Magnesium carbonate precip, ok to use?

[ajger]

Looking for some advice from any chemistry wizards...

I tried to bring up mag and alk on a new batch of mixed water using baked baking soda and Brightwell Magnesion-P, the result was heavy precipitation. The Magnesion contains magnesium sulfate, and the baked baking soda is sodium carbonate. From what I can gather online, mixing these two results in magnesium carbonate precip, which is basically chalk? : link

So my question is, if i let the water sit in the mix container for a week or so without anything stirring it, would the precip be likely to settle to the point it could be siphoned out, and would the remaining water be ok to use? Another option might be to run it through fine floss for a week using a submerged pump. I just want to ensure the reaction wouldn't have created any other unwanted chems.

 

[Randy Holmes-Farley]

Let's back up. it is somewhat unlikely that you precipitated magnesium carbonate, although it may be possible.

More likely is calcium carbonate.

What exactly did you add in terms of amounts to what salt mix? Few if any salt mixes need additional alkalinity added to them.

This has more on various precipitates:

What is that Precipitate in My Reef Aquarium?
http://reefkeeping.com/issues/2005-07/rhf/index.htm

 

[ajger]

Randy, thanks for the quick reply. I'm in process of switching from Seachem to a 50/50 IO/Oceanic mix, the ratios of this batch were ~ 40% Seachem, 60% IO/Oceanic, SG was at 1.025. Mag tested at 1200, CA at 480, dKH 7.5, PH 7.95. To the 30 gallons mixed volume, I added 2 level tsp of BBS then ~400 ml of mixed Magnesion, mixed roughly at their recommended 4 teaspoons per 8 ozs. Best guess was that should have brought the mag into the high 1300 range. I retested mag and got 1320, and dKH at 10. With a minute after adding the mag, I could no longer see the bottom of the 30 gal Brute trashcan. I'll retest tomorrow after letting it sit overnight.

I'm running a small pump with several layers of fine floss on the output and it's clearing up rapidly. The Koralia mixing pump and heaters I had in there were covered with a fine white talc like residue which rinsed off easily.

As a test, I added ~ 1/4 tsp of BBS to a cup of RODI water, stirred until it was clear, then added a couple of ml of mixed magnesion. It immediately went very cloudy.

 

[Randy Holmes-Farley]

That baked baking soda might have pushed the pH too high. next time, try unbaked baking soda. I think you can let it settle and use it.

 

[Randy Holmes-Farley]

As a test, I added ~ 1/4 tsp of BBS to a cup of RODI water, stirred until it was clear, then added a couple of ml of mixed magnesion. It immediately went very cloudy.

Yes, forming magnesium hydroxide, at least initially, maybe followed by magnesium carbonate but that shouldn't happen in seawater where the pH is not high enough for that unless you push it too high. The precipitate in the salt water is more likely to be calcium carbonate, but might also include magnesium hydroxide and possible but less likely, magnesium carbonate.