Water vapor-pressure lowering

Phosphoric Acid Fuel Cell. Concentrated phosphoric acid is used for the electrolyte ia PAFC, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor (see Phosphoric acid and the phosphates), and CO poisoning of the Pt electrocatalyst ia the anode becomes more severe when steam-reformed hydrocarbons (qv) are used as the hydrogen-rich fuel. The relative stabiUty of concentrated phosphoric acid is high compared to other common inorganic acids consequentiy, the PAFC is capable of operating at elevated temperatures. In addition, the use of concentrated (- 100%) acid minimizes the water-vapor pressure so water management ia the cell is not difficult. The porous matrix used to retain the acid is usually sihcon carbide SiC, and the electrocatalyst ia both the anode and cathode is mainly Pt. 

Just as at low pH, concentration mechanisms substantially increase attack. The two principal mechanisms of concentration are evaporation and condensation. Evaporation increases solute concentrations of compounds with vapor pressures lower than water (such as caustic compounds). Condensation increases concentration of aggressive gases such as ammonia. 

Vapor pressure lowering is a true colligative property that is, it is independent of the nature of the solute but directly proportional to its concentration. For example, the vapor pressure of water above a 0.10 M solution of either glucose or sucrose at 0°C is the same, about 0.008 mm Hg less than that of pure water. In 0.30 M solution, the vapor pressure lowering is almost exactly three times as great, 0.025 mm Hg. 

A solution contains 82.0 g of glucose, QH Oe, in 322 g of water. Calculate the vapor pressure lowering at 25°C (vapor pressure of pure water =... 

Strategy First (1), calculate the number of moles of glucose (MM = 180.16 g/mol) and water (MM = 18.02 g/mol). That information allows you to find (2) the mole fraction of glucose. Finally (3), use Raoult s law to find the vapor pressure lowering. 

Effects of vapor pressure lowering. Because a nonvolatile solute lowers the vepor pressure of a solvent, the boiling point of a solution will be higher and the freezing point lower than the corresponding values for the pure solvent Water solutions freeze below 0°C at point A and boil above 100°C at point B. 

E7.14 Estimate the vapor pressure lowering and the osmotic pressure at 293.15 K for an aqueous solution containing 50.0 g of sucrose (Mi = 0.3423 kg-mol"1) in 1 kg of water. At this temperature, the density of pure water is 0.99729 g em"3 and the vapor pressure is 2.33474 kPa. Compare your results with those given in Table 7.3. 

Hydrogen peroxide, H202, is a syrupy liquid with a vapor pressure lower than that of water and a boiling point of 152<>C. Account for the differences between these properties and those of water. 

The eoexistence of laumontite and wairakite is common in zone (1). If the saturated water vapor pressure is equal to 0.3 of total pressure (Zeng and Liou, 1982), the temperature for equilibrium reaetion (1—23) and saturated water vapor pressure are estimated to be 170°C and 230 bar, respectively (Liou, 1971b). Zeng and Liou (1982) have shown that yugawaralite is stable at less than 230°C and a total pressure of 500 bar, under the condition of quartz saturation. However, if the activity of Si02 is not unity, the boundary for reactions (1-24) and (1-25) may shift to lower temperatures. Liou (1971a) studied the equilibrium for reaetion (1-26) and showed that the equilibrium... 

The equilibrium water vapor pressure over the materials in hydrate forms is greater than that over the materials in hydroxide forms. Elimination of water vapor and hence drying of the material occurs when the ambient water vapor pressure in the system is lower than the equilibrium water vapor pressures given by the above equations. To effect drying, there are two options. 

The other option available for drying involves subjecting the material to reduced pressures. By decreasing the ambient pressure, for example, by evacuation, to a level lower than the equilibrium water vapor pressure over the material, drying can be implemented. This option is particularly important when heating affects the material, in addition to drying it. 

SI water= ethylene glycol water where APwater = vapor pressure lowering of water, H20... 

Point 4 above may be correct, but only if the partial water vapor pressure of the product at the product temperature is larger than the existing partial water vapor pressure in the chamber, otherwise the chamber pressure has to be lowered. Since the energy input during secondary drying is not decisive, it would be safer to use a chamber pressure as small as the condenser temperature allows. 

Lindsay, W. T., Jr. Liu, C. T. "Vapor Pressure Lowering of Aqueous Solutions at Elevated Temperatures" Office of Saline Water, U.S. Government Printing Office,... 

As a consequence of the action of the condenser, the water vapor pressure Pp2 at the exit of the condenser is always lower than that at the entrance for (2.23) to be fulfilled, the partial pressure of air Pp2 at the exit must be higher than at the entrance Pp, (see Fig. 2.43), even when no throttle is present. 

The lower limit of the water vapor partial pressure is determined through the compression ratio of the Roots pump at the backing pressure, w/hich is determined by the saturation vapor pressure of the condensed water. Also, in this region the intermediate condenser must be able to reduce the vapor partial pressure to at least 60 mbar. The stated arrangement is suitable -when cooling the condenser with water at 15 °C - tor water vapor pressures between about 4 and 40 mbar. 

Figure 7.18 gives the ratio (K /K)s4 s4 of the calculated equilibrium constants for solution-phase ammonium nitrate compared to the solid salt product at various temperatures and water activities. As the water activity, i.e., water vapor pressure above the solution, increases, the equilibrium constant falls. That is, at higher relative humidities, relatively less HNO, and NH, are found in the vapor phase at equilibrium. This may be why relatively more ammonium nitrate in particles collected on filters evaporates at lower RHs compared to higher ones. 

While water is a major component of tropospheric particles, and hence largely determines the surface tension (y), organics found in particles may act as surfactants (see Chapter 9.C.2). In this case, their segregation at the air-water interface could potentially lead to a substantial surface tension lowering of such particles, which would lead to a lower equilibrium water vapor pressure over the droplet (Eq. (BB)) and hence activation at smaller supersaturations. This possibility is discussed in more detail in the next section. 

As we have seen in Chapter 9, there are a variety of dissolved solutes in atmospheric particles, which will lower the vapor pressure of droplets compared to that of pure water. As a result, there is great interest in the nature and fraction of water-soluble material in atmospheric particles and their size distribution (e.g., Eichel el al., 1996 Novakov and Corrigan, 1996 Hoffmann et al., 1997). This vapor pressure lowering effect, then, works in the opposite direction to the Kelvin effect, which increases the vapor pressure over the droplet. The two effects are combined in what are known as the Kohler curves, which describe whether an aerosol particle in the atmosphere will grow into a cloud droplet or not under various conditions. 

ACTIVITY COEFFICIENT. A fractional number which when multiplied by the molar concentration of a substance in solution yields the chemical activity. This term provides an approximation of how much interaction exists between molecules at higher concentrations. Activity coefficients and activities are most commonly obtained from measurements of vapor-pressure lowering, freezing-point depression, boiling-point elevation, solubility, and electromotive force. In certain cases, activity coefficients can be estimated theoretically. As commonly used, activity is a relative quantity having unit value in some chosen standard state. Thus, the standard state of unit activity for water, dty, in aqueous solutions of potassium chloride is pure liquid water at one atmosphere pressure and the given temperature. The standard slate for the activity of a solute like potassium chloride is often so defined as to make the ratio of the activity to the concentration of solute approach unity as Ihe concentration decreases to zero. 

DELIQUESCENCE. When a substance absorbs moisture upon exposure to the atmosphere, the substance is said lo be deliquescent. At ordinary temperatures the vapor pressure of water varies as shown in Table I. If the solution of a substance in water has a lower water vapor pressure than that of the atmosphere at the given temperature, water vapor condenses in the solution from Ihe atmosphere until the water vapor pressure of the solution equals the water vapor pressure of the surrounding atmosphere. 

Figure A2.1.2 Effect of temperature on vapor pressure measurement. The upper curve is the vapor pressure of pure water, pw°. The lower curve is a system whose partial water vapor pressure, pw, is always a constant fraction of the vapor pressure of pure water. See text for details.

The influence of temperature on the measurement is best illustrated by referring to Figure A2.1.2. Here, the upper curve represents the vapor pressure of pure water, pw°, as a function of temperature, and the lower curve represents a system whose partial water vapor pressure is always a constant fraction, a, of the vapor pressure of pure water. The points E and G representpw° at temperatures T and T2, respectively. The points F and H represent pw at temperatures 7 , and T2, respectively. At both Tx and T2, the ratio of the vapor pressures [pw/pw°]r is a (i.e., the ratios of the lines BF/BE and the lines DH/DG are both equal to a). 

The third important loss is by air convection inside the distiller. This air circulation is a necessary accompaniment of water distillation. The higher the basin temperature, the higher is the water vapor pressure and the lower the ratio of air to water vapor in the atmosphere of the distillation unit. Factors tending to maximize basin temperatures will therefore reduce this heat loss because of the lower concentration of air in the atmosphere of the distiller enclosure. But as radiation loss increases with rise in basin temperature, these two losses cannot be simultaneously minimized by temperature change. Any effort toward reducing convection in the distiller would be undesirable, because this is the only significant mechanism for water distillation. 

The boiling-point elevation of a solution relative to that of a pure solvent depends on the concentration of dissolved particles, just as vapor-pressure lowering does. Thus, a 1.00 m solution of glucose in water boils at 100.51°C at 1 atm pressure (0.51°C above normal), but a 1.00 m solution of NaCl in water boils at... 

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