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Which ingredients in my homemade deodorant recipe are causing it to froth and overflow its container? Is there something I can add to stop this reaction?

Ingredients:

  • Magnesium oil (made from magnesium chloride flakes and distilled water)

  • Baking soda

  • Witch hazel

  • Aloe Vera gel

  • Arrowroot powder

  • Essential oils (lemon, orange blend and lavender)

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  • $\begingroup$ Hello and welcome to Chemistry.SE. If you have any questions about the site you can visit the help center. Regarding your questions, the reaction is likely between the citric acid components of the essential oils (from citrus fruits like lemons and oranges) and the baking soda (sodium carbonate). I doubt that you can get rid of the baking soda and still get the deodorant properties you want. Can you list the order that you mix the components and at what point it starts to froth? $\endgroup$ – airhuff Mar 14 '17 at 16:58
  • $\begingroup$ This is a hard question to answer remotely... $\endgroup$ – Zhe Mar 14 '17 at 16:59
  • $\begingroup$ @airhuff The essential oils are likely added in too small of a quantity to cause this much effervescence. $\endgroup$ – hBy2Py Mar 14 '17 at 17:06
  • $\begingroup$ I start with the magnesium oil, then add witch hazel, aloe gel, baking soda, arrowroot powder. I add the essential oils last. It starts frothing when I add the baking soda. $\endgroup$ – Lisa Mar 14 '17 at 17:15
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    $\begingroup$ @airhuff Normally would have just let it go, but the question asks about gas production and that would take an extra equivalent before foaming occurred. Didn't want to seem petty. $\endgroup$ – SendersReagent Mar 14 '17 at 20:53
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The most likely culprit is something acidic reacting with the baking soda (sodium bicarbonate).

Try mixing separately:

  • Baking soda and witch hazel

  • Baking soda and aloe vera gel

My guess is that one of these two has an acidic buffer in it that's causing the baking soda to turn into a science fair volcano on you.

You could try adding some calcium carbonate to the mix (crush up an antacid tablet, or buy powder from somewhere). $\ce{CaCO3}$ is more basic than $\ce{NaHCO3}$, so it might neutralize enough of the acid that it avoids the foaming.

Sodium carbonate, often sold as "washing soda", is essentially a more-basic version of baking soda and might also work. You have to be careful not to add too much $\ce{Na2CO3}$, though, because it could make the product too alkaline and thus irritating to the skin. In particular, do not substitute washing soda for all of the baking soda—only add the amount of washing soda needed to prevent the frothing.

There are harsher chemicals that you could use to neutralize the acidity, but I wouldn't recommend trying them in a household project like this.

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  • $\begingroup$ Witch hazel is a likely candidate. It seems to have adstringent properties due to tannins. These might be acidic enough to react with the baking soda. $\endgroup$ – Klaus-Dieter Warzecha Mar 14 '17 at 16:49
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    $\begingroup$ I just tried mixing the baking soda with the witch hazel and aloe gel separately and it looks like the aloe gel is the culprit. i never would have guessed! I just checked the ingredients and some of them look suspicious. I'm going to add some calcium carbonate as suggested above or just exclude the aloe gel when I make my next batch. Thanks!! $\endgroup$ – Lisa Mar 14 '17 at 17:10
  • $\begingroup$ @Lisa Happy to help! If this solves your problem, marking my answer as accepted is all the thanks I need. :-) Good luck! $\endgroup$ – hBy2Py Mar 14 '17 at 17:15
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    $\begingroup$ Interesting find with the aloe. The juice of aloe has a pH ~8.5 and is sold as a "natural" antacid. They must put something in the gel that brings that down. Anyway, +1 for the instructions that resolved that out so quickly! $\endgroup$ – airhuff Mar 14 '17 at 17:19
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    $\begingroup$ @Lisa The ascorbic acid is probably what's doing it. They most likely added it as an antioxidant/preservative. $\endgroup$ – hBy2Py Mar 14 '17 at 17:30

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