectic and hydration phenomena, which have usually been hidden from earlier observers because they appear at low temperatures. As one example, consider the mixture water-tert-butanol shown in Figure 17. A stable hydrate of composition A-2H20 and lA = 0.55°C is clearly shown, and there are also indications of several peritectic transitions and metastable hydrates. Similar complexities have been described for aqueous solutions of methanol, ethanol and propanol. ... 

Although most hydrates exhibit a whole-number-ratio stoichiometry, an unusual case is the metastable hydrate of caffeine, which contains only 0.8 moles of water per mole of caffeine. Only in a saturated water vapor atmosphere will additional amounts of water be adsorbed at the surface of the 4/5-hydrate to yield a 5/6 hydrate [59]. 

The binary aqueous systems phase diagrams in the range of the crystallization of TBA carboxylate and isocarboxylate polyhydrates are given in Figures 1 and 2 respectively, and some properties of the hydrates are shown in Table I. Two hydrates with stable crystallization branches form in the TBA acetate - water system. One stable and several metastable hydrates have been discovered in each of the rest of the systems (from 1 (TBA formate) to 8 (TBA o-butyrate)). 

Kinetic factors. Though many hydrates are not stable thermodynamically, they are not far from this state and exist as metastable hydrates. It is interesting that even the presence of the stable phase as seed did not always cause the crystallization of this very phase from the solution. The equilibrium in the solution does not seem to establish instantly. 

In CAC, the CA reacts with water to form a series of calcium aluminate hydrates. These include CAHjq, C2AHg, C3AH6, and AH3 (an amorphous phase). The metastable hydrates, CAHjq and C2AHg, convert to C3AH6. The following scheme summarizes the eonversion reactions. 

A number of other hydrates of potential stratospheric importance have been reported in the literature. All these hydrates are metastable with respect to NAT or SAT, but the formation of metastables preceeding the stable phase is a thermodynamically common feature according to Ost-wald s rule . Most prominent metastable hydrates are nitric acid dihydrate (NAD), sulfuric acid hemihexahydrate (SAH), nitric acid pentahy-... 

Here the case is similar to NAT. Again, model results suggest for the dominant reaction CIONO2+HCI on SAT a significant discrepancy (up to a factor of 20) between the various measurements. Work by Hanson and Ravishankara indicates that the reaction CIONO2+HCI on NAD is similarly fast as that on NAT. There are no studies of chemical reactions on any of the other metastable hydrates. 

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