Carbon Dioxide Bicarbonate And Carbonate

Background

The reactive inorganic forms of environmental carbon are carbon dioxide (CO2), bicarbonate (HCO3), and carbonate (CO32). Organic carbon, such as cellulose and starch, is made by plants from CO2 and water during photosynthesis. Carbon dioxide is present in the atmosphere and in soil pore space as a gas, and in surface waters and groundwaters as a dissolved gas. The carbon cycle is based on the mobility of carbon dioxide, which is distributed readily through the environment as a gas in the atmosphere and dissolved in rain water, surface water, and groundwater. Most of the earth's carbon, however, is relatively immobile, being contained in ocean sediments and on continents as minerals. The atmosphere, with about 360 ppmv (parts per million by volume) of mobile CO2, is the second smallest of the earth's global carbon reservoirs, after life forms which are the smallest.

On land, solid forms of carbon are mobilized as particulates mainly by weathering of carbonate minerals, biodegradation and burning of organic carbon, and burning of fossil fuels.

Solubility of CO2 in Water

Carbon dioxide plays a fundamental role in determining the pH of natural waters. Although CO2 itself is not acidic, it reacts in water (reversibly) to make an acidic solution by forming carbonic acid (H2CO3), as shown in Equation 3.9. Carbonic acid can subsequently dissociate in two steps to release hydrogen ions, as shown in Equations 3.10 and 3.11:

CO2 + H2O o h2co3

As a result, pure water exposed to air is not acid-base neutral with a pH near 7.0 because dissolved CO2 makes it acidic, with a pH around 5.7. The pH dependence of Equations 3.9-3.11 is shown in Figure 3.2 and Table 3.1.

Observations From Figure 3.2 and Table 3.1

• As pH increases, all equilibria in Equations 3.9-3.11 shift to the right.

• As pH decreases, all equilibria shift to the left.

• Above pH = 10.3, carbonate ion (CO32-) is the dominant species.

• Below pH = 6.3, dissolved CO2 is the dominant species.

• Between pH = 6.3 and 10.3, a range common to most environmental waters, bicarbonate ion (HCO3) is the dominant species.

FIGURE 3.2 Distribution diagram showing pH dependence of carbonate species in water.

TABLE 3.1

pH Dependence of Carbonate Fractions (From Figure 3.2)

fraction as CO2 fraction as HCO,- fraction as CO32-

8.34

essentially 1.00 0.50 0.01 essentially 0 essentially 0

essentially 0 0.50 0.98 0.50 essentially 0

essentially 0 essentially 0 0.01 0.50 essentially 1.00

The equilibria among only the carbon species (omitting the display of H+) are

CO2(gas, atm) o CO2(aq) o H2CO3(aq) o HCO3- (aq) o CO32- (aq). (3.12) These dissolved carbon species are sometimes referred to as dissolved inorganic carbon (DIC).

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