Carbonic acid species

Figure 9 The distribution of carbonic acid species at 20° C as a function ofpH in seawater of salinity 35 (solid lines) and pure water (dotted lines)...

Sec. 5.3 Carbon Dioxide and Carbonic Acid Species in Natural Waters... 

CARBON DIOXIDE AND CARBONIC ACID SPECIES IN NATURAL WATERS... 

We will now take a more rigorous approach to this question and compute the distribution diagram for carbonic acid species in water as a function of pH, but ignore ion activity coefficients. First let us define the total carbonate, Cj, where... 

Only carbonic acid species and strong acids or bases are present. 

The buffer capacity due to carbonic acid species may be computed assuming either constant total carbonate, CV, which implies a closed system, or for constant CO2 pressure, which suggests an open system. We will first assume constant Cr and constant Ca with an HCl titrant. The mass-balance equations are... 

Figure 5.10 A linear plot of the buffer capacity of carbonic acid species as a function of pH for Cj= lO M showing that the maximum buffer capacity equals 0.58 Cj, and occurs at pH = pAT,(H2C03) = 6.35. The lower curve is the buffer capacity of water, /Sh,o-...

Next, consider the buffer capacity of carbonic acid species at constant COj pressure. In the previous calculation /3 was obtained by taking the derivative of an equation for in the proton condition that had been obtained from the solution charge-balance equation. We will instead base the calculation of ji directly on the equation for Cg, differentiating in parts. Thus, we write... 

Figure 5.11 A log plot of the buffer capacity due to carbonic acid species for = 10 M (see Fig. 5.10) at saturation with respect to calcite for = I0 M and for equilibrium between the clays illite and kaolinite. The lower curve is...

Hagmeyer G., Gimbel R. (1993), Rejection of carbonic acid species in (reverse osmosis) and nanofiltration, Proc. of AWWA Membrane Technology Conf., Baltimore, Aug 93, 251-257. 

Meta.1 Conta.mina.nts and Ash. Alkali metals form basic oxides that are very reactive toward acidic species such as the acid gases, siHcates, and alurninates. These form stable salts with acid gases if the off-gas contains such gases. Sodium, the most common of these metals, prefers to form chlorides ahead of sulfates. Sodium carbonate only forms in the absence of haHdes and sulfur oxides, SO. There usually is too Htde NO present to form nitrates (see Sodium compounds). 

The solubility of AS2O3 in water, and the species present in solution, depend markedly on pH. In pure water at 25°C the solubility is 2.16 g per lOOg this diminishes in dilute HCl to a minimum of 1.56g per lOOg at about 3 m HCl and then increases, presumably due to the formation of chloro-complexes. In neutral or acid solutions the main species is probably pyramidal As(OH)3, arsenious acid , though this compound has never been isolated either from solution or otherwise (cf. carbonic acid, p. 310). The solubility is much greater in basic solutions and spectroscopic evidence points to... 

Upon the irradiation the nitrous acid ester 1 decomposes to give nitrous oxide (NO) and an alkoxy radical species 3. The latter further reacts by an intramolecular hydrogen abstraction via a cyclic, six-membered transition state 4 to give an intermediate carbon radical species 5, which then reacts with the nitrous oxide to yield the 3-nitroso alcohol 2 ... 

Sometimes we need to know how the concentrations of the ions present in a solution of a polyprotic acid vary with pH. This information is particularly important in the study of natural waters, such as rivers and lakes (Box 10.1). For example, if we were examining carbonic acid in rainwater, then, at low pH (when hydronium ions are abundant), we would expect the fully protonated species (H2C03) to be dominant at high pH (when hydroxide ions are abundant), we expect the fully deprotonated species (C032 ) to be dominant at intermediate pH, we expect the intermediate species (HC03, in this case) to be dominant (Fig. 10.20). We can verify these expectations quantitatively. 

To see how the concentrations of species present in a solution vary with pH, we take the carbonic acid system as an example. Consider the following proton transfer equilibria ... 

FIGURE 10 JO The fractional composition of the species in carbonic acid as a function of pH. Note that the more fully protonated species are dominant at lower pH. 

We have found expressions for the fractions, /, of species in a solution of carbonic acid. They are easily generalized to any diprotic acid H2A ... 

There are several acidic species present in any solution of a polyprotic acid. The solution of carbonic acid in Example contains the following concentrations of acidic species ... 

It was considered that an aqueous solution of bicarbonate contained carbonic acid, carbonate, and carbon dioxide in addition to bicarbonate [67CC799 71 JCS(C) 1501 ]. Experiments were conducted to determine the carboxylating species and it was shown that carboxylation could not be achieved with carbon dioxide nor with carbonate. It was thus concluded that the likely carboxylating species was bicarbonate anion and that the process could be visualized as involving addition of the protonated species (124) to the bicarbonate anion, giving the observed product (125) (Scheme 16). 

Carbonate also forms a mononuclear complex (132). Using an (ionic strength and temperature adjusted) value of log /32 for the formation of carbonic acid, CO + 2H+ C02(aq) + H20, of 15.92 (11), log K for the equilibrium Be2+ + C03 BeC03(aq) was derived as 5.4. The other carbonato species formed were assigned the formulas Be3(0H)2(HC03)3+ and Be5(0H)4(C03)4+, since the Raman spectra of the coordinated carbonate appeared to be different in the two complexes, but the structures proposed for these species are not imme-... 

Carbonic acid, H2C03(aq), never exists as a pure compound it only exists as a species in aqueous solution, where it dissociates in just the same way as ethanoic acid in Equation (6.1) to form a solvated proton and the HCOj(aq) ion. Note how we form a solvated proton H30+(aq) by splitting a molecule of water, rather than merely donating a proton. Carbonic acid is, nevertheless, a Lowry-Brpnsted acid. 

Carboranes with five carbon atoms are rare. In the uncomplexed form the nido-1 -boranediyl-2,3,4,5,6-pcntamcthyl-2,3,4,5,6-pentacarbahexaborane(6) 72 is not known, its stabilization is possible with Lewis acidic species such as Br+, Fe(CO)4, BCI3, and BC12R (Scheme 3.2-37). 

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. 

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