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Magnesium (Epsom Salts) In Hot Tub

18 posts in this topic

Hello everyone!

I searched through the forum and didn't find any recent posts about using Magnesium in a Hot Tub/Spa. The benifits on transdermal magnesium have been studied and proven. Most people are also deficient in magnesium in their bodies. I know I feel so much better after Epsom Salt Baths. My questions are: Does it harm the hot tub? And why do more people not use magnesium in their hot tubs?

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because at the high concentrations that would be used it would interact with the total alkalinity and also the calcium hardness and could precipitate out as magnesium and calcium scale. Magnesium is one of the components of total water hardness, the other being calcium. magnesium scale is much softer than calcium scale but can still be problematic for pumps, heaters, etc.

Then again, you can put whatever you want into your own tub If things mess up you just have to pay for any possible repairs.

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The solubility product of magnesium carbonate is high at around 1x10^(-5) (some sources say 2.38x10^(-6) for trihydrate; one outlier source said 3.5x10^(-8)) compared to calcium carbonate at around 4.8x10^(-9). This means the magnesium level (in molar units) can be about 2000 times higher than the saturated calcium carbonate level before scaling would occur (or about 7 times if one believes the outlier source). Magnesium hydroxide is the other precipitate that can form, but the solubility product is 1.5x10^(-11) so even at a pH of 8 the magnesium concentration would have to be 15 moles/liter so extraordinarily high (at pH 9 it's 0.15 moles/liter but that's still around 15,000 ppm total hardness).

The main issue with magnesium hardness is that, like calcium, it reduces the surfactant properties of soap and precipitates magnesium stearate with solubility 30 mg/L (this along with calcium stearate with solubility 40 mg/L form "soap scum"). This is not normally an issue in pools and spas since soap is not used and is actually to be avoided since foaming is undesireable. In fact, calcium is intentionally added to spas to inhibit foaming (but the quantities of soap are usually small enough to avoid soap scum) and magnesium would have the same foaming inhibition effect.

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I have some Spa Side aromatherapy crystals by Cover Valet that are made for hot tubs and spas, and the main ingredients are epsom salts, sea salt, and (of course) fragrance. These come with a little scoop that holds about an ounce, and the instructions call for adding one scoop to the water, which is probably nowhere near the concentration that you are asking about. I can't say that I've noticed feeling any better after using them in the spa, but the fragrance does do a nice job of masking the chlorine smell.

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Thanks for the responses!

Waterbear: I'll have to look into the correct concentration, but if my memory serves me the concentration they tell you to put into an Epsom Salt bath is roughly between 400-600grams per 600L, which would be about 10Kg on a 400Gallons hot tub if you went on the lower end with 400grams. That is quite a bit. The baths were for 12 minutes at 50-55 degrees celcius, which I believe is warmer than most tubs.

chem geek: wow very techical, so laymens terms......adding magnesium shouldn't be a big issue? I guess it would depend on how much I add right?

spidey9: Yes you are right, that amount of magnesium probably wouldn't do much at all...when I think they are talking about putting 10Kg into a 400G hot tub.

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400 grams in 600 liters is 667 mg/L (ppm) and even less in total hardness units so is not horribly high for magnesium so should not be an issue. A 400 gallon tub is 1514 liters so would need 400*1514/600 = 1009 grams or roughly 1 kilogram. I'm not sure how you got 10 kilograms but you are off by a factor of 10.

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400 grams in 600 liters is 667 mg/L (ppm) so is not horribly high for magnesium so should not be an issue. A 400 gallon tub is 1514 liters so would need 400*1514/600 = 1009 grams or roughly 1 kilogram. I'm not sure how you got 10 kilograms but you are off by a factor of 10.

haha :-) I made a typo there...its supposed to be 400grams in 60L, not 600L. So my math is right, but my typing is sloppy! :-)

Would the form of magnesium matter? Espsom Salt (Magnesium Sulfate) vs Magnesium Chloride?

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The form would affect the weight used to get to the same magnesium level and for magnesium sulfate you'd need to know if it was anhydrous or heptahydrate, etc. The sulfates could be a potential problem with plaster pools or possibly grout between tile. Well, 400 grams in 60 liters would result in 6666 mg/L (ppm) magnesium and that's pretty high. Probably still not enough for scaling but it's pretty high. It'd probably be better if some spa user or dealer with experience with such high levels were to respond if there are issues here.

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Using Magnesum Chloride would be about 1/20th the cost of Epsom Salts, and it appears as it's better absorbed transdermally. Sourcing it locally is a bit of a challenge though.

Surprisingly, from the google searches I've done, using Magnesum in spas doesn't seem too common. Considering all the health benifits magnesium supplementation has, I thought more spa owners would have tried this.

Any spa users or dealers with experience can chime in!! :-)

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One thing I can think of at the very high thousands of ppm level is that this is similar to having very high salt levels in terms of conductivity and therefore potential metal corrosion. Though normally TDS isn't something of concern, if you intentionally increase it to such very high levels, then that is something to worry about, especially for corrosion. I bet spa manufacturers may even say their warranties are voided if you increase TDS that high. Spas aren't necessarily designed to resist metal corrosion as strongly as some pools. This may be particularly true if the heat exchanger in the spa heater is copper instead of cupro-nickel or titanium.

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Thanks chem geek! I'll have to look into the corrosion potential of the water once it hits that high of level. It's a 10 year old hot tub, so warranty would likely be over....plus the original owners didn't leave any of the paperwork. I don't know if I have a heat exchanger, I just replaced the heater on it a couple weeks ago.

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Another point of concern would be pump seals. If the tub is 10 yrs old the seals are most likely Buna N. Newer tubs ,specifically those designed for SWCG will have "Viton" seals. Yes the seals are only about $20 but they're a PIA to change

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No interaction. Think of it as just being another kind of salt.

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Is there a hot tub that moves the water by air where the salt wouldn't come in contact with the metal ?

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Is there a hot tub that moves the water by air where the salt wouldn't come in contact with the metal ?

No. The circulation of the water is done by moving the water. There is air added to aeration jets, but they also have water.

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Thanks! I have recently learned the many benefits of magnesium. Just wondered if I could add it to my hot tub. I will not be trying that.

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