The Inorganic Carbon Complex: Alkalinity, Acidity, CO₂, pH, Total Inorganic Carbon, Hardness, Aluminum

Abstract

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 CO₂ dissolves in water, carbonic acid is formed: and pH is lowered.

Most minerals are only slightly soluble in water, and appreciable amounts usually are not dissolved. For example, limestone (calcium carbonate, CaCO₃) is but slightly soluble in pure water. Limestone is much more soluble, however, in water containing carbonic acid, whereby the CaCO₃ is changed to calcium bicarbonate:

The dissolved CO₂ has a parked 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 (SO₂) into the atmosphere from industrial activities, (SO ^2-₄ ) is becoming the dominant anion in precipitation of large geographical areas. In those areas the geo-chemical relationships as described above may be altered appreciably.

Inorganic carbon as dissolved CO₂ and HCO ^-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 CO₂ by most organisms and by influxes of CO₂ and HCO ^-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 situations does inorganic carbon become a limiting factor to photosynthesis of planktonic algae.

Natural waters exhibit wide variations in this relative acidity and alkalinity, not only in actual pH values, but also in the total amount of dissolved material producing the acidity or alkalinity. The concentrations of these compounds and the ratios of one to another determine the observed pH and the capacity for buffering of a given body of water. The lethal effects of most acids appear when pH < 5.5 and of most alkalis near pH 9.5, although the tolerances of many organisms are considerably more restricted within these pH extremes. Thus, the buffering capacity of natural waters to resist changes in pH can be of great importance to the maintenance of life.