Carbonic acid ionization constants

As with other dicarboxyhc acids the second ionization constant of carbonic acid IS far smaller than the first... 

The concentration of tme carbonic acid (H2CO2) is negligible in comparison to dissolved carbon dioxide, eg, only 0.3% of the latter is hydrated to carbonic acid at 25°C. The ionization constant is a composite constant representing both the CO2 hydration reaction, iC, and ionization of tme H2CO2, ifj = ifjj QQ /(I + K). Temperature-dependent equations for and are (29)... 

Carbon dioxide, the final oxidation product of carbon, is not very reactive at ordinary temperatures. However, in water solution it forms carbonic acid [463-79-6] H2CO2, which forms salts and esters through the typical reactions of a weak acid. The first ionization constant is 3.5 x 10 at 291 K the second is 4.4 x 10 at 298 K. The pH of saturated carbon dioxide solutions varies from 3.7 at 101 kPa (1 atm) to 3.2 at 2,370 kPa (23.4 atm). A soHd hydrate [27592-78-5] 8H20, separates from aqueous solutions of carbon dioxide that are chilled at elevated pressures. 

Kryukov, P.A. Starostina, L.I. Tarasenko, S.Ya Pavlyuk, L.A. Smolyakok, B.S. Larionov, E.G. "Ionization Constants of Carbonic Acid, Hydrogen Sulfide, boric Acid, and Sulfuric Acid at High Temperatures," Mezhdunar. Geokhim., Koagr. (Dokl.) 1st, 1971, 186-98 C.A, 1974, 84 (69193). 

Carbonates. Carbonic acid H CO, is present to the extent of (1.27 i of the total CO present in ihe solution that is formed by dissolving CO in H 0 ai room temperature. The CO may be expelled fully upon boiling. The solution reads with alkalis to form carbonates, e.g. sodium carbonate, sodium hydrogen carbonate, calcium carbonate, calcium hydrogen carbonate. The acid ionization constant usually cited for carbonic... 

Total salinity based on K2 (second ionization constant for carbonic acid) and Kj (solubility product constant for CaCC>3) (in practice measured as total dissolved solids)... 

The compound can only be a carboxylic acid no other class containing only carbon, hydrogen, and oxygen is more acidic. A reasonable choice is HC=CCOzH C-2 is sp-hybridized and therefore rather electron-withdrawing and acid-strengthening. This is borne out by its measured ionization constant Ka, which is 1.4 X 10 2 (pATa 1.8). 

Hamed, H. S. and Davis, R. The ionization constant of carbonic acid in water and the solubility of carbon dioxide in water and aqueous salt solutions from 0 to 50, J. Am. Chem. Soc. 2030-2037 (1943). 

Determination of hydroxyl ion in the presence of carbonate Owing to the small second ionization constant of carbonic acid K2 = 5 x 10" ), the titration of hydroxyl ion alone is not feasible in the presence of an appreciable concentration of carbonate ion. For example, a 0.01 M solution of sodium hydroxide has a pH of about 12, and a 0.01 M solution of sodium carbonate a pH of about 11.2. Thus the pH change in a titration of 0.01 M sodium hydroxide in the presence of 0.01 M sodium carbonate would be only about 0.8 pH unit. 

Some less reliable H bond interpretations of dissociation data have been made. For example, Dippy (526) proposed that a G—H- -O bond from 7 carbon to C=0 formed a ring in C4-Cg acids, to account for the fact that their ionization constants do not fall as rapidly as do those of C2 and Cs acids. A case in aromatic compounds is given by Arnold and Sprung (64). Some later work (907) makes use of the same idea to explain the Ka of a partially fluorinated valeric acid, (CF8(GH2)sGOOH). This scheme of H bonding hzis not been substantiated, and there is still controversy—the discussion at the end of one of Katch2ilsky s papers is illustrative (1089). 

J. Walker and W. Cormack found the ionization constant of carbonic acid [H][HC03 J—A)2[H2C03]. J. Kendall s value for 2... 

Ionization constants of 2-carboxypiperazine and the three dicarboxylic acids have been determined (1684), and the nucleophilic reactivity of piperazine compared to other amines in reactions with l-chloro-2,4-dinitrobenzene has been measured (1685). The rate of quaternization of l-ethoxycarbonyl-4-methylpiperazine with allyl bromide (/ = 2.25 1/mol min) and methyl iodide (10A = 1.22) have been measured in acetone-water solution (1686). The composition and structure of the 2-methylpiperazine-carbon disulfide complex has been investigated it was a mixture of l-dithiocarboxy-3-methylpiperazine (138) and the 2-methylpiperazine salt of l,4-bis(dithiocarboxy)-2-methylpiperazine (1687). 

As mentioned in Section IV.A, Nelson and Yang [494] have proposed a surface complexation model to describe the effect of pH on adsorption equilibria of chlorophenols on activated carbon. To account for the well known suppression of uptakes as pH increases, they introduce the ionization constants of separate acidic (A) (e.g., carboxyl) and basic (B) (e.g., pyrone) functional groups on the carbon surface ... 

Write the acid ionization equation and ionization constant expression for the hydrogen carbonate ion (HCO ). 

On the other hand, in -fluoroalkyl groups (with one uncompensated C-F dipole moment) fluorination usually reduces lipophilicity. Another important instance of fluorine substitution reducing lipophilicity is in compounds with a-fluorocarbonyl groups. In these fluorination can increase the electrophilicity of the carbonyl carbon atom to such an extent that the formation of stable, polar hydrates occurs this again reduces the lipophilicity significantly (Scheme 4.13). In a-fluorocar-boxylic acids and fluorinated phenols the lipophilicity is also reduced by the increased ionization constant which results from the negative inductive —la) effect of fluorine [16]. 

As the number of carbon atoms increases in an acid, ionization decreases, and hence the ionization constant (Ka) decreases. Because of the decrease in the concentration of hydrogen ions, acidic strength decreases as shown in Table 3. 

Carboxylic acids are the most acidic class of compounds that contain only carbon, hydrogen, and oxygen. With ionization constants on the order of 10 (p/fa 5), they are mnch stronger acids than water and alcohols. The case should not be overstated, however. Carboxylic acids are weak acids a 0.1 M solution of acetic acid in water, for example, is only 1.3% ionized. 

The First Ionization Constant of Carbonic Acid. Using a method similar in principle but differing in details from that just described, Maclnnes and Belcher T have determined the first and second ionization... 

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