Reef Chemistry Question of the Day #3

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #3


Consider some of the primary positively-charged ions in seawater:

potassium
calcium
sodium
magnesium
strontium
lithium

What is chemically similar about calcium, magnesium, and strontium that is not true of any of the others, and why is that important to reef keepers?

This question should be easy to medium in difficulty, so don't overthink it, but it does require some chemistry knowledge of these metals.

Good luck!

 

[johnanddawn]

They are +2 cations and can therefor replace one another is precipitates like limestone. If my geology serves me right I think the difference between limestone and dolomite is the calcium is replaced with magnesium after ppt. Ie calcium carbonate becomes magnesium carbonate.
Not sure if this happens in live coral skeletons?

 

[leptang]

the same group generally have the same electron configurations in their valence shell. My best guess would be to help keep alkalinity.

 

[redfishbluefish]

The troika of elements you have chosen are Group 2 alkaline earth elements with two electrons in their valence shell (+2). Accordingly, they will react similarly in forming bonds.


It's important to reefkeepers because the three guys love the carbonate ion carring a -2 charge.....it's a marriage!

 

[hart24601]

I think others nailed it, but I love these questions of the day!

 

[DRThompson]

+1...love these questions!

 

[Randy Holmes-Farley]

Glad you folks appreciate them!

Answer:

As RedFishBlueFish mentions, calcium, magnesium and strontium all carry a +2 charge. The others carry a +1 charge.

That implies that much of the chemistry of the three is similar. In particular, they all form carbonates, such as calcium carbonate and strontium carbonate.

The fact that they look so similar means that they can all be incorporated almost interchangeably into coral skeletons and abiotic precipitation of calcium carbonate. Specifically, magnesium and strontium get into calcium carbonate in place of the calcium. Magnesium also gets onto growing calcium carbonate crystal surfaces and messes up the growing crystal, and that is why magnesium allows calcium carbonate to be supersaturated in seawater.

There are certainly important differences, and these have many implications for reef keepers (such as magnesium not being very soluble in limewater (kalkwasser) despite the fact that both strontium and calcium are soluble in it).

We'll have more questions relating to these issues in the future as they are important to us in many ways.