Carbonic Acid and Carbonates

Carbon dioxide is moderately soluble in H2O at atmospheric pressure. The resulting solution is moderately acidic because of the formation of carbonic acid (H2CO3)  

Carbonic acid is a weak diprotic acid. Its acidic character causes carbonated beverages to have a sharp, slightly acidic taste. 

Although carbonic acid cannot be isolated, hydrogen carbonates (bicarbonates) and carbonates can be obtained by neutralizing carbonic acid solutions. Partial neutralization produces HCO3, and complete neutralization gives CO3. The HCO3 ion is a stronger base than acid (fCj, = 2.3 X = 5.6 X 10 ). The carbonate ion is 

The principal carbonate minerals are calcite (CaC03), magnesite (MgC03), dolomite [MgCa(C03)2], and siderite (FeC03). Calcite is the principal mineral in limestone and the main constituent of marble, chalk, pearls, coral reefs, and the shells of marine animals such as clams and oysters. Although CaC03 has low solubility in pure water, it dissolves readily in acidic solutions with evolution of CO2  

Because water containing CO2 is slightly acidic (Equation 22.67), CaC03 dissolves slowly in this medium  

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Protonated and diprotonated carbonic acid and carbon dioxide may also have implications in biological carboxylation processes. Although behavior in highly acidic solvent systems cannot be extrapolated to in vivo conditions, related multidentate interactions at enzymatic sites are possible. 

In Chapter 5. we noted that COj readily dissolves in seawater to form several inorganic carbon species. TDIC is defined as the sum of the concentration of those species, i.e., the sum of the carbonate, bicarbonate, carbonic acid, and carbon dioxide concentrations. 

However, this simple picture only applies to gases that do not undergo reactions in the boundary layers. For gases that do react, for example through hydration and acid-base reactions, the net flux depends on the simultaneous movement of all the solutes involved, and the flux will not be the simple function of concentration expressed in Equation (3.25). An example is CO2, which reacts with water to form carbonic acid and carbonate species-H2C03, HCOs and COs . The situation is complicated because the exchange of H+ ions in the carbonate equilibria results in a pH gradient across the still layer, and it is therefore necessary to account for the movement of H+ ions across the still layer as well as the movement of carbonate species. The situation is further complicated in the case of CO2 by the kinetics of hydration and dehydration, which may be slow in comparison with transport. 

The human body is a remarkable machine. It relies on a variety of safeguards to keep blood pH constant. Our blood constitutes a buffer system — meaning, it has components that can react with excess base or excess acid. Carbon dioxide, which is produced by the metabolism of food, dissolves in blood to produce carbonic acid, and carbonic acid can neutralize any excess base. The bicarbonate ion, also present in blood, will promptly take care of any surplus acid. The level of carbon dioxide in the blood adjusts to a body s rate of respiration. If blood pH drops — which actually means that the blood has... 

One major concept applicable to problems dealing with the behavior of carbonic acid and carbonate minerals in seawater is the idea of a "constant ionic media". This concept is based on the general observation that the salt in seawater is close to constant in composition, i.e., the ratios of the major ions are the same from place to place in the ocean. Seawater in evaporative lagoons, pores of marine sediments, and near river mouths can be exceptions to this constancy. Consequently, the major ion composition of seawater can generally be determined from salinity. It has been possible, therefore, to develop equations in which the influences of seawater compositional changes on carbonate equilibria can be... 

The blood buffering system depends on two critical equilibria. Garbon dioxide reacts with water to form carbonic acid and carbonic acid reacts with water to form hydronium ion and bicarbonate ion, as shown here. 

Because excited triplet states are less likely to have their unpaired electrons on the same atoms than are singlet states (see reference 24), the triplet states tend to have biradicaloid character. For a more detailed discussion of the acid-base properties of photoexcited organic molecules, including carbon acids and carbon bases, see Wan, P. Shukla, D. Chem. Rev. 1993, 93, 571. 

Effervescent powders contain, besides the active substance, a combination of an acid and a carbonate or bicarbonate. When the powder is added to a glass of water, carbonic acid and carbon dioxide are formed and the latter produces effervescence. During this chemical reaction often a soluble sodium salt of the active substance is formed. Moreover, the effervescence serves as a natural stirring process, which may enhance the dissolution rate. 

From Table 2.4 it can be seen that each successive increase in oxidation state increases the number of bonds between carbon and oxygen and decreases the number of carbon-hydrogen bonds. Methane has four C—H bonds and no C—O bonds carbonic acid and carbon dioxide both have four C—O bonds and no C—H bonds. 

An important component of acid-base homeostasis. It exists in equilibrium with carbonic acid and carbon dioxide as follows ... 

The diagram for carbon in Fig. 83 displays a very small domain of stability. It is thermodynamically possible for carbons to be easily oxidized to carbon dioxide, carbonic acid, and carbonates. Reduction of carbon may lead to the formation of methane, methyl alcohol and other organic substances. However, the energetically possible reactions are strongly irreversible [2] and do not occur under normal conditions of pressure and temperature. Schmidt [24] reported a corrosive destruction of carbon electrodes when a critical potential was exceeded during the reduction of O2. The carbon electrodes were not impregnated with metallic electrocatalysts. The critical potential depended upon the extent to which an oxygen layer was present (compare section 5 in chapter VIII). 

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