Reef Chemistry Question of the Day #7

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #7

Why can't you add magnesium chloride to limewater (kalkwasser) to get a supplement that adds calcium, alkalinity and magnesium?


A. The calcium will precipitate as calcium chloride.
B. The magnesium will suppress the calcium solubility, leaving much less calcium in solution.
C. The chloride will clog the drip tip.
D. The magnesium will precipitate as magnesium hydroxide.

Good luck!















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[UNCFAN]

"D"

 

[mike007]

I think D.

 

[jt17]

I pick D

 

[beaslbob]

'd'

Kinda like the time I added the magnesium supplement to the calcium chloride part of the diy two part. got this white chunk at the bottom. calcium sulfate I think.

 

[revhtree]

Love these!

 

[Randy Holmes-Farley]

Kinda like the time I added the magnesium supplement to the calcium chloride part of the diy two part. got this white chunk at the bottom. calcium sulfate I think.

Yes, that's right. If you added any magnesium sulfate, that would lead to big time calcium sulfate precipitation.

Even adding magnesium chloride to magnesium sulfate can lead to calcium sulfate precipitation, because calcium is often a significant impurity in magnesium chloride.

 

[Randy Holmes-Farley]

Love these!

Thanks.

But no answer to the question?

 

[ritter6788]

D. Even though there is a supplement sold out there that claims to be kalk + mag.

 

[Randy Holmes-Farley]

D. Even though there is a supplement sold out there that claims to be kalk + mag.

Yes, I know. It is very misleading. Last time I checked, it didn't even have any more magnesium in it than comes as an impurity in bulk calcium hydroxide that I have used, but they tout it as if it is a new invention that will be useful.

 

[revhtree]

Thanks.

But no answer to the question?

Sorry forum chemistry is my strong suit! LOL!

 

[choff]

E) All of the above

 

[DRThompson]

D, great way to make milk of magnesia

 

[Randy Holmes-Farley]

Lots of correct responses! Excellent. Good thing I made today's question (#8) much harder.

Yes, the answer is D, the milk of magnesium problem. Neither A, B, nor C actually happen.

Magnesium hydroxide is far less soluble than is calcium hydroxide, so any magnesium added into the solution (or naturally present in the calcium hydroxide) encounters all of those hydroxide ions, and precipitates from solution.

I've analyzed calcium oxide from the Mississippi Lime Company (which I used in the past to make limewater/kalkwasser). It naturally had about 0.25% by weight magnesium and 65.5% calcium by weight. That gives about 0.0038 for the Mg/Ca ratio.

When I analyzed the limewater solution made from this calcium oxide, it ended up with a ratio of 0.000028 for Mg/Ca.

So the magnesium is greatly depleted in the limewater due to precipitation, by a factor of more than 100.

In other words, less than 1% of the magnesium added to limewater (kalkwasser) actually stays in solution.

 

[Cory]

What if magnesium acetate was added?

 

[Randy Holmes-Farley]

What if magnesium acetate was added?

Same problem with magnesium hydroxide precipitate.

The acetate will stay in solution.

 

[Cory]

Are you saying it wouldnt work? And that the magnesium would precipitate and leave acetate in the limewater?

 

[JimWelsh]

That's what he said.

 

[Randy Holmes-Farley]

Right, I don't see any use to it.