Reef Chemistry Question of the Day #18

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #18

Most reef aquarists know that magnesium is important for maintaining calcium and alkalinity in a supersaturated state in seawater. That is, there is more calcium and carbonate in solution than "should" remain at equilibrium without precipitating calcium carbonate.

How does magnesium accomplish this role?

Good luck!










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[Kyuss414]

When calcium carbonate starts to precipitate, the magnesium interferes by binding itself to the outside of the growing calcium carbonate cluster. This blocks any more calcium carbonate from binding itself to the cluster since it's only drawn to other calcium carbonate and not magnesium, so the calcium carbonate cluster cannot grow large enough to the point of falling out of solution.

 

[leptang]

It blocks calcium from precipitate in the water

 

[NanaReefer]

^^What they said

 

[Randy Holmes-Farley]

It blocks calcium from precipitate in the water

But how?

 

[hart24601]

It blocks the calcium crystal formation? The Mg ion interacts preventing the calcium ion from forming with the carbonate?

 

[leptang]

But how?

the magnesium binds to calcium carbonate formation changing the structure of calcium carbonate and stopping further growth of the calcium

 

[beaslbob]

because magnesium is a good guy?



(magnesium interfers with the calcium carbonate lattice?)

 

[beaslbob]

If it doesn't precipitate how could it be super saturated anwyay?

 

[Randy Holmes-Farley]

If it doesn't precipitate how could it be super saturated anwyay?

I'll explain that with the answer.

 

[Randy Holmes-Farley]

Good answers to a hard question, folks. Several were exactly correct!

Here's a section of an article explaining the answer in more detail:

Calcium Carbonate and Magnesium
Finally, we come to magnesium's role in the calcium carbonate system. The situation for magnesium is appreciably more complex than for pH and alkalinity, but we can continue our same analysis to understand it qualitatively. When solid calcium carbonate is put into seawater, it doesn't just undergo the sorts of "on" and "off" dynamics as calcium and carbonate ions discussed above. Other ions can get into the crystal structure in place of either of these ions. In seawater, magnesium ions get into calcium carbonate crystals in place of calcium ions. Strontium ions may also do so, but their numbers are far lower than magnesium's (about 600 times lower) so they are less likely to become incorporated.

Figures 8 and 9 show how magnesium in solution gets onto and actually into a thin layer of calcium carbonate surface put into seawater. Even though magnesium carbonate itself is soluble enough that it will not precipitate from normal seawater, in a mixed calcium and magnesium carbonate structure, its solubility is lower. So solid, pure calcium carbonate (Figure 8) is rapidly converted to a material with a coating of calcium and magnesium carbonate (Figure 9).

This coating has some very important effects. The primary effect is that it makes the surface no longer look like calcium carbonate, so calcium and carbonate ions that land on it no longer find the surface as inviting as before. The magnesium ions have altered the surface in a way that does not hold calcium and carbonate as strongly, and so the "off" rate of any newly landing calcium and carbonate ions is higher (Figure 10). Consequently, even if the driving force to deposit calcium carbonate is still there, the magnesium has gotten in the way and doesn't allow it to happen (or keeps it from happening as fast).

The extent to which magnesium gets onto calcium carbonate surfaces depends strongly on the amount of magnesium in solution. The more there is, the more it gets onto the surfaces. If magnesium is lower than normal, then it may not adequately get onto growing calcium carbonate surfaces, allowing the deposition of calcium carbonate to proceed faster than it otherwise would, potentially leading to increased abiotic precipitation of calcium carbonate from seawater onto objects such as heaters and pumps. Often the inability to maintain adequate calcium and alkalinity despite extensive supplementation, and the precipitation of significant amounts of calcium carbonate on heaters and pumps, are signs that the water has inadequate magnesium.

 

[Randy Holmes-Farley]

If it doesn't precipitate how could it be super saturated anwyay?

That's a good question, and it gets at the difference between equilibrium chemistry and kinetically driven chemistry. In other words, what the chemicals would do if given long enough (infinite time) to reach equilibrium, and what they actually do in a reasonable period of time.

For example, air and wood are not at equilibrium. If you wait long enough, the oxygen in the air will degrade the wood just like burning. But it might take many, many years. Applying heat gives it some activation energy to reach equilibrium faster (e.g., burning), but the process is the same.

So in our magnesium situation, the magnesium getting onto the growing calcium carbonate crystal is blocking the easy kinetic path between the small CaCO3 crystal and a larger crystal. One can experimentally show the effect of magnesium by making artificial seawater without it and studying the precipitation and dissolution of calcium carbonate. Likewise, one can study it with magnesium, and at different levels of calcium and carbonate to see the role magnesium plays.

FWIW, there are other effects of magnesium that also alter these experiments, such as magnesium ion pairing to carbonate in seawater, effectively reducing the free concentration of carbonate by about a factor of 2, but that effect is distinct from the one I described above and is taken into account in still describing seawater as supersaturated with respect to calcium carbonate.

 

[beaslbob]

because magnesium is a good guy?



(magnesium interfers with the calcium carbonate lattice?)

If I had said surface instead of lattice would have I been more correcter?

 

[Randy Holmes-Farley]

If I had said surface instead of lattice would have I been more correcter?

Both answers are essentially correct.

 

[beaslbob]

Both answers are essentially correct.

Awww gosh

Dee whillkders

I got one correct?


Hey:

the difference between equilibrium chemistry and kinetically driven chemistry

Is that the difference betweek sugar in iced tea in Iowa where you have to keep stirring it up

and sweet tea here in alabama where the brew the tea with sugar already added so it dissolves due to heat?


well kinda anyway.

 

[Randy Holmes-Farley]

Hey:

the difference between equilibrium chemistry and kinetically driven chemistry

Is that the difference betweek sugar in iced tea in Iowa where you have to keep stirring it up

and sweet tea here in alabama where the brew the tea with sugar already added so it dissolves due to heat?


well kinda anyway.

Only ice tea I know about is Long Island Ice Tea.