More About Calcium and Alkalinity

Do you have questions, want to discuss the issues raised in this column, or read the comments of other aquarists and the answers from columnist Craig Bingman? You can do so by going to:Biochemistry Interactive.
This month I will discuss a question from John Metaxas regarding calcium and alkalinity balance in a captive reef. These questions seem to have vexed reef aquarists since they became aware that carbonate alkalinity and calcium was rapidly depleted by calcification in a growing reef aquarium.

There are very robust equilibrium relationships between calcium and alkalinity that can guide us. This question is exceptional in that the calcium and total alkalinity measurements seem to have been made with precision and they are in precise quantitative agreement with the predictions of those equilibrium relationships.

I hope you can find a few moments to help me with a question/situation I have with Alk/Ca++ imbalance. Currently, I run a fairly large Ca++ reactor (MTC Pro-Cal) on my 220-gallon reef tank (mixed long-polyped and small-polyped stony corals and soft corals). The unit is capable of handling a large reef tank. Right now, I am putting into the unit approx. two bubbles per second of CO2 and running a flow rate of around 1.5 to 2 liters per hour. The alkalinity of the effluent is around 40dKH and pH =6.5.

My question has to do with the levels in the reef tank. Currently, the alkalinity runs from about 3.5 to 4.5 milliequivalents per liter (mEq/L; around 10 to 12dKH). However, no matter what the alkalinity reads, the calcium level never rises above 370 parts per million (ppm). Sometimes it rides around 350 ppm. It barely changes as the alkalinity moves higher or lower. Magnesium (Mg++) levels are around 1100 ppm. Tests are run using Salifert Test kits.

Can you offer any explanation for this? I have a 12-year old Bachelor of Science degree in Chemistry (not used in 10-plus years), so be gentle.
Thank you in advance.

Dissolution of calcium carbonate supplies calcium ions and carbonate alkalinity in the correct ratios to support the biogenic formation of calcium carbonate in the tank.

The relevant equilibrium equations are:

CaCO3 + CO2 + H2O
calcium reactor
<----> Ca++ + 2(HCO3-)
calcium reactor product
<----> CaCO3 + CO2 + H2O
coral skeleton formation

Figure 1
The effect of this equilibrium reaction is indicated in Figure 1 (“the pipe”). If you use only balanced supplements (limewater, C-Balance, B-Ionic, calcium salts of organic acids) then the system will slide along a line parallel to the central diagonal line through this figure (in the figure, mM = 然) For example, if you started at 0 然 Ca++, 0 mEq/L carbonate alkalinity, a calcium carbonate/CO2 reactor, you will move along the diagonal line in that plot.

So, for every calcium ion produced, there are two equivalents of carbonate alkalinity — two bicarbonate ions. (You can write the above in terms of just calcium and carbonate, but at seawater pH, there is more bicarbonate than carbonate, and bicarbonate is certainly the dominant form in calcium reactor product.) So, a 1 mEq/L change in alkalinity will change the calcium ion concentration by 0.5 然.

40 milligrams per liter (mg/L) = 1然 Ca++

so, an alkalinity change of 1 mEq/L implies a Ca++ ion change of only 20 mg/L, as long as only balanced supplements are in use and calcification is the only drain on alkalinity (no net nitrification). If your alkalinity changes from 3.5 to 4.5 mEq/L, the expected Ca++ concentration change would be from 350 to 370 mg/L, exactly as you observed. Good job! If you want to increase the calcium concentration beyond that, I would use an appropriate amount of calcium chloride to effect the change.

Figure 2
Typical seawater has about 10 然 (400 mg./L) calcium ion present. The baseline alkalinity is approximately 2.0 mEq/L total carbonate alkalinity. All of these numbers are approximate and bounce around a bit depending on whose assay one takes as grail. This set of starting conditions gives rise to Figure 2 and if you wish to get back to natural seawater conditions, the correction is obvious. The system has an excess of carbonate alkalinity, and is deficient in calcium.

Although you have said that things are going well in your reef now, if you wish to correct that imbalance, you should add calcium chloride or do water exchanges with a salt that is known to give full-strength seawater concentrations of calcium ion when mixed to full-strength salinity. Most people will be able to plot the state of their system in this map and devise the appropriate course of action. At least they will be able to determine if their system will ever cross near-seawater values for calcium and alkalinity given the current “vector” of their system parameters.

Future columns may deal with the details on correcting these imbalances, using algebraic and graphical techniques. Best wishes,


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