Water vapour pressure, 4.18

The variation in n- or p-conductivity as a function of water pressure at constant oxygen activity appears to be a good method for determining the proton contribution to defect formation. When protons correspond to the majority defects, they can dominate the conductivity and the transport number may be determined. 

In the preceding chapter, protonic conduction has been classified according to the relation between electrical properties and water partial pressure. The stoichiometry and conductivity of intrinsic conductors are almost water pressure independent while those of surface conductors vary strongly with humidity. In the latter case, it appears as if a solid solution (gel) between the skeleton compound (rigid framework) and the liquid water is formed. In the former case, on the other hand, several phases 

The role of defects in the conductivity process was already discussed in the first papers concerning protonic conductors, those of Bjerrum on ice and of O Keefe Perrino on KH2PO. It was suggested that defects are formed following the reactions 

The most recent studies of Sharon Kalia of H and P diffusion coefficients confirm the role of phosphate ion rotation in the conductivity process . The existence of extrinsic and intrinsic regions, however, has not been established with certainty, which leaves the discussion about the effective mechanism open. 

A similar interpretation has been suggested for KH2PO, H OCIO and CsHSO  

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Often, in drying inorganic salts, the final material that is required is a hydrate. In such cases, the purified substance is left in a desiccator to equilibrate above an aqueous solution having a suitable water-vapour pressure. A convenient range of solutions used in this way is given in Table 17. 

The complex preparative interrelationships occurring in the sodium polyphosphate system are summarized in Fig. 12.21 (p. 531). Thus anhydrous NaH2p04, when heated to 170" under conditions which allow the escap>e of water vapour, forms the diphosphate Na2H2p207, and further dehydration at 250" yields either Maddrell s salt (closed system) or the cyclic trimetaphosphate (water vapour pressure kept low). Maddrell s salt converts from the low-temperature to the high-lemperaturc form above 300", and above 400" reverts to the cyclic... 

This is defined as the percentage ratio of the water vapour pressure in the atmosphere compared to that which would saturate the atmosphere at the same temperature. Alternatively, the difference in temperature between the ambient atmosphere and that to which it would have to be cooled before moisture condensed from it, is also used as a measure of moisture content. This difference in temperature is called the dew point depression. The actual temperature at which condensation takes place is known as the dew point. The relative humidity is then expressed as ... 

For tropical storage with average water-vapour pressure 3-2kN/m ... 

When the evaporator begins to cool a cold store, surplus moisture in the air in the room will condense on the coil and, if cold enough, will freeze. This will continue until the water vapour pressure inside... 

Moisture can be removed from any material which is to be dried, by passing air over it which has a lower water vapour pressure. Also, in removing this moisture, the latent heat of evaporation must be supplied, either directly by heating, or by taking sensible heat from the airstream which is carrying out the drying process. 

Robinson [230] has developed a specimen chamber for use in the scanning electron microscope whereby the surface charging of insulators is reduced by a relatively high water vapour pressure (1 kPa). 

The Smith—Topley (S—T) effect is the characteristic variation of isothermal dehydration rate (da /df)D with prevailing water vapour pressure (PHzo) shown in Fig. 10. (da/df)D first decreases with increasing PH2oi later rises to a maximum value and thereafter diminishes towards the zero rate of water loss that is achieved at the equilibrium dissociation pressure. For many hydrates, the reduction in (da/df)D from that characteristic of reaction in a good vacuum to that at PHzo 0.1 Torr is large (X 0.1) and the subsequent maximum may be more or less sharp. Since the reaction rate is, in general, represented by... 

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.)...

Fig. 12. Schematic representation of variations in dehydration rates (ft) with prevailing water vapour pressure (Ph2o) These examples include Smith—Topley behaviour and indicate correlations with phase stability diagrams. (After Bertrand et al. [596], reproduced with permission, from Journal of Inorganic and Nuclear Clemistry.)...

Fig. 13. Plot of variations of activation energy ( /kJ mole"1) with water vapour pressure (PHjO/Torr) for dehydration of calcium sulphate. Data from Ball et al. [281,590, 591] who discuss the significance of these kinetic parameters. Dehydrations of CaS04 2 H2O, nucleation ( ), boundary (o) and diffusion (e) control Q-CaSC>4 5 H2O, diffusion control, below (X) and above (+) 415 K j3-CaS04 5 H20, diffusion control ( ).

This deceleratory reaction obeyed the parabolic law [eqn. (10)] attributed to diffusion control in one dimension, normal to the main crystal face. E and A values (92—145 kJ mole-1 and 109—10,s s-1, respectively) for reaction at 490—520 K varied significantly with prevailing water vapour pressure and a plot of rate coefficient against PH2o (most unusually) showed a double minimum. These workers [1269] also studied the decomposition of Pb2Cl2C03 at 565—615 K, which also obeyed the parabolic law at 565 K in nitrogen but at higher temperatures obeyed the Jander equation [eqn. (14)]. Values of E and A systematically increased... 

Mineral Water vapour pressure, PH2o (Torr) Activation energy, E (kJ mole-1) Preexponential factor, logioA (molecules m-2 s-1) Temperature range (K) Ref. 

The equilibrium relationships found by Sorrell (1977) were valid only for room temperature (22+2 °C) and, because samples were allowed to cure in sealed containers, for equilibrium water vapour pressures determined by the assembly of phases present. The phases which exist under such conditions were quite unequivocally found to be 4 1 5 and 1 1 2. However Sorrell pointed out that it is entirely possible that lower hydration states of either phase could be stable at higher temperatures or lower humidities. In particular the 4 1 4 phase (Feitknecht, 1933) may well be such a phase, particularly as one of the five waters of hydration is known to be held only loosely in the structure. Indeed, Sorrell reported that he observed a slight shoulder on the larger dehydration peak of the DTG curve of the 4 1 5 phase that might be assigned to the loss of this first water molecule. He did not, however, succeed in isolating or characterizing a 4 1 4 phase. 

Boiling will commence when the sum of the organic partial pressures and the water vapour pressure is equal to the total pressure, or in terms of the mole fractions... 

At a given ambient water vapor pressure (usually the level found in the open atmosphere), the temperature of the material is raised so that the equilibrium water vapor pressure over the hydrated material is higher than the ambient water vapour pressure. Generally, heating up to 400 °C is sufficient to remove all the water of crystallization from materials. This removal of water yields a material which may contain some more strongly bound water. To remove this water, the material requires to be heated to a higher temperature (400-600 °C) so that the equilibrium water vapour pressure exceeds the ambient water vapour pressure. For near-complete removal of the last traces of water, temperatures as high as 1000 °C may be required. In addition to the heat required to raise the temperature of the material, heat is also required for the evaporation of water, which is an endothermic process. The enthalpy of evaporation increases as the water content, and hence the equilibrium water vapor pressure, decreases. 

Key component of the system is the dehumidifier shown in Figure 267. The absorptive dehumidifier has to cool the salt solution sufficiently to guarantee a low water vapour pressure. A small specific solution flow has to be distributed uniformly over the dehumidifier surfaces to achieve a high energy storage capacity. Furthermore the dehumidifier has to withstand the corrosive forces of the salt solution and has to be build of inexpensive materials which can easily be manufactured. 

Some vapour pressure data at 20°C are reported in Table 1.3. In Fig. 1.1, the temperature dependence of vapour pressure of some substances is reported. A special attention is to be devoted to water vapour pressure which is very high. 

Water vapour makes a sizeable contribution, and probably the largest, to radiation trapping and as the temperature increases the water vapour concentration increases. Temperature rises as a result of increased water vapour concentration and hence a mechanism for a positive feedback in the greenhouse effect that might lead to a runaway greenhouse effect. When the vapour pressure for water reaches saturation, condensation occurs and water rains out of the atmosphere this is what happens on Earth and Mars. On Venus, however, the water vapour pressure never saturates and no precipitation occurs and the global warming continues to increase. Thus Venus suffers from extreme temperatures produced by both its proximity to the Sun and the presence of water vapour and carbon dioxide in its atmosphere. 

Willemer, H. Water vapour pressure, its influence on the freeze-drying process and its control. 40th Annnual Congress of the International Association for Pharmaceutical Technology, Abstracts 1-67, Medpharm GmbH, Scientific Publishers, Stuttgart, Marz 1994... 

Other methods of surface area determination depend, in general, on adsorption under well defined conditions of various solute molecules of known dimensions (Sposito, 1984 Davis and Kent, 1990). Some of these are dipole molecules so that dipole interactions with the surface or H-bonding are involved. Water adsorbed at a fixed relative water vapour pressure (e. g. 0.2) to provide a monolayer is one example (Torrent et ah, 1990). An organic dipole frequently used for soils is ethylene glycol monoethylether (EGME) (Carter et al., 1965). The main problem with these dipole molecules lies in their mutual association which may lead to localized adsorption beyond a monolayer (capillary condensation), particularly on porous material. 

The solubility of hematite was measured in NaOH/NaCl (0.007-2 m) at between 60 and 300 °C at saturated water vapour pressure with the dissociation reaction being described by ... 

Water activity (uw) is defined as the ratio between the water vapour pressure exerted by the water in a food system (p) and that of pure water (p0) at the... 

Dry substances containing water of crystallization in desiccators over a drying agent and see that the water vapour pressure over it (Table 2) corresponds to the water vapour pressure over the crystal-lohydrate being dried. 

Substance Water vapour pressure, mmHg (20 C) Substance Water vapour pressure, mmHg (20 C)... 

Water-jet pumps can be used wherever there is an adequate pressure of the water in the mains 2 at or 196.2 kPa). The ultimate vacuum depends both on the design of the pump and the water temperature and cannot exceed the water vapour pressure at the given temperature (Appendix 1, Table 13). 

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