Quote:
Originally posted by Mickadee
... my alk and its 5.5 so what does this mean and how do I correct it/ ... its usually from a drop in PH so I buffer and it lasts about 3 to 4 months before it starts to drop again. The last buffer that I added was about a month ago...
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In a tank that has live rock and is adequately lighted, organisms that that have the proper mechanisms and physiology use CO2 in the water column to make sugars in the presence of special pigments in the cell structure. The process (photosynthesis) effectively removes CO2 from the water column so long as there is adequate light and proper physiological conditions in the water column. This supplies the energy that green plants and many xanthophyll- and chlorophyll-containing marine organisms need to make the foods that they need to survive.
In many species of corals and octocorals, a small dinoflagellate lives in the coral tissue that has photosynthetic capabilities as well, and it is responsible for supplying a portion of the metabolic needs of many of these said organisms in a symbiotic relationship where the coral (animal) tissue gets both energy (and other needs for skeletalization) and has some of its metabolic wastes removed (CO2 and some nitrogenous wastes) while the coral's zooxanthellae (plant) is supplied with the raw materials (animal waste and CO2) needed for the photosynthesis of carbohydrates, allowing the dinoflagellates to survive in a protected environment. During the daylight hours, the plant portion of the coral organism as well as any macroalgae, microalgae,
coralline algae, and photosynthetic protists (like
Cyanobacterium spp., etc.) removes CO2 from the water column. This has the effect on the water column of driving the pH up during the day (decreased CO2 removes the carbonic acid it forms from the water column, and the buffer system then allows for a slow increase in the pH of the solution).
This continues for as long as the lights supply adequate energy to the organisms; as soon as the lights go out, photosynthesis shuts down and now both animals
AND plants begin to metabolize the sugars available to them for energy. This then begins the production of CO2 as the metabolic product of oxidation of carbohydrates (sugars), which then forms carbonic acid in seawater and begins to donate protons (which drives the pH down, makes the water column
more acidic). This is why we see the diurnal pH cycle in seawater and aquaria, although it is more pronounced in closed aquarium systems, especially those with heavy photosynthetic populations. These changes in alkalinity are not due to consumption of the responsible ions, but rather a shift in equilibrium of a set of ionic relationships.
If We want to understand the mathematics behind
why there is a relationship between pH and alkalinity in Seawater, we have to know that whenever the concentration of CO2 in seawater changes, the pH will have a corresponding change as well. If we ignore those changes in the concentration of CO2 in seawater, then there is a set and straightforward relationship based on the amount and degree of ionization of the components of alkalinity that Chris has listed in the above response.
Without changes in the concentrations of atmospheric CO2 (or artificial manipulation via reactors, etc) leading to changes in the pH, then the pH will always have the same relationship with carbonate and bicarbonate. For the most part, total carbonate alkalinity accounts for 96% to 97% of the total alkalinity (A sub c), unless you're using Seachem's heavy borate salt (I have a problem with this product, but I guess I can save that for another time)
)
At a pCO2 of 350 PPM in seawater at 75 F, an alkalinity of 2.2 to 2.4mEq/l of alkalinity will almost always correspond with a pH of 8.2 or so. In NSW, alkalinity rises sharply for small increases in pH. This becomes more pronounced as pH exceeds the 8.0 mark. This phenomenon is based on the forementioned relationship between the CO2 in the atmosphere and the solubility of carbonic acid (dissolved CO2 in seawater). When atmospheric pCO2 goes up, so does the pCO2 of NSW, when it goes down in the atmosphere, it does the same in seawater. This concept extends to the relationships that exist between carbonic acid and both the bicarbonate ion and the carbonate ion as well, such that as pH increases, there is a shift in concentrations of these substances that favors the formation of carbonate ions in solution: The higher the pH, the more formation of Carbonate ion (and bicarbonate) we see. This increase occurs to such an extent that if we view a chart with alkalinity on the vertical axis and pH levels on the horizontal axis, we see a graph that starts off flat in the low ranges (3 to 4) of pH, but reaches a slope of near 45 degrees at 8.2 or 8.3 pH, then goes to near vertical at pH above 9.5 or so. This relationship is seen strongly in NSW, but is demonstrated in freshwater as well, although to a lesser extent (based on pKa of these substances in SW being higher than those pKa's in FW). This is always true in SW
so long as the water column is in equilibrium with atmospheric CO2. Interestingly enough, for every doubling of atmospheric CO2, there is a corresponding drop of 0.3 to 0.35 pH units.
Systems that have either an excess of CO2 (due to poor circulation, systems with poor surface area to tank depth ratios, or use of poorly adjusted CO2 reactors) or a deficiency of CO2 (due to excessive limewater use/dose/rapid infusion) will have this relationship (graph) skewed either up or down based on the particular situation and how it is remedied. Those that use CO2 reactors may raise the alkalinity to high levels, but have a correspondingly low level of pH. This would be a circumstance where there is an artificially high level of CO2 in the water column due to the reactor. Removing this excess CO2 through increased agitation will raise the pH, but maintain the high level of alkalinity. We have skewed the curve to our favor by removing the excess CO2 while maintaining the alk we have worked so hard to supply (this usually means either turning down the water flow through the reactor or slowing the delivery rate of the CO2 gas.) I can explain this in detail, but for the sake of brevity, just take it for granted that such relationships exist, and that they can be controlled and manipulated to our favor. Alkalinity is somewhat consumed during this process to help control the shifts in pH, although this is more an application (reversible) of
Le Chatelier's principle, as this is not a consumption of alkalinity, but rather a shift in the balance of the products vs. reactants (see Chris's equilibrium above).
For the major consumption of alkalinity, we have to look at another physiological activity of marine organisms: calcification.
Depending on who's model of calcification we use, CO2 can be consumed for the purpose of making bicarbonate so that Corals can make calcium carbonate via supplementation by photosynthesis,
OR calcification provides excess CO2 during calcification that supplements photosynthesis... (this goes even deeper than that, and it goes to whether you subscribe to
cis-calcification or
trans-calcifications models.) Regardless of your model/theory; inorganic Carbon is needed in the form of either carbonate, bicarbonate, or CO2 derived from one of these ions. This inorganic carbon is then combined with Calcium to make the skeleton or sclerites (most likely bicarbonate when you look at the stoichemistry, although I am sure most of it is enzymatically driven, such that this does not become such a big deal...). THIS is where most of the consumption of alkalinity occurs in closed marine systems.
Calcareous algae (coralline) are the largest consumers in most home aquaria, followed by halimeda and the corals (depending on specie and populations). Even if you don't keep corals, there is a large amount of consumption occurring in your system by coralline algae on your live rock. Adding alkalinity supps once a month is definitely much too infrequent. You should be checking your alk every week, and if the system is deficient every week, increase your supplementation method until your alk becomes normal. It would be better to test on a daily basis for a while until you find where your consumption level is, then supplement with BOTH calcium and alkalinity to maintain that consumption level (with once a week testing)
Sorry for the long post, hope this helps.
