Reef Chemistry Question of the Day #141 Magnesium Solubility

Randy Holmes-Farley · Sep 18, 2015

Reef Chemistry Question of the Day [HASHTAG]#141[/HASHTAG]

Consider a common magnesium supplement made from 10 parts of magnesium chloride and 1 part of magnesium sulfate. Both salts are highly soluble in fresh water so one can easily make a very concentrated solution for dosing.

Now that you have your stock solution, you are wondering what else you might be able to mix into it to dose at the same time.

You do some experimenting and find that something you wanted to add in a very concentrated solution didn't work out, resulting in precipitation of solids.

Which of the following could NOT have been what you added, because it wouldn't result in a precipitate? (ignore for the moment whether you personally want to dose this or not; people have dosed all of these)

You may elect more than one, if you think it appropriate

A. Calcium chloride
B. Muriatic acid (hydrochloric acid)
C. Sodium hydroxide
D. Acetic Acid (primary ingredient of vinegar)
E. Sodium carbonate
F. Ethanol (primary ingredient of Vodka)

For the most credit, explain your rationale for the answers

Good luck!

.

JimWelsh · Sep 18, 2015

I think the following are true: The calcium ions from calcium chloride will combine with sulfate to precipitate calcium sulfate. The hydroxide ions from sodium hydroxide will precipitate magnesium hydroxide. Sodium carbonate will precipitate magnesium carbonate, and perhaps even sodium sulfate.

I suspect that if sufficiently pure ethanol were used, it may cause precipitation of either magnesium chloride or magnesium sulfate, because of the reduced solubility of these compounds in ethanol, depending on the ethanol-to-water ratio.

I believe that HCl will cause no precipitate, because it will simply increase the chloride concentration, and acidify the solution. I also believe that acetic acid will not cause a precipitate because magnesium acetate is quite soluble in water.

All of the above are my (somewhat under-educated) guesses.

Shahin Ruyani · Sep 18, 2015

I had it.

Shahin Ruyani · Sep 18, 2015

F and D because Both are weak acids and will hydrogen bond to the water and will not react with the ions in solution, form precipitates or even ionize much at all.

Lynn52 · Sep 18, 2015

F. Ethanol. The solubility of any ion is dependent on the other ions disolved in the solution. Solubility product. A, C, E are all salts and will decrease the solubility of any other salt in the solution.
Decreasing the pH with HCl or Acetic acid will increase the solubility of MgCl2 and MgSO4.
Ethanol is a solvent not a solute.

beaslbob · Sep 19, 2015

<<<<<<conducting experiment to see if it's ethanol

Randy Holmes-Farley · Sep 19, 2015

beaslbob said:
<<<<<<conducting experiment to see if it's ethanol

And what did you find?
Drinking it won't yield the answer.

Randy Holmes-Farley · Sep 22, 2015

Good job folks! (well, except Beaslbob who must have consumed his experiment rather than reporting back with the ethanol results

)

Some of the answers are clear, and some are a but fuzzy since it may depend on exactly how much of what you add.

A. Calcium chloride will result in a precipitate of calcium sulfate. This is largely why, in DIY recipes, mixing fully dissolved magnesium chloride (with a known impurity level of calcium chloride) with fully dissolved magnesium sulfate frequently results in a small amount of precipitation.

B. Muriatic acid (hydrochloric acid) won't result in a precipitate.

C. Sodium Hydroxide will result in precipitation of magnesium hydroxide, which is quite insoluble and is why you cannot mix magnesium into limewater (kalkwasser).

D. Acetic acid (vinegar) is a bit less clear than most of the other choices. Magnesium acetate is very soluble, so that wouldn't precipitate under most conditions. Using vinegar, you won't get a precipitate because you are actually adding much more water than acetic acid. If you added so much pure acetic acid that it became largely an acetic acid solution and not mostly a water solution, might magnesium chloride or magnesium sulfate precipitate? Certainly, those salts will be much less soluble in pure acetic acid than pure water, but you are necessarily adding some water from the original solution and diluting the magnesium chloride with the added acetic acid. I expect that if you start with a saturated solution of magnesium chloride/sulfate in water and add pure acetic acid, you'll get a precipitate. Exactly how much of the magnesium salts will dissolve in how much acetic acid/water is something that can likely only be deduced by experimentation.

E. Sodium carbonate will result in precipitation of magneisum hydroxide and magnesium carbonate.

F. Ethanol (primary ingredient of Vodka) is complicated, like acetic acid, and the answer likely depends on the concentration of the ethanol. Adding 100% ethanol to a saturated solution of magnesium chloride/sulfate in water will probably result in precipitation because those salts are much less soluble in pure ethanol than in pure water. I don't think that adding vodka in any amount will produce a precipitate because vodka is typically 60% water and 40% ethanol. But as with acetic acid, exactly how much of the magnesium salts will dissolve in how much ethanol/water is something that can likely only be deduced by experimentation.