Constant Humidity Solutions

A saturated solution of sodium chloride can be used as a constant-humidity solution at 25°C, a relative humidity of 75% is produced. A specification for sodium chloride is contained in the Food Chemicals Codex (FCC). 

Wexler, A. (1994) in Constant Humidity Solution.s,Llde, D. P. (Ed.) CRCHandbookof Chemistry and Physics 75th Edition, Boca Raton, FL. 

Confidence intervals, A-88 to 90 Constant Humidity Solutions, 15-33 Constantan... 

Table 11.4 Solutions for Maintaining Constant Humidity Table 11.5 Concentration of Solutions of H2SO4, NaOH, and CaCi2 Giving Specified Vapor Pressures and Percent Humidities at 25°C Table 11.6 Relative Humidity from Wet and Dry Bulb Thermometer Readings Table 11.7 Relative Humidity from Dew Point Readings...

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. 

Equilibrium moisture content of a hygroscopic material may be determined in a number of ways, the only requirement being a source of constant-temperature and constant-humidity air. Determination may be made under static or dynamic conditions, although the latter case is preferred. A simple static procedure is to place a number of samples in ordinaiy laboratoiy desiccators containing sulfuric acid solutions of known concentrations which produce atmospheres of known relative humidity. The sample in each desiccator is weighed periodically until a constant weight is obtained. Moisture content at this final weight represents the equilibrium moisture content for the particular conditions. 

A rational development of models for moisture uptake begins with a description of the experimental procedure used to determine moisture uptake as a function of time. The first step in the experiment is to control the relative humidity to which a sample will be exposed. One technique to control humidity is to use saturated salt solutions. When placed in a closed system and held at a constant temperature, a saturated aqueous salt solution will provide a constant humidity (RH0) within that system. Table 1 lists relative humidities that will be maintained over various saturated salt solutions [14],... 

A saturated aqueous solution in contact with an excess of the solute when kept in an enclosed space will maintain a constant humidity at a given temperature. 

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. 

Spotting device 1-p.l disposable micropipet (e.g., Drummond Microcap), I-pi microsyringe, or (semi)automatic sample spotter Constant humidity chamber containing a cup of saturated NaCl solution 100°C oven... 

To provide constant humidity, Chromarods that have been spotted with a lipid sample are incubated in a constant humidity chamber for a fixed period of time (10 to 15 min) to reach equilibrium before being transferred to a developing tank. A saturated sodium chloride solution is widely used to provide constant humidity, as saturated solutions of inorganic salts give... 

The simple piezoelectric mass detection systems and the more sensitive surface acoustic wave (SAW) devices operate well in a dry or constant humidity gaseous environment (26,27), but they suffer from loss of signal in aqueous media. In the former case, this is largely a non specific response to water at the surface, and in the latter case it is associated with serious loss of the surface Rayleigh wave to the bulk solution. 

D Choice A is a restatement of Le Chatelier s Principle. B is a definition of equilibrium. A constant humidity (Choice C) occurs if the rate of vaporization and condensation are equal, indicating equilibrium. No solid is present in an unsaturated solution. If solid is added, all of it dissolves indicating a lack of equilibrium. D would be true for a saturated soution. 

Further increase of humidity has no appreciable effect on moisture content until a water activity is reached equal to that associated with a saturated solution of the substance in water at the temperature of the analysis. The relative humidity associated with this activity will be referred to as RHj. In this case, RH = 91% relative humidity, as the solubility of 50wt% is X2 = 0.09. On a mole fraction basis, and, assuming ideal solution behavior, x = 1 — X2 = Pi/Ri. In this hypothetical case, the moisture content of the saturated solution when all the solid is dissolved is 100% therefore, at 91% RH the monohydrate sorbs water from the atmosphere until dissolution is complete. The fact that constant humidity... 

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