The Solubility of Gases in Liquids

Review by Markham and Kobe 1941) on The Solubility of Gases in Liquids  

Attention was drawn to the lack of discipline in the presentation of data up to that time. It had been common practice to observe random pressures near 1 atm and then adjust to 1 atm by means of Henry s law. There was sometimes confusion over partial and total pressure, and there was confusion over the Bunsen and Ostwald coefficients. 

Much data in the literature, especially for earlier dates, are for gases in the lower half of the range of boiling points/1 atm, and so they refer to absolutely small values of Xa( = Na). Operational procedures, therefore, have involved the measurement of a volume of gas A, and this has usually been expressed as an absorption coefficient. The matter of total, pr, or partial pressure, p, must be considered. To equate Pa to 

Pr may involve an error which for certain purposes may be neglected. Attempts to allow for the vapor pressure ps of the original liquid S and thereby arrive at Pr—ps-PA should be carefully scrutinized in every case because ps will be lowered as Xa increases at the different Pa- In the present exposition, Pa is the partial pressure of A, i.e., the pressure due to A. Furthermore, I deem it important to bring into the pattern data for Pa 1 atm, or even 2 atm. 

Briefly, the primary data may be recognized as the volume of gas A measured at a stated t°C and Pa (approximately 1 atm). Now this volume may contain the number of moles of A absorbed at that fC and that Pa, or absorbed at the same or different t°C and a different (could be much greater) Pa which prevailed at the absorption. 

Air is somewhat soluble in water at room temperature (20°C) one liter of water dissolves 19.0 ml of air at 1 atm pressure. (The amount of dissolved air decreases with increasing temperature.) If the pressure is doubled, the solubility of air is doubled. This proportionality of the solubility of air to its pressure illustrates Hemy s lan f which may be stated in the following way At constant temperature, the partial pressure in the gas phase of one component of a solution is, at equilibrium, proportional to the concentration of the component in the solution, in the region of low concentration. 

This is equivalent to saying that the solubility of a gas in a liquid is proportional to the partial pressure of the gas. 

The solubilities of most gases in water are of the order of magnitude of that of air. Exceptions are those gases which combine chemically with water or which dissociate largely into ions, including CO2 (solubility 1,713 ml 1 at 0°C), H2S (4,670), and SO2 and NH3, which are extremely soluble. 

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Bunsen (1855), to whom we owe the first accurate measurements of the solubilities of gases in liquids, expressed his results in terms of an absorption coefficient /3, which he defined as the volume of gas, reduced to 0° C. and 76 cm., dissolved by 1 c.c. of the liquid at any given temperature under the same pressure. If v c.c. of gas are dissolved by Y c.c. of liquid at a temperature 6 and pressure p cm., the volume reduced to normal conditions is... 

Many definitions are used to express the solubility of gases in liquids, but usually the equilibrium law is defined as ... 

The solubility of most solids increases with increasing temperature. However, the solubility of gases in liquids decreases with increasing temperature. For example, if you open a cold bottle of soda and a warm bottle of soda, more gas is released by the warm soda. This is the basis of thermal pollution, in which the solubility of oxygen in stream or lake water is decreased if the water is polluted by heat. 

In this chapter, you learned about solutions. A solution is a homogeneous mixture composed of a solvent and one or more solutes. Solutions may be unsaturated, saturated, or supersaturated. Solution concentration units include percentage, molarity, molality, and mole fraction. The solubility of solids in liquids normally increases with increasing temperature, but the reverse is true of gases dissolving in liquids. The solubility of gases in liquids increases with increasing pressure. 

Furthermore, the solubility of gases in liquids decreases with rising temperature. Accordingly, the concentration of a dissolved gas in water can be reduced simply by vacuum degassing or by heating. Alternatively, one can resort to specific chemical methods of removal.18,19... 

Bassam Z. Shakhashiri, "Effect of Temperature and Pressure on the Solubility of Gases in Liquids," Chemical Demonstrations, A Handbook for Teachers of Chemistry, Vol. 3 (The University of Wisconsin Press, Madison, 1989) pp. 280-282. 

Volatilization of ammonia from aqueous solutions. When an aqueous solution of ammonia (ammonium hydroxide) is warmed, some of the dissolved ammonia volatilizes in the gaseous form. This result is to be anticipated in light of the manner in which the solubility of gases in liquids is generally influenced by an increase in temperature. In the use of this method, the quantity of heat supplied should be such that a minimum quantity of water is vaporized. 

W. Swope, H. Andersen, A molecular dynamics method for calculating the solubility of gases in liquids and the hydrophobic hydration of inert-gas atoms in aqueous solution. J. Phys. Chem. 88, 6548 (1984)... 

Increases in pressure increase the solubility of gaseous solutes, but have little effect on solid solutes. Similarly, decreases in pressure decrease the solubility of gases in liquids and have little effect on solid solutes. 

The correct answer is (B). Gases are most soluble in liquids at low temperature. This is unlike most solids, which become less soluble at lower temperatures. In addition, high external pressures increase the solubility of gases in liquids. For solids, pressure has a negligible effect. 

The solubility rules for common salts. The solubility of gases in liquids—Henry s law. The partition of a solute between two solvents. 

We can deduce an important result, as to the solubility of gases in liquids, from this theorem. A solution of this kind can be regarded as a hquid mixture of two substances A and B, in which the partial pressure pf, is the pressure of the dissolved gas over the liquid. Hence, if Raoult s law holds, we have... 

Describe factors involved in the solubility of gases in liquids. 

The application of Henry s law in the calculation of the effect of pressure on the solubility of gases in liquids is illustrated in Example 2.5. 

Discuss the meaning of Henry s law and use it to calculate the solubilities of gases in liquids (Section 11.6, Problems 57-60). 

The resistance to mass transfer according to (1.221) and (1.223) is made up of the individual resistances of the gas and liquid phases. Both equations show how the resistance is distributed among the phases. This can be used to decide whether one of the resistances in comparison to the others can be neglected, so that it is only necessary to investigate mass transfer in one of the phases. Overall mass transfer coefficients can only be developed from the mass transfer coefficients if the phase equilibrium can be described by a linear function of the type shown in eq. (1.217). This is normally only relevant to processes of absorption of gases by liquids, because the solubility of gases in liquids is generally low and can be described by Henry s law (1.217). So called ideal liquid mixtures can also be described by the linear expression, known as Raoult s law. However these seldom appear in practice. As a result of all this, the calculation of overall mass transfer coefficients in mass transfer play a far smaller role than their equivalent overall heat transfer coefficients in the study of heat transfer. 

The solubility of gases in liquids is often treated as an equilibrium process. Take the dissolution of carbonyl sulphide (OCS) as an example ... 

Increases in pressure increase the solubility of gaseous solutes, but have little effect on solid solutes. Similarly, decreases in pressure decrease the solubility of gases in liquids and have little effect on solid solutes. There are four main coUigative properties, or properties of a solvent that are affected by the presence of a solute vapor-pressure reduction, boiling-point elevation, freezing-point depression, and osmotic pressure. 

Equilibrium data are presented in a variety Of ways. Frequently, the solubilities of gases in liquids in which they are sparingly soluble are given in terras of Henry s Law constant H. Henry s Law states simply that lbe solubility of a gas in a liquid is directly proportional to its partial pressure in the gas phase that... 

Coalescenceis especially typical in concentrated emulsions. In such systems coalescence mainly determines the lifetime of emulsions prior to phase separation. In finely dispersed emulsions, both dilute and concentrated, the average size of drops may noticeably increase due to Ostwald ripening. At the same level of dispersion Ostwald ripening of emulsion droplets is a slower process than mass transfer of bubbles in foams [60]. This is due to a rather low interfacial energy, and consequently, low difference in chemical potentials of substance in droplets of different size, as well as due to a lower mutual solubility of liquids as compared to the solubility of gases in liquids. 

The study of vapor-liquid equilibria (Sec 10.1) of the solubility of gases in liquids (Sec. 11.1), and of the solubility of solids in liquids (Sec. 12.1), all involve nonsimple, mixtures. To see why this occurs, consider the criterion for vapor-liquid equilibrium ... 

The solubility of gases in liquids is increased and that of solids is usually decreased by raising the pressure. Therefore, the solid solute of a saturated solution may precipitate during the generation of pressure and is no longer accessible for the reaction. The viscosity of liquid increases approximately twice every kilobar. This effect is particularly important for reactions containing diffusion-controlled steps. Finally, the compressibility of liquids is usually small compared to that of... 

The liquid acts as a barrier to free migration of gas. The rate of diffusion of gas is brought to a steady state when the rate of diffusion of the gas into the liquid on the upstream side is just equal to the rate at which the gas is diffusing out of the membrane on the downstream side. Various steady-state conditions may exist according to pressure (the solubility of gases in liquids increases with pressure), temperature (the solubility of gases in liquids decreases with temperature but the rate of diffusion increases), and other controllable factors. The inieirelaiionships of these factors can be predicted from Ficl s laws of diffusion and the gas laws. 

The dependence of the solubility of gases in liquids on their pressure at a constant temperature is expressed a.s Henry s Law... 

Another very early discovery (Henry, 1803) involved the solubility of gases in liquids. It was found that the amount of gas that dissolved in a liquid in contact with it was directly proportional to the pressure on the gas (Figure 11.3). Thus... 

Equation (14.28) states that the solubility Xj of a volatile constituent is proportional to the partial pressure of that constituent in the gaseous phase in equilibrium with the liquid. Equation (14.28) is used to correlate the data on solubility of gases in liquids. If the solvent and gas do not react chemically, the solubility of gases in liquids is usually small and the condition of diluteness is fulfilled. Here we have another example of the physical significance of the partial pressure. 

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