Bicarbonate species

We now consider as an example an analysis (Table 15.1) of water from Mono Lake, California. The reported alkalinity of 34 818 mg kg-1 as CaC03 is equivalent to 700 meq of acid or 350 mmol of H2SO4. Since at this pH carbonate and bicarbonate species are present in roughly equal concentrations, we can quickly estimate the total carbonate concentration to be about 30 000 mg kg-1. We take this value as a first guess and model the titration with react... 

The authors ascribed the high activity of Au/thoria to a Schottky junction effect between the metal and oxide leading to an enhancement of active OH groups associated with oxygen deficiencies. The authors did observe formation of formate, carbonate, and bicarbonate species, but are still elucidating which, if any, of the species are important for the mechanism. 

No protonated pyridine (which resonates at 174 ppm) was found on initial measurement. The author attributes the subsequent appearance of this signal (on exposure to air) to the formation of carbonate or bicarbonate species which serve as counterions to protonated pyridine. 

Alternatively, a true C02 adduct may not be formed, and instead an isomer of the M(I) bicarbonate species M(C0)(HC03)(PPh3)2 may be generated (see below). 

Formation of Carbon Dioxide Complexes. As mentioned in the introduction, our initial interest in synthesizing the PCy3 complexes was in their potential for binding C02. However, except for the formation of peroxycarbonate and carbonato complexes from IrCl(02)(PCy3)2 (44), which is well-established chemistry for some platinum metal peroxide complexes (42) (but, to our knowledge, not with PCy3 systems), we have not been able to isolate any C02 complexes or even carbonate or bicarbonate species which are formed sometimes in the presence of adventitious water (16). 

The dissolution reaction in Eq. 3.59b can be regarded as an example of a ligand-promoted process, in that adsorbed bicarbonate species are likely to play a role as intermediates in the kinetic analysis of the reaction.5 Ligand-promoted dissolution reactions are a principal basis for the reductive dissolution processes described in Section 3.4 (see Eq. 3.46). The sequence of steps is analogous to that in proton-promoted dissolution ... 

The C02 effect has been explained (191) in terms of the formation of alkali carbonate or bicarbonate species, where the cation is replaced by a proton derived from a water molecule. This proposal does indeed account for both the observed increase in activity and decrease in selectivity which is similar to proton-exchange forms. 

The precipitate, CaCO, is indicated by a downward pointing arrow,i. As shown, the two bicarbonate species in the reactant side are converted to the two carbonate species in the prodnct side. The carbonate ion shown unpaired will pair with whatever cation the OH was with in the reactant side of the reaction. 

In the previous reaction, the number of references is 6. Thus, the equivalent mass of alum is Al2(S04)3 XH2O/6 = 57.05 + 3x and that of the calcium bicarbonate species is 3Ca(HC03)2/6 = 81.05. The other alkalinity sources that can be used are lime, caustic soda, and soda ash. Lime is used in the discussion that follows. Also, alkalinity requirements are usually expressed in terms of CaC03. Therefore, we also express the reactions of alum in terms of calcium carbonate. The respective chemical reactions are ... 

The activation of carbon dioxide was studied over a zirconium dioxide catalyst via infrared spectroscopy and 0-labeled reactants. The carbon dioxide adsorbed on the surface as either a carbonate or a bicarbonate species. The carbonate species formed as a result of CO2 Interaction with lattice oxygen. The bicarbonate species formed from CO2 interaction with a hydroxyl group. There was no direct interconversion between the carbonate and the bicarbonate. It is proposed that the bicarbonate can be converted to the formate via molecular CO. 

Cooling the zirconia from 450 to 25°C in H2/CO2 resulted in the formation of carbonate and bicarbonate species. (Additional TPD studies, not shown, established that formate and methoxide species did not form during the adsorption of CO2 > under the conditions reported here.) The use of H2 0 to exchange groups on the... 

Figure 4 also shows the formation of carbonates from CO2 and a lattice oxygen. The nature of the surface sites involved in carbonate formation were not revealed in these studies. The results for [ 0] incorporation into the carbonate and bicarbonate species, following C 02 adsorption, suggest that these species are formed at different sites and that there is no direct conversion of the carbonate into the bicarbonate. 

Fig. 7 Possible structures of zinc bicarbonate species supported by the [12]aneN3 and [12]aneN4 ligands.

IR spectra of adsorbed CO2 on MgY zeolite exhibit a strong infrared band around 1630 cm with a shoulder on high frequency side, and a weak broad band centered at about 1450 cm. No IR bands were detected between 1300-1400 cm. According to the literature [14,15], bicarbonate species formation involves surfece hydroxyl groups. Bicarbonates show a C-OH bending mode at 1220 cm as well as a symmetric and asymmetric O-C-O stretching at 1480 cm and 1650 cm, respectively. The IR spectra on MgY are consistent then with the... 

Reactive adsorption of CO2 with the participation of surface hydroxyls that are basic in nature results in the formation of bicarbonate species. The OH modes of surface bicarbonates are detected in the interval between 3625 and 3605 cm, while the deformation COH modes are positioned around 1236—1220 cm . Both modes are sensitive to H/D exchange. Additional bands reported are the symmetric and antisymmetric C—O stretches at 1670—1595 and 1490—1370 cm respectively (481). Although the positions of these bands have been proposed to contain information on the basicity of the hydroxyls, it was remarked that they depend on too many factors to allow unambiguous conclusions (56,480). 

Much more interesting than this single example, however, would be to see how the activities or concentrations of H2CO3, HCO, and CO vary as a function of pH. Natural solutions contain many components in addition to CO2 and water, and so the pH can be quite different from the one we have just calculated. To do this, we simply choose specific pH values from 0 to 14 and solve for the activities of the three carbonate species. The result is shown in Figure 16.3. This diagram makes it easy to see which species is dominant (has the largest concentration) at any given pH. For example, in seawater, with a pH of about 8.1, carbonate is present almost entirely as the bicarbonate species. 

---