Carbonate ion

Clearly such bonding would produce two different carbon-oxygen bond distances (p. 48) but in fact all bonds are found to be identical and intermediate in length between the expected C=0 and C—O bond distances. We conclude, therefore, that the true structure of the carbonate ion cannot be accurately represented by any one diagram of the type shown and a number of resonance structures are suggested (p. 50). 

As in the case of NH4 the charge is distributed over the whole ion. By considering each multiple bond to behave spatially as a single bond we are again able to use Table 2.8 to correctly deduce that the carbonate ion has a trigonal planar symmetry. Structures for other covalently-bonded ions can readily be deduced. 

As with the hydroxides, we find that whilst the carbonates of most metals are insoluble, those of alkali metals are soluble, so that they provide a good source of the carbonate ion COf in solution the alkali metal carbonates, except that of lithium, are stable to heat. Group II carbonates are generally insoluble in water and less stable to heat, losing carbon dioxide reversibly at high temperatures. 

The carbonate ion is planar and can be regarded as a resonance structure between the three forms given below (see also p. 44) ... 

Hydroxide ion lies below phenol m Table 1 7 hydrogen carbonate ion lies above phe nol The practical consequence of the reactions shown is that NaOH is a strong enough base to convert phenol to phenoxide ion but NaHCOs is not... 

Many important biochemical reactions involve Lewis acid Lewis base chemistry Carbon dioxide is rapidly converted to hydrogen carbonate ion m the presence of the enzyme carbonic anhydrase... 

The systematic name for bi carbonate ion is hydrogen carbonate Thus the system atic name for sodium bicar bonate (NaHCOs) is sodium hydrogen carbonate... 

See Adsorption, LIQUID separation Aluminum compounds, aluminum oxide (alumina) Carbon, activated carbon Ion exchange Molecular sieves Silicon... 

Physical Properties. Physical properties of importance include particle size, density, volume fraction of intraparticle and extraparticle voids when packed into adsorbent beds, strength, attrition resistance, and dustiness. These properties can be varied intentionally to tailor adsorbents to specific apphcations (See Adsorption liquid separation Aluminum compounds, aluminum oxide (alumna) Carbon, activated carbon Ion exchange Molecular sieves and Silicon compounds, synthetic inorganic silicates). 

Molten Carbonate Fuel Cell. The electrolyte ia the MCFC is usually a combiaation of alkah (Li, Na, K) carbonates retaiaed ia a ceramic matrix of LiA102 particles. The fuel cell operates at 600 to 700°C where the alkah carbonates form a highly conductive molten salt and carbonate ions provide ionic conduction. At the operating temperatures ia MCFCs, Ni-based materials containing chromium (anode) and nickel oxide (cathode) can function as electrode materials, and noble metals are not required. 

Weak to moderate chemiluminescence has been reported from a large number of other Hquid-phase oxidation reactions (1,128,136). The Hst includes reactions of carbenes with oxygen (137), phenanthrene quinone with oxygen in alkaline ethanol (138), coumarin derivatives with hydrogen peroxide in acetic acid (139), nitriles with alkaline hydrogen peroxide (140), and reactions that produce electron-accepting radicals such as HO in the presence of carbonate ions (141). In the latter, exemplified by the reaction of h on(II) with H2O2 and KHCO, the carbonate radical anion is probably a key intermediate and may account for many observations of weak chemiluminescence in oxidation reactions. 

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. 

The alkalinity is determined by titration of the sample with a standard acid (sulfuric or hydrochloric) to a definite pH. If the initial sample pH is >8.3, the titration curve has two inflection points reflecting the conversion of carbonate ion to bicarbonate ion and finally to carbonic acid (H2CO2). A sample with an initial pH <8.3 only exhibits one inflection point corresponding to conversion of bicarbonate to carbonic acid. Since most natural-water alkalinity is governed by the carbonate—bicarbonate ion equiUbria, the alkalinity titration is often used to estimate their concentrations. 

Precipitation softening processes are used to reduce raw water hardness, alkalinity, siHca, and other constituents. This helps prepare water for direct use as cooling tower makeup or as a first-stage treatment followed by ion exchange for boiler makeup or process use. The water is treated with lime or a combination of lime and soda ash (carbonate ion). These chemicals react with the hardness and natural alkalinity in the water to form insoluble compounds. The compounds precipitate and are removed from the water by sedimentation and, usually, filtration. Waters with moderate to high hardness and alkalinity concentrations (150—500 ppm as CaCO ) are often treated in this fashion. 

The lime or lime—soda process results in the precipitation of calcium as calcium carbonate and magnesium as magnesium hydroxide. The solubiUties of these compounds are shown in Figure 4 as functions of pH. When lime is used alone, only the carbonate hardness is reduced. The carbonate hardness is present as calcium or magnesium bicarbonate. The additional use of soda ash can reduce the noncarbonate hardness by providing additional carbonate ion. The reactions involved in the various steps of the process are Hsted below ... 

Since the effluent from a softening unit is usually supersaturated with calcium carbonate at the usually high pH values, it is necessary to reduce the pH to a value that allows the solution to be exactiy saturated for the calcium-ion and carbonate-ion concentrations present. The relationship is... 

Modifications of the basic process are undersoftening, spHt recarbonation, and spHt treatment. In undersoftening, the pH is raised to 8.5—8.7 to remove only calcium. No recarbonation is required. SpHt recarbonation involves the use of two units in series. In the first or primary unit, the required lime and soda ash are added and the water is allowed to settie and is recarbonated just to pH 10.3, which is the minimum pH at which the carbonic species are present principally as the carbonate ion. The primary effluent then enters the second or secondary unit, where it contacts recycled sludge from the secondary unit resulting in the precipitation of almost pure calcium carbonate. The effluent setties, is recarbonated to the pH of saturation, and is filtered. The advantages over conventional treatment ate reductions in lime, soda ash, and COg requirements very low alkalinities and reduced maintenance costs because of the stabiUty of the effluent. The main disadvantages are the necessity for very careful pH control and the requirement for twice the normal plant capacity. 

In both cases, the carbonate ion concentration increases and eventually equiUbrates in the system, releasing carbon dioxide in the stripping column and thereby reducing product purity. Hence, a small caustic wash tower is employed to remove any carbon dioxide that is Hberated in the stripper. 

These observations consummated in a growth model that confers on the millions of aligned zone 1 nanotubes the role of field emitters, a role they play so effectively that they are the dominant source of electron injection into the plasma. In response, the plasma structure, in which current flow becomes concentrated above zone 1, enhances and sustains the growth of the field emission source —that is, zone 1 nanotubes. A convection cell is set up in order to allow the inert helium gas, which is swept down by collisions with carbon ions toward zone 1, to return to the plasma. The helium flow carries unreacted carbon feedstock out of zone 1, where it can add to the growing zone 2 nanotubes. In the model, it is the size and spacing of these convection cells in the plasma that determine the spacing of the zone 1 columns in a hexagonal lattice. 

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