The Solubility of Carbon Dioxide in D2O and in Mixtures of D2O and H2O
The system CO2-DO has not been studied by many workers. The first solubility measurements were made by Curry and Hazelton (1) in 1938, a few years after the discovery of deuterium. Since then only three research groups have reported solubility data for CO2 in heavy water. Salomaa et al. (3) measured the solubility of CO2 in pure H2O, pure D2O and in mixtures of H2O and D2O at 298.15 K and atmospheric pressure using the same method as Curry and Hazelton. Kratochvil et al. (5) report solubility values in pure D2O at three temperatures (303.15 K, 323.15 K, and 353.15 K) and partial pressures of CO2 between 55 kPa and 3972 kPa. The data from Refs. (1), (3) and (5) are used in this evaluation. At high temperatures and pressures there exists one set of measurements by Kapitanov et al. (6). Unfortunately, there is not enough information in Kapitanov’s paper for the calculation of mole fraction solubilities, and these data will not be handled further in this evaluation.
To represent the temperature dependence of the mole fraction solubility of CO2 in D2O at 101.325 kPa partial pressure of gas the data from Refs. (1), (3) and (5) were fitted to Eq. 1.
ln x1 = A + B / (T/ 100 K) + C ln (T/ 100 K)
(1)
The best fit for the five points in the temperature range 298 K to 353 K gave Eq. (2),
ln x1 = –34.0577 + 49.6909 / (T/ 100 K) + 9.1455 ln(T/ 100 K)
(2)
where x1 is the mole fraction solubility of CO2 at 101.325 kPa partial pressure of gas. The fit in line x1 gave a standard deviation of 1.1 %. Smoothed values of the mole fraction solubility and changes in the thermodynamic functions on solution are shown in Table I at 5 K intervals. The thermodynamic properties were calculated from the smoothing equation according to the following equations:
ΔS°1 = R [A + C ln (T/ 100 K) + C]
(3)
ΔH°1 = R (-100B + CT)
(4)
Figure 1 shows the temperature dependence of the mole fraction solubility for carbon dioxide in D2O at 101.325 kPa partial pressure of gas. The curve was obtained from the smoothing Eq. 2. For comparison, the temperature dependence of the mole fraction solubility for CO2 in H2O is presented in the same figure as a dotted curve which was obtained from the smoothing equation given by Carroll and Mather in the CO2 -H2O section of this volume (“The Solubility of Carbon Dioxide in Water at Low Pressure”, Eq. (1)).
In Figure 2 the pressure dependence of the mole fraction solubility of carbon dioxide in D2Oat three different temperatures is presented.
The only reported data for the solubility of CO2 in mixtures of D2Oand H2O are the data of Salomaa et al. (3). In the original paper the Ostwald coefficients are given. To calculate the molality of CO2 in mixed solutions the H2O -D2O density data are needed. Salomaa has unpublished density data which are worth publishing in this connection. In his measurements, Salomaa used the procedure described by Kirshenbaum (2). The values for the density of L2O (L = H,D) at 298.15 K are listed in Table 2.
By the method of least squares Eq. (5) is obtained from the data of Table 2. The linear correlation between ρ(L2O) and x(D; L2O) is excellent (correlation coefficient r = 0.999995), and at x(D; L2O) = 0 the density values extrapolate well to the density of pure H2O at 298.15 K, as they should.
ρ(L2O/g cm–3 = (0.10852 ± 0.00015) × x(D;L2O) + (0.99704 ± 0.00012)
(5)
Using a real gas molar volume of 24.34 dm3 mol-1 and the L2O densities obtained from Eq. (5), the evaluator calculated the molalities of CO2 in the mixtures of H2O and D2O from the Ostwald coefficients given in (3). In Figure 3 the molality of CO2 at 298.15 K and 101.325 kPa partial pressure of CO2 is presented as a function of the deuterium isotope mole fraction x(D;L2O) in the solvent water.
Comparison of the smoothed mole fraction solubilities of CO2 in D2O (from Table 1) with corresponding solubilities of CO2 in H2O (Carroll & Mather, CO2 - H2O section, “The Solubility of Carbon Dioxide in Water at Low Pressure”, Table 1, Eq. (1)) shows that between 298.15 K and 323.15 K the solubility of CO2 in D2Ois 1.5 - 2 % lower than the solubility of CO2 in D2O. The difference increases with increasing temperature, and at 353.15 K the mole fraction solubility of CO2 in D2O is about 9 % lower than in H2O (see also Fig. 1). This differs from most simple nonpolar gases for which the mole fraction solubility in D2O is higher than in H2O by about 5 - 15 % at 288.15 K to 318.15 K and 101.325 kPa partial pressure of gas (8, 9).
The experimental values of Kratochvil et al. (5) predict a higher value for the solubility of CO2 in D2O at 298.15 K and 101.325 kPa than the value for the solubility of CO2 in D2O. However, the two independent measurements (1, 3) at 298.15 K and 101 .325 kPa, which are in good agreement with each other, support the value of x1 = 0.611 • 10-3 for the solubility of CO2 in D2O, which is 1.6 % lower than the solubility in H2O. The systematic solubility measurements of Salomaa et al. (3) in both isotopically pure waters and in their mixtures show that the solubility of CO2 decreases with increasing deuterium content of water (Fig. 3) supporting the result that CO2 is more soluble in H2O than in D2O at 298.15 K and 101.325 kPa. The measurements of Kratoehvil et al. (5) at 323.15 and 353.15 K and at higher pressures also indicate a higher solubility of CO2 in H2O than in D2O.
Because of the rather small number of measurements on the solubility of carbon dioxide in D2O, the results are classed as tentative until confirmed by additional measurements.