ฉันรักการแปลThe carbon dioxide/ bicarbonate/ carbonate buffering system
The initial pH of water is determined by the type of dissolved compounds that it accumulates, although it may well be chemically altered by the water company before it reaches your tap. However, once it is in the pond or aquaria, water pH is also influenced by other factors such as plant and animal respiration and plant photosynthesis. Without some form of buffering these natural activities would cause huge diurnal swings in pH.
The most common buffering system is the the carbon dioxide/ bicarbonate/ carbonate buffering system. Essentially it stabilises pH by mopping up excess hydrogen ions and then releases them again as levels drop, so that the hydrogen concentration, and therefore the pH, stays fairly constant.
CO2 + H2O H2CO3 HCO- + H+ CO32- (solid) + 2H+
What this equation tells us (from left to right) is that carbon dioxide, excreted by fish and plants, dissolves in water to form carbonic acid (H2CO). If pH levels increase, that is the water becomes more alkaline (say from plant photosynthesis), then the carbonic acid dissociates to form bicarbonate and hydrogen ions (HCO3- + H+). Hydrogen ions are acidic-forming ions and will therefore counteract the alkalinity increase. If the pH continues to increase, the bicarbonate will dissociate to form solid carbonate and release yet more hydrogen ions (CO32- (solid) + 2H+), to counteract the increased alkalinity. The solid carbonate is the chalk layer covering the pond bottom and walls (or the kitchen kettle). If pH levels start to fall the process is reversed. At a normal pond pH of 7-8 some of all of the above species will be present, with bicarbonate dominating. Carbonate will predominate above pH 9.
The buffering capacity of water depends on the total amount of bicarbonate and carbonate present. Water that has low levels of these ions will quickly exhaust its ability to counteract pH fluctuations.