The Aqueous Solubility of Gases

Recall that solubility is defined as the maximum amount of solute that will dissolve in a given quantity of solvent at a specific temperature. Temperature affects the solubility of most substances. In this section we will consider the effects of temperature on the aqueous solubility of solids and gases, and the effect of pressure on the aqueous solubility of gases. 

The relationship between temperature and the aqueous solubility of gases is somewhat simpler than that of solids. Most gaseous solutes become less soluble in water as temperature increases. If you get a glass of water from your faucet and leave it on the kitchen counter for a while, you will see bubbles forming in the water as it warms to room temperature. As the temperature of the water increases, dissolved gases become less soluble and come out of solution—resulting in the formation of bubbles. 

The absolute values of the solubilities of gases are not at present calculable from any general law, although W. M. Tate (1906) finds in the case of aqueous solutions a relation with the viscosities of the solution (/x ), and water (/x0), the critical temperatures of the gas (T0), and of water (T. ), and the absorption coefficients ... 

Solubility of the reactants and products in the catalyst-containing aqueous phase is another factor to be considered. The solubility of >C3 terminal olefins rapidly decreases with increasing chain length [7] as shown in Table 4.3. The solubihty data in the middle colunm of Table 4.3 refer to room temperature, therefore the values for ethene through 1-butene show the solubility of gases, while the data for 1-pentene through 1-octene refer to solubihties of liquids. For comparison, the solubihties of hquid propene and 1-butene are also shown (third colunm), these were calculated using a known relation between aqueous solubihty and molar volume of n-alkenes [60]. 

Volatile irritants such as ammonia and chlorine initially cause constriction of the bronchioles. These two gases are water soluble, are absorbed in the aqueous secretions of the upper airways of the respiratory system, and may not cause permanent damage. Irritant damage may however lead to changes in permeability and edema, the accumulation of fluid. Some irritants such as arsenic compounds cause bronchitis. 

A.6 Solubility of gases. The solubility of gases in aqueous media is described by an equilibrium constant known as the Henry s Law constant, fCH. The value fCH relates the amount of gas in an aqueous phase (mol dm 3) to the partial pressure of the gas (atmosphere) at a given temperature. For carbon dioxide JCH would be defined as... 

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)... 

The units used to express solubilities of gases, e.g. Henry s law coefficients, Ostwald coefficients and Bunsen coefficients, have to be converted to the relevant solution standard state (p. 213). Such solubilities (Battino and Clever, 1966 Wilhelm and Battino, 1973) are valuable in the analysis of kinetic data. For example, the solubility of a neutral solute in a range of aqueous mixtures can provide some indication of the variation of the chemical potential of a neutral reactant because, from eqn (11), 8m AGe = 8mnf. Where the pure solute is a liquid or solid, it is often convenient to chose the pure solute as a standard state, represented by the symbol, ° in eqn (12). Similar comments apply to the related thermodynamic quan-... 

The concentrations of the various forms of C02 present in an aqueous phase are temperature dependent and extremely sensitive to pH (the concentrations also depend on the presence of other solutes, which presumably is a small effect for the cell wall water). For instance, the equilibrium concentration of C02 dissolved in water divided by that of C02 in an adjacent gas phase, Cc 7cccv decreases more than two-fold from 10°C to 40°C (Table 8-3 the decreasing solubility of C02 as the temperature increases is a characteristic of dissolved gases, which fit into the interstices of water, such space becoming less available as molecular motion increases with increasing temperature). This partition coefficient is not very pH dependent, but the equilibrium concentration of HCO3- in water relative to that of dissolved C02 is markedly affected by pH. In particular, C02 dissolved in an aqueous solution can interact with water to form carbonic acid, which then dissociates to form bicarbonate ... 

The data have, however, been obtained for only a few systems by Onda et al. (J), Joosten and Danckwerts (2), and Hikita et al. (3). These authors also proposed methods for estimating the solubility of gases in aqueous mixed-salt solutions from the corresponding data for each salt in the systems. These studies are extensions of the empirical method proposed by van Krevelen and Hoftijzer (4) on the basis of the following modified Setschenow Equation. 

The estimating methods by van Krevelen and Hoftijzer (4) and Onda et al. (I) are based on the linear relationship between log(L0/L) and salt concentration. When these methods are used to estimate the solubility of gases in aqueous salt solutions over a wide range of salt concentration, however, the estimates are sometimes in serious error, as shown in Figures 3 and 4. 

Dalton s meteorological observations led him to believe that aqueous vapor exists separately from the other constituents of the air, also that gases themselves are made up of distinct particles. His work on the solubility of gases in water led him to undertake to determine the relative weights of ultimate particles of bodies. ... 

Henry s law constants, as other thermodynamic constants, are valid for ideal solutions ideally, the expression should be written in terms of activities and fugacities. Since activity coefficients for uncharged species are much smaller than those for ions, we can use expressions such as equation 3 for diluhj solutions (fresh water) and atmospheric pressures. However, corrections anj necessary for seawater and concentrated solutions. Since activity coefficients for molecules in aqueous solution become larger than 1 (salting out effect), the solubility of gases in concentration units is smaller in the salt solution than in the dilute aqueous medium. 

The solubility of a gas in a mixture of solvents is a problem of interest in many industrial applications. One example is the removal of acidic compounds from industrial and natural gases. The solubility of a gas in a binary mixture containing water has particular importance because it is connected with the solubility of gases in blood, seawater, rainwater, and many other aqueous solutions of biological and environmental significance. Therefore, it is important to be able to predict the gas solubility in a mixture in terms of the solvent composition and the solubilities in the individual constituents of the solvent or in one pure component and a selected composition of the mixed solvent. 

For comparison, we selected the solubilities of gases in aqueous binary solvents because, as noted in ref 6, the prediction of the Henry s constant for such mixtures is the most difficult and the available methods are not reliable. The results of the calculations are presented and compared in Table 1 and Figure 1 with the Krichevsky equation and an empirical correlation for aqueous mixtures that provided the best results among the existing expressions. 

The aqueous mixtures of polymers (PEG and PPG) were selected for comparison with the theory, because accurate data [4,5] regarding the solubility of argon (Ar), methane (CH4), ethane (C2H6) and propane (CsHg) in the individual constituents and the polymer + water mixtures are available. In addition, the above polymers and water are miscible in all proportions and solubility data [4,5] are available for the entire composition range. The theoretical approach regarding the solubility of gases in polymer + water mixed solvents can be extended to the correlation of their solubility in mixed solvents formed of water and pharmaceuticals, proteins, biomolecules, etc. 

The solubilities of gases, liquids, and solids in multi-component solvents constitute important issues in science and technology. The aqueous multicomponent solutions represent a meaningful example because the overwhelming majority of solutions of biological and environmental interest are aqueous multicomponent solutions. 

Another method suggested by the authors for predicting the solubility of gases and large molecules such as the proteins, drugs and other biomolecules in a mixed solvent is based on the Kirkwood-Buff theory of solutions [18]. This theory connects the macroscopic properties of solutions, such as the isothermal compressibility, the derivatives of the chemical potentials with respect to the concentration and the partial molar volumes to their microscopic characteristics in the form of spatial integrals involving the radial distribution function. This theory allowed one to extract some microscopic characteristics of mixtures from measurable thermodynamic quantities. The present authors employed the Kirkwood-Buff theory of solution to obtain expressions for the derivatives of the activity coefficients in ternary [19] and multicomponent [20] mixtures with respect to the mole fractions. These expressions for the derivatives of the activity coefficients were used to predict the solubilities of various solutes in aqueous mixed solvents, namely ... 

The authors of the present paper have shown previously [21-31] that the fluctuation theory of solution can provide a new approach to the solubility of gases, drugs, protein, etc., in binary and multicomponent aqueous mixed solvents. 

In the present paper, the method which the authors employed previously to derive an expression for the solubility of various proteins in aqueous solutions, has been extended to the solubility of gases in mixtures of water + strong electrolytes. One parameter equation for the solubility of gases has been derived, which was used to represent the solubilities of oxygen, carbon dioxide and methane in water -i- sodium chloride. In additions, the developed theory could be used to examine the local composition of the solvent around a gas molecule. The results revealed that the oxygen, carbon dioxide and methane molecules are preferentially hydrated in water-i-sodium chloride mixtures. A similar result was obtained for the water -i- methane -i- sodium chloride by molecular dynamics simulations [72]. 

Aqueous solubility is a fundamental, chemical-specific property. It is defined as the concentration of a chemical dissolved in water when that water is both in contact and at equilibrium with the pure chemical. (For the moment, consider the chemical to be in either liquid or solid form solubility of gases is discussed in Section 1.8.2.) Although aqueous solubility is temperature dependent, it does not vary greatly for a given chemical over the typical range of temperatures encountered in the environment. 

Henry s Law constant is equal to the ratio of the vapor pressure of a species over the operating system pressure. If the number of stages required in a separation decreases with increasing Henry s Law constant, how can the Henry s Law constant be increased (i.e., of what thermodynamic variable is vapor pressure a function) Laboratory experiments were performed to assess the feasibility of separating ethylene from ethane. It was determined that the equilibrium solubilities of ethylene and ethane in an acidic copper(I) aqueous solution were similar. The rates of uptake of the two gases into the aqueous solution were measured independently and it was found that the rate of ethylene absorption is several times greater than that of ethane. Is this an example of an equilibrium- or a rate-controlled separation and why ... 

---