BIOCHEMISTRY OF AQUARIA
BY CRAIG BINGMAN Ph.D.

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Expanding the Limits of Limewater:
Adding Organic Carbon Sources

Several previous columns have examined various techniques for maintaining calcium and alkalinity in reef aquaria. Among the techniques covered have been limewater, calcium carbonate/carbon dioxide reactors and various “ionic” supplements. The methods that are easiest to use provide both calcium and alkalinity in appropriate ratios to support the formation of calcium carbonate.

I’ve gone on record here and several other places with my belief that limewater is probably the most economical method for maintaining these parameters in a reef aquarium, especially in aquaria with moderate calcification rates. Once an appropriate dosing mechanism is in place, it is probably one of the simplest as well. However, some aquarists experience difficulties with limewater. Beyond simply not being able to get enough limewater into the system to support rapid calcification rates, some aquarists find that limewater can increase the system pH to significantly higher than natural seawater values. This issue frequently arises when the limits of limewater are being extended by dosing a somewhat milky suspension of limewater or when measures are taken to increase the rate at which water evaporates from the system.

Limewater supplies calcium in the form of calcium ions and alkalinity in the form of hydroxide ions. In order for this alkalinity to be useful in calcification, it must be converted into carbonate alkalinity. This usually happens spontaneously in the aquarium, because the hydroxide ions neutralize carbon dioxide from respiration in the aquarium or atmospheric carbon dioxide dissolving in the water. However, when you are really punching limewater into a system with a low fish load, the rate at which carbon dioxide becomes available from respiration or the atmosphere doesn’t keep pace with the rate of limewater addition and the system pH can increase dramatically.

Nilsen has championed a system that uses a reactor that produces milky limewater and uses bottled carbon dioxide, along with a pH controller and solenoid valve, to provide metered amounts of carbon dioxide on demand to the system, when respiration and exchange with atmospheric gas isn’t sufficient to keep up with the carbon dioxide demand of the hydroxide ions being added from the limewater reactor. This system undoubtedly works, as he and others have maintained thriving coral exhibits for extended periods of time using this methodology. However, it is a “high tech” option, and involves both a pH controller and a CO2 delivery system. This apparatus is moderately expensive, although no more expensive than a calcium carbonate (CaCO3/CO2 reactor and CO2 delivery system, and, like any other technical equipment, it can break. Regardless of whether the CO2 metering apparatus fails in either the “on” or “off” positions, it could be a really bad day for your aquarium. Is there any other “low-tech” way carbon dioxide could be added along with limewater?

Let’s deal with the trivial notion of just dissolving carbon dioxide in limewater and then dripping it into your aquarium. If you think this won’t work out very well, you are correct. Initially, all that would be accomlished is the formation of some solid calcium carbonate in your limewater bucket. And, it wouldn’t really help your corals unless you kept shooting carbon dioxide into the bucket until the calcium carbonate formed redissolved. Then you would have something like the product of a calcium carbonate/carbon dioxide reactor in the bucket. If you kept it tightly sealed and dispensed it with a dosing pump, then you might be able to make that work, but it sounds like a nuisance to me. That would be basically a “batch” CaCO3/CO2 reactor. (I haven’t tried this, and accordingly I don’t know how it would compare to a CaCO3/CO2 reactor.)

Anyway, this wasn’t what I had in mind. A different, and I think more productive, route is to spike the limewater with a source of organic carbon. As the limewater is gradually dosed into the aquarium, the organic compound we place in the limewater would serve as a food source for bacteria and perhaps other organisms in the aquarium. As they oxidize the added organic compound, carbon dioxide is generated. And presto, we now have some extra carbon dioxide in the system — and that’s exactly what we needed.

What could we use? Well, there are a great number of possibilities. The ones I’ve tried are acetic acid, glucose and calcium gluconate. As you are no doubt aware, vinegar is a dilute solution of acetic acid dissolved in water. And yes, it seems to be perfectly safe when used the way I’m about to describe. Glucose and calcium gluconate also had no discernible negative impact on my system when used in the way I’m about to describe.

How much of these things should we add? Well, rather than try to calibrate this empirically, we can anticipate in advance how much carbon dioxide will be needed to neutralize the hydroxide ions in limewater and supply an amount of dissolved organic carbon sufficient to supply a known fraction of that carbon dioxide demand. We start with the concentration of saturated limewater at 25 degrees Celsius (77 degrees Fahrenheit), 0.0203 moles per liter. Each unit of calcium hydroxide would require two units of carbon dioxide to form calcium bicarbonate or one unit of carbon dioxide to form calcium carbonate. Which should we choose? I actually think that adding less dissolved carbon is probably desirable, so we will tentatively adopt one mole of carbon dioxide equivalents per one mole of calcium hydroxide as the maximum amount of dissolved organics that we would like to add in this way.

TABLE I
CompoundNumber
of Carbons
Molar Mass
grams per mole (g/M)
Mass per Mole-C
(g/mole-C)
Maximum
Concentration
grams per liter (g/L)
Milliliters (ml)
Vinegar per liter (L)
Acetic acid260.0530.030.6112.2 ml/L
Glucose6180.1630.030.61NA
Calcium
gluconate
12430.3835.870.73NA

Table I gives the vital statistics for the three carbon sources under discussion. As you can see, complete oxidation of about 12 milliliters of vinegar per liter of limewater would provide enough carbon dioxide to balance the inorganic carbon removed from the aquarium when the calcium and alkalinity in a liter of saturated limewater is converted into calcium carbonate. Again, this is the maximum concentration of acetic acid that one would add, and, in practice, I would suggest adding no more than a quarter of this quantity to see how your system tolerates it. The acetic acid would be added at the same time as the lime is added to the water, and it should all be mixed together well. The acetic acid neutralizes some of the calcium hydroxide and forms calcium acetate, or actually largely dissociated calcium ions and acetate ions.

It should be noted that you will be able to dissolve substantially more calcium hydroxide than usual in this way. If you use the maximum concentration of acetic acid listed in Table I, you will be able to get about 36 percent more calcium hydroxide to dissolve than if you mixed it normally. That may be a substantial help to some people in and of itself. Of course, as the calcium concentration goes, up, you would need to add more acetate to get one organic carbon per calcium ion in the mixture. Twelve milliliters of acetic acid per liter of saturated (spiked) limewater will give about 0.73 organic carbon atoms per calcium atom, which will be sufficient to reduce the maximum daily pH the reef attains. (A mixture of acetic acid and calcium hydroxide that has one organic carbon per calcium ion will have a calcium concentration of roughly 0.032 molar, or about 50 percent more concentrated than untreated limewater.)

Even starting at one quarter the maximum dosage, which I’m suggesting you try first, the concentration of calcium in this spiked limewater will be about 9 percent higher than in unspiked limewater. The pH of all of these mixtures will also be somewhat lower than a pure solution of calcium hydroxide in water. And you really should make saturated solutions when you do this, and perhaps check the pH of the solution as well, to make sure it isn’t very low.

There is no reason to use anything other than white vinegar (made for human consumption) for this purpose. You can buy pure acetic acid from chemical supply stores, but a source of “Food Chemical Codex” (FCC) grade acetic acid is as close as your local grocery store. I doubt your corals want cider, balsamic or herb-flavored vinegars either. Just plain old white vinegar should do very nicely and is what I’ve been using. While you are at the store, you might also want to check for pickling lime and stock up: canning season in the northern hemisphere is rapidly coming to a close and pickling lime tends to be seasonally available.

One other note: the actual acidity of vinegar varies. So start out with no more than a quarter of the maximum dose (no more than 4 milliliters per liter) and make sure you make a saturated solution of limewater. And if vinegar in your country customarily has more than 5-percent acetic acid, e-mail me and I’ll be happy to run the numbers for your local concentration.

Glucose won’t allow you to increase the number of calcium and alkalinity equivalents added, because it won’t neutralize any hydroxide ions. Adding calcium gluconate will allow more calcium and alkalinity to dissolve in the mixture than pure limewater, but if you only want to add one mole of organic carbon per mole of calcium in the mixture, this enhancement will be substantially lower than one can achieve by adding acetic acid to limewater (you would get about 8 percent more calcium in gluconate-treated calcium hydroxide at one mole of carbon per mole of calcium). Of course, calcium gluconate itself is rather freely soluble in water, and Seachem markets a very concentrated liquid calcium gluconate additive. One could add small amounts of this to limewater as well.

So, we see that the three organic carbon additives have different properties. Spiking with acetic acid can allow one to substantially increase the amount of calcium and alkalinity that goes into the aquarium without adding a lot of extra carbon in the process. Calcium gluconate can also increase the amount of calcium and alkalinity going into the system, but because gluconate carries so much carbon per calcium ion, one can’t get nearly as far with it as with calcium acetate without adding an excess of total carbon to the system. Glucose is fine for adding carbon, but does nothing to increase the strength of limewater.

I think this technique will be of interest primarily to two groups of aquarists: those who can’t quite get enough limewater into their systems, and those who find the system pH going higher than desired when they dose limewater. For some people who just can’t quite make limewater work, adding small quantities of white vinegar might be enough to allow limewater to carry all of the calcification demand of your aquarium.

This technique may also be of interest to a third group — individuals who find that their systems tend to accumulate nitrate. The added organic carbon could serve as a fuel for denitrification in the live rock and sand of the aquarium. Of the few people who have tried this, some have reported that the nitrate concentration in their tanks has decreased somewhat since they started spiking their limewater with a dissolved carbon source.

Another side benefit would be that a source of organic carbon is being slowly added to the aquarium over the course of the day. This may benefit some animals, although I don’t know whether any of these three possible carbon sources are better or worse for the types of animals you would like to keep in your aquarium.

Again, for anyone who wants to try this on their own reef: I haven’t had any problems with my system attributable to adding moderate quantities of dissolved organic carbon to limewater. If you want to try this, start out with about a quarter of the “maximum” concentration of one of these three carbon sources and if things look good you might work up from there.

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