The Effect of Pressure on Solubility

When you open a carbonated drink, you can observe the effect of pressure on solubility. Figure 8.13 shows this effect. Inside a soft drink bottle, the pressure of the carbon dioxide gas is very high about 400 kPa. When you open the bottle, you hear the sound of escaping gas as the pressure is reduced. Carbon dioxide gas escapes quickly from the bottle, since the pressure of the carbon dioxide in the atmosphere is much lower only about 0.03 kPa. The solubility of the carbon dioxide in the liquid soft drink decreases greatly. Bubbles begin to rise in the liquid as gas comes out of solution and escapes. It takes a while for all the gas to leave the solution, so you have time to enjoy the taste of the soft drink before it goes flat. ... 

Do you crack your knuckles The sound you hear is another example of the effect of pressure on solubility. Joints contain fluid. When a joint is suddenly pulled or stretched, the cavity that holds the fluid gets larger. This causes the pressure to decrease. A bubble of gas forms, making the sound you hear. You cannot repeatedly crack your knuckles because it takes some time for the gas to re-dissolve. 

The discussion in the preceding paragraph may now be used to examine the effect of pressure on solubility that is to say on conditions away from the eutectic point. We now have crystals of 2 in equilibrium with a solution consisting of 1 and 2. We need now only consider the second equation of (22.24). Under isothermal conditions... 

The first experimental determinations of the effect of pressure on solubility were made by E. von Stackelberg, who obtained values which are in qualitative agreement with Braun s law and the theorem of Le Chatelier, as the following table shows —... 

Eq. (1.43) is the most general form of the solubility equation. In most situations (though not all) the effect of pressure on solubility is negligible so that the last term on the right-hand side of the equation can be dropped. In addition, the heat capacity term can also usually be dropped from the equation. This yields... 

Bottlers use the effect of pressure on solubility in producing carbonated beverages, which are bottled under a carbon dioxide pressure greater than 1 atm. When the bottles are opened to the air, the partial pressure of CO2 above the solution decreases. Hence, the solubility of CO2 decreases, and C02(g) escapes from the solution as bubbles (4 FIGURE 13.16). 

The effect of pressure on solubility was calculated independently by F. Braun and J. J. Thomson. ... 

Pouring root beer into a glass illustrates the effect of pressure on solubility. The escaping CO2 produces the foam. 

The effect of pressure on solubility plays a prominent role only where gases are involved and has little effect in the cases of liquids and solids. 

Le Chatelier s principle was introduced in Section 12.3, where it was used to determine the effect of pressure on solubility of gases in liquids. 

The phase behaviour of water and carbon dioxide has been studied by various authors, so literature data in wide ranges of temperature and pressure is available. Selected data for the relevant pressure and temperature conditions are presented in Fig. 15.9 [15,16]. CO2 has the lowest solubility in water barely exceeding 6 wt% at 313 K and 20.3 MPa. The effect of pressure on solubility weakens above 5 MPa as depicted by the near vertical isotherm progress. The composition of the light phase shows around 4 wt% water vapour at 1.1 MPa and 373 K. 

The effect of pressure on the solubility of chlorine ia hydrochloric acid has been reported for pressures varying from about 100 to 6500 kPa (1—6.5 atm) (20). At pressures above 200 kPa, there is a linear dependence of pressure on the solubility in the acid concentration range of 0.1—5.0 N. 

Another consequence of the effect of pressure on gas solubility is the painful, sometimes fatal, affliction known as the bends. This occurs when a person goes rapidly from deep water (high pressure) to the surface (lower pressure), where gases are less soluble. The rapid decompression causes air, dissolved in blood and other body fluids, to bubble out of solution. These bubbles impair blood circulation and affect nerve impulses. To minimize these effects, deep-sea divers and aquanauts breathe a helium-oxygen mixture rather than compressed air (nitrogen-oxygen). Helium is only about one-third as soluble as nitrogen, and hence much less gas comes out of solution on decompression. 

The data of Smith [35] is reported graphically in Fig. 11 and shows the effect of pressure on the solubility of adamantane in various supercritical solvents (carbon dioxide, methane, and ethane) at 333 K. 

It may be added here that Le Chatelier s principle is quite general in nature, and that its applicability is not restricted only to chemical equilibria. It can also be applied to physical equilibria, as for example, to explain qualitatively the effects of temperature and pressure on solubility or the effect of pressure on the melting of a solid. 

A more detailed analysis of the effect of pressure on the solubility of gases is planned for the future. 

Increasing the operating pressure of MCFCs results in enhanced cell voltages because of the increase in the partial pressure of the reactants, increase in gas solubilities, and increase in mass transport rates. Opposing the benefits of increased pressure are the effects of pressure on undesirable side reactions such as carbon deposition (Boudouard reaction) ... 

Figure 9.11 shows the effect of pressure on the solubility of H2O in an albitic melt at r = 1100 °C, based on the experiment of Burnham and Davis (1974). At low H2O amounts, a marked linear correlation between fugacity of H2O in the gaseous phase and the squared molar fraction of H2O component in the melt is... 

Lastly, figure 9.16 shows the effects of pressure on the solubility of argon in... 

Figure 9J6 Effects of pressure on solubility of argon in silicate melts. From White et al. (1989). Reprinted with permission of The Mineralogical Society of America.

Another factor that differentiates the solubility of gases from solids and liquids is the effect of pressure. The effect of pressure on gas solubility was studied extensively by a contemporary and close associate of John Dalton named William Henry (1775-1836). Henry s Law states that the solubility of a gas is directly proportional to the partial pressure of that gas over the solution. Stated mathematically, Henry s Law is c = kP, where c is the concentration of the dissolved gas in moles per liter, k is Henry s law constant for the solution, and P is the partial pressure of the gas above the solution. Henry s Law is demonstrated every time a carbonated beverage is opened. During the carbonation process, carbon dioxide is dis-... 

The equations are sometimes useful, but it must be borne in mind that the formulae are equations of continuous curves, whereas actual solubility curves are not usually continuous except over limited ranges of temp, determined by the stability of particular phases—e.g. hydrates. The effect of pressure on the solubility of sodium chloride has been previously indicated. C. Moller showed in 1862 that the solubility is increased by 20 and by 40 atm. press. The thermal expansion of salt soln. was also found by G. C. Schmidt to be more regular than with water. W. C. Rontgen and J. Schneider, and V. Schumann, have measured the compressibility of soln. of potassium and sodium chlorides. 

Studiengesselschaft Kohle m.b.H. (2) reported the effect of temperature on solubility level in supercritical gas. The solubility is highest within 20 K of the critical temperature and decreases as temperature is raised to 100 K above the critical temperature. At temperatures near the critical temperature, a sharp rise in solubility occurs as the pressure is increased to the vicinity of the critical pressure and increases further as the pressure is further increased. Less volatile materials are taken up to a lesser extent than more volatile materials, so the vapor phase has a different solute composition than the residual material. There does not seem to be substantial heating or cooling effects upon loading of the supercritical gas. It is claimed that the chemical nature of the supercritical gas is of minor importance to the phenomenon of volatility amplification. Ethylene, ethane, carbon dioxide, nitrous oxide, propylene, propane, and ammonia were used to volatilize hydrocarbons found in heavy petroleum fractions. 

Nonlinear, pressure-dependent sorption and transport of gases and vapors in glassy polymers have been observed frequently. The effect of pressure on the observable variables, solubility coefficient, permeability coefficient and diffusion timelag, is well documented (1, 2). Previous attempts to explain the pressure-dependent sorption and transport properties in glassy polymers can be classified as concentration-dependent and "dual-mode models. While the former deal mainly with vapor-polymer systems (1) the latter are unique for gas-glassy polymer systems (2). 

The magnitude of the effect of pressure on activity coefficients is much less than is the case for solubility products (see previous discussion). The errors in ignoring the compressibility term for activity coefficients are, at most, 2-5% in a pressure range up to 1000 bars (Millero 1983 Krumgalz et al. 1999). For the broad-scale FREZCHEM model, these errors are acceptable. 

While few things are absolutely insoluble, some solutes are much more soluble in a given solvent than others. The solubility of a solute is the amount of the solute that will dissolve in a given amount of solvent at a given temperature. For example, sodium chloride is quite soluble in water and has a solubility of 39.5 g per 100 mL of water at 25°C. We ll talk later about the effect of temperature on solubility. Oxygen, on the other hand, is not very soluble in water, with a solubility of 42 mg per 100 mL of water at 25°C and a pressure of 1 atm. We ll talk later about the effect of temperature and pressure on the solubility of gases. 

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