A possible explanation of how borates work to stabilize pH

[Guest]

I was soaking in my hot tub this afternoon and I was thinking about what I know about borates and came up with this theory of how they stabilize pH. The claims made for borates have proven to be true in actual tests in several pools, including my own. They include reduced sanitizer demand, more stable pH, and "sparkling" water. The reduced sanitizer demand is easily explained by their algaestatic properties but the actual reduction in sanitizer demand is much more than expected so perhaps something else is going on here also. The "sparkling water" effect could be possibly explained by the borates not allowing any nascient algae blooms so the water is clearer than without. By lowering one of the things necesary for algae growth it just won't grow and that is going to be more effective than killing it after it starts with the sanitizer. We all know dead algae will cloud a pool so perhaps if it never gets a chance to start growing in the first place the water will have more "sparkle" then water that is relying on the chlorine to kill any algae that starts to grow. Even a tiny amount of dead and oxidized algae might be enough to deaden the "sparkle" of the water.

First we need to look at the way that borates act as an algae preventative. They do this by reducing the amount of carbon dioxide in the water. Carbon dioxide is necessary for photosynthesis. I suspect this occurs because any H+ ions in the water react with the borates and form boric acid instead of reacting with the carbonates and bicarbonates to form carbonic acid. Carbonic acid is simply carbon dioxide dissolved in water. I must stress that this is only supposition and I have not looked at the chemistry involved yet nor investigated if the formation of boric acid is more likely than the formation of carbonic acid when borates are present.

If the above IS true and the secondary borate/boric acid buffer in the water is more active than the primary carbonate/bicarbonate/carbonic acid buffer then less carbonic acid will form when pH is dropped and that will lead to less outgassiing of carbon dioxide. The outgassing of carbon dioxide is what causes pH to rise. If there is less formed in the water then the pH will not rise as fast. This could explain the more stable pH that many, if not all, users of borates have reported. This is in line with Chemgeek's recommendation of keeping the pH at 7.6 and lowering the TA to help stabilize pH by lessening the outgassing of carbon dioxide when borates are not used. By doing this you are minimizing the formation of carbonic acid in the water.

I would appreciate any feedback on my theory since the actions of borates have never been properly explained anywhere that I have been able to find.

 

[chem geek]

This document supports the theory of borates, which is mostly boric acid at the near neutral pH of most water, as an algaecide (and even inhibits the growth of some bacteria). As for how this works, I quote one part of this document below.

The authors therefore concluded that the delay in cell division is probably a direct effect of boron on cytokinesis, as nuclear division is unaffected.

There also appears to be effects on photosynthesis which, obviously, inhibits all kinds of energetic processes in the cell. There was nothing in this document to indicate that the borates affect carbon dioxide intake or reduce the carbon dioxide contents of the water.

In my spreadsheet where I calculate all of the chemical species, it is true that the Boric Acid / Tetrahydroxyborate pH buffer reaction does handle a small portion of the pH buffering of acid and a large portion of the pH buffering of base. That is, it is much better at buffering a rise in pH than a drop in pH. Because Total Alkalinity measures all species that can accept a hydrogen ion (proton), the borates in water only increase measured TA by a relatively small amount since most of the buffer is in the form of Boric Acid. In fact, at a pH of 7.5 there is about 40 times as much Boric Acid as Tetrahydroxyborate. The equilibrium is as follows (I first show the reaction of what happens when sodium tetraboarate decahydrate is added to the water):

Na₂B₄O₇•10H₂O --> 2Na⁺ + 4B(OH)₃ + 2OH^- + 3H₂O
Sodium TetraBorate Decahydrate --> Sodium Ion + Boric Acid + Hydroxyl Ion + Water

B(OH)₃ +H₂O <--> B(OH)₄^- + H⁺
Boric Acid + Water <--> Tetrahydroxyborate + Hydrogen Ion

I cannot find anything where there is a direct reaction between the borates and carbonates. Obviously, they both participate in pH buffering and involve hydrogen ion in doing so, but once pH is adjusted the species operate independently. There is a very small interaction in forming an ion pair between calcium and tetrahydroxyborate, but this is a tiny factor and can be ignored -- it's far less in concentration than the calcium and bicarbonate, hydroxyl, and neutral carbonate ion pairs and all of those combined can usually be ignored (though they are in my spreadsheet, nevertheless).

In other words, the amount of carbonic acid in the water is determined solely by the total amount of carbonates in the water and the pH. The borates have nothing to do with it except for the minor effect of contributing to TA so that perhaps someone lowers TA to compensate and THAT will lower carbonate alkalinity. 50 ppm Borates at a pH of 7.5 only increase TA by 5.7 ppm so not that much. The following reactions shows the carbonate buffer system and the reactions of carbon dioxide equilibrium with air (technically, CO₂(aq) and H₂CO₃ are not measured or accounted for separately and only a small fraction is actually the acid but due to their equilibrium that is independent of any other species, they are treated together and in my spreadsheet I just call it H₂CO₃):

CO₂(g) <--> CO₂(aq)
Carbon Dioxide Gas <--> Aqueous Carbon Dioxide

CO₂(aq) + H₂O <--> H₂CO₃
Aqueous Carbon Dioxide + Water <--> Carbonic Acid

H₂CO₃ <--> HCO₃^- + H⁺
Carbonic Acid <--> Bicarbonate Ion + Hydrogen Ion

HCO₃^- <--> CO₃^2- + H⁺
Bicarbonate Ion <--> Carbonate Ion + Hydrogen Ion

Another way to look at it is that with an additional pH buffer the same amount of acid or base will cause the pH to move less, but at any given pH the amount of carbonic acid is solely determined by the total carbonates in the water, independent of borates. Since we see a dramatic drop in the rise in pH with the addition of borates, then the slowdown in rising pH is certainly due to the borate buffer system which is very strong against rising pH BUT the borate buffer system has NO effect on the amount of carbonic acid and therefore should not be affecting the rate of outgassing. So though you should see a lower RATE of pH rise, you should also see that you need to add exactly the same amount of acid OVER TIME to restore pH -- that same buffering that the borates does in making the pH rise slowly from the outgassing also applies in the reverse direction when you add acid to lower the pH. So something else must be going on to reduce the amount of acid needed over time (not the frequency of addition due to pH rise, but the total quantity of acid per long period of time). So far, we only have the algaecide theory which allows one to lower the SWG output time and we KNOW that lowers aeration.

How much did you lower the SWG output time after adding the Borates? If it was half, then that would be about half the outgassing so in theory would result in about half as much total acid being required -- BUT the borates make the frequency of such acid addition spaced out over a much longer period of time.

Richard

 

[Guest]

This patent description describles the CO2 lowering effect of sodium tetraborate by forming carbonates and bicarbonates in the water. I have also seen other references to this.

 

[chem geek]

The sentence in the patent is:

The tetraborate solutions plus dissolved carbon dioxide with produce sodium carbonate plus sodium bicarbonate thereby lowering the CO₂ concentration in the water and making it difficult for algae to germinate in the water.

However, what they mean by this is that adding tetraborate is adding a base to the solution to make the pH rise. At higher pH it is true that there is less CO₂ concentration in the water. It's the change in pH that causes this and has nothing to do with tetraborate itself. One could use lye / sodium hydroxide and achieve the same effect. This effect of higher pH slows down carbon dioxide outgassing, but we already knew that.

The patent also says the following:

The tetraborates have also been shown to react with dissolved halides in the water to give off free halogen. Finally, the tetraborates increase the solubility of most other salts that may be added to the standing water system.

Now that's an interesting statement since I'm not sure what dissolved halides they are referring to that are bound together. Sodium Chloride is a halide salt, but in water it freely dissociates into the free halogen: chloride ion. Do they mean that it converts to chlorine gas? I don't see that happening at all. As for increasing the solubility of most other salts, that may be due to ionic strength effects, but I'm not sure. It could just refer to higher pH which may make some salts dissolve more readily in water, but again I'm not sure. We know that this is NOT true for calcium carbonate which precipitates at higher pH.

Now there could certainly be some mechanism whereby Boric Acid or its derivatives somehow affect the carbonate buffer system other than via pH, but I have not found any such connection.

The other flaw in the logic of the patent is that pools are intentionally over-carbonated already. One can clearly reduce TA and reduce total carbonates and therefore reduce carbon dioxide in the water. However, since algae grow readily in ponds and lakes and streams where there appears to be more of an equilibrium concentration of carbonates in the water, I don't think algae really need that much CO₂ to survive. This whole idea of "starving" algae by depriving them of dissolved carbon dioxide sounds like a bunch of bunk to me. On the other hand, if it truly did somehow "shield" or combine with carbonic acid or bicarbonate such that it "appeared" to be lower in concentration, then that could lower the dissolved carbon dioxide content and reduce outgassing. I just don't see the mechanism. If you can find it, please post it. I've done several searches and can't come up with anything.

I did find this link that describes how boric acid inhibits the oxidation of carbohydrates, but that doesn't answer the question about carbonates (which are completely different).

I also found this document that relates Total Alkalinity to the ability of oceans to absorb carbon dioxide and this is done in a formula that determines the "homogeneous buffer factor". All this is saying is that when there is more pH buffering in water then if there is an increase in carbon dioxide in the air, then more can be dissolved into the water. The reason is that the pH will drop less than it normally would when the CO₂ dissolves into the water (this is the reverse of outgassing). It is true that water that has a boric acid buffer system will be able to absorb more carbon dioxide, but this relationship between carbonates and borates is connected solely through pH (hydrogen ion in the equations). In the pool system, we have a FIXED and predetermined pH that is our target so all of the carbonate species quantities are also fixed. At a given pH and with a certain total of carbonates there is a known and calculated amount of carbonic acid. It is true that with Borates in the pool that the pool can release carbon dioxide through outgassing and not rise in pH as much, but we already knew that.

If we start with standard pool water at a pH of 7.5, CYA of 30, but a TA of 50 (instead of 100), then a certain amount of CO₂ added will cause the pH to drop to 6.9 and we increase the CO₂(aq) amount by a factor of 5.2. If we take this same amount of CO₂ but start with a TA of 100, then the pH drops only to 7.1 and we increase the CO₂(aq) amount by a factor of 2.8. If we do the same thing but add 50 ppm Borates (and acid to still start at a pH of 7.5) so our TA is now 115 for the same carbonate alkalinity, then adding the CO₂ makes the pH drop to 7.2 and has the CO₂(aq) amount increase by a factor of 2.2. So the increased buffering essentially puts less of our added gaseous carbon dioxide in the form of aqueous carbon dioxide and instead puts more of it into the form of bicarbonate and this happens with ANY form of pH buffer since it is the pH, and the pH alone, that determines the relative amounts of carbonate, bicarbonate and carbonic acid / aqueous carbon dioxide (this latter carbonic acid and aqueous carbon dioxide pair are NOT dependent on pH, which is part of the reason they are simply treated as if they were a single compound since they don't interact with anything else including hydrogen ion).

This link shows a nice fraction graph of the carbonate buffer system as well as equations for it. I just don't see how the Borates affect this system except via changing the pH.

Richard

 

[TheOne]

Are the borax calculations correct in the bleachcalc tool? It seems like its high.

What is the current dosing amount of borax per 1000 gal to raise from 0ppm to 50ppm borate level? Also how much acid do you add per 1000 gal to negate the ph rise?

 

[Guest]

The borax calculaton in bleach calc is wrong. It will raise the borate level by less than 1/3 of the amount the calculation says it will if you follow it's dosing recommendations. (It calculates that you only need 3.3 oz of borax to raise 1000 gallons to 10 ppm instead of 12 oz!) To raise each 1000 gallons to 50 ppm you need 3lbs 11 oz of borax (59 oz) and will need 30 oz of muriatic acid to neutralize the pH rise. For maintenance to replenish the borate level when it starts to drop you need 12 oz (3/4 lb) of borax and 6 oz of acid (3.4 cup) to raise 1000 gallon 10 ppm. Not exact but close enough for government work!
Note that each box of borax contains 72 oz (4 3/4 lbs).

If you are going to be adding borax I would recommend getting the AquaChek borate test strips. I have found them online from various sources. They provide enough precision to get you into the proper 30-50 ppm range. If you have a Bioguard or Proteam dealer around you can have them test for Optimizer (Biogaurd) or Suporeme (Proteam) if you don't want to test yourself. They used to use a reagent test but many of them have gone to the strips because the reagent test is expensive and complicated to do.

Edit: Chemgeek's speadsheet does have a borax calculation that is accurate. I don't have the link handy but I am sure he will post it when he reads this!!

 

[chem geek]

If you are a brave soul, here's a link to the Pool Equations spreadsheet (actually, a ZIP file of it that you need to unzip). The Borates are in two sections in the spreadsheet -- it's one of the "Base to add to increase pH" options and it's also further down under Borates if you are adding them to add Borates to your pool as an algaecide or pH buffer and not just to raise the pH (you still need to set the type of Borates in the "Base" section). For this latter use of borates, setting your goal Borate level, entering in the quantity under Borates and then your target pH and then clicking on the "Calculate Acid/Base/TA" button is the most common thing to do to figure out how much acid you'll need to add along with the Borates.

Note that the density numbers I have for Borax and sodium carbonate (pH Up) and some other items are too high -- they are for solids, not powder or granules (i.e. they aren't bulk density). The weight amounts are correct. I'm getting some real-world density numbers on this thread at the Pool Forum and will update the spreadsheet after I get such data. In the meantime, just use a postage scale or similar method to weigh the chemical (or you can modify the densities in the spreadsheet -- in the CONSTANTS section after line 154, but you'll need to Tools-Protection-Unprotect Sheet... first (there is no password).

 

[Guest]

I use a small kitchen scale (diet scale) to measure my pool chems. It allows me to zero out any container (usually a 4 cup plastic measuring cup) I put on it (an important consideration) and will weigh up to 4 lbs, graduated in ounces, at a time. (It also has a metric scale on it) It was inexpensive. The ones that will weigh larger amounts were on the expensive side so I passed on them.

 

[TheOne]

The borax calculaton in bleach calc is wrong. It will raise the borate level by less than 1/3 of the amount the calculation says it will if you follow it's dosing recommendations. (It calculates that you only need 3.3 oz of borax to raise 1000 gallons to 10 ppm instead of 12 oz!) To raise each 1000 gallons to 50 ppm you need 3lbs 11 oz of borax (59 oz) and will need 30 oz of muriatic acid to neutralize the pH rise. For maintenance to replenish the borate level when it starts to drop you need 12 oz (3/4 lb) of borax and 6 oz of acid (3.4 cup) to raise 1000 gallon 10 ppm. Not exact but close enough for government work!
Note that each box of borax contains 72 oz (4 3/4 lbs).

If you are going to be adding borax I would recommend getting the AquaChek borate test strips. I have found them online from various sources. They provide enough precision to get you into the proper 30-50 ppm range. If you have a Bioguard or Proteam dealer around you can have them test for Optimizer (Biogaurd) or Suporeme (Proteam) if you don't want to test yourself. They used to use a reagent test but many of them have gone to the strips because the reagent test is expensive and complicated to do.

Edit: Chemgeek's speadsheet does have a borax calculation that is accurate. I don't have the link handy but I am sure he will post it when he reads this!!

Ok for a sanity check for a 15000 gal pool with 0 ppm borate levels...

15 * 59 oz borax = 885 oz or about 55 lbs borax (11.5 boxes at 4.75lb each) to raise to 50 ppm

15 * 30 oz muriatic acid = 450 oz or about 3.5 gallons of acid to counterbalance the borax.

Is there a need to premix the borax in water first? A gradual addition of borax is probably preferable correct?

 

[chem geek]

The Borax numbers look correct except that I get 422 ounces or 3.3 gallons of Muriatic Acid to compensate for the pH. As waterbear said, "close enough for government work."

Of course, you add a little Borax, then a little acid, in proportion. As for premixing, I don't think that's necessary if you pour slowly over a return jet with the pump running. I just used the Borax on Saturday and found it quickly dissolved.

 

[Guest]

I added half the borax needed and brushed until it was dissolved....it went pretty fast....then I added half the acid needed. I then added the rest of the borax and brushed and then the rest of the acid. This is the way Proteam says to put in Supreme, which is their expensive borax!

 

[rhondave69]

Last year residual

Just had Optimizer levels checked at Bioguard dealer and they say 50ppm.
Last year was 20lbs ProTeam.
Water volumn is 5300 gal.
We drained 9 in over winter.
Dealer wants to sell another 20lbs ProTeam.
I just want to maintain 50 ppm.

 

[CaOCl2]

Re: Last year residual

Just had Optimizer levels checked at Bioguard dealer and they say 50ppm.
Last year was 20lbs ProTeam.
Water volumn is 5300 gal.
We drained 9 in over winter.
Dealer wants to sell another 20lbs ProTeam.
I just want to maintain 50 ppm.

Yeah what's that titration test they use? Anyone know where to get the reagents? No reply from taylor on if/when they'll come out with a Borates kit.

 

[Guest]

Proteam used to have a titration test for borates available but they don't seem to anymore. It was complicated and lengthy to perform (not ot mention expensive...I think it was around $70), which is probably why they went to the strips. Ditto for Bioguard. LaMotte had borate test Unit Dose Vials for use with some of their colorimeters if you feel like shelling out about $1000. I'll just make due with the strips for now. The color changes are subtle but you get used to them after a while. You do need to read them in sunlight because artificial like will give inaccurate results!

 

[CaOCl2]

Thanks for the reply. As the story goes, Pool Store Clerk didn't believe me when I told him TMT borax was the same thing as Optimizer. I returned to the pool store with a sample and he proceeded to test the water for borates, a titration test, after adding the first reagent the water turned yellow, added a second then it turned clear, then a third and 27 drops later then blue endpoint was reached, 54 ppm.

I'd rather use that he has than my AquaChek strips but then again, pool maintenance is no longer a hobby for me ...it has become a symptom.

 

[Guest]

The Proteam test is a bit more complicated. This linkwill take you to their page describing it. As you can see the test has several steps. I have not been able to find this test kit anywhere and I am not sure if it is still available.

Edit: I did a little digging and this test (and the Bioguard test) seem to be just the standard mannitol titration test for boric acid. This test is usually done with a pH meter but they seem to use an indicator similar to the way TA is tested.

 

[CaOCl2]

The Proteam test is a bit more complicated. This linkwill take you to their page describing it. As you can see the test has several steps. I have not been able to find this test kit anywhere and I am not sure if it is still available.

Edit: I did a little digging and this test (and the Bioguard test) seem to be just the standard mannitol titration test for boric acid. This test is usually done with a pH meter but they seem to use an indicator similar to the way TA is tested.

Thanks for the reply. I want to get my hands on a recent edition of the Standards Methods book, hopefully eBay will have one for me soon.

 

[waterhog]

adding borax&muriatic acid

wow i guess my figures arent as bad as i thought they were ,i figured to go from o to 50 ppm for borates id need 49.75 lbs and 405 ozs of muriatic acid 3.16gals for my 13500 gal ag i thought that couldnt be right it seems like so much to add does anyone agree with my figures?

 

[chem geek]

For your 13,500 gallon pool it would take 49.68 pounds of 20 Mule Team Borax and 379.66 ounces of Muriatic Acid. So your Borax number was correct, but your acid was a little high, but that's probably just from the rough rounding that was done for the recommendations.

The reason it takes so much weight of Borax to get to 50 ppm is that the 50 ppm is parts per million of Boron and Borax is composed (by weight) of 47% water (in a polyborate crystal), 29% Oxygen, 12% Sodium and only 11% Boron. After Borax dissolves in water, the Boron in the water is mostly in the form of Boric Acid which if measured as ppm then the 50 ppm would be 286 ppm (which is its true contribution to TDS, though isn't measured on most TDS tests since Boric Acid is neutral).

 

[waterhog]

thanks for getting me stright again,it just sounded like i was doing my math way off base as the amounts are so high,having a hard time convincing my better half,why we would want to add all this to a beautiful sparkling pool,that she maintains while i'm away and thats unfortunately most of the time :lol: i just want it to all stay that way for her with all the trouble we had with bacugoo for 4 seasons whew what a mess....