Fishaholic said:
BReefCase wrote the best reason for using Vinegar I have seen. it is as follows:
When Calcium Hydroxide solution (Kalkwasser) is slowly dripped into your aquarium, it ...converts it to Bicarbonate ions
Ca++ + 2(OH-) + 2(CO2) <==> Ca++ + 2(HCO3-)
If you drip too fast or if there is not enough Carbon Dioxide available in the water, your shiny new Bicarbonate ions will be converted to Carbonate ions (a bad thing), like this:
Ca++ + 2(OH-) + 2(HCO3-) <==> Ca++ + 2(CO3--) + 2 H2O
Actually this is a local phenomenon associated with locally high pH concentration that is driven by increases in pKa of the Carbonate/bicarbonate buffer system. Increasing the pKa of Carbonate at this point increases it's proportion of the reactivity, creating the precipitation of calcium carbonate as it exceeds the solubility limit of seawater at that pH. The reason that it rapidly disappears is it's dilution (assuming rapidly moving water) and the subsequent acidification of the microenvironment in which it is dripped into as the solution is distributed into the larger volume of the water column. This limit can, however, be exceeded if the pH is driven to an extreme (greater than around 9.0 to 9.5), and would occur in the mixing container (pH of around 12.0 to 12.4 at 25C) if a suitable acid (either vinegar or dissolved CO2) were not present.
...So, too rapid addition of Kalk may actually cause the Calcium and Alkalinity in your tank to go DOWN instead of UP, like this:
Ca++ + 2(HCO3-) + Ca++ + 2(OH-) <==> 2 CaCO3 + 2 H2O
In the above reaction, a Calcium ion and two Bicarbonate ions from the aquarium combine to form solid calcium carbonate -- the white stuff you are getting in your tank...
Not true...
This would be true if the reactions were driven to form carbonate in solution as the primary product, and that its pKa were to predict that it would be the most reactive species of ion in solution, but in actuality, both in calculations and in actuality (see
Chemical Oceanography by Frank J Millero , CRC press, 1996) predict reactions at the final dilution and pH of marine aquaria (at ppH of 8.1) to be driven by CO2, and that normal atmospheric replenishment (
usually) is not a rate limiting step. CO2 will form carbonic acid initially, which almost immediately dissociates to mostly bicarbonate ion (87%), loose one proton to form some Carbonate ion (13%) and a very small amount will remain as undissociated carbonic acid (less than 1%). The biggest advantage of using kalk alone is that when supplying the calcium ion as the hydroxide, and with 2 moles of hydroxyl ion for every mole of calcium, we naturally drive the reaction to the bicarbonate, which is the predominant ion responsible for the buffer capacity of seawater. However, this assumes that our goal is to improve alkalinity levels in the tank. If we wish to drive the system to improve Ca levels in the water column; lower pH and less buffering, along with an additional ionic specie to further aid in solubilizing the Ca++, is necessary. This is when the use of acetic acid is most useful.
This (precipitation of Calcium Carbonate) can happen even with a slow drip of Kalk if there is not enough CO2 in your water -- something you can't easily control.
It is possible to use up all the CO2 in a seawater system, especially in systems that have heavy macroalgae growths and high temps with poor circulation and low alkalinity, but in most healthy systems this is NOT a rate limiting step. The rate of CO2 gas exchange between the water column and the environment can be a rate limiting step when either the organisms in the tank (macroalgae and zooxanthellae) are growing rapidly or the atmosphere above a system has been depleted of CO2, but usually not.
Another factor to consider when evaluating these species of ions is that carbonate forms ion pairs with a multitude of monovalent and divalent metals besides calcium. More than half of the carbonate forms an ionic pair with magnesium, and this ionic pair, unlike other ionic pairs, moves around together with such an affinity between each other that they are unavailable to react with most other species of ions in seawater. Approximately another 20% forms pairs with sodium and other species of ions in addition to calcium. This leaves around 14% of carbonate as free carbonate available to react and participate in the carbonate/bicarbonate buffer system. In addition, borate and some organic substances have the ability to act as proton donators/acceptors based on pH and their pKa, and can affect the entire equilibrium as well.
...after all the cool Calcium ion chemistry is over, the leftover Acetate ions from the broken-down Vinegar leaves you with free organic Carbon in the water that feeds the bacteria in your tank so that it converts more poisonous Nitrates to NO2 gas (a very good thing)...
Interestingly enough, on several boards where I found individuals that use this system, there are posts that the down side is sandbed clumping and, paradoxically, either a disappearance of nuisance algae, or a sudden bloom of uncontrollable (most notably cyanobacteria) nuisance algae. The blooming seems to be worst in systems that do no use skimmers, understandable in view of the increase in DOC of not only acetate, but whatever other organic substances distill over with the acetic acid when making vinegar. In those systems that do use skimmers, it seems to temporarily increase skimmate output, albiet thin skimmate, possibly increasing the removal of DOCs.
By the way, you should check your pH before and after you do this the first few times to make sure it is not affected by the process. It should not be a problem. Also, if you don't already have them, get and learn to use Salifert test kits for Calcium, Alkalinity, and Magnesium. The levels of all of these are related and affected by dripping Kalk.
Definitely a plus here, The use of vinegar at stoichiometric amounts would be definitely in the best interest of those undertaking the use of vinegar to solubilize the calcium. I would recommend this only for those systems that have heavy requirements for calcium, as it is one of the best methods I have seen to wring out some extra calcium from kalkwasser. This would especially be true for systems that have inadequate evaporative losses (i.e., glass covers or systems in high humidity environments/climates) such that inadequate amounts of limewater can be added for replacement. In addition, acetic acid will drive more of the carbonate to bicarbonate by adding a proton to the ion. This will allow for further utilization of local phenomenon in solution, but I am not sure to what extent this occurs (heh, maybe I will spend all day NEXT Saturday working on THIS...) This is the only way that can account for an increase in buffer capacity using vinegar with kalk: when acetic acid is added to a solution with bicarbonate alone, the bicarbonate ion will donate a proton back to the acetate, and provide free protons to solution, driving the equilibrium back to acetic acid. This usually results in the solubility of CO2 exceeding the limit of seawater, and effervescence can occur, depending on the concentration of each species. (heh, try adding vinegar to baking soda in a cup, but do this in the sink!) It is this removal of carbonate species from the solution that allows for more solubilization of calcium carbonate to occur, and higher saturation rates of calcium can be achieved with this method, but at a cost of gaining the acetate ion in solution. The presence of this weak acid is negated by the bicarbonate it produces, so we are
still using up alkalinity to provide more calcium to the solution. The upshot of this is that I would recommend this method only for those systems that have so many organisms uptaking Ca++ that the concentration of Ca++ cannot be maintained above 300 PPM; and then, only with some alk supplementation (i.e., some B-Ionic part I (alk) or a buffer builder such as sodium bicarbonate/socium carbonate 8:1)
One of the best discussions for this method follows. It is an article that Craig Bingham wrote for Aquarium Fish Magazine:
Craig Bingman's Acetic acid and Limewater article
In previous discussions with Craig, there has been some concern about the ultimate goal of depending so much on the numbers to get where we want for our creatures. Keep in mind that for calcification and skeletalization to occur, that the ultimate source of carbon is usually bicarbonate or carbonate in the water column (depending on whether you subscribe to Ted McConnaughey's model or the Tom Goreau model of calcification in Scleractinians). Regardless of the source, this carbon is necessary in
2:1 molar amounts with Calcium in order for these organisms to build skeletons. These
two ions are just as important as sunlight and zooxanthellae for corals to thrive. Sorry for such a long post, I am currently working on some experimental setups using vinegar in varying amounts with Kalk to see if predictions match actuality for these reactions based on seawater with differing pH.
On a personal note, I am having problems with adding an acid, even though it is in equimolar reactive amounts, to increase alkalinity. I guess that it depends on the goal you want, whether you want more Ca++ in solution or equal amounts of Ca and alkalinity.
Heh, the girlfriend is threatening to disown me at this point! I have much more, and I have the math available for those that want to follow the material, but it is more than I want to try and format for vBull.