Acid, Bicarbonate, and Carbonate

Carbonic Acid, Bicarbonate, and Carbonate A. Solubility AND Equilibria of CO2 in Solution 

Carbon dioxide is a symmetric linear molecule with zero dipole moment. Hence its interaction with a dipolar molecule like H2O is weak. However, it is moderately soluble in water and in many other organic solvents. The solubility equilibrium can be described by Eq. (1)  

The rate of reversible hydration of carbon dioxide has been studied extensively and a critical evaluation of different techniques used has been documented by Kern (21). In the absence of an externally added catalyst the hydration reaction can be described as follows  

Below pH 7.4 (i.e., the pH of blood), Eq. (9) predominates and H2CO3 is an intermediate existing less than 2% of the total dissolved CO2. The diffusion controlled rate constant for the protonation of HCO3 based on the coulombic concept is ca. 6.5 x 10 M s in agreement with the experimental value reported by Eigen (22) as 4.7 x 10 s  

Paneth and O Leary 26a) investigated the isotope effects on the dehydration of HCO3 in water and D2O at 1, 22, and 24 °C. The values for h jh ) reported by them are 1.0171 zb 0.0006 (1°C, H2O), 1.0151 zb 0.0008 (22 °C, H2O), and 1.0178 zb 0.0005 (24 °C, D2O) earlier reported value 1.0147 zb 0.0007 at 24 °C, H2O 26b). Model calculations were also made by them. They concluded that the mechanism of HCO3 dehydration is stepwise [see Eq. (12)]. The first step involves the protonation of bicarbonate by hydronium ion to the zwitterionic intermediate, H20 -C02, which then decomposes to yield the products (CO2 and H2O) the rates of protonation and decomposition are similar. 

Whether the formation of the zwitterionic intermediate occurs by way of carbonic acid ( 2 ) or directly by the reaction of the hydronium ion with HCO3 still remains uncertain from the kinetic considerations. 

The base hydrolysis of C02 [see Eq. (10)] has been extensively studied and a comparative account of the rate constant k2 has been presented by Palmer and van Eldik (3). k2 is 105 times higher than k] under comparable conditions despite the fact that C02 is a symmetrical and uncharged molecule with zero dipole moment. The values of 2(av (M-1 s-1), AHfav) (kJ mol-1) and ASfavl (J K-1 mol-1) at 25.0 °C and 7=0 are (8.0 0.6) x 103, 54.4 2.4, and 12 8, respectively. k2 also shows a minor ionic strength dependence (log k2 = 3.772 (0.213 0.013) 

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The formation and degradation of planktonic POC and PIC influence pH and C.A. as follows. The remineralization of POC produces CO2, which is rapidly hydrolyzed to carbonic acid, bicarbonate, and carbonate via the reactions given in Eqs. 5.53 through 5.57. Carbonic acid and bicarbonate are both weak acids, so their dissociation generates H. This acid enhances the dissolution of PIC through the following reaction ... 

The equilibrium state between carbonic acid bicarbonate and carbonate ions is established rapidly. The solution of C02 and formation of H2C03 take place over a longer period. However, since many authors dispute the availability of H2C03, a summarized concentration of C02 and H2C03 should be considered by denoting it as [C02]. Let us introduce some notations to characterize the total concentration of... 

The distribution of the carbon-containing species (carbonic acid, bicarbonate and carbonate) depends upon pH, as shown in Fig. 3.28 (see Box 3.4). Seawater is slightly alkaline, and its pH rarely falls outside the range 7.5-8.5 owing to the buffering effect of the equilibrium between bicarbonate and carbonate ions. Consequently, seawater contains negligible carbonic acid, and OH ions are more abundant than H+, so the main equilibria can be represented by Eqns 3.9a and 3.9b, which are combined to give Eqn 3.9c ... 

Fig. 3.28 Summary of the approximate equilibration of carbonic acid, bicarbonate and carbonate in water of varying pH. (DIC = dissolved inorganic carbon.)...

New three-dimensional models of sulfuric acid, carbonic acid, bicarbonate, and carbonate are added. 

Amino-2-hydroxybenzoic acid is manufactured by carboxylation of 3-amiaophenol under pressure with ammonium carbonate at 110 °C (182) or with potassium bicarbonate and carbon dioxide at 85—90°C (183) with subsequent acidification. 

Alkalinity. The alkalinity of a water sample is its acid-neutrali2ing capacity. Bicarbonate and carbonate ions are the predominant contributors to alkalinity in most waters, and their chemical equiUbria generally maintain the pH of 5—9. The presence of enough hydroxide ion to affect the alkalinity determination in natural waters is rare. SiUca, borate, or phosphate do contribute to the overall alkalinity if present in large enough quantities. 

Aikaiinity Bicarbonate (HCOs" ), carbonate (COs , and hydroxyl (OH ), expressed as CaCOs Foaming and carryover of solids with steam embrittlement of boiler steel bicarbonate and carbonate produce CO2 in steam, a source of corrosion Lime and lime-soda softening, acid treatment, hydrogen zeolite softening, demineralization, dealkalization by anion exchange, distillation, degasifying... 

At equilibrium, the concentration of H+ will remain constant. When a strong acid (represented by H+ or HA) is introduced into solution, the concentration of H+ is increased. The buffer compensates by reacting with the excess H ions, moving the direction of the above reaction to the left. By combining with bicarbonate and carbonate ions to form the nonionic carbonic acid, equilibrium is reestablished at a pH nearly the same as that existing before. The buffer capacity in this case is determined by the total concentration of carbonate and bicarbonate ions. When no more carbonate or bicarbonate ions are available to combine with excess H+ ions, the buffer capacity has been exceeded and pH will change dramatically upon addition of further acid. 

Equation 8.26 predicts a concentration of CO2 in one litre of water at 25°C of 3.32 x 10-2 moll-1 at a pressure of 1 bar. The pH of the oceans is related to the amount of dissolved C02 but also to the equilibria controlling the formation of carbonic acid and the bicarbonate and carbonate ions ... 

Dissolved carbon dioxide produces carbonic acid, which ionizes to bicarbonate and carbonate ions, the reactions for which are shown in Figure 5.2 (equations 1-3). This reaction sequence is extremely important because bicarbonate is a counterion to many cations, is active in buffering the soil solution, and is involved either directly or indirectly in many soil chemical reactions. Bicarbonates are generally more soluble than carbonates, which are generally insoluble. Adding acid to carbonates or bicarbonates results in the release of carbon dioxide and the formation of the salt of the acid cation. The acid is thus neutralized. 

In practice, the alkalinity of seawater is determined by titrating a sample until the pH drops to 3, well below the bicarbonate equivalence point (pH 4.5). Although most of the titratable negative charge is contributed by bicarbonate and carbonate, the other weak bases present in seawater do consume some acid above and below the bicarbonate... 

Carbon dioxide is a symmetrical, linear triatomic molecule (0 = C=0) with a zero dipole moment. The carbon-to-hydrogen bond distances are about 1.16A, which is about 0.06A shorter than typical carbonyl double bonds. This shorter bond length was interpreted by Pauling to indicate that greater resonance stabilization occurs with CO2 than with aldehydes, ketones, or amides. When combined with water, carbonic acid (H2CO3) forms, and depending on the pH of the solution, carbonic acid loses one or two protons to form bicarbonate and carbonate, respectively. The various thermodynamic parameters of these reactions are shown in Table I. 

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. 

In this method, the crystallized and anhydrous carbonates are mixod in such proportions as to leave a slight excess of water over what is necessary for the water of crystallization of the whole salt, when it is converted entirely into bicarbonate and carbonic acid is admitted to the bottom of the mixture by a tube proceeding from a pneumatic apparatus similar to a gasometer, so constructed that there can never be any waste of gas, The absorption is completed every twenty-four hours. 

By analogy with the carbonic acid system, OCS hydrates to form H2O.OCS in aqueous solution, and the bicarbonate and carbonate like anions OCS.OH and OCS.O2 known collectively as monothiocarbonates (MTC). The entire carbonate analog system is shown in Figure 1, and it should be clear that MTC species are likely to serve as intermediates in both overall hydrolysis channels CL). Several pieces of information point to a lack of reversibility to OCS, or, schematically, to... 

Carbon dioxide, C02, is a fairly small molecule with acidic properties, which has frequently been used as a probe molecule for basic surface sites and as a poison in catalytic reactions. As shown in the following, C02 adsorption onto oxide surfaces leads to a variety of surface species such as bicarbonates and carbonates that coordinate to surface metal ions in various ways. The type of the coordination influences the symmetry of these ligands so that different surface species held by distinct surface sites can be distinguished by means of their infrared absorptions (162). The characteristic infrared (and Raman) bands of C02 and possible surface species are summarized in Table VI. The wave-number range below 1000 cm"1 was usually not accessible in studies on adsorbed C02 because of the strong absorption of the oxides at lower wave numbers. 

Alkalinity is defined as a measure of the proton deficit in solution and should not to be confused with basicity. Alkalinity is operationally defined by titration with a strong acid to the carbonic acid end point. This is known as the titration alkalinity (TA). Seawater contains weak acids other than bicarbonate and carbonate that are titrated, and therefore TA is given as... 

Hydroxyl radicals are extremely reactive, short Hved and unselective transient species. The mean lifetime of OH radicals depends on their chemical environment and was estimated to be in the order of 10 ps in the presence of dissolved natural organic matter, bicarbonate and carbonate (Hoigne, 1998). Pryor (1986) estimated the half-life of hydroxyl radicals in the presence of Hnoleate (C18H31O2, the conjugate base of linoleic acid) at T=37°C to be in the order of nano seconds. 

The chemical analyses tabulated in this article identify "alkalinity" as a property of the water rather than a simple constituent. Alkalinity has been more broadly defined as "capacity for acid neutralization" (12,13). Common practice in water analysis is to report alkalinity in terms of bicarbonate and carbonate concentrations, although other ionic species also may contribute by reacting with the titrating acid. 

Effervescent tablets are uncoated tablets that generally contain acid substances and carbonates or bicarbonates, and that react rapidly in the presence of water by releasing carbon dioxide. They are usually dissolved or dispersed in water before administration. ... 

Because of the ubiquitousness of carbonate rocks and the equilibrium reactions of CO2, bicarbonate and carbonate are present as bases in most natural waters. In addition, small concentrations of the bases borate, phosphate, arsenate, ammonia, and silicate may be present in the solution. Volcanoes and certain hot springs may yield strongly acid water by adding gases like HCl and SO2. 

The pH of water is an important factor for AOP application in the water phase as the hydroxyl radical concentration is a function of pH. pH controls the equilibrium of carbonate, bicarbonate, and carbonic acid present in water carbonate and bicarbonate both scavenge hydroxyl radicals with rate constants of 3.9 x 10 M s and 8.5 x 10 M s , respectively. Thus, acidic pH is better for water with high carbonate and bicarbonate alkahnity (greater than 400 mg/L as CaCOg). However, the effect of pH is system-specific for example, generally UV/H2O2 is more effective at low pH, while UV/O3 is more effective at slightly basic pH. 

Atmospheric carbon dioxide is dissolved in seawater where it forms carbonic acid, which dissociates into bicarbonate and carbonate ions, respectively (reaction 1). Dissociation is strongly dependent upon pH. 

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