Measurement of Vapor Pressure

The accurate determination of relative retention volumes and Kovats indices is of great utility to the analyst, for besides being tools of identification, they can also be related to thermodynamic properties of solutions (measurements of vapor pressure and heats of vaporization on nonpolar columns) and activity coefficients on polar columns by simple relationships (179). 

I.r. spectra 4,13) and measurements of vapor pressure 4) indicate strong inter-molecular hydrogen bridging. In contrast to concentrated H2SO4 difluorophosphoric acid is a base and can be titrated conductometrically 14) ... 

Sinke, G. C. (1974) A method for measurement of vapor pressures of organic compounds below 0.1 torr. Naphthalene as reference substance. J. Chem. Thermodyn. 6, 311-316. 

Sato, N., Inomata, H., Arai, K., Saito, S. (1986) Measurements of vapor pressures for coal-related aromatic compounds by gas saturation method. J. Chem. Eng. Jpn. 19, 145-147. 

EXCESS GIBBS FUNCTION FROM MEASUREMENT OF VAPOR PRESSURE... 

An attractive alternative to the direct measurement of vapor pressure is the use of gas chromatographic retention to estimate p (e.g., Hinckley et al., 1990). This method is based on the evaluation of the partitioning behavior of a given compound between the gas phase (i.e., the mobile phase) and a bulk organic phase (i.e., the stationary phase) at different temperatures. The method hinges on the selection of an... 

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. 

Under equilibrium conditions, the partial vapor pressure of water above an aqueous sample is defined by thermodynamic parameters. The measurement of vapor pressure, therefore, is linked to thermodynamic principles, though in a real system, which may or may not be at equilibrium, the vapor pressure is not necessarily that which is to be expected at equilibrium. In order to select a methodology for determining vapor pressure or related properties, the limitations imposed by any requirement for equilibrium and the consequences of a departure from equilibrium must be understood. The purpose of this commentary is to clarify the generic constraints that exist when one attempts to determine the water vapor pressure of a sample. 

The Basic Protocol describes the determination of water activity of a product using a chilled mirror dew-point water activity meter. Dew point is a primary measurement of vapor pressure that has been in use for decades (Harris, 1995). Dew-point instruments are accurate, fast, simple to use, and precise (Richard and Labuza, 1990 Snavely et al., 1990 Roa and Tapia de Daza, 1991). In a dew-point instrument, water activity is measured by equilibrating the liquid-phase water in the food sample with the vapor-phase water in the headspace, and then measuring the vapor pressure of the headspace. The basic principle involved in dew-point determinations of vapor pressure in air is that air may be cooled without change in water content until it saturates. The dew-point temperature is the temperature at which the air reaches saturation. It is determined in practice by measuring... 

Water activity (aw) is the ratio of the partial vapor pressure of water above a solution to that of pure water at the same specific temperature. It plays an important role in evaluating the microbial, chemical, and physical stability of foods during storage and processing. The vapor pressure in the headspace of a food sample can be measured directly by a manometer. A manometer has one or two transparent tubes and two liquid surfaces where pressure applied to the surface of one tube causes an elevation of the liquid surface in the other tube. The amount of elevation is read from a scale that is usually calibrated to read directly in pressure units. Makower and Myers (1943) were the first to use this method to measure vapor pressure exerted by food. Later, the method was improved, in terms of design features of the apparatus, by various scientists (Taylor, 1961 Labuza et al., 1972 Lewicki, 1987). Trailer (1983), Lewicki (1989), and Zanoni et al. (1999) used a capacitance manometer instead of a U-tube manometer for the measurement of vapor pressure. Lewicki et al. (1978) showed that the precision and reproducibility of the method can be improved by the simultaneous measurement of the water vapor pressure and temperature of the food sample. The method is reviewed in detail by Rizvi (1995) and Rahman (1995). 

Widiatmo, J.V., Tsuge, T., Watanabe K. (2001) Measurements of vapor pressures and PVT properties of pentafluoroethyl methyl ether and 1,1,1-trifluoroethane. J. Chem. Eng. Data 46, 1442-1447. 

In Eq. (2.1), Uo represents the energy of the crystal at the absolute zero as a function of the distance r between nearest neighboring atoms, where to represents the value of this distance in equilibrium, when U0 has its minimum value. The quantity L is the energy required to break up the metal into atoms at the absolute zero, or the latent heat of vaporization at the absolute zero, a quantity that can be found from experimental measurements of vapor pressure, as we saw in Chap. XI. The quantity a is an empirical constant. We shall determine L from the experimental value of the vapor pressure, ro from the experimental density at the absolute zero, and a from the compressibility. To compare with our formulation of Chap. XIII, we expand Eq. (2.1) in Taylor s series, finding... 

Which of the following processes must exist in equilibrium with the evaporation process when a measurement of vapor pressure is made ... 

DETERMINATION OF ACTIVITY COEFFICIENTS BY MEASUREMENT OF VAPOR PRESSURES... 

Methods of measurement have been reviewed (Kuntz and Kauzmann, 1974 McLaren and Rowen, 1951 Poole and Finney, 1986). Hydration levels are often established by isopiestic equilibration of protein samples against concentrated salt or sulfuric acid solutions of known water vapor pressure. A difficulty with this method is the long equilibration time, possibly several days, which is likely a consequence of the sample size (typically 100 mg or larger). Wilkinson et al. (1976) have described an automated sorption isotherm device transducers are used for the measurement of vapor pressure and sample weight. Gascoyne and Pethig (1977) used a resonating quartz crystal microbalance to study the hydration of bovine serum albumin and other proteins. Rao and Bryan (1978)... 

Measurement of Vapor Pressure. A number of experimental methods for the measurement of vapor pressure are available. Two of these methods will be briefly described since they illustrate the physical meaning of vapor pressure. 

Buback, M., and Franck, E.U. Measurements of vapor pressure and critical data of ammonium halides. Ber. Bunsenges. Phys Chem., 1972, 76, p. 350-4. 

Figure 13-12 A representation of the measurement of vapor pressure of a liquid at a given temperature. The container is evacuated before the liquid is added. At the instant the liquid is added to the container, there are no molecules in the gas phase so the pressure is zero. Some of the liquid then vaporizes until equilibrium is established. The difference in heights of the mercury column is a measure of the vapor pressure of the liquid at that temperature.

Formation of vapor includes all the processes in which gas is created from a system of condensed phases. Measurement of vapor pressure is closely connected to the determination of equilibrium between the gaseous and liquid or solid phases. 

Values of the water activity and osmotic coefficient for NaCl solutions in water for concentrations up to 6 m are shown in fig. 3.3. First, it is apparent that the vapor pressure of water decreases continuously with increase in salt concentration as one would expect. The value of at the most dilute concentration considered (0.1 m) is 0.99665. Measurements of vapor pressure lowering for lower concentrations are extremely difficult. The concentration dependence of the osmotic coefficient 9 reported in the same figure is quite different from that of the water activity. This quantity at first decreases, and then rises to values which are significantly... 

ASTM D-2503 Test Method for Estimation of Molecular Weight (Relative Molecular Mass) of Hydrocarbons by Thermoelectric Measurement of Vapor Pressure. 

In the measurement of vapor pressure, it is essential that the sample be thoroughly degassed even a small amount of dissolved air or other volatile component will distort the measurement. Degassing usually involves either distillation of the sample or placing the sample (sometimes after freezing) under vacuum. 

Measurement of Vapor Pressure and Boiling (or Sublimation) Point... 

Models are often developed to explain certain kinds of data, ignoring other kinds that also might be pertinent. The initial development of Pitzer s equations (33.34) for activity coefficients in concentrated solutions was focused on explaining measurements of vapor pressure equilibrium and of electromotive force (emf). The data could be explained by assuming that the electrolytes examined were, at least in a formal sense, fully dissociated. Later work using these equations to explain solubility data required the formal adoption of a few ion pair species (30). Even so, no speciation/activity coefficient model based on Pitzer s equations is presently consistent with the picture of much more extensive ion-pairing based on other sources, such as Smith and Martell s (35) compilation of association constants. This compilation is a collective attempt to explain other kinds of data, such as electrical conductance, spectrophotometry, and acoustic absorption. 

The value of 12,965 for the molal heat of vaporization at 0 K was found by Weinstock et al. [W2] to provide the best correlation of measurements of vapor pressure, density of each phase, and heat capacity of each phase through the Qapeyron equation. 

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