Vapor pressure aqueous salt solutions

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... 

Only a few solids have vapor pressures near atmospheric at safe temperatures, among them COz, UF , ZrCL(, and about 30 organics. Ammonium chloride sublimes at 1 atm and 350°C with decomposition into NH3 and HC1, but these recombine into pure NH4C1 upon cooling. Iodine has a triple point 113.5°C and 90.5 Torr it can be sublimed out of aqueous salt solutions at atmospheric pressure because of the entraining effect of vaporized water. 

Although Johnson and Furter (1,2), among others, observed a surprising insensitivity of k to mixed-solvent composition in many alcohol-water-inorganic salt systems, such does not appear to be the case with ammonium bromide-ethanol-water. A linear dependence of k with x was observed and is demonstrated in Figure 4. The slope of this dependence is 2.63 and the intercept with the y-axis occurs at approximately a value of unity. This extrapolated salt effect when x = 0, that is, with water as the single solvent, is consistent with Raoult s Law in that the vapor pressure of the aqueous salt solution depends directly on the salt concentration. However the same behavior has not been observed for the ammonium chloride-ethanol-water system (3) as seen in Table VIII its salt effect parameter shows essentially no dependence on the liquid composition. Therefore the two systems differ in this respect. 

Lowering of Vapor Pressure by Salts in Aqueous Solutions... 

In the isopiestic method, the vapor pressure of a solution is determined by equilibrating it with that of a reference solution (aqueous NaCl is often used) whose vapor pressure as a function of salt concentration and temperature is well known. This method can provide osmotic coefficients accurate to better than 1% over a wide range of conditions, but it is accurate only for electrolyte molalities above approximately 0.1 mol kg. The necessary equilibration may take several days, and specialized apparatus is required for measurements significantly above room temperature. 

Thermochemical data from either measurements or computations on organic compounds, including vapor pressures and solubility in organic and aqueous salt solutions. Data on cluster properties of organics and the sulfuric acid-water-ammonia system are necesary for understanding nucleation. 

LOWERING OF VAPOR PRESSURE BY SALTS IN AQUEOUS SOLUTIONS... 

Freezing Point Depression Dissolution of a salt in water lowers the vapor pressure over the solution. A direct result of this is the depression of the freezing point of water. In order to quantify this change, assume that our system has constant pressure and contains air and an aqueous salt solution in equilibrium with ice. According to thermodynamic equilibrium the chemical potential of water in the aqueous and ice phases will be the same, /a, = fiu,-If Ou, is the activity of water in solution then... 

In 1952, F.A. Long and W.F. McDevit (S15) presented the results of their extensive study of the activity coefficients of nonelectrolytes in aqueous salt solutions. Their results for the molar activity coefficients of undissociated nonelectrolytes in salt solutions were based mainly on solubUity, distribution and vapor pressure measurements. They noted that since the activity coefficient for any species i could be expressed as a power series to show the effects the concentrations of all solutes j in the solution ... 

A saturated aqueous solution in contact with an excess of a definite solid phase at a given temperature will maintain constant humidity in an enclosed space. Table 11.4 gives a number of salts suitable for this purpose. The aqueous tension (vapor pressure, in millimeters of Hg) of a solution at a given temperature is found by multiplying the decimal fraction of the humidity by the aqueous tension at 100 percent humidity for the specific temperature. For example, the aqueous tension of a saturated solution of NaCl at 20°C is 0.757 X 17.54 = 13.28 mmHg and at 80°C it is 0.764 X 355.1 = 271.3 mmHg. 

The high solubility of the salt and resultant low water vapor pressure (58) of its aqueous solutions ate usehil ia absorption air conditioning (qv) systems. Lithium bromide absorption air conditioning technology efficiencies can surpass that of reciprocal technology usiag fluorochlorocarbon refrigerants. 

Lithium Iodide. Lithium iodide [10377-51 -2/, Lil, is the most difficult lithium halide to prepare and has few appHcations. Aqueous solutions of the salt can be prepared by carehil neutralization of hydroiodic acid with lithium carbonate or lithium hydroxide. Concentration of the aqueous solution leads successively to the trihydrate [7790-22-9] dihydrate [17023-25-5] and monohydrate [17023-24 ] which melt congmendy at 75, 79, and 130°C, respectively. The anhydrous salt can be obtained by carehil removal of water under vacuum, but because of the strong tendency to oxidize and eliminate iodine which occurs on heating the salt ia air, it is often prepared from reactions of lithium metal or lithium hydride with iodine ia organic solvents. The salt is extremely soluble ia water (62.6 wt % at 25°C) (59) and the solutions have extremely low vapor pressures (60). Lithium iodide is used as an electrolyte ia selected lithium battery appHcations, where it is formed in situ from reaction of lithium metal with iodine. It can also be a component of low melting molten salts and as a catalyst ia aldol condensations. 

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). 

The formula weight of this compound is 1173.7. The crystals are red-orange equiaxed triclinic prisms, stable indefinitely in the dry state in air. The streak is yellow. The flotation density is 2.37 g/mL and the pH of 5% aqueous solution (24°) is 7.2. The hydrate vapor pressure is 0.3 torr at 25°. The solid may be dehydrated without further decomposition at 80° in vacuo in 2 hours. Loss of NH3 is rapid above 180° in air. The salt may be pyrolyzed to pure V2Os in air at 350-400° in about 2 hours. The solubilites are given below. 

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