Calcium, Magnesium, and Ionic Imbalance
When we talk about ionic imbalance and its relationship to pH, we are talking about the concentration of magnesium, calcium, boron, and carbonate ions.
Calcium (Ca2+) has a positive charge. Carbonate (CO32-) has a negative charge. Because of these opposite charges, these ions naturally pair with one another (similar to two magnets). When this happens with these two particular ions, the result is calcium carbonate. In a closed reef system, this is not what one wants. The calcium and carbonate are now bound to one another making each unavailable, and thus, unusable. Calcium levels drop, which means corals are being deprived of necessary calcium, and carbonate levels drop.
So what does this mean for pH? As carbonate is removed from the system, it is no longer present to act as a buffer. The acid dissociation constant, pKa, of the system drops closer and closer to the value of water, as more carbonate is removed. The pH also drops, approaching 7, the pH of pure water.
Why doesn’t this occur in nature?
Magnesium (Mg2+) also has a positive charge, and in the ocean, magnesium is present in much greater quantities than calcium (approximately 3:1) and is thus able to preferentially compete with calcium for the bond with carbonates. This is called ionic competition.
It stands to reason that if there is a higher concentration of magnesium ions than calcium ions, there is a greater probability of carbonates pairing with magnesium rather than calcium. For illustrative purposes, imagine a very crowded room of strangers where you are looking for a few friends; you are much more likely to bump into strangers than your friends. The same is true for carbonate and magnesium.
Typically reef aquariums will, in fact, have higher concentrations of magnesium ions than calcium, but more often than not, they still fall woefully short of the 3:1 ratio found in nature. This is called ionic imbalance and can be the result of a number of factors.
So why is it better for carbonates to pair with magnesium? Unlike the nearly irreversible bond between calcium and carbonates, the bond between magnesium and carbonates is weaker, thus, they will continually pair and then dissociate, and both will continue to be available. Calcium and carbonate also pair and disassociate however the rate at which this occurs is far lower than the rate for magnesium and carbonate.Their pairing prevents the pairing of calcium and carbonate; calcium remains available, carbonate remain available and pH remains stable.
Boron performs a similar function, in that it can pair with calcium to form a calcium-borate complex, Ca(B(OH)4)2In this case, the borate is screening the charge on the calcium, and not on the carbonate; the complex formed is like the magnesium-carbonate complex: easily broken and reformed. So boron effectively prevents calcium from bonding with carbonate, yet the calcium remains readily available. This is why we include borate in our ions™.