Alkalinity, total hardness, carbonate hardness, pH, carbon dioxide, the carbonate-bicarbonate system, and calcium carbonate are all terms used to describe some component or process that is part of the buffering system present in seawater. - Seawater contains many mineral salts. These salts are present as ions of the elements that compose the salt, such as positive ions (cations) of sodium and negative ions (anions) of chlorine that form the salt, sodium chloride. Seawater also contains carbon dioxide gas. When dissoved in water, carbon dioxide reacts with the hydrogen of water to form a weak acid, carbonic acid (H2CO3). if excess carbon dixoide is in the water, carbonic acid levels increase beyond the point where it can be quickly utilized in the carbonate-bicarbonate system, and pH levels drop. if too much carbon dioxide is taken from the water, carbonic acid decreases and pH levels quickly rise. Addition or deletion of carbon dioxide beyond atmospheric equilibrium temporarily changes pH but does not change the alkalinity (carbonate composition) of the water. Carbonic acid forms negatively charged carbonate (CO3 to 2nd -) and bicarbonate (HCO3-) ions. Carbonate in turn joins with calcium to form calcium carbonate (CaCO3). The chemical reactions that form bicarbonate, carbonate, and calcium carbonate are equilibrium reactions; that is they can go back and forth depending on conditions such as increase or decrease of carbon dioxide, pressure, and temperature. Under normal physical and biological conditions, the carbonate-bicarbonate ions act as a bank that automatically takes up excess carbonic or other acids, or forms more carbonic acid if carbon dioxide is lost. This is the buffer system that maintains the normal pH of seawater at about 8.2. Bicarbonate is most important in the normal pH range of seawater. Calcium is an important element present in seawater and is part of this system. Calcium is improtant for building strong coral skeletons, mollusk shells, and algal support, and has numerous other biological uses as well. - As calcium carbonate is removed from seawater by chemical and biological processes (mostely utilized in the shells of tiny planktonic animals), more bicarbonate forms from carbonate, more carbon dioxide is assimilated into the buffer system, and pH remains constant. in the sea, carbon dioxide flows into the system in areas of high biological activity and water run off from land, accumulates in the depths, and is released back into the atmosphere in areas where deep sea waters well upward to the surface. The carbonate-bicarbonate buffer system equilibrates back and forth and pH stays relatively constant. - In the aquarium system organic acids from decomposition of organic matter and animal metabolism are constantly being added to the water. these acids constantly withdraw carbonate and bicargbonate from the buffer bank. Calcareous gravels can only add to the buffer bank by adding dissolved calcium carbonate if the pH falls below 7.5 to 7.8 which is too low to maintain a system in optimum condition. The marine aquaist must battle to filter out organics before they make a withdrawal from the buffer bank and must add carbonate-bicarbonate through chemical addition or water change when a decrease in pH and or redox potential indicates the need.
