Randy's Magnesium Recepie: What's that white sediment settling to the bottom?

[Nathan]

Hi Randy,

What is the white sediment settling to the bottom of my container?

I am using a 0.5 gallon glass milk jug. I used half of the amount you suggested in your article since I am making half a gallon not one full gallon.

Recipe #1, Part 3A
http://reefkeeping.com/issues/2006-02/rhf/index.php

For MgCl I am using MAG deicing salt in the pellet form (looks like small hail stones, or styrafoam balls).
http://www.meltsnow.com/msds-mag-pellets.htm

For MgSO4 I am using Equaline Epsom Salt (heptahydrate). The company logo is:

I used HOT RO/DI water to dissolve the mixture but even so it took a long time for all of the salts to dissolve.

So, that was last night, but this morning I come downstairs and see that there is sediment on the bottom of the jug. Is this normal, and what is that sediment and should I be dosing that sediment as well?

Thanks!

-Nathan

 

[Nathan]

By the way, I found this neat photo of the Dead Sea Works evaporation pans at the lower part of the Dead Sea:

 

[Randy Holmes-Farley]

Cool picture.

If the solution was clear and then got cloudy, I expect it is calcium sulfate. If so, it is Ok to dose it.

The magnesium chloride hexahydrate from the Dead Sea Works has about 6.1% as much calcium as magnesium (by weight).

So the Part 3A is about 1800 ppm calcium (0.045 M calcium) and 70,000 ppm sulfate (0.73 M sulfate).

The Ksp of calcum sulfate is 9.1 x 10-6 M, and there the solubility product is 0.045 x 0.73 = 0.03.

So the solution is way above the solubility limit of the calcium sulfate.

I'll add a statement to the article indicating that the solid is not a concern.

 

[Nathan]

The solution was not clear when I first mixed it up. It was milky; and even after letting it sit for an hour or so it was still milky but beginning to separate.

I did shake it up before adding it to the tank.

However, after I dose it to the tank it goes clear after a few seconds. Must be that calcium sulphate dissolving, right?

-Nathan

 

[Randy Holmes-Farley]

Yes. It will likely only get totally clear if you dissolve the two parts independently. Once mixed it goes a bit cloudy again.