Carbon dioxide aqueous-phase equilibrium

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... 

EQUILIBRIUM DISTRIBUTION OF CARBON DIOXIDE BETWEEN THE GAS PHASE AND AQUEOUS SOLUTION... 

Experimental results are presented for high pressure phase equilibria in the binary systems carbon dioxide - acetone and carbon dioxide - ethanol and the ternary system carbon dioxide - acetone - water at 313 and 333 K and pressures between 20 and 150 bar. A high pressure optical cell with external recirculation and sampling of all phases was used for the experimental measurements. The ternary system exhibits an extensive three-phase equilibrium region with an upper and lower critical solution pressure at both temperatures. A modified cubic equation of a state with a non-quadratic mixing rule was successfully used to model the experimental data. The phase equilibrium behavior of the system is favorable for extraction of acetone from dilute aqueous solutions using supercritical carbon dioxide. 

The system carbon dioxide - acetone - water was investigated at 313 and 333 K. The system demonstrates several of the general characteristics of phase equilibrium behavior for ternary aqueous systems with a supercritical fluid. These include an extensive LLV region that appears at relatively low pressures. Carbon dioxide exhibits a high selectivity for acetone over water and can be used to extract acetone from dilute aqueous solutions. 

Addition of a solute to the aqueous phase changes the D/H and 180/16Q ratios in the free water since newly formed hydration spheres selectively take hydrogen and oxygen isotopes. This in turn results in the change in the D/H and 0/" 0 ratios in the water vapor or the 0/ 0 ratio in the carbon dioxide in equilibrium with the free water, which is considered to have an energy state similar to pure water. 

A number of different solid phases can precipitate from a solution of ammonia and carbon dioxide in water. As an example, the equilibrium of ammonium bicarbonate with an aqueous solution is written in the form (13) ... 

Phenol, a common priority pollutant, was extracted from two environmental matrices, soil and water, using near critical and supercritical carbon dioxide. The primary objective of this study was to determine the distribution of the contaminant between the soil or water and the supercritical phase, and the effect of soil moisture and co-solvents on the distribution coefficients. Static equilibrium extractions were performed on dry and wetted soil contaminated with 1 wt.% phenol and on water containing 6.8 wt.% phenol. Supercritical carbon dioxide (with and without en-trainers) was chosen as the solvent for the study. An appropriate entrainer for dry soil extractions (methanol) ffiffered from that found for aqueous extractions (benzene). However, soil moisture was found to have a significant impact on the effectiveness of en-trainers for soil extractions of phenol. Entrainers appropriate for extracting wetted soil were found to be the same as those advantageous for aqueous extractions. Benzene was also extracted from dry and wetted soil to investigate the extractability of a hydrophobic compound. 

A detailed description of the experimental apparatus and procedure used for the aqueous study are given elsewhere (Roop and Akgerman, Ind. Eng. Chem. R., in review) Static equilibrium extractions were carried out in a high pressure equilibrium cell (300 mL Autoclave). After the vessel is initially charged with 150 mL of water containing 6.8 wt.% phenol and supercritical carbon dioxide (and a small amount of entrainer, if desired), the contents were mixed for one hour followed by a two hour period for phase separation. Samples from both the aqueous phase and the supercritical phase were taken for analysis and the distribution coefficient for phenol calculated. 

Kramer [48] calculated the equilibrium constant to be 3.5 X 10" and the pressure of hydrazoic acid is 1.9 X 10- mm at a carbon dioxide pressure of 1 mm. Aqueous hydrazoic acid in equilibrium with the gas phase at 25°C would be 2.3 X 10" M with respect to hydrazoic acid. The equilibrium constants were measured [48] for militaiy-grade azide in 50 50 alcohol-water solutions and are summarized in Table I. 

For pH <5, the dissolved carbon dioxide does not dissociate appreciably and its effective Henry s law constant is, for all practical purposes, equal to its Henry s law constant. For a gas-phase C02 mixing ratio equal to 330 ppm, the equilibrium aqueous-phase concentration is 11.2 pM (Figure 7.4). As the pH increases to values higher than 5,C02 H20 starts dissociating and the dissolved total carbon dioxide increases exponentially. However, even at pH 8, Hq0i is only 1.5 M atm-1, and practically all the available carbon dioxide is still in the gas phase. The aqueous-phase concentration of total carbon dioxide increases to hundreds of pM for alkaline water. 

Carbon Dioxide/Water Equilibrium 345 Sulfur Dioxide 348 Ammonia/Water Equilibrium 353 Nitric Acid/Water Equilibrium 355 Equilibrium of Other Important Atmospheric Gases Aqueous-Phase Reaction Rates 361 S(IV) to S(VI) Transformation and Sulfur Chemistry 363... 

In water, the following chemical carbon-IV species exist in equilibrium carbon dioxide (CO2), carbonic acid (H2CO3), bicarbonate (HCOJ) and carbonate (COf ). Additionally, the phase equilibriums with gaseous CO2 and a possible solid body such as CaCOs and MgCOs have to be considered. Free carbonic acid is not isolated but the structure 0=C(OH)2 in aqueous solution has been confirmed. Often the expression CO2 H2O is also used for carbonic acid. The sum of the dissolved carbonate species is denoted as total DIG and is equivalent with other terms used in literature ... 

In the current study we are mainly interested in describing the gas solubility in pure water, under two-phase equihbrium (H-Lw) conditions. Gases of interest to this study include methane and carbon dioxide, and we report results mainly for the case of methane. To this purpose we couple different published thermodynamic models that are based on (i) the van der Waals-Platteeuw (vdWP) theory [9, 10] from Statistical Thermodynamics to describe three-phase (H-Lw-V) equihbria, (ii) Equations of State (EoS) for fugacity calculations, and (iii) models of gas solubihty in the aqueous phase. The considered approach is described in detail by Tsimpanogiannis et al., [11]. The authors conducted an extensive review of experimental and theoretical studies related to the solubility of gases in the aqueous phase under hydrate equilibrium conditions. Here, we report additional results that were not included in the original publication. 

---