Carbon dioxide water-atmosphere equilibrium

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. 

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... 

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...

It is necessary to draw attention to the variable pH of water which may be encountered in quantitative analysis. Water in equilibrium with the normal atmosphere which contains 0.03 per cent by volume of carbon dioxide has a pH of about 5.7 very carefully prepared conductivity water has a pH close to 7 water saturated with carbon dioxide under a pressure of one atmosphere has a pH of about 3.7 at 25 °C. The analyst may therefore be dealing, according to the conditions that prevail in the laboratory, with water having a pH between the two extremes pH 3.7 and pH 7. Hence for indicators which show their alkaline colours at pH values above 4.5, the effect of carbon dioxide introduced during a titration, either from the atmosphere or from the titrating solutions, must be seriously considered. This subject is discussed again later (Section 10.12). 

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. 

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. 

Recent fuel processor performance is summarized in Table 4. The fuel processors were operated at atmospheric pressure, and the water and methanol feed mixture was about 60 wt % methanol. The typical composition of the reformate stream was 72— 74% hydrogen, 24—26% carbon dioxide, and 0.5—1.5% carbon monoxide on a dry gas basis. The carbon monoxide levels were significantly below equilibrium (5.4% at 350 °C), but they still require additional cleanup for use in fuel cells. The fuel processor efficiency was calculated using eq 5 and was reported to be greater than 80%. It is interesting to note that increasing the power 5-fold, from 20 to 100 W, only resulted in a 50% increase in volume and a 33% increase in mass. 

If the pH of natural rain were measured, you might expect a pH of around 7.0. Because a pH of 7.0 indicates neutral conditions, many people assume this to be the pH of rain. The theoretical pH of pure rainwater is actually about 5.6. Pure rain is acidic due to the equilibrium established between water and carbon dioxide in the atmosphere. Carbon dioxide and water combine to give carbonic acid ... 

The equilibrium constant is called the Henry s law constant for carbon dioxide, because Henry s law states that the solubility of a gas in a liquid is proportional to the pressure of the gas.) The acid dissociation constants listed for "carbonic acid in Appendix G apply to C02(aq). Given that I co, in the atmosphere is 10 3 4 atm, find the pH of water in equilibrium with the atmosphere. 

To a good first approximation, the Great Lakes fit a model involving the equilibrium of calcite, dolomite, apatite, kao-Unite, gibbsite, Na- and K-feldspars at 5°C., 1 atm. total pressure with air of PCo2 = 3.5 X 10" atm. and water. Dynamic models, considering carbon dioxide pressure and temperature as variables (but gross concentrations fixed), show that cold waters contain excess carbon dioxide and are unsaturated with respect to calcite, dolomite, and apatite, whereas warm waters are nearly at equilibrium with the atmosphere but somewhat supersaturated with respect to calcite, dolomite, and apatite. 

Figures 5 and 6 show the results of the calculations, and they are compared with the actual data distribution as shown by a dashed bounding line. With both calcite and dolomite, colder waters match conditions of carbon dioxide pressure greater than atmospheric, and high temperature conditions match carbon dioxide pressure nearly the same as the atmosphere. It appears the degree of mixing and the rate of carbon dioxide diffusion is of prime importance when considering approach to liquid, gas, and solid equilibrium.

Carbonate equilibria in an open system. What is the pH of water in equilibrium with atmospheric C02 gas To answer such a question involves a knowledge of acid-base chemistry, the use of Henry s Law constant for the solubility of carbon dioxide and the use of the ENE to calculate the proton concentration of the equilibrium solution. The details of the equilibrium constants used are detailed below. 

The most actively cycled reservoir of carbon is atmospheric C02 (it constitutes 0.034% of the atmosphere). Carbon dioxide dissolves readily in water and is in direct equilibrium with dissolved inorganic forms of carbon (H2C03, HCO, and CO7-, see Section 6.2.1.3). Once there, it may precipitate as solid calcium carbonate (limestone). Corals and algae encourage this reaction and build up limestone reefs in the process, but a much larger portion in the deep sea equilibrates only at the slow rate of... 

Figure 26-18 shows the carbon cycle. Carbon dioxide in the atmosphere is in equilibrium with an enormous quantity that is dissolved in oceans, lakes, and streams. Some of this dissolved CO2 was once in the form of calcium carbonate (CaC03), the main component of the shells of ancient marine animals. The shells were eventually converted into limestone, which represents a large store of carbon on Earth. When the limestone was exposed to the atmosphere by receding seas, it weathered under the action of rain and surface water, producing carbon dioxide. Some of this CO2 was released into the atmosphere. This process continues today. 

Carbon dioxide dissolved in water leads to the formation of carbonic acid and a consequent increase in H. Ponnamperuma (1967) has calculated that water at 25°C, in equilibrium with the normal concentration of CO2 in the earths s atmosphere (0.03% by volume), will attain a pH of 5.63. The weathering action of this weak acid over geologic time is well known to geologists (Krauskopf, 1967). Ponnamperuma s calculations also indicate that increased atmospheric CO2 concentrations will result in further decreases in pH, down to pH 3.97 with one amosphere of CO2. Respiratory CO2 concentrations in soil atmospheres can be 10 to 100 times greater than the normal 0.03% in the earth s atmosphere (Stotsky, 1972). Thus, pH values considerably lower than 5.63 can be achieved through respiration. Similarly, respiratory activity in shallow waters and tidal flats, especially at night when photosynthetic CO2 assimilation is halted, can cause a marked decrease in pH (Oppenheimer and Master, 1965). 

A river has a pH of approximately 6. If the river water is in equilibrium with atmospheric carbon dioxide (which has a pressure, Pcc,2, °f approximately 10-3 5 atm), what are the concentrations of carbonate system species in the water ... 

In some surface waters (and shallow groundwaters) depleted in dissolved oxygen because of their organic-matter content, there is an inverse correlation between the O2 consumed by aerobic decay and respiration and the increase in CO2 found in the water over its equilibrium atmospheric value. This would be expected if changes in the dissolved oxygen and carbon dioxide contents of the water are controlled chiefly by respiration and aerobic decay (reaction 2 ). The ratio of the CO2 produced to the O2 consumed is called the respiratory coefficient, or,... 

Investigations of the state of carbonate equilibrium of water which is transiting the vadose zone have not been numerous. The equilibrium COj (carbon dioxide partial pressure) and state of saturation with respect to calcite was studied by Holland et al. (1964) in Indian Echo Cave, Pennsylvania, and Luray Caverns, Virginia, and by Thrailkill (1971) in Carlsbad Caverns, New Mexico. These investigations showed that seepage water entering the cave was in equilibrium with a f cO2 much higher than that of the normal atmosphere and was often supersaturated with respect to calcite. 

Distilled water. Distilled water in equilibrium with air contains a small amount of atmospheric carbon dioxide and has therefore an acid reaction. Normal air contains approximately 0.03 volume per cent of carbon dioxide. The distribution coeffi-... 

The flow of carbon in the marine part of the biochemical subcycle involves buffering by the large reservoir of dissolved C (39 T t), most of which is in the form of dissolved inorganic carbon (DIC). Whereas terrestrial plants take up carbon dioxide directly from the atmosphere in gaseous form, aquatic plants utilize the C02 dissolved in water, and it is for this reason that the photosynthesis and respiration flux arrows in Fig. 6.1 do not point directly to marine biota. A dynamic equilibrium exists whereby C02 molecules are constantly exchanging between the atmosphere and oceans (where it... 

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