I run a small herbal wellness business and one of the products i sell are bath salts containing baking soda. The base ingredients are Epsom salts, Dead Sea salts, magnesium chloride, baking soda, coconut milk powder, and fractionated coconut oil. I’ve made and sold this recipe for over a year and because i buy my ingredients in bulk i haven’t bought new ingredients recently. All of a sudden, some kind of chemical reaction is happening (which i assume is coming from the baking soda), and after i seal the bags they blow up like a balloon and then eventually explode. I cannot for the life of me figure out what is causing the baking soda to react. I’ve noticed when I’m mixing up batches that there is a fizzing sound that has never been there before, and I’ve wondered if the salts have somehow absorbed water. But water doesn’t really cause baking soda to react like this. Thanks for the help!

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    $\begingroup$ Try the other ingredients one by one against the baking soda. $\endgroup$ – Ivan Neretin Jun 18 '20 at 16:41
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    $\begingroup$ Magnesium chloride will certainly pick up water from the water vapor in air. Perhaps then the coconut milk powder is then producing an acid, maybe lactic acid (?), and then the baking soda and acid react to yield carbon dioxide. This is just my best guess: I never even heard of coconut milk powder. $\endgroup$ – Ed V Jun 18 '20 at 16:43
  • $\begingroup$ Coconut milk powder is basically just dried full fat coconut milk. It contains essential fatty acids and lauric acids (medium chain). Would these be the types of acids to react with baking soda? And why is it just now happening after a year of production? Thanks for the comments y’all. $\endgroup$ – Lauren Ellis Jun 18 '20 at 16:52
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    $\begingroup$ All acids do react with baking soda. Plenty of substances are transformed into acids by a long standing in contact with the air. The best example is the wine (solution of ethanol) which gets slowly transformed into vinegar by contact with air. And vinegar is a solution of acetic acid. $\endgroup$ – Maurice Jun 18 '20 at 17:02
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    $\begingroup$ I think the water pickup is key: dry baking soda and dry citric acid, for example, do nothing. But with moisture, they react to yield carbon dioxide, water and the other stuff. So the reaction gets going faster, etc. I second Ivan’s suggestion: try the coconut powder and baking soda mixture in a baggie, add a little water, and seal the baggie. $\endgroup$ – Ed V Jun 18 '20 at 17:06

I suspect the seasonal introduction of humidity is the root of the problem, which is readily taken up by the magnesium chloride. Then, the following possible reaction:

$\ce{ MgCl2 (aq) + 2 NaHCO3 (s) -> 2 NaCl (aq) + Mg(HCO3)2 (aq)}$

Now, aqueous Magnesium bicarbonate exists only in solution (so decomposes on drying), is not stable on standing (per my experience), or warming (whereupon it also decomposes):

$\ce{ Mg(HCO3)2 (aq) -> MgO (s) + H2O (l) + 2 CO2 (g)}$

where the pressurized buildup of carbon dioxide gas is likely causing a problem.

Solution: Try adding a drying agent or removing the very hygroscopic Magnesium chloride from your formulation.

  • $\begingroup$ Thank you so much! Do you have any suggestions just to get the ball rolling in my mind of what to look for in a drying agent? $\endgroup$ – Lauren Ellis Jun 18 '20 at 17:26
  • $\begingroup$ If the $\ce{MgCl2}$ has already taken up a lot of water, there may not be much you can do---you may just have to buy new $\ce{MgCl2}$. Once you have new material, store it in an airtight container with a bunch of water absorber packets (e.g., silica gel). You should be able to find the absorber packets at various stores... search for things like "desiccant" or "silica gel packet". $\endgroup$ – hBy2Py Jun 18 '20 at 17:53
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    $\begingroup$ I am afraid that this equation has nothing to do with reality. Magnesium bicarbonate would decompose to magnesium carbonate not magnesium oxide. It may happen but only at very high temperatures. $\ce{ Mg(HCO3)2 (aq) -> MgO (s) + H2O (l) + 2 CO2 (g)}$ $\endgroup$ – M. Farooq Jun 19 '20 at 1:26
  • $\begingroup$ In line with the comment above I do not think that the equations shown are involved. This is as to say that Mg bicarbonate is just more igroscopyc than other ingredients already present in the mix. As stated in other comments it is likely an acid has been formed by the coconut part upon storage. OP You could check the pH of it. $\endgroup$ – Alchimista Jun 19 '20 at 8:56

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