I often see tons of questions from folks that realize that there is a strong relationship between good calcium levels and the ability to have calcifying corals thrive. This is more than a coincidence, as corals both hermatypic and ahermatypic utilize calcium and carbonate from the water column to form their skeletons (in the case of
hermatypic corals) or spicules ( in the case of ahermatypic corals). There are very well-linked relationships between the levels of calcium and the alkalinity of seawater which we strive to mimic in our aquaria. In seawater, alkalinity is primarily controlled by carbonate and bicarbonate ions, with smaller contributions from borate/boric acid pairs and silicate, phosphate, and some organic acids in descending levels of importance. Depending on their concentrations in seawater, these substances either absorb or release protons (hydrogen ions, responsible for the "acidity" of an aqueous system), thereby dampening or altogether preventing shifts in the pH of seawater. At low, more acidic pH, these substances absorb hydrogen ions. At higher , more alkaline pH, these substances release hydrogen ions. If alkalinity (buffer capacity) of the water column is low, then the daily swing seen in home aquaria (and seawater in the wild) will be more pronounced, resulting in a lower low in the mornings and a higher high in the evenings (beginning and end of the photoperiods, respectively).
pH in the tank is strongly linked to carbon dioxide in the tank, both dissolved CO2 from the atmosphere (should be constant) and endogenous CO2 released from animal and plant respirations. The rate of CO2 production in a closed system is dependant primarily on the rate of respiration in the tank. Loosely, it is equivalent to the consumption of oxygen and carbohydrates as they are converted into CO2 and water to liberate the energy in the carbon-carbon bonds for ADP to ATP conversion for the creatures in the tank. More exotic reactions concern those in the anaerobic environment for the sand bed (denitrification cycles where nitrogen gas is formed as the result of oxygen atoms being stripped of the nitrate molecule, but that is another story). Those reactions contribute little to the overall fate of carbon in our systems for this discussion. An extra source of dissolved CO2 in many tanks is the
calcium reactor, but the reactors utilize the carbonate part of the aragonite substrate to provide soluble carbonate and bicarbonate as they are diluted into the water column, and may degas off as part of the cycle (heh, this gets more and more complex, I may address the issue in another thread sometime in the future.) When the lights go out, photosynthesis in macroalgae and the zooxanthellae shuts down, and they shift their respirations to net consumers of oxygen and net producers of CO2. This will drive the pH down at night, resulting in the cycle we often see in marine aquaria where pH is low in the mornings, then slowly goes up diring the day to a peak when the photoperiod ends. This is one of the shifts that is slowed by the presence of the buffer system.
If you use one of the balanced additives (B-Ionic, Restore, C-Balance, Kalkwasser) and you have started with one of the better synthetic mixes for your seawater, then you have an equilibrium already in place that basically provides 1 mM of calcium ions for every two bicarbonate ions. Although the buffer system is a mix of carbonate and bicarbonate ions, the primary ion in this reaction in seawater at the temps and pH we use is bicarbonate, and an equilibrium exists between the two of them anyway. As each ion of calcium is consumed or added from/to the dynamic equilibrium, 2 ions of bicarbonate are also used. As 40 milligrams per liter (mg/L) of Calcium is equal to 1 micromole of calcium ions, it would follow that an alkalinity change from 3.5 to 4.5 mEq/L would only result in a Ca++ change from 350 to 370 mg/L. This is why many folks must add more of the Ca++ part of these additives to boost the calcium levels above the predicted levels. Another means of doing the same thing wuld be to use a Calcium Chloride additive to raise the Ca levels, but calcium chloride has the disadvantage of supplying hydrogen ions as well
Deciding which product you want to use in your buffer system will depend on whether the intent of the product is to correct Ca++ only or to correct Ca++ AND alkalinity. Most products that contain only CaCl2 will only push up calcium levels, and then only for a short period of time. In addition, most of our need for calcium would be in somewhat alkaline (pH of 8.0 to 8.4) ranges, and we try to avoid additions of substances that produce acids or use up alkalinity. CaCl2 in the presence of water dissociates into Calcium oxide and hydrochloric acid, NOT GOOD!:
CaCl2 + 2 H2O <--> Ca(OH)2 + 2 HCl
and
Ca(OH)2 <--> CaO and H2O
Calcium is very insoluble in seawater (heh, look at it in terms of it’s levels, …ppm!!!!) and administering it to the seawater levels we have in quantities of more than 400 or so ppm will end up with precipitation of that excess (buffer systems and magnesium allow us to exceed this absolute limit through supersaturation of systems). Factors that affect the solubility of calcium in seawater mostly depend on the anion associated with the solute, the temperature of the aqueous phase, and the pH and buffering capacity of that solution. Calcium for the most part has a positive temperature coefficient of solubility, that is, as temperature increases in the solution, calcium becomes more soluble (want more info? Check out Le Chatelier’s principle) However, this becomes much more complex as the temp rises over around 85 F, as there is a degassing of CO2 from solution, more on this later. Pressure affects solubility, but because liquids and solids are, for all purposes in the aquaria, incompressible, it has little affect on Ca++ concentrations, although it does affect CO2 levels in seawater. When thinking of CaCO3, the equilibrium that is established by the dissociation into CaO and CO2 can be drastically affected by the removal or addition of CO2 to the solution by animal respiration or other biological processes (i.e. plant processes in the absence of sunlight…) including barometric changes in the atmosphere (think hurricane lows for a scale of change). When CO2 levels go up in seawater, we see:
CaCO3 + CO2 + H2O <--> Ca(HCO3)2
AND
CaCO3 + H2O <--> Ca(OH)2 + H2CO3
Which increases the solubility of Calcium by forming another anion/cation pair (a new AND separate equilibrium equation) with Bicarbonate, which is much more soluble than CaCO3 alone. This also leads to the increases in buffering capacity/alkalinity that we desire for our aquaria. By increasing the concentrations/availability of the Bicarbonate ion, we increase that alkalinity factor so desired by out little critters. We now have (starting with CaCO3 or Ca(OH)2 instead of CaCl2:
CO2 + H2O <-->H2CO3 <--> HCO3- + H+
And
Ca(HCO3)2 <--> Ca++ + 2 (HCO3--)
And
Ca(OH)2 + 2CO2 <--> CaCO3 + H2O
All in dynamic equilibrium (and now giving buffer capacity and calcium):
CO2 + H2O <-->H2CO3(carbonic acid) <-->(H+) + HCO3- (Bicarbonate)
(H+) + HCO3- (Bicarbonate) + Ca(OH)2 <-->2 H2O + CaCO3 !!!
This is the basis of the carbonate/bicarbonate buffer system in seawater. Additions of CaCO3 on the right end (by adding Calcium as the carbonate) or through the use of Kalkwasser ( Ca(OH)2 or CaO ) will not only improve calcium levels, but will improve the alkalinity/buffer capacity of our water column. The solubility or rather, the INSOLUBILITY of Calcium Carbonate is a possible sink (source of removal) for calcium from this dynamic equilibrium, so that continuous additions of calcium in the presence of carbonate from CO2 without adequate buffering will do little to increase calcium levels in a system. In a similar fashion, addition of calcium in ANY form without adequate buffering in a system will do little bu drop calcium out of solution as the carbonate salt. This is often see by aquarists when they cannot get their calcium levels above 300 ppm, and although it may have other causes, it is most often due to inadequate pH and buffer capacity of the seawater column. Calcium ions are also removed by biological action (skeletal deposition by stony corals) and by significant levels of phosphate, which is much more insoluble than Carbonate, to form both dibasic and tribasic Calcium Phosphate (if you want to see the chemistry, you’ll have to email me on this one, a little more complicated than I want to type and format for this thread). In addition, biological processes that produce organic acids (DOM decomposition, etc) will reduce the amounts of bicarbonate as well, and lead to increases in dissolved carbon dioxide (leading to the formation of carbonic acid and a proton). The USEFUL additives are either CaCO3 (still hard to get to dissolve unless you use a Calcium reactor) or Ca(OH)2/CaO (best) to get your calcium levels up..
trt tells me this is too long, part 2 to follow
