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I'm using an additive of magnesium sulfate to complement my fertilizer. The additive lists the following ingredients: Magnesium sulfate additive

I know that it does NOT contain magnesium oxide, because due to historical / commercial reasons ingredients of phosphorus, potassium or magnesium are indicated as mass percentages of the oxides with the element in question. Also the merchant confirmed that the source of magnesium is magnesium sulfate.

I noticed the precipitation of a white powder at the bottom of my reservoir. I suspect it to be magnesium carbonate. The mains water contains the following chemicals. The dose of magnesium sulfate is within the range recommended. The precipitation occurs even if I do NOT use any fertilizer, so I think It is a reaction with chemicals in mains water supply.

Chemicals in mains water Chemicals in mains water 2

Is the precipitate magnesium carbonate? How can I prevent this precipitation? What are factors influencing it besides the availability of hydrogen carbonate?

EDIT:

User AJKOER suggested that the precipitate is calcium sulfate. I don't think that is the case because:

  1. The solubility of calcium sulfate is given at 2.1g / liter @ 20° C (https://en.wikipedia.org/wiki/Calcium_sulfate). This equates to about 2.1 * 0.71 ~ 1.48g sulfates per liter. (mass of SO4 / mass of CaSO4 = 0.71)
  2. I'm adding only about 92.1 mg / liter sulfates from magnesium sulfates
  3. The mains water contains only 13.3mg / liter sulfates
  4. 92.1 mg / liter + 13.3mg / liter <<<< 1480 mg / liter
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  • $\begingroup$ Can downvoters elaborate how to improve the question $\endgroup$ Aug 15, 2020 at 10:00
  • $\begingroup$ There are plenty of unusual things here going on, it is more likely that we are on the wrong path due to insufficient data on the mentioned product. First the pH looks suspicious, since Mg is not that basic compared to alkali metal bases, so the pH should be somewhere below 7. I guess there is more Mg than sulfuric acid stoichiometrically speaking, so that is probably reacting with the (bi)carbonates in the water or atmospheric carbon dioxide. $\endgroup$ Aug 16, 2020 at 17:15
  • $\begingroup$ @AndrewKovács But you would generally agree that a reaction with carbonates is more likely than a reaction with sulfates? Because If I add magnesium sulfate and in consequence reduce the calcium levels this would be an exercise in futility because I'm already low on calcium.. $\endgroup$ Aug 16, 2020 at 17:38
  • $\begingroup$ In this case of scenario since the pH is high YES it is very possible. $\endgroup$ Aug 16, 2020 at 18:38
  • $\begingroup$ I will run an experiment where I add only potassium sulfate to the same tap water. If no precipitation occurs I can conclude that the precipitation which occurs after adding magnesium sulfate is indeed magnesium carbonate..Or some other form including magnesium $\endgroup$ Aug 16, 2020 at 18:41

2 Answers 2

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The white precipitate is more likely insoluble white calcium sulfate.

Per a reference on the making of fertilizer, to quote:

Calcium, magnesium, and sulfur are also important materials in plant growth. They are only included in fertilizers in small amounts...

Net ionic reaction:

$\ce{ Ca^2+ (aq) + SO4^2- (aq) -> CaSO4 (s) }$

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  • $\begingroup$ The origin of calcium in your net ionic reaction is not yet obvious because it is not explicitly mentioned in the OP's question. I speculate it is by your reasonable reasoning that tab water contains a varying concentration of $\ce{Ca^2+}$ ... The trigger to edit your answer however was the notation of $\ce{Ca(2+)}$ which I brought into a more mhchem-like syntax with the circumflex sign ^. Maybe this collection of examples in ChemSE meta is interesting for you: chemistry.meta.stackexchange.com/questions/86/… $\endgroup$
    – Buttonwood
    Aug 15, 2020 at 8:56
  • $\begingroup$ I don't think the precipitate is calcium sulfate because the calcium sulfate levels are too low. See the edit to my question.Am I missing something? $\endgroup$ Aug 16, 2020 at 10:10
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AJKOER suggested that the precipitate is calcium sulfate.

However this seems unlikely due to the following reasons:

  1. The solubility of calcium sulfate is given at 2.1g / liter @ 20° C (https://en.wikipedia.org/wiki/Calcium_sulfate). This equates to about 2.1 * 0.71 ~ 1.48g sulfates per liter. (mass of SO4 / mass of CaSO4 = 0.71)

  2. I'm adding only about 92.1 mg / liter sulfates from magnesium sulfates

  3. The mains water contains only 13.3mg / liter sulfates

  4. 92.1 mg / liter + 13.3mg / liter <<<< 1480 mg / liter

It seems more likely that the precipitate is indeed magnesium carbonate.

  1. The water contains 181mg / liter hydrogencarbonate, which is the highest concentration of a molecule in the mains water.
  2. The solubility of magensium carbonate is much lower than the solubility of calcium sulfate. (0.14g / liter (MgCO3) vs. 2.1g / liter (CaSO4))
  3. I'm adding about 26.9mg / liter Mg from MgSO4
  4. The mains water contains 7.5mg / liter Mg
  5. The solubility of magnesium carbonate is 0.14g / liter. This equates to 0.14 * 0.29g = 0.04g / liter magnesium.
  6. 26.9 mg / liter + 7.5mg / liter = 34.4mg / liter mg
  7. 34.4mg / liter Mg < 40mg / liter (solubility of magnesium carbonate with respect to magnesium)
  8. There is 181mg / liter hydrogencarbonat which equates to about 181 * 0.93 = 178 mg / liter carbonate (CO3 2-)
  9. Speculation: The high availability of hydrogencarbonate facilitates precipitation of magnesium carbonate.

Note: Not a chemist here so maybe I made some gross mistake in my reasoning, please point it out!

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