Carbon Dioxide in Water
Carbon dioxide is a colorless, odorless, noncombustible gas constituting about .04% of normal air.
Carbon dioxide is highly soluble in water. The source of free carbon dioxide in water is seldom that from the air phase because CO2 is a product of aerobic and anaerobic decomposition of organic matter and it is intimately bound in the complex carbonate equilibria (see bicarbonates and carbonates).
The dissolved carbon dioxide reacts with water to form carbonic acid. CO2 + H2O {;} H2CO3 which in turn disassociates to H2CO3 §;} H+ + HCO3- such that [H+][HCO3-]/[H2CO3] = 3.5 x 10’7 at 18°, thus at pH 8, the ratio of carbonic acid to bicarbonate ions is only 0.286, but at pH 7 it is 0.86 and at pH 6 it is 2.86. Below pH 4.3 almost all the bicarbonate is converted to carbonic acid. Carbonic acid can be toxic even at pH values that are not in themselves harmful. Hence, pH is not a reliable index of dangerous CO2 pollution.
Free carbon dioxide in domestic water is of no significance because it appears to have no direct physiological effect. In fact, soft drinks and beer are highly charged with CO2.
The concentrations of dissolved CO2 and carbonic acid in water have a marked effect on fish. In their migration, fish tend to respond to slight gradients of carbon dioxide tension and to avoid concentrations of 1 to 6 mg/L.
It is doubtful if any fresh water fish can continue to live throughout the year in water with an average CO2 content as high as 12 mg/L. Concentrations of 20 mg/L will quickly prove fatal to the more sensitive species. For example, the lethal limit for trout has been reported as 45 mg/L.
Free carbon dioxide in excess of 20 mg/L may be harmful to fish in normal fresh water, but when the dissolved oxygen content drops to 3 to 5 mg/L, lower CO2 concentrations may be detrimental.
The presence of carbon dioxide may at times have beneficial effects for fish because CO2 lowers the pH On the other hand, lowering the pH would increase the toxicity of cyanides.
The sensitivity of fish to carbon dioxide appears to decrease directly with the increase in temperature.
In U.S. waters that support good fish fauna, ordinarily 5% have less than 0.1 mg/L, 50% have less than 1.5 mg/L, and 95% have less than 5 mg/L of free carbon dioxide.
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