Hydration at

Deliquescence and efflorescence. A substance is said to deliquesce (Latin to become liquid) when it forms a solution or liquid phase upon standing in the air. The essential condition is that the vapour pressure of the saturated solution of the highest hydrate at the ordinary temperature should be less than the partial pressure of the aqueous vapour in the atmosphere. Water will be absorbed by the substance, which gradually liquefies to a saturated solution water vapour will continue to be absorbed by the latter until an unsaturated solution, having the same vapour pressure as the partial pressure of water vapour in the air, is formed. In order that the vapour pressure of the saturated solution may be sufficiently low, the substance must be extremely soluble in water, and it is only such substances (e.g., calcium chloride, zinc chloride and potassium hydroxide) that deliquesce. 

Propylene is hydrated at ca 540 K under pressure. Conversions are 60—70% and selectivity to 2-propanol is 99%. 

Selected physical properties of sodium thiosulfate pentahydrate are shown in Table 1. The crystals are relatively stable, efflorescing in warm, dry air and dehquescing slightly in moist air. They melt in their water of hydration at 48°C and can be completely dehydrated in a vacuum oven at this temperature, or at atmospheric pressure at 105°C. Anhydrous sodium thiosulfate can also be crystallised direcdy from a 72% solution above 75°C. It decomposes at 233°C ... 

In hydrations at ordinary temperatures (27) pure C S and P-C2S, corresponding to the aHte and beHte phases ia Pordand cements, respectively, react with water to form calcium hydroxide and a single calcium siHcate hydrate (C—S—H). Using cement chemists notation... 

Table 7. Cement Phases Hydrated at Normal Temperatures ...

Hydration at Ordinary Temperatures. Pordand cement is generally used at temperatures ordinarily encountered in constmction, ie, from 5 to 40°C. Temperature extremes have to be avoided. The exothermic heat of the hydration reactions can play an important part in maintaining adequate temperatures in cold environments, and must be considered in massive concrete stmctures to prevent excessive temperature rise and cracking during subsequent cooling. 

Citric acid monohydrate [5949-29-1] has a molecular weight of 210.14 and crystallizes from cold aqueous solutions. When gendy heated, the crystals lose thek water of hydration at 70—75°C and melt in the range of 135—152°C. Rapid heating causes dehydration at 100°C to form crystals that melt sharply at 153°C. Citric acid monohydrate is available in limited commercial quantities since most appHcations now call for the anhydrous form. 

The presence of H S should not be overlooked in the determination of susceptibility of a gas to form hydrates. At concentrations of 30% or greater, hydrates will form in hydrocarbon gases at about the same temperature as in pure H2S. 

The addition of water across carbon-carbon double bonds, a reaction thoroughly investigated by Lucas and Taft, requires strong activation and is catalyzed by hydrogen ions and hydroxyl ions. Addition of water across the 0= =0 bond of aldehydes has also been studied kinetically. Whereas chloral and formaldehyde are largely hydrated (at equilibrium in dilute aqueous solution), acetaldehyde and other... 

Some petroleum geologists believe that there may be more methane trapped in hydrates than what is associated with natural gas reserves. However, as an energy source, there is considerable uncertainty whether this methane can ever be recovered safely, economically, and with minimal environmental impact. The Russians have experimented with the use of antifreeze to break down hydrates at some onshore locations in Siberia. But perhaps a more promising approach would be to pipe warm surface water to the bottom to melt the hydrates, with a collector positioned to convey the gas to the surface. Another approach might be to free methane by somehow reducing the pressure on the methane hydrates. 

The radii av a2 and coordination numbers zv z2 follow from x-ray analysis (cf. Section I.B), and aQ/2 — 1.25 A corresponds to Pauling s van der Waals radius of 1.40 A for a covalently bound oxygen atom.25 The value of eQlk — 166.9°K was chosen to obtain agreement between calculated and experimental values of the equilibrium vapor pressure of argon hydrate at 0°C. 

TABLE V. Dissociation Pressures and Degrees of Occupation of Gas Hydrates at 273°K30... 

Preparation. It is prepd by dissolving pure Sr nitrate in an excess of perchloric ac, the excess ac neutralized with Sr carbonate, all solids centrifuged off, and the soln chilled until crysts appear (Ref 2). The anhyd salt can also be prepd by drying the hydrate at 250°, yield 50% (Ref 3) or the addn of anhyd perchloric ac to a soln of Sr ion in anhyd trifluoroacetic ac, the pptd solid filtered, and the excess acid removed in a vacuum (Ref 7)... 

Cadmium Picrate. [C6H2(N02)30] 2Cd, mw 569.70, N 14.76%, yel powd which explds violently at 336—41°. Was obtained by heating one of its hydrates at 80—150°. Silberrad and Phillips (Ref 2) reported on the hepta- and pentahydrates, while Hopper (Ref 3) reported on the octahydrate. Sensitivity of the anhydr salt is 12" vs 14" for TNT, as detd with the PicArsn app using a 2kg wt. The value for the octahydrate is 35", making it less sensitive than BlkPdr dust... 

The rate-acidity profile for pyrimidin-2-one indicated reaction on the free base but since the derived second-order rate coefficient is 104 times greater than that for 2-pyridone, and the acidity dependence in the H0 region was also greater, the slope of log kt versus —H0 plot being 0.45, cf. 0.15 for 2-pyridone reaction was, therefore, postulated as occurring via a covalent hydrate, hydration taking place at the 4 position. Methyl substitution increased the rate as expected and N-methyl substitution produced a larger effect than 4,6-dimethyl substitution and this may be due to alteration of the amount of covalent hydration at equilibrium. The data... 

The spherical fine nickel powders have been prepared fiom aqueous NiCU and hydrazine hydrate at various temperatures wife ethanol-water solvent by the conventional and ultrasonic hydrothermal reduction method. The induction time decreased wife inrareasing fee reaction temperature in both fee method, but was relatively shorter in fee ultrasonic mefeod. Compared to the conventional one, the surface morphology and particle size of fee sample obtained by the ultrasonic method was much smooth and regular in spherical shape and was much small, respectively. Therefore, the tap density of the sample obtained by fee ultrasonic mefeod was relatively higher than feat obtained by fee conventional one. 

Figure 1. Fremature infant, well hydrated at birth...

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