CO2-carbonic acid system

It should be kept in mind that, in spite of these major variations in the CO2-carbonic acid system, virtually all surface seawater is supersaturated with respect to calcite and aragonite. However, variations in the composition of surface waters can have a major influence on the depth at which deep seawater becomes undersaturated with respect to these minerals. The CO2 content of the water is the primary factor controlling its initial saturation state. The productivity and temperature of surface seawater also play major roles, in determining the types and amounts of biogenic carbonates that are produced. Later it will be shown that there is a definite relation between the saturation state of deep seawater, the rain rate of biogenic material and the accumulation of calcium carbonate in deep sea sediments. 

In all of the examples given, a temperature of 25°C and 1 atm total pressure are assumed. The task will be to calculate the concentration of all of the carbonic acid system components, pH, At, CO2, Pcc>2< Ca2+ concentration, ionic strength, and the appropriate ion activity coefficients. The values of the thermodynamic constants used in these calculations were calculated from appropriate equations in Chapter 1. Concentrations are on the molal scale, and results for each case are summarized in Table 2.4. 

Because At is known (0) and cannot change upon addition of only CO2, the equation for At can be readily written for the addition of CO2. Using the equations for the equilibrium relations in the carbonic acid system, it is possible to write an equation for alkalinity in terms of Pc02 (known) and mH+ (our unknown). 

The distribution of CO2 and the associated carbonic acid system species in the upper ocean (here loosely defined as waters above the thermocline and generally only a few hundred meters in depth) is primarily controlled by the exchange of CO2 across the air-sea interface, biological activity, and circulation of the ocean, mainly through vertical mixing processes. Other factors, such as the temperature and salinity of the water, can also contribute to variations by influencing the solubility of CO2 in seawater and the equilibrium constants of the carbonic acid system. 

Aside from mineral stabilities, the behavior of the CO2-H2O system with increasing P and T is also important to an understanding of the deep burial diagenesis of carbonate rocks. One reaction of interest, which represents the summation of K0 and Ki (see Chapter 1) for the carbonic acid system, is... 

A description of acid-base balance involves an accounting of the carbonic (H2C03, HCOh COa", and CO2) and noncar-bonic acids and conjugate bases in terms of input (intake plus metabolic production) and output (excretion plus metabolic conversion) over a given time interval. The acid-base status of the body fluids is typically assessed by measurements of total CO2 plasma pH and PCO2, because the bicarbonate/carbonic acid system is the most important buffering system of the plasma. Occasionally, measurement of total titratable acid or base, or other acid and base analytes (e.g., lactate and ammonia [NH3]) is necessary to determine the etiology of an acid-base disorder. 

The decrease in alkalinity and the increasing magnitude of the decrease associated with deeper sediment samples can be explained in part by reactions (6.3) and (6.4) and the known increase in FeS below the sediment-water interface at NWC (Part I). It can be shown that the small effect of sulfide oxidation on pH is in agreement with the magnitude of the alkalinity change and the experimental conditions. Because of vigorous aeration, the experimental jars can be considered to be held at a constant CO2 pressure (by the end of the experiment) so that if the carbonic acid system is controlling pH, the relation... 

In general, natural waters contain alkalinity as well as weak acids and weak bases. Calculation of the pH of a water from Aik and weak acid and weak base concentrations requires that mass conservation equations, the electroneutrality condition, and mass action equations for all weak acids and weak bases be solved simultaneously. In natural waters, however, the weak acids of the carbonate system are often present at significantly higher concentrations than other weak acids or weak bases. Cj is the sum of the concentration of dissolved CO2, carbonic acid, and the concentrations of the two anions produced when carbonic acid ionizes (HCOs and COa ). Various tools have been developed to simplify the analysis of waters dominated by Aik and Cj. One of these tools is the Deffeyes diagram... 

Under the conditions of temperature and ionic strength prevailing in mammalian body fluids, the equilibrium for this reaction lies far to the left, such that about 500 CO2 molecules are present in solution for every molecule of H2CO3. Because dissolved CO2 and H2CO3 are in equilibrium, the proper expression for H2CO3 availability is [C02(d)] + [H2CO3], the so-called total carbonic acid pool, consisting primarily of C02(d). The overall equilibrium for the bicarbonate buffer system then is... 

The carbonic acid-bicarbonate buffer system has a of 6.1, yet is still a very effective buffer at pH 7-4 because it is an open buffer system, in which one component, CO2, can equilibrate between blood and air. 

The answer is C. Ingestion of an acid or excess production by the body, such as in diabetic ketoacidosis, may induce metabolic acidosis, a condition in which both pH and HCOj become depressed. In response to this condition, the carbonic acid-bicarbonate system is capable of disposing of the excess acid in the form of CO2. The equilibrium between bicarbonate and carbonic acid shifts toward formation of carbonic acid, which is converted to COj and HjO in the RBC catalyzed by carbonic anhydrase, an enzyme found mainly in the RBC. The excess CO2 is then expired by the lungs as a result of respiratory compensation for the acidosis (Figure 1-2). The main role of the kidneys in managing acidosis is through excretion of H" rather than CO2. 

Temperature and pressure variations in natural systems exert major influences on carbonate mineral solubility and the distribution of carbonic acid chemical species. For example, the solubility of calcite decreases with increasing temperature, as does the solubility of CO2 gas in water. These two effects on solubilities can lead to precipitation of calcite as a cement in a marine sediment-pore water system that undergoes moderate burial. 

High pressure continuously operated reactor. The design of the continuously operated apparatus is shown in Figure 2. An air operated high pressure pump delivered CO2 in the system. The gaseous fluid was dried when passing through columns packed with molecular sieves. The flow rate of C02 was 1.0 L per min. Equimolar solution of substrates (oleic acid and oleyl alcohol) was pumped into the system with an HPLC pump. Carbon dioxide and substrates were equilibrated in the saturation column. The reaction was performed in a... 

The atmosphere is a major source of soil acidity. Even in unpolluted environments rainwater is slightly acidic, having a pH of about 5.7 due to the dissolution of atmospheric CO2 to form the weak carbonic acid (see Worked example 5.4). The CO2 concentration in the partially enclosed soil pore system can be significantly higher (typically up to about 10 times) than in the free atmosphere due to respiration of soil microorganisms and plant roots. This results in a lower pH. In areas affected by industrial pollution, sulfur dioxide and nitrogen oxides dissolve in rainwater to produce sulfuric and nitric acids (acid rain), which are both strong acids and cause even more acidity. 

These reactions undoubtedly verify the special importance of the CO2/H2O system for the efficiency of AOPs. The omnipresent bicarbonate or carbonate ions can compete successfully for OH radicals, especially at low loads of organic matter. The composition of the CO2 (H2C03)/HC0i/C03 system in water is strongly dependent on the pH of the solution (Fig. 6-15). When carbon dioxide is dissolved in water, only a small amount (ca. 0.1%) reacts to form carbonic acid (H2CO3). In fact, most of the undissociated acid is actually present as C02(aq) at pH values lower than 4.3. At a pH of 8.2 the system consists solely of bicarbonate ions and at higher pH values above 12 the carbonate ion is mainly present. For a detailed discussion of the CO2/water system, see Sigg and Stumm (1996). 

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