Reef Chemistry Question of the Day #184 Precipitating Additives

[Randy Holmes-Farley]

Reef Chemistry Question of the Day [HASHTAG]#184[/HASHTAG]

Which of the following common reef tank additives cannot be mixed due to the likelihood of a solid precipitate forming?

A. Calcium hydroxide dissolved in water (kalkwasser) plus magnesium chloride dissolved in water
B. Calcium chloride dissolved in water plus magnesium chloride dissolved in water
C. Sodium bicarbonate dissolved in water plus sodium carbonate dissolved in water
D. Calcium chloride dissolved in water plus vinegar

Assume that each of them is basically pure so that you are not assessing the precipitation of impurities.

Good luck!

































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[Cory]

Id say A cannot be mixed together

 

[cmcoker]

A

 

[Drauka99]

A

 

[beaslbob]

wild guess---d

 

[JonasRoman]

A

 

[Randy Holmes-Farley]

And the answer is...A. Calcium hydroxide dissolved in water (kalkwasser) plus magnesium chloride dissolved in water

Very good folks, except Beaslbob who guessed wrong.


All of the chemicals would be compatible except magnesium in limewater. It precipitates as magnesium hydroxide because magnesium hydroxide is much less soluble than calcium hydroxide.

I discuss it and do some experiments here:

Aquarium Chemistry: Magnesium And Strontium In Limewater ? Advanced Aquarist | Aquarist Magazine and Blog
http://www.advancedaquarist.com/issues/dec2003/chem.htm

from it:

In a previous article on the solubility of metals in limewater,13 I showed a graph of the theoretical solubility of magnesium as a function of pH (below).

At the pH of limewater (low 12's) the solubility is between 0.01 and 0.001 ppm. The experimental solubility here is a tad higher (0.017 ppm), presumably for one of two reasons: some particulates of magnesium hydroxide may have been present in the solution which are detected as soluble magnesium when in fact, it is not. A second possibility is that the solution has simply not reached thermodynamic equilibrium, and the theoretical limit to solubility has not yet been reached.. Nevertheless, the point is that it is expected that magnesium hydroxide will precipitate from such a solution, and this did, in fact, happen. The magnesium in solution was depleted by a factor of more than a hundred compared to what would have been in solution had it all been soluble. In the next section of this article, the precipitate on the bottom of the limewater reservoir was tested to show this extra magnesium.

 

[beaslbob]

...

Very good folks, except Beaslbob who guessed wrong.
...

.

and beaslbob done learned something today.

 

[JimWelsh]

When your Calcium test kit has a step where you add a clear liquid reagent to the sample, and that step makes the sample get cloudy, the reagent added is typically a hydroxide of some sort (NaOH, KOH, etc.). That step is making the magnesium unavailable to the EDTA titration by precipitating it as Mg(OH)2.