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Boric Acid Usage.


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Hi,

I'm about to do a full water change and want to fully convert to Dichlor/bleach from day one. I've found Boric acid powder on eBay (99.9% purity) which I intend to buy and use however, I could do with some clarity on exactly how to add the Boric acid.

What the best order to balance the water IE which chemicals do I balance first and in which order?

I think I read somewhere that adding Boric acid will automatically reduce the PH level when added to the water. If so, what PH should I be targeting before adding the Boric acid ?

Am I right in thinking the Boric acid sort of locks the PH level ?!

Thanks.

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Yes, it will be harder to move the pH or TA after adding the boric acid so you first want to lower the TA via acid addition and aeration at lower pH (usually around 7.0) first. Then after your TA is at 50 ppm, have the pH rise by aeration (without adding acid) to around 7.7 though it's not critical and 7.5 is fine as well. Then add the boric acid which will slightly lower the pH, probably to around 7.5 (or 7.3 if you start with 7.5). You can use The Pool Calculator to calculate dosing.

You should compare your boric acid price against what you can get at AAA Chemicals or The Chemistry Store.

Your pH will still tend to rise, but less than if you hadn't lowered the TA or used the boric acid.

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I actually stopped at Ace on Friday to get Boric Acid and some other cleaning items.

When I asked them to show me their muriatic acid, it is in a bottle that is labelled muriatic acid and is sold as such.

Then, when I asked them for Boric Acid, they sell it as: Harris Boric Acid Roach Powder which is 99% Boric Acid. The attached MSDS states it is 99.0% pharmaceutical grade A Boric Acid. (the MSDS states remaining 1% of components are non-toxic / non-hazardous)

Before I pour 2 bottles of roach killer into my freshly cleaned hot tub... can somebody confirm that this Harris Boric Acid Roach Powder is an acceptable source for the boric acid that is discussed in the forums. Thanks!!

http://pfharris.com/msds/Harris%20Boric%20Acid%20Roach%20Powder%20with%20Lure.pdf

ps: THANKS to those on this forum that are very knowledgable about chemicals / chemistry and help the rest of us that have basic questions. On other forums for other parts of my life, I am more of an "answer-er" than a "question-er". So I greatly understand how much work it is. THANKS!

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Yes, that's the stuff. Concentration is everything. Boric acid kills insects not only because of the higher concentration you use, but because they have no way to excrete boron from their bodies -- unlike mammals who can excrete boron (within limits). Also, the boric acid does not get absorbed through your skin and you won't be drinking quarts of spa water.

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Chemgeek,

On a fresh refill my TA tends to be lower than 50ppm in the first place. Is there any reason why I can't get the levels of TA and PH right by purely using chemicals as opposed to aeration ? I'm not totally clear around aeration therefore if possible it would be simpler for me to use chemicals only.

Thanks.

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The aeration is only necessary if you need to lower the TA. The aeration accelerates the process. Adding acid lowers both pH and TA, but carbon dioxide outgassing (which is sped up by aeration) raises the pH with no change in TA. So the combination of the two lowers the TA. If you were to just use acid alone, you couldn't lower the TA fast enough without having the pH get dangerously low that could cause corrosion of equipment or harm spa surfaces. Lower pH will naturally have faster outgassing, but aeration speeds this up so you can do the process at a relatively mild pH of 7.0 or thereabouts.

If your TA is already at 50 ppm, then you don't need to adjust it. You can just add the boric acid, assuming your pH is 7.5 to 8.0 since boric acid is slightly acidic (will drop pH from 7.5 to 7.3, usually).

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Thanks.

If I adjust my TA and PH to 50ppm and 7.6 respectively and CH to 100-150ppm (as Arctic Spa's recommend) I've calculated my SI to be between -0.2 and -0.4 at 37 degrees.

Do I need to be concerned with my SI at these levels, if so, what preventative action should I take ?

Thanks.

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Are you talking about an acrylic spa? If so, then you don't need to worry about the saturation index. That index which is about saturating the water with calcium carbonate is only relevant if you have plaster or grout surfaces (i.e. that contain calcium carbonate) to protect or perhaps fiberglass that has a gelcoat containing calcium carbonate. Metal corrosion is not directly related to the saturation index and forming a "protective layer" in rapidly changing water flow and temperature is nearly impossible. Read this link for more info on factors affecting corrosion (mostly low pH, dissolved oxygen, strong oxidizers).

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Chemgeek,

My Boric acid powder has arrived, the tub has new water, PH is 7.7ppm, TA is 67ppm, CA is 125ppm and FC is 10ppm (first fill) am I good to add the Boric acid ?

I've looked at the pool calculator and it shows for a 330 gallon tub I need 429g of Boric acid. I've got no means of testing the level once added, my Boric acid powder says H3BO3 min assay 99.9% is 429g right for the Boric acid I have ?

My 5 year old son likes to go under water in the tub and inevitably swallows water at times, is this a concern using Boric acid ?

Thanks

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If you are using Dichlor-then-bleach and if your pH tends to rise while on bleach, then you might want to lower your TA to 50 ppm before you add the boric acid. It's easier to adjust the TA before the borates are in there -- otherwise it will take a long time for aeration to raise the pH back up for the last step. Other than that, you look good to go for the boric acid addition and yes, you've got the right stuff, but I calculate 357 grams for the Boric Acid. The Pool Calculator says 13 ounces weight if I'm in U.S. units. If I switch to metric and use 1249 liters (automatically calculated), then it says 357 grams. I'm not sure how you are getting 429.

See Are Borates Safe? for more info on safety. The EPA established the 50 ppm limit using a factor of 100 Margin of Exposure (MOE) to account for cross-species variability since safety experiments were done on dogs and other animals and not on humans and to account for individual variability within a species (i.e. the smaller sample size of the experiments). A child weighing 40 pounds (18.15 kilograms) would need to consume 1.6 mg per day which with 50 ppm Borate water is 32 ml (a little more than 1 fluid ounce) of spa water every day to be at the EPA limit, and that's with that factor of 100 and this limit is at the no observed adverse effect, not when first symptoms are seen. Remember that boron is in foods and mammals have the ability to excrete excess boron so the issue is exceeding the body's limit for removing excess boron that is consumed.

On the other hand, it's probably not the best thing in the world to be dunking one's head into spa water regularly, especially if the spa is used regularly. Spas are a low water of volume, unlike residential pools, so accumulate a lot of partially oxidized organic substances, especially if you don't have an ozonator. If you have the choice, I'd keep the head out of the water. It's not a serious problem, just a relative one compared to pools where huge amounts of dilution make things easier to manage and is why you don't need to replace your pool water every so many months.

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Whoops. I used U.S. gallons and yes there is a difference. The Pool Calculator also lets you use Imperial units (so Imperial Gallons) and when I do that then 330 Imperial Gallons is 1500 liters (not 1480; 330*4.546 = 1500). I then get 429 grams which is what you got. Sorry for my confusion on this.

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No problem. I appreciate its a daft thing to say however, I wanted to check as adding 429g of any chemical into the tub felt a bit unusual compared to normal quantities of chemicals I have to add !

I've managed to get TA to 47ppm and PH to 7.9ppm.......the next step is to add the Boric acid !!

Thanks again.

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Yes, it is a larger amount compared to other chemicals. It's closer to what you do when increasing calcium hardness (150 ppm CH in 1500 L would be 250 g of anhydrous or 331 g of dihydrdate). However, because 50 ppm borates are measured in units of 50 mg/L Boron which has a molecular weight of 10.811 g/mole while boric acid has a molecular weight of 61.83 g/mole, 50 ppm borates as ppm Boron is the same as 286 mg/L of boric acid. (286 mg/L) * (1500 L) / (1000 mg/g) = 429 g. So the amount is deceptive because of the units of measurement convention for boron.

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At a pH of 50 ppm borates (which is measured in units of ppm Boron), it increases TA by 5 ppm at a pH of 7.5. At a pH of 8.0, it is 15 ppm TA while at 7.0 it is 2 ppm TA. So for practical purposes you can usually ignore its effects on TA.

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All chemicals are now added including Boric acid, I've tested the water again the TA is 47ppm however, my PH is 7.8, is that ok or too high ?

If its too high, with 50ppm Borate I've calculated I need 20g of acid to get the PH to 7.6 (pool calculator). I assumed it would need more acid than that due to the Boric acid, can you confirm my calculation ?

I guess I will then need to add some Baking soda to bring TA back in line ?

Thanks.

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If there were no borates, then it would only take 5 grams of acid so the borates do buffer the pH. Your calculation is correct.

I'd leave the TA where it is and see what happens to the pH. If the pH doesn't rise over time, then you can bump up the TA some; otherwise, just leave it where it is. You want the TA to be on the low side when using bleach since you don't want or need carbon dioxide outgassing that occurs at higher TA and raises the pH.

If you are going to be using Dichlor first, however, then 10 ppm FC from Dichlor after the chlorine gets used up would lower the TA by 3.5 ppm so after using it you can test your TA again and can raise it up to 50 ppm if it's much lower than that.

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Up to you. It's not a problem at 7.8 given the low TA and CH (i.e. no risk of calcium carbonate scaling). You can try lowering it to 7.6 and see if it tends to rise. Sometimes the pH likes to settle in at a somewhat higher level and it's easier not to fight it.

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Ok, think I will leave it then. One last question, I'm currently at the stage of adding Dichlor to raise my CYA. Over the last four days I've raise my FC to the following levels.

Initial water fill 0.28-10 FC

Day 3 (3-6 FC)

Day 4 (4-6 FC)

Dat 5 (4-6 FC)

Having checked my CYA on day 6 I expected my CYA to be around 16ppm however, it was only 6ppm is that unusual ?

Thanks.

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It could be test error. The CYA test isn't super-accurate. It wouldn't hurt for you to add some more Dichlor if you are worried about it. Even if the measurement error is +/- 10 ppm you just want to be in the 30-40 ppm ballpark for CYA (50 won't be a problem, but 100 might be).

You can also double-check that your Dichlor is actually nearly pure and isn't something like 60%. Some products aren't as pure and have fillers or other ingredients.

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