A possible explanation of how borates work to stabilize pH

[TheOne]

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.

Unfortunately so far I havent seen any positive affects regarding lower acid usage after adding Borax to my pool. I added the borax a couple of weeks ago and still have ph creep that appears to be the same rate as before. I dont have a borate test kit but did add the recommended amount from above posts. Hope it works out for others though.

 

[chem geek]

The Borates should most definitely change the amount of pH rise, though they will not change the total amount of acid needed to be added unless you change your SWG output setting. So is that what you are saying that you are seeing -- that the pH rises by a smaller amount, but it still takes the same amount of acid to restore the pH to where it was? So perhaps you now add acid less frequently but it requires the same quantity over time?

The way we think that Borates additionally lowers the amount of acid needed over time is by killing algae so that chlorine doesn't have to so that you end up lowering your SWG output. Have you noticed that your FC level is now higher at the same SWG setting so that you can lower your SWG output to get the same FC level as before? If you lower your SWG level (on-time) then that should lead to less acid usage.

Richard

 

[TheOne]

Oh I see what you mean. My FC has been a bit high >5ppm as I have been slowly letting my CYA drop. My CYA is back down to about 60ppm from 110. I am going to test lower my swcg output from 50% to 45% and see if I can slowly drop my FC and lower my acid requirement.

Its not hard work just would be nice to not have to add acid so often. My pH generally rises .2 to 8.0 after about 3 days. I add about 2 cups of muriatic acid to bring it down to 7.8. It seems easier to keep my pH on the high side. At least that is what I am trying for now.

 

[mas985]

Unfortunately so far I havent seen any positive affects regarding lower acid usage after adding Borax to my pool. I added the borax a couple of weeks ago and still have ph creep that appears to be the same rate as before. I dont have a borate test kit but did add the recommended amount from above posts. Hope it works out for others though.

I too added Borax about two months ago. 16 boxes for 21,000 gallons and have not experienced any less acid usage. I have about the same rise in PH and still add about 1/2 gallon of acid per week which is what I did last year as well.

Chlorine levels seem to be about the same as last year.


TheOne,

What type of pool do you have? Plaster, fiberglass, vinyl.

 

[TripleB4me]

What are your TA levels? I thought with the proper TA the resistance to pH swings would be enough. I have a fairly high TA, ~ 125 ppm and a high CH ~225 ppm, due to my fill water and need to add only about 2 cups of acid once a week in a 25000 gallon pool.

 

[chem geek]

Mark,

That is really strange since the same outgassing of carbon dioxide in a pool without borates that causes the pH to go from 7.5 to 7.91 would only go to 7.67 with 50 ppm Borates. The same amount of acid would be needed to restore the pH to 7.5 in either case unless there was less aeration from lowering SWG output. Of course, if the pH didn't rise by very much before you added acid each week, then the difference might not be very noticeable. Having to add the same amount of acid over time is more readily measurable and understandable if the SWG output could not be lowered.

Perhaps in some pools, such as yours, the amount of potential algae growth is low so having the Borates as an algaecide doesn't help reduce chlorine consumption so doesn't let you lower your SWG output (which would lower aeration).

Richard

 

[chem geek]

TripleB4me,

The pH buffering from TA or Borates has nothing to do with the amount of acid you need to add. pH buffers simply make the pH change less with the same amount of acid or base, but they do not change the amount of acid or base needed to restore pH. Basically, the buffer works both ways -- making it harder for the pH to rise (i.e. taking more base or outgassing) AND making it harder to lower the pH back down again (i.e. by adding acid). So a pH buffer may reduce the frequency of acid addition because it will take longer for the pH to rise, but when you do add the acid, you have to add the same amount over that time period as you would have with less of a pH buffer (so if you add it half as often, you add twice as much acid each time so that the total amount of acid per unit time is the same).

Now that said, it is also true that high TA itself contributes to rising pH. This is because the largest part of TA is the bicarbonate in the water. The pool is intentionally over-carbonated and just like a lovely tasty carbonated beverage, it will outgas carbon dioxide into the air. When this occurs, the pH rises (with no change in TA). So, ironically, lowering the TA level will lower the amount of acid you need to add over time. It will also slightly lower the rate of pH rise, but since a lower TA is less of a buffer the effect on the rate is only noticeable (for some) when the TA gets really low. This chart shows that the amount of pH rise over time should be reduced by about 15% which won't be measurable.

You can try lowering your TA to 80 or below (using the acid and aeration procedure described elsewhere including this post) to see if it changes the amount of acid you need to add. In theory, according to this chart, at a pH of 7.5 lowering your TA from 125 to 80 would reduce the amount of acid that needs to be added by 42% (assuming your CYA level is near 30 ppm).

Richard

 

[TheOne]

I have a plaster (aggregate finish) pool. My TA is at 100ppm. I havent had a problem with algae, at least it hasnt been visible if there was any. I also have an ozone oxidizer running as well. Pool was perfectly clear before and after adding the 11.5 boxes of borax.

 

[chem geek]

TheOne,

I would lower your TA level to 80 (see procedure mentioned in my previous post) or even lower depending on CYA level (if you go lower, you may need to increase your CH somewhat to compensate or set a target pH of 7.7 instead of 7.5). We usually recommend doing that first and then adding Borates if the lower TA isn't enough. The higher (even "normal") TA is a source of rising pH along with the aeration from an SWG. Do you have an SWG and if not, what is your source of chlorine (i.e. bleach/chlorinating liquid, Trichlor tabs, Cal-Hypo, etc.)?

The combination of the lower TA (and higher pH if not already at 7.7) with Borates has both a lower acid amount added and less frequent addition required. The former reduces mostly the acid amount while the latter reduces the amount of pH rise, but between these two effects the first one is more dramatic. The effect with the Borates only seems to be more visible if it lets you lower your SWG on-time; otherwise, its buffering effect isn't seen unless the pH rise was rather high in the first place.

The lowering of TA and/or the raising of the pH target level has always worked to reduce the amount of acid needed over time (for pools with rising pH and hypochlorite sources of chlorine), but it does not always result in a noticeably slower rate of pH rise. In theory, the combination of the lower TA with the Borates should be enough to make the rate of pH rise noticeably slower. We'll see if Mark sees that as well (if his TA gets lowered) or if his pool is from the Twilight Zone. Same with your pool.

Richard

 

[TripleB4me]

chem geek,

Thanks for the explanation. I was trying to understand how adding borates were similiar to the TA effect of acting as a pH buffer. It seems as if dumping pounds of borax didn't change the amount of acid needed or the frequency of adding it.

 

[chem geek]

TripleB4Me,

So the total acid amount not changing is understood. As for the frequency, that's less clear in your case. Keep in mind that at a lower pH, the Borates have less of a buffering effect. So the same amount of outgassing that causes the pH to rise from 7.2 to 7.40 without Borates goes to 7.35 with 50 ppm Borates so probably not measurable. Starting at 7.5, a rise to 7.70 without Borates would go to 7.60 with 50 ppm Borates so possibly just barely measurable. Starting at 7.8, a rise to 8.00 without Borates would go to 7.86 so more likely to be measurable. Of course, at higher pH there should be less outgassing anyway and that overwhelms the Borate buffering difference with pH.

The Boric Acid pH buffer is well understood so at this point I assume that your lack of seeing a difference in pH rise is due to its subtlety since it isn't a huge effect unless the rise is large enough and at higher pH. Of course, there could always be something wrong in the theory, but this is much more standard chemistry than the issues surrounding SWG efficiency and CYA protection of chlorine from sunlight (where multiple explanations are plausible and experiment must determine which is most important or dominant).

Richard

 

[TripleB4me]

Thanks again. Is borax the only way to add borates?

 

[mas985]

My problem has been keeping TA low. I can lower it to 70 ppm with a heavy acid dose but in a week or two it is back up to 100-110 ppm. I will keep an eye on this to see if it continues.

Evaporation has been quite heavy recently which could explain the TA bump but even if it were as high as 6" in a week, probably unrealistic, the dilution would still be 10:1 for the fill water so if the fill water TA is 120, it should only add 5 ppm. I am at loss at this point.

As for algae, the only time I had a problem with algae is when I had the chlorinator off for over a week. CL was 0 for a couple of days and it started showing up. I don't think there is a lot of algae contribution in my pool although this year there seems to be a lot more stuff floating in the pool.

 

[Guest]

Thanks again. Is borax the only way to add borates?

Yes. you can use regular borax (sodium tetraborate decahydrate) or one of the products sold for pools like Supreme, Optimizer, or Endure (sodium tetraborate pentahydrate). the only difference would be the amount needed to reach 30-50 ppm.

 

[Guest]

My problem has been keeping TA low. I can lower it to 70 ppm with a heavy acid dose but in a week or two it is back up to 100-110 ppm. I will keep an eye on this to see if it continues.

Are you aerating to outgas the CO2 and raise your pH? If you are not then the TA will climb back up when the pH risesand the carbonic acid converts back into bicarbonates and carbonates.

 

[mas985]

Waterbear,

I am not aerating but according to your Acid Column Myth article:

Carbonic acid will not revert back into alkalinity unless a base (alkaline) material such as soda ash is added to the water. Therefore, there is no need to remove carbon dioxide from the water to achieve the desired results of lowering alkalinity.

Am I reading this wrong or is it really saying it is not necessary to aerate in order to lower alkalinitiy.

 

[chem geek]

You need to see the context of the quote. Basically, all it is saying is that the outgassing of carbon dioxide only raises the pH with NO CHANGE in alkalinity. That is a true statement. Outgassing ALONE will not change the TA. However, outgassing (from aeration to accelerate the process) PLUS addition of acid will lower the TA. Basically, the following procedure for lowering TA shows what happens to pH and TA with each step:

ACTIVITY .......... pH .... TA ... (The following assumes 6.8 is the lowest measurement on the pH test kit)
==================

Acid ..................... - ........ - ... Add enough acid to bring pH down to 7.0

Aeration ............. + ....... 0 ... Aerate until pH rises to 7.2
Acid ..................... - ........ - ... Add enough acid to bring pH down from 7.2 to 7.0 (you may continue to aerate while you do this)
------------------------------------
Aeration & Acid .. 0 ....... - ... Continue this combination (cycling of the two above) until TA is at the target you want

then AFTER you have reached your target TA,

Aeration ............. + ....... 0 ... Aerate until the pH rises to your target pH (say, 7.5).

==================
Net of Above ....... 0 ........ -

Technically, you could just aerate and add acid at whatever pH that you want, but the process goes faster at lower pH since that makes the outgassing go faster since the pool is more out of equilibrium with the air in that case (because at lower pH more of the carbonates are in the form of carbonic acid and dissolved carbon dioxide species). Hope that helps explain the situation.

Richard

 

[TheOne]

TheOne,

... Do you have an SWG and if not, what is your source of chlorine (i.e. bleach/chlorinating liquid, Trichlor tabs, Cal-Hypo, etc.)?...


Richard

Yes I use a Pentair IC40 SWCG for my chlorine generation. I will see if I can possibly get my TA down a bit.

 

[mas985]

Richard,

I understand the steps of alkalinity reduction you laid out. What I am now confused about is what happens if you do not aerate or aerate very slowly (eg. with a SWG). In other words, does the rate of aeration change the process? Does it need to be done quickly such that TA does not rise? My assumption, perhaps wrong, was that a SWG aerates enough to cause the PH rise but not TA.

 

[chem geek]

The aeration from the SWG is conceptually no different than aeration from any other source such as waterfalls, fountains, or an air compressor. Technically, it is a rather efficient form of aeration since the hydrogen gas bubbles are small and they contain no carbon dioxide in them (initially). In other words, I would not consider having an SWG turned on as being slow aeration. On the contrary, it is quite vigorous which is why most users with SWG systems find a rather strong tendency for the pH to rise.

As for what happens when there is little aeration, in this case there is still outgassing of carbon dioxide and still a rise in pH, but it is much slower. In fact, in some pools it is so slow as to be effectively zero or swamped by other factors that affect pH. Of course, I'm assuming a hypochlorite source of chlorine in what I am describing since such sources are approximately pH neutral when considering addition and usage of the chlorine (obviously the pH goes up when chlorine is added and goes down when it gets used up so the timing of pH tests relative to such chlorine addition/consumption is important -- or put another way, measure the pH when the FC level is the same).

As for your question regarding having aeration happen quickly to prevent the TA from rising the answer is no. It does not matter how quickly the aeration occurs. The physical process of having carbon dioxide leave the pool water causes the pH to rise with no change in TA. It doesn't matter how quickly or slowly this occurs. The physical transfer of dissolved carbon dioxide, CO₂ from the water into the air causes the following shifts to occur in equilibrium:

CO₂(aq) --> CO₂(g)
Dissolved Carbon Dioxide in Water --> Carbon Dioxide Gas in Air

H₂CO₃ --> CO₂(aq) + H₂O
Carbonic Acid --> Dissolved Carbon Dioxide in Water + Water

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

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

Notice two things about the above reactions. First is that there is a reduction in hydrogen ion. That is, by definition, what causes the pH to rise since higher pH means a lower quantity of hydrogen ion. Second, notice that every component of alkalinity (bicarbonate ion and carbonate ion) that gets removed also removes a hydrogen ion. The definition of alkalinity is not only any substance that can accept a hydrogen ion (above a limiting pH of the alkalinity test -- around a pH of 4.5), but hydrogen ion itself is considered to have negative alkalinity (technically, it is the difference between hydroxyl ion OH^- and hydrogen ion H⁺ that is the alkalinity from dissociated water).

This is confusing because we've always said that pH and TA move together. That is only true when adding an acid (adding hydrogen ion) or a base (adding hydroxyl ion or equivalently removing hydrogen ion). With carbon dioxide outgassing, we are removing BOTH a positive contributor to alkalinity (CO₃^2-) AND a negative contributor to alkalinity (H⁺) that exactly cancel each other out (the carbonate counts twice for alkalinity and carbonic acid has two hydrogen each counting once towards negative alkalinity).

I know this is confusing. I'm not sure how to explain it any better.

Richard