I am considering adding borates to my hot tub using Borax. All of the instructions I have found say to use Muriatic Acid with the Borax to balance the PH. Is there any reason I can't use Dry Acid instead? I already have Dry Acid readily available as PH Down. The amounts of Muriatic Acid and Dry Acid to balance the PH rise of the Borax appear similar on the Pool Calculator.

The best and easiest way is to use boric acid

If you want to use Borax, muriatic acid is better than using dry acid. Dry acid (sodium bisulfate) adds too much sulfate to the water.

Welcome to TFP!

A spa uses such relatively small quantities that you can just purchase some ProTeam® Gentle Spa you can get at Pool Geek and elsewhere since that is essentially boric acid. Or if you can find boric acid in small quantities shipped at a reasonable price, that's fine too. See The Chemistry Store, for example, but that seems more expensive. [EDIT] It's actually not -- James corrected me below. [END-EDIT]

A 425 gallon hot tub will take 1 pound of boric acid. You should get enough for several fills of the tub since you will periodically drain and refill. You can get 7 pounds for $13.44 (plus shipping) at the chemistry store. So, that works out to be around two dollars per fill.

The poolgeek price for 2 pounds is $13.78 (plus shipping), which is 7 dollars per fill. So, more expensive.

The $1.84 price at poolgeek is for the 6 ounce size.

http://www.chemistrystore.com/search.cg ... boric+acid

Thanks JamesW and Chem Geek for the responses. I will get some boric acid. I have some follow up questions.

In addition to being a PH buffer, does pure boric acid have the same qualities of reducing skin and eye irritation as Borax or Gentle Spa? It seems counter intuitive that any of these products whose main ingredient is an acid would do that.

Regarding the Dry Acid, if the sulfates are a concern why is it commonly used to reduce PH in hot tubs instead of Muriatic Acid? I thought the Muriatic Acid was damaging to acrylic. Why isn't that a concern when using it to establish the borate buffer?

Boric acid is used in eye washes, though at higher concentrations at around 1% (1750 ppm). It is a weak acid which in buffered water isn't going to have any "acid" effect on eyes or anything else. Even a strong acid such as Muriatic Acid added to the water does not cause a problem -- it lowers the pH, but it is the resulting final pH that determines whether there is an effect -- the type of acid used or its strength is not relevant.

When added to the water, boric acid is identical to adding Borax and Muriatic Acid (except the latter results in more sodium chloride salt). Gentle Spa IS boric acid with a small amount of Borax to make it a little more pH neutral, but pure boric acid is only slightly acidic in buffered pool water and again that's not relevant anyway. Adding 50 ppm borates using boric acid to water with a TA as low as 50 ppm would lower the pH from 7.5 to 7.15 so not that much considering how much is added. Basically, most of the boric acid remains as boric acid in the water rather than dissociating to become borate ion. At a pH of 7.5, 96.2% is boric acid while 3.8% is borate ion.

Dry acid is commonly used in hot tubs because the quantities of acid needed are small and it's more convenient, but the same concerns for sulfates apply there as well. It's just less likely to find as much stainless steel where the high sulfates might be an issue for accelerating corrosion or for there to be splash-out on cement or stone where magnesium sulfate salt recrystallization pressure could crack such surfaces. Muriatic acid is only damaging if CONCENTRATED acid comes into contact with surfaces. When diluted in water, the type of acid is not relevant to damage (except for side effects such as sulfates vs. chlorides) and it is the final resulting pH that is what is relevant.

So if adding 20 Mule Team Borax and Muriatic Acid to a spa or pool for borates, one should add them slowly over a return flow with the pump running. For a spa, with the small quantities that are used, you have the option of simply mixing them together in a bucket of spa water since once combined the pH should be close to neutral so adding the combo would be safer.

Thanks for all the great information Chem Geek. One more question.

Some reported benefits of borates are silky feeling water and reduced skin and eye irritation. Based on what you have said, I understand boric acid in this application will not be an irritant, but how does it reduce irritation compared to not using it?

Check out http://www.dudadiesel.com for good prices on granular technical grade boric acid. They have good prices and offer free shipping on 1,5,15 & 25 pound sizes. I ordered a 55 pound container from them last year for my pool and was very happy with their customer service. Good luck.

I don't think it does, or at least not much. ProTeam® Gentle Spa presumably has "Fresh fragrance and moisturizers added" so I would assume that it is the moisturizers that may reduce skin irritation and give the water a silkier feel. The borates do seem to lower the surface tension of the water so that may also make the water feel more "silky". It also gives the water more of a "sparkle", again most likely due to the lower surface tension.

When applying borates to a spa using borax + acid, you're adding a relatively large amount of acid; for my spa, I've settled on using 2c borax + 7 oz muriatic (31%). For controlling pH over time, I use dry acid, but only a couple teaspoons at a time. The sulfates from that amount of dry acid don't have a chance to become annoying before it's time for a refill. Using dry acid instead of muriatic when adding borates would be more likely to become a sulfate problem.
--paulr