Crystalline hydrates

The traditional definition of a zeolite refers to microporous, crystalline, hydrated aluminosilicates with a tliree-dimensional framework consisting of comer-linked SiO or AlO tetrahedra, although today the definition is used in a much broader sense, comprising microporous crystalline solids containing a variety of elements as tetrahedral building units. The aluminosilicate-based zeolites are represented by the empirical fonmila... 

Potassium Phosphates. The K2O—P20 —H2O system parallels the sodium system in many respects. In addition to the three simple phosphate salts obtained by successive replacement of the protons of phosphoric acid by potassium ions, the system contains a number of crystalline hydrates and double salts (Table 7). Monopotassium phosphate (MKP), known only as the anhydrous salt, is the least soluble of the potassium orthophosphates. Monopotassium phosphate has been studied extensively owing to its piezoelectric and ferroelectric properties (see Ferroelectrics). At ordinary temperatures, KH2PO4 is so far above its Curie point as to give piezoelectric effects in which the emf is proportional to the distorting force. There is virtually no hysteresis. 

Over 50 acidic, basic, and neutral aluminum sulfate hydrates have been reported. Only a few of these are well characterized because the exact compositions depend on conditions of precipitation from solution. Variables such as supersaturation, nucleation and crystal growth rates, occlusion, nonequilihrium conditions, and hydrolysis can each play a role ia the final composition. Commercial dry alum is likely not a single crystalline hydrate, but rather it contains significant amounts of amorphous material. 

Bromine is moderately soluble in water, 33.6 g/L at 25°C. It gives a crystalline hydrate having a formula of Br2 <7.9H2 O (6). The solubiUties of bromine in water at several temperatures are given in Table 2. Aqueous bromine solubiUty increases in the presence of bromides or chlorides because of complex ion formation. This increase in the presence of bromides is illustrated in Figure 1. Kquilibrium constants for the formation of the tribromide and pentabromide ions at 25°C have been reported (11). 

Hypobromites, the salts of hypobromous acid, do not keep well because they gradually disproportionate to bromide and bromate. Solutions are best prepared as needed from bromine and alkafl with cooling. Because disproportionation is catalyzed by cobalt, nickel, and copper (70), these impurities should be avoided. SoHd alkaline earth hypobromites, or more properly, bromide hypobromites such as calcium bromide hypobromite [67530-61 CaBr(OBr), have been known for many years, but the pure crystalline hydrates sodium hypobromite pentahydrate [13824-96-9] NaOBr 5H20, and potassium hypobromite tribydrate [13824-97-0], KOBr 3H20, were not described until 1952 (71). Hypobromites are strong bleaching agents, similar to hypochlorites. 

Chlorine dioxide is soluble in water, forming a yellow to yeUow-green color solution that is quite stable if kept cool and in the dark. Various crystalline hydrates of chlorine dioxide have been described including a hexahydrate (19), an octahydrate (20), and an orange colored decahydrate (21). The partition coefficient between water and CIO2 gas is about 21.5 at 35°C and 70.0 at 0°C (22). Data on the solubiUty of chlorine dioxide in water at various... 

Recrystd from Me2CO, H2O or Et0H-Et20 and dried. Soluble in H2O (60%) and EtOH (5%) but insol in Et20 and slightly in CHCI3. The hydrochloride has m 300 (from Mc2CO). The free base is a viscous liquid which forms a crystalline hydrate with m 59 and [a] -28° (c 2.7, H2O). Readily hydrolysed in dilute acid or base. [Meinwald J Chem Soc 712 7953 Fodor Tetrahedron 1 86 1957.]... 

Lithium orthophosphates are unimportant and differ from the other alkali metal phosphates in being insoluble. At least 10 crystalline hydrated or anhydrous sodium orthophosphates are known and these can be grouped into three series ... 

There has been considerable discussion about the extent of hydration of the proton and the hydroxide ion in aqueous solution. There is little doubt that this is variable (as for many other ions) and the hydration number derived depends both on the precise definition adopted for this quantity and on the experimental method used to determine it. H30" has definitely been detected by vibration spectroscopy, and by O nmr spectroscopy on a solution of HF/SbFs/Ha O in SO2 a quartet was observed at —15° which collapsed to a singlet on proton decoupling, 7( 0- H) 106 Hz. In crystalline hydrates there are a growing number of well-characterized hydrates of the series H3O+, H5O2+, H7O3+, H9O4+ and H13O6+, i.e. [H(0H2) ]+ n = 1-4, Thus... 

Of the anhydrous dihalides of iron the iodide is easily prepared from the elements but the others are best obtained by passing HX over heated iron. The white (or pale-green) difluoride has the rutile structure the pale-yellow dichloride the CdCl2 structure (based on cep anions, p. 1212) and the yellow-green dibromide and grey diiodide the Cdl2 structure (based on hep anions, p. 1212), in all of which the metal occupies octahedral sites. All these iron dihalides dissolve in water and form crystalline hydrates which may alternatively be obtained by dissolving metallic iron in the aqueous acid. 

When aluminium is immersed in water, the air-formed oxide film of amorphous 7-alumina initially thickens (at a faster rate than in air) and then an outer layer of crystalline hydrated alumina forms, which eventually tends to stifle the reaction In near-neutral air-saturated solutions, the corrosion of aluminium is generally inhibited by anions which are inhibitive for iron, e.g. chromate, benzoate, phosphate, acetate. Inhibition also occurs in solutions containing sulphate or nitrate ions, which are aggressive towards iron. Aggressive anions for aluminium include the halide ions F ,... 

The relation between moisture content and vapor pressure (or relative humidity) at constant temperature is expressed by an isotherm. Figures 1 and 2 show typical isotherms for a crystalline hydrate sodium carbonate and a food material (potato). 

There have been few attempts to classify decomposition reactions of solids. Gamer [64] made only the broad distinction between endothermic processes (which are often reversible and include dissociation of crystalline hydrates and carbonates) and exothermic processes (which are usually... 

Dissociation of crystalline hydrates and carbonates of group Ha metals... 

The kinetics and mechanisms of dehydration of crystalline hydrates are considered in Chaps. 4 and 5. It is, however, relevant to mention here that the conditions during dehydration can influence the reactivity of the anhydrous salt (or lower hydrate) formed. Thus, anhydrous solids produced under high vacuum are often amorphous, while those prepared in the presence of appreciable amounts of water vapour may be crystalline [281-283,374],... 

There have been many instances of examination of the effect of additive product on the initiation of nucleation and growth processes. In early work on the dehydration of crystalline hydrates, reaction was initiated on all surfaces by rubbing with the anhydrous material [400]. An interesting application of the opposite effect was used by Franklin and Flanagan [62] to inhibit reaction at selected crystal faces of uranyl nitrate hexa-hydrate by coating with an impermeable material. In other reactions, the product does not so readily interact with reactant surfaces, e.g. nickel metal (having oxidized boundaries) does not detectably catalyze the decomposition of nickel formate [222],... 

This behaviour has been identified from microscopic measurements for growth of nuclei during the dehydration of crystalline hydrates [431], the... 

Water [579] is present in the structure of true crystalline hydrates [580] either as ligands co-ordinated with the cation (e.g. [Cu(OH2)4]2+ in CuS04 5 H20) or accommodated outside this co-ordination sphere within voids left in anion packing, further stabilized by hydrogen bonding (e.g. the remaining water molecule in CuS04 5 H20). 

Every water molecule in a crystalline hydrate has, as its nearest neighbours [579], two proton acceptors and at least one electron acceptor. Where only a single electron acceptor is present, co-ordination of the H20 molecule is approximately planar trigonal, and, when two are present, tetrahedral co-ordination is adopted. Large deviations from these configurations seldom occur. Classification [579—582] of the water molecules in hydrates, on the basis of co-ordination of the lone pair orbitals, has been discussed further [579,581] and modified [580] (see Fig. 9 and Table 9). For example, the water in CuS04 5 H20 is located in three different environments two H20 molecules are in Class 1, type D two are in Class 1, type J, and the remaining one is in Class 2, type E. 

Fig. 9. Environment of a water molecule in a crystalline hydrate see also Table 9. (Redrawn from ref. 580, p. 3572 reproduced, with permission, from Acta Crystallo-graphica.)...

Classification of water molecules in crystalline hydrates on the basis of co-ordination environment... 

Representative kinetic data for dehydrations of crystalline hydrates and some comparisons with predictions of the Polanyi—Wigner equation [eqn. (19)]... 

Fig. 10. Schematic representation of variations in rate of dehydration with prevailing water vapour pressure for certain crystalline hydrates. This is an example of Smith Topley behaviour. (Based on observations [64] for the dehydrations of CUSO4 5 H20 and MnC2C>4 2 H20.)...

The following survey of the kinetics of dehydration of crystalline hydrates includes examples of the various types of behaviour commonly observed and unusual systems which merit inclusion in a general review. In this section (and also those which follow), the content is inevitably the product of selection since every relevant article cannot be individually cited. 

Release of water from the crystalline hydroxides (dehydroxylation) differs from the dehydration of a crystalline hydrate (Sect. 1) in that product release must be preceded by chemical interaction between anions. 

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