Carbon dioxide equilibrium concentration

Diffusion coefficients can be estimated with the aid of the mathematical description of the diffusion of carbon dioxide from the paint film (Scheme II). Film thickness, saturation concentration and carbon dioxide equilibrium concentration are known. The emission curves of carbon dioxide calculated by the model have been fitted with the actual emission curves in Figure 7. In this case carbon dioxide is not formed chemically. 

In Fig. 3.21, we compare the percent difference between model and experimental estimates of carbon dioxide gas concentrations for NaCl solutions in equilibrium with C02-6H20. Only 9% of the carbon dioxide comparisons are off by > 10% (Fig. 3.21). A large part of this variability is clearly attributable to variation in measured gas concentrations. For example in the Dholabhai et... 

Fig. 3.21. Percent difference between model-calculated and experimental carbon dioxide gas concentrations for NaCl systems in equilibrium with CO26H2O, where % Diff. = [model — expt./expt.] x 100. Reprinted from Marion et al. (2006) with permission...

The case is obvionsly similar for dissolved carbon dioxide whose concentration is essentially determined by the equilibrium of the aquatic carbonate phases. As for the alkalinity, no remarkable variations were found in measured values, even at high concentrations, when the measurements were performed successively on adjacent parts of the same sediment core (cf. alkalinity profile shown in Fig. 3.1). 

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. 

Except as an index of respiration, carbon dioxide is seldom considered in fermentations but plays important roles. Its participation in carbonate equilibria affects pH removal of carbon dioxide by photosynthesis can force the pH above 10 in dense, well-illuminated algal cultures. Several biochemical reactions involve carbon dioxide, so their kinetics and equilibrium concentrations are dependent on gas concentrations, and metabolic rates of associated reactions may also change. Attempts to increase oxygen transfer rates by elevating pressure to get more driving force sometimes encounter poor process performance that might oe attributed to excessive dissolved carbon dioxide. 

Table 21.22 Saturated solubilities of atmospheric gases in sea-water at various temperatures Concentrations of oxygen, nitrogen and carbon dioxide in equilibrium with 1 atm (lOI 325 N m ) of designated gas...

Effects of Cold Gas Recycle and Approach to Equilibrium. Product gases resulting from various CGR ratios were analyzed (Table XI). For the experiments tabulated, a decrease in the cold recycle ratio resulted consistently in increases in the product gas concentrations of water vapor, hydrogen, and carbon dioxide and a decrease in methane concentration. These trends may be noted in experiment HGR-12 as the CGR ratio decreased from 8.7 1 to 1.2 1, in experiment HGR-13 as it increased from 1.0 1 to 9.1 1, and in experiment HGR-14 as it decreased from 3.0 1 to 1.0 1. These trends indicate that the water-gas shift reaction (CO + H20 —> C02 + H2) was sustained to some degree. Except for the 462-hr period in experiment HGR-14, the apparent mass action constants for the water-gas shift reaction (based on the product gas compositions in Table XI) remained fairly constant at 0.57-1.6. These values are much lower than the value of 11.7 for equilibrium conversion at 400°C. In... 

The space velocity was varied from 2539 to 9130 scf/hr ft3 catalyst. Carbon monoxide and ethane were at equilibrium conversion at all space velocities however, some carbon dioxide breakthrough was noticed at the higher space velocities. A bed of activated carbon and zinc oxide at 149 °C reduced the sulfur content of the feed gas from about 2 ppm to less than 0.1 ppm in order to avoid catalyst deactivation by sulfur poisoning. Subsequent tests have indicated that the catalyst is equally effective for feed gases containing up to 1 mole % benzene and 0.5 ppm sulfur (5). These are the maximum concentrations of impurities that can be present in methanation section feed gases. 

C. J. and Schloss, A. L. (1997). Equilibrium responses of global net primary production and carbon storage to doubled atmospheric carbon dioxide Sensitivity to changes in vegetation nitrogen concentration, Global Biogeochem. Cycles 11,173-189. 

The factor a has been estimated by measuring the concentration of carbon dioxide in the gas phase and in the film at equilibrium. 

The factor a = [COzJfilmfCOzJalr (at equilibrium) is calculated from a determination of the saturation concentration of carbon dioxide in the paint film. 

Carbon dioxide fugacity fcOi- CO2 fugacity (/coa) of ore fluids is estimated based on CO2 concentration of fluid inclusions analyzed. By using equilibrium constant of the reaction, C02(g) + H2O = H2CO3, and assuming uh20 to be unity, /CO2 can be estimated. 

Acid-base reactions of buffers act either to add or to remove hydrogen ions to or from the solution so as to maintain a nearly constant equilibrium concentration of H+. For example, carbon dioxide acts as a buffer when it dissolves in water to form carbonic acid, which dissociates to carbonate and bicarbonate ions ... 

The carbon dioxide resulting from combustion of the particulate organic carbon can be passed through the analyser, producing a single spike on a recorder, or it can be pumped in a loop through the analyser, until an equilibrium concentration is reached in the loop. Since the output of the analyser is nonlinear, the latter technique has been favoured by some investigators [46]. 

The radiocarbon ratio also evolves very rapidly from its initial value of -50 to an average value of about -8 per mil. This evolution is not a consequence of evaporative concentration but, instead, of an approach to equilibrium with atmospheric carbon dioxide. Average surface seawater contains significantly less radiocarbon than does the atmosphere because its isotopic composition is affected by exchange with the deep ocean as... 

Figure 6-6 also shows the variation in the partial pressure of carbon dioxide in equilibrium with the lagoon s waters. The average value of this pressure exceeds the atmospheric value, 1, so on average, carbon dioxide is evaporating from the lagoon. The evaporation rate is greatest at times of maximum alkalinity and bicarbonate concentration and minimum carbonate ion concentration. 

Figure 6-8 shows how the partial pressure of carbon dioxide in equilibrium with surface water oscillates in phase with the fluctuations in precipitation rate, saturation state, and temperature. The oscillations in alkalinity and bicarbonate concentrations have shifted in phase by about 90° because these quantities decrease when precipitation and evaporation are removing carbon from the system at above-average rates. 

In the condensation of alkyl methyl ketones with esters, the primary hydrogen is the one lost as in the reactions previously discussed with carbon dioxide, aldehydes, etc. The reaction is with the more rapidly formed and less hindered ion rather than with the ion that would be present in higher concentration at equilibrium. 

Several computer-based techniques have been developed for more specific applications. Truesdell (45) describes a computer program for calculating equilibrium distributions in natural water systems, given concentrations and pH. Edwards, et al. (31, Z2) have developed computer programs for treating volatile weak electrolytes such as ammonia, carbon dioxide and hydrogen sulfide systems however, in their present state these programs (presumably) do not accommodate metallic species in solutions. 

Once the spontaneous direction of a natural process is determined, we may wish to know how far the process will proceed before reaching equilibrium. For example, we might want to find the maximum yield of an industrial process, the equilibrium solubility of atmospheric carbon dioxide in natural waters, or the equilibrium concentration of a group of metabolites in a cell. Thermodynamic methods provide the mathematical relations required to estimate such quantities. 

Even if equilibrium had been achieved before the water mass sank, the observed water mass concentrations could exhibit apparent deviations from equilibrium, if the atmospheric pressures have changed or if the water mass temperature has been altered. Thus, deepwater that was last at the sea surface hundreds of years ago could potentially have equilibrated with the atmosphere at much different partial pressures of gases such as methane and carbon dioxide whose atmospheric levels have risen over time. For example, the partial pressure of CO2 in dry air has risen from about 280 ppm in the 1860s to 385 ppm in 2008. 

Vertical concentration profiles of (a) temperature, (b) potential density, (c) salinity, (d) O2, (e) % saturation of O2, (f) bicarbonate and TDIC, (g) carbonate alkalinity and total alkalinity, (h) pH, (i) carbonate, ( ) carbon dioxide and carbonic acid concentrations, and (k) carbonate-to-bicarbonate ion concentration ratio. Curves labeled f,p have been corrected for the effects of in-situ temperature and pressure on equilibrium speciation. Curves labeled t, 1 atm have been corrected for the in-situ temperature effect, but not for that caused by pressure. Data from 50°27.5 N, 176°13.8 W in the North Pacific Ocean on June 1966. Source From Culberson, C., and R. M. Pytkowicz (1968). Limnology and Oceanography, 13, 403-417. 

Carbon dioxide has a dominant effect on the dissolution of carbonate minerals, such as calcite and dolomite (Table 2.1). If a carbonate mineral dissolves in water that is equilibrated with a constant source of CO, then the concentration of dissolved carbonic acid remains constant and high. However, when calcite dissolution is accompanied by consumption of carbonic acid and a continuous source of CO is not maintained, the reaction proceeds further to achieve equilibrium. 

Carbonic anhydrase presents an instructive case where the catalytic efficiency is so great (kcat > 10 s- ) that proton transfer becomes rate-limiting. The rate was found to depend on the concentration of the protonated form of buffers in the solution. Indeed, Silverman and Tu adduced the first convincing evidence for the role of buffer in carbonic anhydrase catalysis through their observation of an imidazole buffer-dependent enhancement in equilibrium exchanges of oxygen isotope between carbon dioxide and water. The effect is strictly on kcat, and is unaffected because the latter is... 

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