Poynting effect

Neglecting vapor-phase corrections and the Poynting effect, the relative volatility between the key components in the presence of the solvent is... 

Solubility in Compressed Gases.— When an inert gas (1) is added to a system in which a liquid or solid (2) is in equilibrium with its vapour, the vapour pressure of the condensed substance changes. In other words, if the total pressure in the system is p and the mole fraction of (2) in the vapour phase is y, the partial pressure py is not equal to pf, the vapour pressure of the pure substance. If the inert gas is not appreciably soluble in the solid or liquid, the change in vapour pressure is attributable to two effects. (/) The chemical potential of the solid or liquid is increased because of the rise in the applied pressure, the so-called Poynting effect. (k) The chemical potential of the vapour is changed because of interactions between the vapour and the inert gas. 

At very low pressures, the two fiigacity coefficients and the Poynting effect are equal to unity, and ... 

Determination, thus, of the activity coefficient value requires the evaluation of the two fugacity coefficients and of the Poynting effect. To this purpose, we recall from Chapters 9 and 11 that at the low pressures of interest here ... 

Determination, thus, of the fugacity coefficients through the virial equation and of the Poynting effect through Eq.9.11.14 allows the evaluation of activity coefficients from the experimental data. An appropriate computer program (GAMMA) is presented in Appendix E. 

S.Values for the Poynting effect can be determined from available or estimated liquid molar volume data. 

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. 

For components near or above their critical temperatures, the liquid volume t>j was evaluated by extrapolation with respect to temperature. For supercritical components, the fugacity f° was also evaluated by extrapolation the effect of pressure was found from the Poynting relation using the previously extrapolated liquid molar volumes. 

The exponential term in equation 8.32 is known as the Poynting correction, and corrects for the effects of pressure on the liquid-phase fugacity. 

The fundamental fact on which the analysis of heterogeneous reactions is based is that when a component is present as a pure liquid or as a pure solid, its activity may be taken as unity, provided the pressure on the system does not differ very much from the chosen standard state pressure. At very high pressures, the effect of pressure on solid or liquid activity may be determined using the Poynting correction factor. 

Gibbs-Poynting equation phys chem An expression relating the effect of the total... 

Figure 12.4 Field lines of the total Poynting vector (excluding that scattered) around a small aluminum sphere illuminated by light of energy 8.8 eV (a) and 5 eV (b). The dashed vertical line in (a) indicates the effective radius of the sphere for absorption of light.

The standard state used for the finite concentration data was usually the pure solvent at the temperature and pressure of the mixture. In most cases, the Poynting correction (pressure effect on the liquid) could be neglected. The end result of this approximation is that saturation pressure appears in the expression for the activity coefficient, but not the system pressure. 

The exponential term is the Poynting correction, which is the effect of pressure on the reference fugacity, and is only important at high pressure. 

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