Vapor Pressure of Saturated Salt Solutions

This table gives the vapor pressure in kPa of water above saturated solutions of some common salts at ambient temperatures. Data on pure water are given on the last line for comparison. 

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Vapor Pressure of Fluids at Temperatures below 300 K Vapor Pressure of Saturated Salt Solutions lUPAC Recommended Data for Vapor Pressure Calibration Enthalpy of Vaporization Enthalpy of Fusion... 

In all such methods, the solvent activity is measured over a range of solute concentrations. For example, solvent activity for a vapor-saturated aqueous salt solution can be easily determined from the decrease in vapor pressure of the salt solution relative to that of pure water, using the apparatus illustrated in Figure 17.7a. This technique... 

Membranes were dried under vacuum at 120°C and placed into closed bottles containing various water vapor pressures. Different saturated salt solutions were used to obtain the desired water vapor pressure. Solubility coefficients were obtained from the gravimetric measurement of the water sorbed into the membrane. 

Water isotherms were determined in the following way Samples of 80-100 mesh coal were evacuated at 100°C. for 2 hours in a glass tube and were then transferred to weighing bottles. The dried samples were exposed in a vacuum desiccator to water vapor from a series of saturated salt solutions with relative pressures ranging from 0.11 to 0.96 (6). The desiccator was kept in an air thermostat maintained at 35 0.5°. It was evacuated to a pressure of less than 5 mm. and then closed for equilibration. Seven days were allowed for equilibration at each sorption point. In previous work (1) this time was found to be sufficient for attaining equilibrium. 

The vapor pressure of an aqueous solution depends on its salt concentration. Curves of the saturation pressure as a function of temperature are reported in Figure 9.10 for freshwater, seawater, and brine with a salinity of 0, 3.45%, and 28%, respectively [28]. Vapor pressure increases quickly with temperature, even though the trend is affected by salinity. Considering solutions at the same temperature, an extremely small vapor pressure difference can be appreciated between freshwater and seawater, but a wider gap can be observed with reference to a high-salinity brine. The reverse vapor compression technology exploits such a pressure difference. 

Physical and Chemical Properties. Ammonium nitrate is a white, crystalline salt, df = 1.725, that is highly soluble in water, as shown in Table 3 (7). Although it is very hygroscopic, it does not form hydrates. This hygroscopic nature compHcates its usage in explosives, and until about 1940, was a serious impediment to its extensive use in fertilizers. The soHd salt picks up water from air when the vapor pressure of water exceeds the vapor pressure of a saturated aqueous ammonium nitrate solution (see Table 4). 

Referring to Figure 8, temperature Tc is the chamber temperature and Ts is the surface temperature at the salt solution/vapor interface. The temperature of the chamber is well defined and is an experimental variable, whereas Ts must be higher than Tc due to condensation of vapor on the saturated solution surface. We can determine Ts by applying the Clausius-Clapeyron equation to the problem. Assume that the vapor pressures of the surface and chamber are equal (no pressure gradients), indicating that the temperature must be raised at the surface (to adjust the vapor pressure lowering of the saturated solution) to Pc (at Tc) = Ps (at Tc). However, there is a difference in relative humidity between the surface and the chamber, where RHC is the relative humidity in the chamber and RH0 is the relative humidity of the saturated salt solution, and we obtain... 

The boundary conditions for the system are (1) that at the surface of the hygroscopic material the partial pressure of water is determined by that of the saturated salt solution (Ps) and (2) that at a characteristic distance from the surface (8) the partial pressure of water vapor is given by the chamber pressure (Pc). 

The heat of vaporization for a sea salt solution boiling at the same temperature and at its own saturation vapor pressure, P2, is the difference between the enthalpy of the vapor per pound, HV2l and the enthalpy per pound of the water in the salt solution, Hl2. 

The desiccator lid was manufactured with a ring stopcock through which evacuation was accomplished. A center tube was sealed on the lid, connecting it to a bulb. The bulb contained the saturated salt solution that maintained a constant water vapor pressure over the specimens. The bulb was isolable by means of a stopcock, so that the saturated salt solution could be stored under vacuum while samples were being changed. A ground joint connection made it possible to remove the whole upper assembly from the lid. Six such desiccators were employed, a different water vapor pressure being maintained in each. The relative... 

Saturated salt solutions and sulfuric acid solutions establish relative humidity by reducing the vapor pressure above an aqueous solution (a colligative effect). Saturated salt solutions at controlled temperature maintain a constant relative humidity as long as excess salt and bulk solution are present. As water is added or removed from the solution, moisture from the head-space will either condense or evaporate (as appropriate), with subsequent dissolution or precipitation of salt to maintain the equilibrium vapor pressure. Because the degree of vapor pressure depression is dependent on the number of species in solution and, further, since the solubility of most salts is somewhat dependent on temperature, the relative humidity generated is also temperature dependent. Hence, use of the same salt at different temperatures can result in different relative humidities. Refs. can be consulted for specific saturated salt solutions that result in defined relative humidities as a function of... 

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