When atmospheric CO2 dissolves in seawater it immediately becomes hydrated and forms carbonic acid. Carbonic acid is divalient, which means it can undergo two deprotonation reactions to form bicarbonate, and carbonate. The co-existence of carbonate and bicarbonate in seawater creates a chemical buffer system that regulates the pH and the partial pressure of CO2 of the oceans. The seawater buffering system works because these compounds shift back and forth in equilibrium reactions when changes in the concentration of hydrogen ions occur, tending to maintain a pH of 8.2
The influence of photosynthesis by plants and respiration by animals and plants, is responsible for the daily variance in pH in aquatic systems, and is especially pronounced in closed systems where the ratio of water volume to living mass is closer to one. Because there is so little free CO2 dissoved in seawater it is quickly exhausted by algae during photosynthesis, and so it is essentially not present when an aquarium is illuminated. When the algae use up available CO2, they can use hydrocarbonates instead, and this can cause the pH to rise sharply if the water is not sufficiently aerated to supply additional atmospheric CO2.
In reef aquariums with surface skimming overflows and protein skimmers, CO2 is usually nearly at equilibrium with the atmosphere. Excesses can occur when CO2 is purposely added to the system or the environment where the tank is located has higher than normal CO2 levels i.e. crowded room or a room that has very little air exchange with the outside,or, use of a calcium reactor. These can result in chronically low pH values despite adequate alkalinity levels. Retention of excess CO2 in the water because of poor circulation, or administration of CO2 by means of a dosing system, causes the pH to decrease. As the CO2 equilibrates with the atmosphere, the pH naturally returns to about 8.2 if the carbonate/bicarbonate buffer system is not depleted.
Conversely, it is possible to have a high pH and a low or normal alkalinity level when administering calcium hydroxide (kalkwasser) solution during the day, since Ca(OH)2 rapidly combines with available dissoved CO2 and forms dissoved calcium bicarbonate. In the absence of the artificial addition of CO2 to replenish what combines with kalkwasser, the pH can rise too high and alkalinity can be depleted.
The affects of algae on CO2 and pH can be balanced with the use of reverse daylight photosynthesis in a refugium attached to the display tank.
Partial excert from full article.
Posted jhnrb
The influence of photosynthesis by plants and respiration by animals and plants, is responsible for the daily variance in pH in aquatic systems, and is especially pronounced in closed systems where the ratio of water volume to living mass is closer to one. Because there is so little free CO2 dissoved in seawater it is quickly exhausted by algae during photosynthesis, and so it is essentially not present when an aquarium is illuminated. When the algae use up available CO2, they can use hydrocarbonates instead, and this can cause the pH to rise sharply if the water is not sufficiently aerated to supply additional atmospheric CO2.
In reef aquariums with surface skimming overflows and protein skimmers, CO2 is usually nearly at equilibrium with the atmosphere. Excesses can occur when CO2 is purposely added to the system or the environment where the tank is located has higher than normal CO2 levels i.e. crowded room or a room that has very little air exchange with the outside,or, use of a calcium reactor. These can result in chronically low pH values despite adequate alkalinity levels. Retention of excess CO2 in the water because of poor circulation, or administration of CO2 by means of a dosing system, causes the pH to decrease. As the CO2 equilibrates with the atmosphere, the pH naturally returns to about 8.2 if the carbonate/bicarbonate buffer system is not depleted.
Conversely, it is possible to have a high pH and a low or normal alkalinity level when administering calcium hydroxide (kalkwasser) solution during the day, since Ca(OH)2 rapidly combines with available dissoved CO2 and forms dissoved calcium bicarbonate. In the absence of the artificial addition of CO2 to replenish what combines with kalkwasser, the pH can rise too high and alkalinity can be depleted.
The affects of algae on CO2 and pH can be balanced with the use of reverse daylight photosynthesis in a refugium attached to the display tank.
Partial excert from full article.
Posted jhnrb
