Vapor pressure of pure compounds

Vapor pressure is the most important of the basic thermodynamic properties affec ting liquids and vapors. The vapor pressure is the pressure exerted by a pure component at equilibrium at any temperature when both liquid and vapor phases exist and thus extends from a minimum at the triple point temperature to a maximum at the critical temperature, the critical pressure. This section briefly reviews methods for both correlating vapor pressure data and for predicting vapor pressure of pure compounds. Except at very high total pressures (above about 10 MPa), there is no effect of total pressure on vapor pressure. If such an effect is present, a correction, the Poynting correction, can be applied. The pressure exerted above a solid-vapor mixture may also be called vapor pressure but is normallv only available as experimental data for common compounds that sublime. 

Methods used to determine the vapor pressure of pure compounds are usually divided into two groups based on pressure ranges to which they apply (1) between 1.0 and 760 torr, and (2) below 1.0 torr. 

The Antoine Eq. (1.8) has been used to represent vapor pressures of pure compounds more than arty of the others because it has several important advantages ... 

While the parameters presented in this series only describe pure compounds, the vapor pressures of pure compounds are essential for describing the phase behavior of mixtures accurately. The simplest equation for describing the phase behavior of mixtures is Raoulf s Law which states that the mole fraction of a component in an equilibrium vapor mixture multiplied by the total pressure equals the mole fraction of that component in the equilibrium liquid mixture multiplied by the vapor pressure. More accurate equations append correction terms to each side of this equation. 

FIGURE 7 Activity coefficients f2 (mixture), and /02 (solution) from vapor-pressure measurements on binary systems of components Yi (chloroform) and Y2 (acetone), p, pf, hypothetical total and partial vapor pressures of the ideal system pf, Raoult s law) Oi Ei = p, O2E2 = PgA vapor pressures of pure compounds Yi (chloroform) and V2 (acetone), p , p , Pg measured total and partial vapor pressures of the binary system chloroform-acetone, hypothetical partial vapor pres-... 

Provide reliable prediction of vapor pressures of pure compounds over a wide temperature range ... 

Henry s law constants for most of the compounds of interest can be found in the literature.54 Figure 18.11 shows Henry s law constants for TCE, EDC and several gasoline compounds.19 These data are derived from water solubility data and the equilibrium vapor pressure of pure liquids at certain temperatures, and may be extrapolated correctly to field design work. Temperature has a major effect on Henry s constant and on stripper performance. Each rise of 10°C in temperature... 

Boublik, T., Fried, V., Hala, E. (1984) The Vapor Pressure of Pure Substances, 2nd revised Edition, Elsevier, Amsterdam, The Netherlands. Bowman, B. T., Sans, W. W. (1983) Determination of octanol-water partitioning coefficient (KqW) of 61 organophosphorus and carbamate insecticides and their relationship to respective water solubility (S) values. J. Environ. Sci. Health B18, 667-683. Bradley, R. S., Cleasby, T. G. (1953) The vapour pressure and lattice energy of some aromatic ring compounds. J. Chem. Soc. 1953, 1690-1692. 

Stull, D.R. (1947) Vapor pressure of pure substances organic compounds. Ind. Eng. Chem. 39(4), 517-560. 

Commonly, gas-liquid partitioning is expressed by the saturated liquid vapor pressure, pi, of the compound i. This important chemical property will be discussed in detail in Chapter 4. Briefly, pi is the pressure exerted by the compound s molecules in the gas phase above the pure liquid at equilibrium. Since this pressure generally involves only part of the total pressure, we often refer to it as a partial pressure due to the chemical of interest. In this case, when there is no more build up of vapor molecules in a closed system, we say that the gas phase is saturated with the compound. Note that because pa. is strongly temperature dependent, when comparing vapor pressures of different compounds to see the influence of chemical structure, we have to use pi values measured at the same temperature (which also holds for all other equilibrium constants discussed later see Section 3.4). 

The vapor pressure of a compound is not only a measure of the maximum possible concentration of a compound in the gas phase at a given temperature, but it also provides important quantitative information on the attractive forces among the compound s molecules in the condensed phase. As we will see below, vapor pressure data may also be very useful for predicting equilibrium constants for the partitioning of organic compounds between the gas phase and other liquid or solid phases. Finally, we should note that knowledge of the vapor pressure is required not only to describe equilibrium partitioning between the gas phase and a condensed phase, but also for quantification of the rate of evaporation of a compound from its pure phase or when present in a mixture. 

Since the vast majority of organic molecules behave almost ideally in the gas phase for temperatures below 200°C, Eq. (7.6) allows us to calculate the vapor pressure of such compounds from the chemical potential difference of the molecule in the ig phase and in the respective liquid state. As a special case, we can express the vapor pressure of the pure compound X as... 

The temperature dependent vapor pressure of pure elements or compounds is frequently used to define volatility. For the adsorbed state the relevant quantity is the desorption pressure, which depends on the temperature and on the surface coverage. The individual crystal lattices with their characteristic binding properties and thus, their standard entropy of the pure solid phase,... 

Pesticides applied indoors vaporize from treated surfaces (e.g. carpets and baseboards) and can be resuspended into air on particles. Many pesticides are semivolatile (saturation vapor pressures between 10 kPa and 10 kPa at 25 °C) and tend to vaporize from treated indoor surfaces. The rate of volatilization will depend on the vapor pressure of the compound, the formulation (solvent, surfactants, microencapsulation, etc.), the ambient and surface temperatures, indoor air movement and exchange rates (ventilation), the type of surface treated and the elapsed time after application. The vapor pressure data for pure pesticides is frequently available and may be of value for assessing the relative importance... 

Strommen MR, Kamens RM (1999) Simulation of semivolatile organic compound microtransport at different time scales in airborne diesel soot particles. Environ Sci Technol 33 1738-1746 Thibodeaux LJ, Nadler KC, Valsaraj KT, Reible RR (1991) The effect of moisture on volatile organic chemical gas-to-particle partitioning with atmospheric aerosols-competitive adsorption theory predictions. Atmos Environ 25A 1649-1656 Thompson JD, Cramer CJ, Truhlar DG (2003) Predicting aqueous solubilities from aqueous free energies of solvation and experimental or calculated vapor pressures of pure substances. J Chem Phys 19 1661-1670... 

Vapor Pressure of Pure Substances. Organic Compounds Industrial and Engineering Chemistry 39, 517 (1947)... 

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