EXERCISE 8
The Inorganic Carbon
Complex: Alkalinity, Acidity,
CO 2 , pH, Total Inorganic
Carbon, Hardness
Precipitation falling on the surface of the Earth as
rain or snow contains a variety of gases. Aerosols
or dust particles also may be dissolved or picked
up from the air. Thus, by the time the water
reaches the Earth's surface, it is no longer pure.
As it flows over or penetrates into plants or the
ground, water dissolves more gases, notably
carbon dioxide, and various mineral substances
with which it comes into contact. When CO 2
dissolves in water, carbonic acid is formed:
H20 + CO 2 p H 2 C0 3 P H+ + HCO;
and pH is lowered.
Most minerals are only slightly soluble in
water, and appreciable amounts usually are not
dissolved. For example, limestone (calcium carbonate, CaC0 3 ) is but slightly soluble in pure
water. Limestone is much more soluble, however,
in water containing carbonic acid, whereby the
CaC0 3 is changed to calcium bicarbonate:
CaC0 3 + H 2 C0 3 P Ca2+ + 2HCO;
(insoluble)
The dissolved CO 2 has a marked effect on the
properties of the water. It forms a weak, carbonic
acid solution that can change the pH of the water
and dissolve minerals, which in turn can increase
alkalinity and impart hardness to the water. The
amount of dissolved salts in water is of major
importance to the maintenance of life and in the
treatment of water for domestic and industrial
use.
In addition to bicarbonates, carbonates, and
hydroxides, other minerals that often dissolve in
water in moderate amounts are silica and the
chlorides, sulfates, and nitrates of calcium, magnesium, sodium, and potassium. With increased
emission of sulfur dioxide (S02) into the atmosphere from industrial activities, SOl- is
becoming the dominant anion in precipitation of
large geographical areas. In those areas the
geochemical relationships as described above
may be altered appreciably.
Inorganic carbon as dissolved CO 2 and
HC0 3 - is the primary source of carbon for
photosynthesis by algae and larger aquatic
plants in natural waters. This utilization is balanced by respiratory production of CO 2 by most
organisms and by influxes of CO 2 and HC0 3 -
from incoming water and from the atmosphere.
The amounts of inorganic carbon available for
use in photosynthesis are adequate in most
natural fresh waters; only under special conditions of soft waters and intensely productive
107
The Inorganic Carbon
Complex: Alkalinity, Acidity,
CO 2 , pH, Total Inorganic
Carbon, Hardness
Precipitation falling on the surface of the Earth as
rain or snow contains a variety of gases. Aerosols
or dust particles also may be dissolved or picked
up from the air. Thus, by the time the water
reaches the Earth's surface, it is no longer pure.
As it flows over or penetrates into plants or the
ground, water dissolves more gases, notably
carbon dioxide, and various mineral substances
with which it comes into contact. When CO 2
dissolves in water, carbonic acid is formed:
H20 + CO 2 p H 2 C0 3 P H+ + HCO;
and pH is lowered.
Most minerals are only slightly soluble in
water, and appreciable amounts usually are not
dissolved. For example, limestone (calcium carbonate, CaC0 3 ) is but slightly soluble in pure
water. Limestone is much more soluble, however,
in water containing carbonic acid, whereby the
CaC0 3 is changed to calcium bicarbonate:
CaC0 3 + H 2 C0 3 P Ca2+ + 2HCO;
(insoluble)
The dissolved CO 2 has a marked effect on the
properties of the water. It forms a weak, carbonic
acid solution that can change the pH of the water
and dissolve minerals, which in turn can increase
alkalinity and impart hardness to the water. The
amount of dissolved salts in water is of major
importance to the maintenance of life and in the
treatment of water for domestic and industrial
use.
In addition to bicarbonates, carbonates, and
hydroxides, other minerals that often dissolve in
water in moderate amounts are silica and the
chlorides, sulfates, and nitrates of calcium, magnesium, sodium, and potassium. With increased
emission of sulfur dioxide (S02) into the atmosphere from industrial activities, SOl- is
becoming the dominant anion in precipitation of
large geographical areas. In those areas the
geochemical relationships as described above
may be altered appreciably.
Inorganic carbon as dissolved CO 2 and
HC0 3 - is the primary source of carbon for
photosynthesis by algae and larger aquatic
plants in natural waters. This utilization is balanced by respiratory production of CO 2 by most
organisms and by influxes of CO 2 and HC0 3 -
from incoming water and from the atmosphere.
The amounts of inorganic carbon available for
use in photosynthesis are adequate in most
natural fresh waters; only under special conditions of soft waters and intensely productive
107