Saturated vapour

Flere is the volume of gas required to saturate the monolayer, V the total volume of gas adsorbed, P the sample pressure, P the saturation vapour pressure and C a constant related to the enthalpy of adsorption. The resulting shape of the isothemi is shown plotted in figure Bl.26.6 for C = 500. A plot of P/V(P - Pq) against P/Pq should give a straight line having a slope (C - )/y C and an intercept The BET surface area is... 

Steam Distillation. Distillation of a Pair of Immiscible Liquids. Steam distillation is a method for the isolation and purification of substances. It is applicable to liquids which are usually regarded as completely immiscible or to liquids which are miscible to only a very limited extent. In the following discussion it will be assumed that the liquids are completely immiscible. The saturated vapours of such completely immiscible liquids follow Dalton s law of partial pressures (1801), which may be stated when two or more gases or vapoms which do not react chemically with one another are mixed at constant temperature each gas exerts the same pressure as if it alone were present and that... 

The model implies that at any pressure below the saturation vapour pressure, the fractions of the surface covered with 1, 2,. .., i molecules will be 6 02,.., 0i respectively, so that the thickness of the adsorbed layer will not be constant throughout. On the specific surface area A, therefore, the total number Z of molecules adsorbed will be... 

As already mentioned, the choice of the supercooled liquid as reference state has been questioned by some workers who use the saturation vapour pressure of the solid, which is measured at the working temperature in the course of the isotherm determination. The effect of this alternative choice of p° on the value of a for argon adsorbed on a number of oxide samples, covering a wide range of surface areas, is clear from Table 2.11 the average value of is seen to be somewhat higher, i.e. 18 OA. ... 

Following the pioneer work of Beebe in 1945, the adsorption of krypton at 77 K has come into widespread use for the determination of relatively small surface areas because its saturation vapour pressure is rather low (p° 2Torr). Consequently the dead space correction for unadsorbed gas is small enough to permit the measurement of quite small adsorption with reasonable precision. Estimates of specific surface as low as 10 cm g" have been reported. Unfortunately, however, there are some complications in the interpretation of the adsorption isotherm. 

The working temperature, 77 K, is well below the triple point of krypton, 116 K, but if the solid is taken as the reference state the isotherm shows an unusually sharp upward turn at the high-pressure end. The usual practice, following Beebe, is therefore to take p° as the saturation vapour pressure of the supereooled liquid (p° = 2-49 Torr at 77-35 K and 27-5 Torr at 90-2 K). 

From the Kelvin equation it follows that the vapour pressure p over a concave meniscus must be less than the saturation vapour pressure p°. Consequently capillary condensation of a vapour to a liquid should occur within a pore at some pressure p determined by the value of r for the pore, and less than the saturation vapour pressure—always provided that the meniscus is concave (i.e. angle of contact <90°). 

Saturated vapour concentration Specific gravity Vapour density (air = 1) Solubility in water... 

The saturated vapour pressure above solid and liquid sulfur is given in Table 15.7. The molecular composition of the vapour has long been in contention but, mainly as a result of the work of J. Birkowitz and others " is uow known to contain all molecules S with 2 < < 10 including odd-numbered specie.s. The actual conccntralion... 

Saturated vapour pressure of HjO at the dew point Saturated vapour pressure of HjO at ambient temp. 

Moisture precipitation Apart from wetting by sea-spray, moisture may either be deposited on a surface by rainfall or dew formation. For a known ambient humidity the dew point can be calculated, using the expression given previously, from standard tables giving the saturated vapour pressure of... 

The relative lowering of the saturated vapour pressure of water is described by the Thomson equation ... 

If air and water are present together in a confined space, a balance condition will he reached where the air has become saturated with water vapour. If the temperature of the mixture is known, then the pressure of the water vapour will be the pressure of steam at this temperature (see also Section 1.3) (Table 23.1). Dalton s Taw of partial pressures (see also Section 1.5) states that the total pressure of a mixture of gases is equal to the sum of the individual pressures of the constituent gases, taken at the same temperature and occupying the same volume. Since the water saturation vapour pressure will remain constant, depending on temperature and not on volume, this pressure can be obtained from steam tables as below. The partial pressure exerted by the dry air must therefore be the remainder. 

Total (standard) pressure = 1013.25 mbar Partial pressure of saturated vapour = 31.66 mbar... 

The theorem also applies to a heterogeneous system, such as a liquid in presence of its saturated vapour, or in presence of the solid. In the former case, vapour is liquefied by compression and gives out its latent heat. Under isothermal conditions this would escape as fast as produced, but if the heat is compelled to remain in the system, it raises the temperature and thereby increases the pressure. If, on the other hand, a mixture of ice and water is compressed, ice melts and the mass is cooled by abstraction of heat. If heat is allowed to enter from outside, so as to restore the original temperature, more ice melts, and the pressure falls by reason of the contraction. 

It has often been supposed that at very high temperatures all gases would behave normally, i.e., would approach a limiting ideal state. As a matter of fact the deviations appear to be influenced by the density of the gas, and disappear at infinitely small densities whatever the temperature may be. Thus a saturated vapour at very low temperatures may behave like a permanent gas, on account of its very small density. 

The methods used for the determination of the density of a saturated vapour will be found in the treatises on Physics (cf. Young, Zeitschr. Physikal. Chem., 70, 620, 1910, who also finds the very simple relation ... 

If a mixture of liquid and saturated vapour is reversibly compressed in a vessel impervious to heat, the corresponding values... 

Clausius (1850), in considering Regnault s data for the latent heat of steam, introduced a new specific heat, applicable to either phase of a saturated complex of two phases, viz., the amount of heat absorbed in raising the temperature of unit mass of a saturated phase by 1°, the pressure being at the same time varied so as to preserve the substance in a saturated state. In the case of a vapour, this is called the specific heat of saturated vapour (a). 

Let cf = specific heat of vapour at constant pressure, cr" = specific beat of saturated vapour,... 

The physical interpretation of this result is that, according to the conditions of pressure and temperature, the fluid to which the equation is applied can exist either in three states with different specific volumes at the same temperature and pressure, or else in only one state (imaginary roots having no physical significance). Case (ii.) corresponds to a gas heated above its critical temperature. In case (i.) the physical interpretation is that the smallest value of v corresponds to the liquid, the largest value of v corresponds to saturated vapour, and the intermediate value corresponds to an unstable state, all at the given temperature. 

This, with the equations of 112, gives us three equations of corresponding states, one for the liquid, one for the saturated vapour, and one for the heterogeneous complex ... 

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