CHEMISTRY AND THE AQUARIUM by RANDY HOLMES-FARLEY

Calcium Carbonate as a Supplement

The primary supplements needed on all reef tanks are calcium and alkalinity.  These are required because corals and other organisms consume them to form calcium carbonate skeletons.  There are many ways to provide these supplements, with each type of supplement having some advantages and some disadvantages.  This article will focus on calcium carbonate itself as a supplement.  After all, that is what corals are depositing, so why not simply add it back?

Of course, if it were as easy as that, few people would buy expensive pieces of equipment to supplement calcium and alkalinity.  They would just buy a truckload of limestone at the local quarry for a few dollars and then just drop a piece or two into the tank every day.  The reason that this addition does not work, obviously, is that calcium carbonate will not dissolve in the water column of normal marine aquaria.  This fact makes perfect sense:  what good would it be for a coral to deposit a calcium carbonate skeleton if it simply dissolved?  [Well, it might actually be a benefit to some corals even if it later dissolved as a way to boost photosynthetic efficiency.  This aspect was discussed briefly in a previous article, and won’t be addressed further here.]

CaCO₃/CO₂ Reactors

One of the ways that many people use calcium carbonate as a supplement is with a carbon dioxide reactor.  These reactors combine CaCO₃, tank water, and carbon dioxide in a reactor that permits dissolution.  Much has been written about these reactors and the processes taking place.  Consequently, even though this is a wonderful process for adding calcium and alkalinity to a reef tank, it will not be further discussed here.

Direct Addition of Solid CaCO₃ to a Marine Aquarium

Some have suggested that these particulate products (whether dry or as a slurry in water) can be added directly to an aquarium to provide calcium and alkalinity.  Unfortunately, that method does not work well.  Calcium carbonate is already substantially supersaturated in seawater and in  reasonably maintained reef tank.  Consequently, adding more solid does not lead to dissolution.  On the contrary, since the water is already supersaturated, addition of solid calcium carbonate can actually lead to a decline in calcium, magnesium, alkalinity, and pH.  First, I’ll provide the theoretical explanation of what will happen, and then I’ll provide some real world evidence for those who don’t generally believe that equations apply to them.

First, some background.  There are a variety of reasons that calcium and carbonate can remain supersaturated in seawater for very long periods of time, and these have been detailed in a previous article.  In short, if any CaCO₃ does begin to precipitate as a solid, the growing crystal surface rapidly becomes clogged (or, in chemistry jargon, poisoned) with magnesium, phosphate, and organic materials.  With these other materials present, the calcium and carbonate no longer find the surface as attractive for precipitation, and crystal growth stops.

When you add finely divided calcium carbonate to seawater, you are suddenly adding a large surface area for new crystal growth.  As soon as it is added, calcium and carbonate begin to precipitate onto the surface, lowering both the calcium and the alkalinity.  At the same time, magnesium is also precipitating as a mixed carbonate, and eventually a surface material is attained that contains both magnesium and calcium, and you no longer have an aragonite surface (this process is detailed by Stephen Spotte in Captive Seawater Fishes; 1992). As the surface changes, it reaches an equilibrium where no more net ions are precipitating or dissolving.  Over a longer time period phosphate and organic materials also coat the surface.  These also reduce the likelihood for additional calcium or magnesium precipitation.

Long before these products came out, I noticed that when adding a large amount of calcium carbonate sand (Southdown aragonite sold at Home Depot) to my tank, the pH dropped immediately on addition.  I recently repeated that experiment by adding some AragaMIGHT to my reef tank water (1 teaspoon in ½ gallon of tank water).  Within 5 minutes the pH had dropped from 8.22 to 8.14.  That drop is not very big, and not likely to be of a concern to anyone by itself.  It is, however, indicative of the precipitation of carbonate onto the fresh, bare aragonite crystal surfaces.  The precipitate is likely to be a mixture of calcium and magnesium carbonate, so the whole process has resulted in a decline in these ions.  The decline is reasonably small (I’d estimate it to be about 0.5 meq/L based on the relationship between pH and alkalinity described previously), and involves a lot more solid per unit volume than would typically be used to supplement a reef tank, but the drop in pH is clearly inconsistent with any net dissolution of carbonate (read alkalinity) from the added material. 

Remember that I claimed that the drop in the supersaturation of CaCO₃ was halted before it was completely eliminated?  The above experiment is evidence of this fact.  If the supersaturation had disappeared completely, the alkalinity would have dropped by a factor of about 2.  Since it did not, the water is still supersaturated, and the precipitation must have been halted prior to reaching saturation of CaCO₃.

Why not just measure calcium and alkalinity in the water before and after addition of solid CaCO₃?  Unfortunately, the presence of CaCO₃ particulates in the water that are delivered with the AragaMIGHT and are not dissolved will confound the interpretation.  For example, in an alkalinity determination the pH is steadily dropped to below pH 5.  At these low pH values, more of these particulates will dissolve (maybe all of them depending on how fast you do the test).  When dissolved, they will be measured as available calcium and alkalinity even though they are not dissolved at normal tank pH.  Similar issues may apply to calcium tests where the complexing agents used may promote dissolution of dispersed CaCO₃.    Consequently, you should not be fooled into thinking that these products can be added directly to a marine aquarium because you measured it and it looks like it works (either in the tank or in a test container).

Dissolution of CaCO₃ in Water Prior to Addition

The best way, in my opinion, to use calcium carbonate as a supplement is to dissolve it in fresh water prior to addition. In this sense, it can be used rather like limewater.  One can rig up an automatic evaporation replacement system using appropriate pumps and float switches, and just use water saturated with CaCO₃ instead of limewater.  Alternatively, one can simply pour the saturated water into the tank each day.  Unfortunately, the fact that you can add it this way is a mixed blessing.  One reason that you can add it this way is that there is so little present that the carbonate does not drive up the pH too much.

So how much goes into solution?  This question is rarely addressed directly, and it is because of one big complication: carbon dioxide from the atmosphere.  In the case of limewater, it is partially destroyed by atmospheric carbon dioxide (producing insoluble CaCO₃ from the dissolved calcium and hydroxide).  In the case of calcium carbonate, however, the solubility is actually increased by mixing with carbon dioxide.  The reason that the solubility is increased is that the carbon dioxide enters the water, becomes carbonic acid (equation 1), and largely combines with carbonate ions to form two bicarbonate ions (equation 2):

1.
2.

 

The net effect is that the concentration of carbonate ions declines: since the solubility of calcium carbonate is governed by the multiplication product of the calcium and carbonate concentrations (equation 3), more calcium carbonate can dissolve to regain saturation.

3. 

Knowing the K_sp and some other constants, it is a textbook calculation to determine how much calcium carbonate can dissolve in pure water in the absence of atmospheric carbon dioxide.  Pankow (Aquatic Chemistry Concepts; 1991) carries out this calculation for calcite (a slightly less soluble form of calcium carbonate than aragonite). 

For those really interested in the chemical details, this calculation is actually much more complicated than it would first appear (i.e., more complicated than for a simple salt like NaCl).   You cannot simply solve equation 3 for [Ca⁺⁺] and [CO₃^--].  You need to take into account the fact that some of the carbonate that comes from dissolution will be converted into bicarbonate (HCO₃^-) and even carbonic acid (H₂CO₃).  This conversion permits more CaCO₃ to dissolve before the carbonate concentration rises too high to dissolve any more.  One also needs to take into account the fact that calcium can exist as CaOH⁺, which effectively lowers the calcium concentration (though not very extensively at pH values below 11).

At one point I had the bright idea of adding aragonite to seltzer (soda water) bought at the grocery store to really boost the solubility and maybe have a nice, liquid additive.  Seltzer has far more carbon dioxide in it than water in contact with normal air (which is why it goes flat when open), and that extra carbon dioxide will cause a great deal more calcium carbonate to dissolve (at 3.5 atmospheres CO₂, the solution would contain more than 10 meq/L alkalinity and would be similar to limewater in potency, but much lower in pH).  If only I had been able to mix them……  Instead, it ended up a science experiment for the kids, with the added aragonite sand providing a perfect surface for the carbon dioxide to turn into the gas phase and erupt from the bottle as a fountain of water, gas, and sand!

The Importance of Particle Size

AragaMIGHT claims that it contains fine particles (10 microns).  Why is particle size important? There are two potential reasons, only the first of which is likely important for this product.

The rate of dissolution of all solids in liquids is dependent on, among other things, the solid/liquid surface area.  Calcium carbonate is generally slow to dissolve, even when far below saturation.  For this reason, maximizing the surface area of the solid is important in order to attain a reasonable amount of dissolution in a reasonable period of time.  Smaller particles have more surface area per unit weight (or volume) and hence dissolve faster. 

How important is this factor?  Compared to a single particle with a diameter of 2 mm (typical sand), a single particle with a diameter of 10 μm will have a surface area about 40,000 times smaller.  A single 2-mm particle is equivalent in weight to about 8 million 10-μm particles, and so even though each one has less surface area, together they have about 200 times the total surface area.  Consequently, with all other things being equal, the smaller particles will dissolve about 200 times faster.  While the exact number is inconsequential, it is clear that the smaller particles have a big advantage in dissolution.

How fast is the dissolution?  “Flash” remineralization claimed by CaribSea for AragaMIGHT may be a bit of hyperbole, as in my dissolution tests it took at least 10 minutes to reach maximum dissolution (measured via pH in pure water).  Still, that is much faster than with larger chunks of aragonite; such as the larger particles present in Southdown aragonite sand or in typical tank substrates, and is plenty fast enough for all of the applications that I propose below.  The Southdown sand, however, does contain a fair amount of fine particles, and if one simply ignores the larger particles and uses enough sand, the water can be saturated with calcium carbonate almost as fast as when using AragaMIGHT.

A second factor relating to particle size is more subtle.  Smaller particles actually have greater equilibrium solubility than larger particles.  This greater solubility largely stems from the fact that atoms on the surfaces of crystals are not as stable as those underneath (after all, they are not attached to other ions on their tops or sides as are ions underneath). The ions at the edges, corners, and other crystal defects are particularly more soluble.  The large surface area relative to the volume for very small particles makes this factor important for them when it is not for larger particles (simply because such edge ions comprise a smaller part of the total ions in a large crystal).  This difference, however, does not typically begin to be detectable until the particles are smaller than 1 μm, and isn’t very important until the particles are below 0.1 μm.  Some of the particles may be below this size in all of the products discussed here, but most are likely not, and the bulk of the volume certainly is not.  Consequently, we can safely ignore this aspect of the importance of particle size when considering the dissolution of aragonite in our tanks.

Recommended Uses for Calcium Carbonate

Here are the ways that I recommend using such products:

1. If you are using a calcium carbonate/CO₂ reactor, and have not yet made the leap to using limewater to help raise the pH, why not use CaCO₃-saturated water as evaporation replacement?  However you replace evaporated water, let the freshwater sit in contact with CaCO₃ prior to adding it to the tank.  If you have a big reservoir that you use, all the better.  Just put some Southdown aragonite sand (or other product) on the bottom when you fill it up, and away you go….It won’t raise the pH nearly as much as limewater, but may be simpler and less expensive, and is likely better than nothing.

2. If you are using any additive other than limewater (which itself uses most or all of the evaporation replacement water) you can incorporate this material into the evaporation replacement.  If you are using baking soda for alkalinity supplementation, this method may help raise the pH (should that be needed).  If you are using expensive two part balanced additives or two part separate additives, you will end up using less of them.  If it is sand that you use with the evaporation replacement water, then the net effect will likely be lower costs over time as you use slightly less of the additives.  Of course, because of the limited potency of this method, we are talking about a 2-10 percent reduction in the use of these other additives, but in a large tank that may be significant to some people.

3. In a tank with a low calcification rate, such as one with few corals or with mostly slow-growing corals, you might even get away with this method alone.  The cost associated with even the commercial products will be fairly low, and using Southdown sand will be almost nonexistent.  Still, it may be adequate to maintain pH, alkalinity, and calcium in such situations.  Moreover, it will never (or almost never) become unbalanced the way separate additions of calcium and alkalinity might become over time.

In each of these applications, it behooves you to permit atmospheric CO₂ to enter the system (exactly the opposite of a limewater setup).  Aerating the container with a bubbler would be optimal as that will also bring in fresh, CO₂-laden air, and will keep the CaCO₃ mixing a bit.  Just leaving it open to the air is better than keeping it closed (unless, of course, you have pets or kids that might consider taking a dip in it!).