Vapour pressure reduction 5.22

Real colligative properties are only found in ideal gases and ideal solutions. Examples are osmotic pressure, vapour pressure reduction, boiling-point elevation, freezing-point depression, in other words the osmotic properties. 

In the past, the basic experiment used to study the general properties of solutions was the measurement of the vapour pressure lowering of the solvent (Raoult 1890). But this method which is, in general, efficient is not really useful when the vapour pressure reduction is small. In such situations, it has been replaced by another method which is quite similar it consists in measuring the osmotic pressure of the solution with respect to the solvent. 

The colligative properties of solutions, e.g. osmotic pressure, boiling point elevations, freezing point depression and vapour pressure reduction, depend on the effect of solute concentration on the solvent aetivity. The ehemieal potential, yt, of a non-electrolyte in dilute solution may be expressed by... 

Couditious. In the calculation, the water is assumed to wet the pore wall completely. In case of partial wetting, the surface tension a is simply replaced in all expressions by the component a cos(0) in the longitudinal direction of the pore, where ff denotes the contact angle. At 25 °C, the following data for water are used in the numerical calculation of the vapour pressure reduction over capillary pores density g = 997.0 kg/m surface tension a = 0.072 N/m molar mass M = 18.02 g/mol. Water vapour is assumed to be an ideal gas. 

Using that RH = p/ps and V(i) = M/q, where p denotes the density of the water, we obtain the Kelvin equation for vapour pressure reduction over capillary pores... 

Discussion. Capillary condensed water in pores is in a state of tension. Due to this, the free energy of the water G is reduced in relation to the free energy of water at the standard pressure of 1 atmosphere. This change of pressure results in a reduction of the equilibrium vapour pressure over capillary-bound water. In practice this phenomenon is seen by the fact that the water vapour of the air is able to condense and be bound in narrow pores although the relative humidity RH < 100 %. As seen from the calculations, the vapour pressure reduction is only significant at pore radii r of the order of magnitude 100 mm and below. 

The resulting solution has a much lower vapour pressure than a solution of iodine in pure water, and consequently the loss by volatilisation is considerably diminished. Nevertheless, the vapour pressure is still appreciable so that precautions should always be taken to keep vessels containing iodine closed except during the actual titrations. When an iodide solution of iodine is titrated with a reductant, the free iodine reacts with the reducing agent, this displaces the equilibrium to the left, and eventually all the tri-iodide is decomposed the solution therefore behaves as though it were a solution of free iodine. 

A) Cold vapour technique. This procedure is strictly confined to the determination of mercury,45 which in the elemental state has an appreciable vapour pressure at room temperature so that gaseous atoms exist without the need for any special treatment. As a method for determining mercury compounds the procedure consists in the reduction of a mercury(II) compound with either... 

By equation (1) of 100 this will cause a reduction of vapour-pressure to the extent ... 

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... 

The production of germanium and silicon is carried out by the reduction of the oxides, SiCh and GeC>2. There is a considerable difference in the stabilities of the oxides, and GeC>2 can be reduced by hydrogen at temperatures around 1100K, but SiC>2 must be reduced by carbon at temperatures around 2300 K. The elements only reach a vapour pressure of 10-6 atmos at 1500 K and 1750 K respectively, and so the reduction of GeC>2 does not lead to any significant vaporization, but this is not so in the production of silicon. The gaseous composition of a 1 2 mixture of Si(>2 and carbon at 2000 K is as follows ... 

The metallothermic reduction of the oxides by La produces the metals Sm, Eu, Tm, Yb, all having high vapour pressures. The reaction goes to completion due to the removal of the rare earths by volatilization from the reaction chamber (lanthanum has a low vapour pressure). The remaining rare earth metals (Sc, La, Ce, Pr, Nd, Y, Gd, Tb, Dy, Ho, Er, Lu) can be obtained by quantitative conversion of the oxides in fluorides, followed by reduction with Ca. The metallothermic reduction of the anhydrous rare earth chlorides could be also used to obtain La, Ce, Pr and Nd. The molten electrolysis can be applied to obtain only the first four lanthanide metals, La, Ce, Pr and Nd, because of the high reactivity of the materials that limits the operating temperatures to 1100°C or lower. 

The depression of freezing point or reduction of the vapour pressure of a solvent is proportional to the molar concentration of the solute. 

Power recovery is generally achieved by the reduction of fluid pressure, sometimes with a contribution from vapour or gas evolution during the pressure reduction. A hydraulic power recovery turbine may be a pump operated with reverse flow. 

The choice of the starting compound of the actinide (oxide or carbide) and of the reductant is determined by the vapour pressure of the actinide metal. For a given temperature, the vapour pressures of the actinide metals (Fig. 2) span a ratio of more than 10 . As the vapour pressure of La is similar to those of Ac, Cm, Pu, only the more volatile... 

The vapour pressure ratio of actinides to noble metals is also the basis of the actinide metal preparation by thermal dissociation of intermetallic compounds. Such intermetallic compounds of An and noble metals can be prepared by hydrogen reduction of a mixture of an An oxide and a finely divided noble metal (Pt, Ir.. in the absence of noble metals, hydrogen reduction of An oxides is impossible. Am and Cm metals have been obtained by thermal dissociation of their intermetallic compounds with Pt and Ir High purity Th and Pa, the least volatile actinide metals, can be prepared by thermal dissociation of their iodides, which form readily by reaction of iodine vapour with car-... 

This applies solely to mercury as it is the only analyte that has an appreciable atomic vapour pressure at room temperature. The 253.7 nm line is usually used for mercury atomic absorption, but the transition is spin forbidden, and relatively insensitive. The 184.9 nm line is potentially 20-40 times more sensitive, but at this wavelength most flame gases and the atmosphere absorb strongly. Thus, flame methods for mercury are not noted for their sensitivity (typical flame defection limits are in the range 1-0.1 pg ml-i). If chemical reduction is employed, mercury can be brought into the vapour phase without the need to use a flame, and defection limits are dramatically improved. 

This chapter shows how a biphasic medium can help in reducing loss of volatile compounds in a gaseous phase exiting from a bioreactor, in comparison with pure aqueous systems. It also emphasises the usefulness of solvents having low vapour pressure (heavy organic solvents or ionic liquids) in the reduction of the release of compounds into the environment. There are, from this point of view, common interests between engineering needs and environmental concerns in the flavouring industry. 

This limitation causes the temperature-limits for the solubility of the initial and end products of the reaction. Normally, at higher pressure levels an increase in temperature also results in higher solubilities of substances in supercritical fluids because the increase in the vapour-pressure of the compounds to be dissolved overcomes the reduction in density. 

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