Carbon dioxide in water

An important solute in water that is associated with both acidity and alkalinity is dissolved carbon dioxide, CO2, which is almost always present in natural water from contact with atmospheric air or as a product of microbial biodegradation of organic matter. Present in the atmosphere at a level of about 390 ppm of dry air (and increasing due to release from the anthrosphere at a rate of almost 2 ppm per year), atmospheric CO2 gas makes rainwater from even a totally unpolluted atmosphere slightly acidic. At 25 C, in water in equilibrium with unpolluted air containing 390 ppm carbon dioxide, the concentration of dissolved C02(aq) is 1.276 x 10 mol/L (M), a value that is used for subsequent calculations in this chapter. 

Dissolved carbon dioxide at high levels can be toxic to aquatic life. Three volcanic lakes in Africa, Lake Nyos and Lake Monoun in Cameroon and Lake Kivu in Rwanda, are saturated with carbon dioxide gas from underground sources. Occasionally, people and livestock are killed by exposure to the gas along the lakeshores. In the worst incident by far, 1700 people and several thousand livestock were asphyxiated in 1986 by an abrupt release of carbon dioxide from the supersaturated bottom layer of Lake Nyos. 

Most dissolved CO2 in water is present as neutral C02(aq) and not as H2CO3 as it is often shown. The following reactions and equations describe the CO2-HCO3-CO3 system in water. Dissolved CO2 in water acts as an acid reacting with water as follows  

This is a chemical equilibrium reaction and the double arrows denote that it is reversible, and it lies primarily to the left as shown by the relatively low value of its equilibrium constant at 25°C  

But it is an extremely weak acid as shown by the very low value of its equilibrium constant 

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The following equilibria apply to a solution of carbon dioxide in water ... 

The solubility of carbon dioxide in water is given in Figure 1 (11). Over the temperature range 273—393 K, the solubiUties at pressures below 20 MPa (200 atm) decrease with increasing temperature. From 30 to 70 MPa (300—700 atm) a solubiUty minimum is observed between 343 and 353 K, with solubihties increasing as temperature increases to 393 K. Information on the solubiUty of carbon dioxide in pure water and synthetic seawater over the range 268 to 298 K and 101—4,500 kPa pressure (1—44 atm) is available (12,13). 

Fig. 1. Solubihty of carbon dioxide in water at various pressures in MPa. To convert MPa to atm, multiply by 10.

Absorption of a gas in a liqiiid, for example, ammonia in water or carbon dioxide in water. 

The solubility of carbon dioxide in water depends on the pressure and temperature. The relationship between temperature and pressure for 3.5 and 5 volumes is shown in Figure 17.4. It will also be affected by the amount of air already dissolved in the water. The raw water is therefore carefully filtered and de-oxygenated under vacuum before the sugars and flavourings are added. 

Massimilla et al. (M5) measured the rate of absorption of carbon dioxide in water from a mixture of carbon dioxide and nitrogen. Used as solid phase were silica sand particles of average equivalent diameter 0.22 mm, or glass ballotini of average equivalent diameter 0.50 and 0.80 mm. Columns of 30-and 90-mm i.d. were used, and the column height was varied from 100 to 1200 mm. 

Weiss, R. F. (1974). Carbon dioxide in water and seawater the solubility of a non-ideal gas. Marine Chem. 2,203-215. 

Janet wants to dissolve carbon dioxide in water. The rate of solvation could be most improved by —... 

Figure 3.1.1 The solubility of gaseous carbon dioxide in water as a function of both temperature and pressure. The CO2 solubility is expressed in terms of the mole fraction of carbon dioxide in the liquid solution.

Both of these facts are employed in the carbonation process of sodas and beer and some sparkling wines. Low-temperature conditions and CO2 pressures of 3 to 4 atm are used to enhance the dissolution of carbon dioxide gas in water. The graph in Fig. 3.1.1 presents the solubility of carbon dioxide in water at various temperatures and pressures. The parameter used to express CO2 solubility is... 

Solubility of Carbon Dioxide in Water at Various Temperatures and Pressures, in Handbook of Chemistry and Physics, 74th ed., ed. David R. Lide (Boca Raton, FL, CRC Press 1993), 6-7. 

Certain oxygen acids have been included in the table for orientation. Two values are given for carbonic acid, the lower value being for the overall acidity of a solution of carbon dioxide in water. Carbonic acid, HaCOa, is actually a fairly strong acid.348... 

A second type of ternary electrolyte systems is solvent -supercritical molecular solute - salt systems. The concentration of supercritical molecular solutes in these systems is generally very low. Therefore, the salting out effects are essentially effects of the presence of salts on the unsymmetric activity coefficient of molecular solutes at infinite dilution. The interaction parameters for NaCl-C02 binary pair and KCI-CO2 binary pair are shown in Table 8. Water-electrolyte binary parameters were obtained from Table 1. Water-carbon dioxide binary parameters were correlated assuming dissociation of carbon dioxide in water is negligible. It is interesting to note that the Setschenow equation fits only approximately these two systems (Yasunishi and Yoshida, (24)). 

Figure 3. Henry s constant for solubility of carbon dioxide in water (O) int l. crit. table (A) calc, from Takenouchi and Kennedy, and Malinin (Q) calc, from Wiebe and Gaddy, Houghton et al. (V) Malinin, low pressure see Ref. 8...

It is difficult to compare the performance of various spray towers since the type of spray distributor used influences the results. Data from Hixson and Scott 33 and others show that KGa varies as G70-8, and is also affected by the liquid rate. More reliable data with spray columns might be expected if the liquid were introduced in the form of individual drops through a single jet into a tube full of gas. Unfortunately the drops tend to alter in size and shape and it is not possible to get the true interfacial area very accurately. This has been investigated by Whitman et a/. 34 , who found that kG for the absorption of ammonia in water was about 0.035 kmol/s m2 (N/m2), compared with 0.00025 for the absorption of carbon dioxide in water. 

Carbonic acid (HjCO ) is produced by dissolving carbon dioxide in water. When formed under pressure, it is the gas used in carbonated drinks. In nature, it dissolves the limestone in caves, resulting in the formation of stalactites and stalagmites. It is corrosive as are other acids, although it is considered a rather weak acid. 

It was around this time that Priestley discovered soda water. Fixed air (carbon dioxide) had long been known to chemists, and Priestley had experimented with it a little. One day he had the idea of trying to dissolve the carbon dioxide in water. He succeeded and found that the water fizzed. Priestley gave some of the soda water to friends and then went on to other kinds of research. Some years later the British Navy expressed interest in the use of Priestley s sparkling water as a remedy for scurvy, but naturally it was unsuccessful. However, soda water quickly became popular in other circles, even earning praise from Lord Byron, who wrote the following stanza on the back of the manuscript of his poem Don Juan ... 

Fig. 13. Typical result for absorption of pure carbon dioxide in water in horizontal cocurrent flow for two liquid rates. Tubing 0.0575 ft. inside diameter 7.7 ft. long test section 15°C. outlet pressure 1 atm. abs.

Probably an example and problems derived from the carbon dioxide-blood buffer system in humans should be in every physical chemistry course. What a rich, complex example this is from Henry s law for the solubility of carbon dioxide in water (blood) to buffer capacity, that is, the rate of change of the law of mass action with proton concentration. The example can be expanded to include nonideal solutions and activities. How many physical chemistry courses use this wonderful and terribly relevant to life example First-year medical students learn this material. 

The hydration rate constant of C02, the dehydration rate constant of carbonic acid (H2C03), and p pK2 values (pTf, =6.03, pTf2 = 9.8 at 25 °C, 7=0.5 M) (63) are such that nearly 99% of dissolved carbon dioxide in water at pH < 4 exists as C02. However, these four different species may be considered as the reactive species under different pH conditions which can react with aqua metal ions or their hydroxide analogues to generate the metal carbonato complexes. The metal bound aqua ligand is a substantially stronger acid than bulk H20 ( )K= 15.7). Typical value of the p of H20 bound to a metal ion may be taken to be 7. Hence the substantial fraction of such an aqua metal ion will exist as M-OH(aq)(ra 1) + species at nearly neutral pH in aqueous medium. A major reaction for the formation of carbonato complex, therefore, will involve pH controlled C02 uptake by the M-OH(" 1)+ as given in Eq. (17). 

Dipole—induced dipole attractions also occur between molecules of carbon dioxide, which are nonpolar, and water. It is these attractions that help keep carbonated beverages (which are mixtures of carbon dioxide in water) from losing their fizz too quickly after they ve been opened. Dipole—induced dipole attractions are also responsible for holding plastic wrap to glass, as shown in... 

Was this your answer The solubility of carbon dioxide in water decreases with increasing temperature. The warm drink will therefore fizz in your mouth more than does the cold one. 

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