?? for Waterbear - re: acid usage

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M

momof2grlz

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LifeTime Supporter
Mar 31, 2007
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Sunny, Humid Central Florida
Waterbear, I saw your response on Maxsrents post about tweaking your water balance to get your acid use down to 6 oz. every six weeks. If you don't mind me asking, how did you reduce your acid use? We have a 6,550 gallon FG with Reliance SWG, which I love. Currently, I'm adding 8 oz. of acid each week, which isn't bad, but I would love to reduce the acid maintenance as much as possible.

We don't have any water features, so aeration isn't really an option to lower the PH.

Thanks.

Chris
 
My fiberglass pool is about the same size as yours (San Juan Cocoa Beach model). I got my acid use down by:
Adding borax to a 50 ppm level see this link
and by lowering my TA to about 80 ppm (some people have gone as low as 60 ppm TA with good results) and finally, by adjusting the pH to 7.6 AND NO LOWER. I add acid when the pH climbs up to 7.8.

Aeration does not lower pH, it raises pH by outgassing carbon dioxide. If your TA is too high you lower the pH to about 7.0 (but not any lower since pool test kits cannot accurately test a pH lower than this). This converts some of the carbonates and bicarbonates we call TA into carbonic acid (carbon dioxide dissolved in water--think club soda). Since it's only the carbonates and bicarbonates that are tested when we test TA we have now effectively lowered our TA. The problem is how to raise the pH without raising the TA. We do this by aerating the water to drive out the carbon dioxide gas (think shaking a bottle of club soda to make it go flat). When the pH reaches 7.6 retest the TA. If it is not low enough repeat the process. (There is a faster way but this way is more failsafe until you get the hang of it.) THIS IS THE ONLY WAY TO LOWER TA THAT ACTUALLY WORKS!

If you don't have any water features to aerate the water you can buy one of those floating fountains that attaches to a return or just get some pvc pipe and fittings and make an elbow to attach to a return to shoot the water into the air so it falls back into the pool.
Check this discussion for ideas on how to do this.
You can even invite all the neighborhood kids to come over and have a 'splash party'.
 
This kinda puzzles me, mainly because with a SWC, if you use it, it produces a small amount of NaOH. The only way to reduce your acid demand over the long term is to either dial your SWC down, or run it less....neither of which is an acceptable solution. I have a 20,000g pool and I use about 32oz acid/week. Keeps my pH happy happy and the pool stays clean. I'm at a loss as to how one could use a SWC and only add 6oz of acid over a 6 week time span.

Did I miss something?

Michael
 
MWsmith2,
Towards the end of last season I followed Chem Geek's advice and dropped by TA down to the 70-80 range. I've got a 20K vinyl with SWC. I run about 50 ppm Borates as well. My CYA at that time was around 80. I keep my salinity around 3200 and FC from 4-6 ppm.

I went from using about 1/2 gallon of Muriatic acid every 5-6 days to needing almost none. I kept pH in the 7.6-7.8 range. Between rain and everything else, I used less than 1 gallon from late Aug to now.

I started using the solar cover a lot just after that, which will reduce off gasing and keep pH in check, so there may be some confounding actions going on, but I was very pleased with the results.

So far this year I haven't used the pool much, but I hope that trend continues. I had to add CYA and more Borax this spring after draining about 12" to fix a leak, but I've got everything balanced now and pH is stable. The cover is also still on. We'll see what happens this summer without the cover, fountain on, etc.

YMMV
 
4 boxes and 50 oz more by weight (about 4 2/3 boxes) and 1 gallon and 1 1/2 qts of acid will get you pretty much there. Add half the borax dissolved about 1/4 box at a time in a bucket of water, then add about half the acid (about 3 quarts would be a good amount), then add the rest of the borax dissovled about 1/4 box at a time then the rest of the acid. Brush the pool well to mix everything and let the pump run for 24 hours. You might get a high salt reading if some of the undissolved borax goes through the salt cell. No big deal, just restart the SWG if it shuts down. Check your pH in a day or two and make final adjustments to get it to 7.6 You might need another half cup or so of acid to do this. Your borate level will now be betweeen 30-50 ppm (closer to 50 ppm than 30 ppm).

I based my calculations for the measurements above on 6000 gallons. It's better to underdose slightly than to overdose. If you want to do it yourself you need 12 oz of borax and 6 oz of acid to raise 1000 gallons 10 ppm or you would need 3 lbs 11 oz of borax and 30 oz of acid to raise 1000 gallons 50 ppm.
 
Michael,

The SWG doesn't actually produce NaOH (sodium hydroxide). It essentially produces the same thing as what bleach introduces except that it doesn't actually add any sodium and it converts the chloride ion to chlorine gas which dissolves in water to produce hypochlorous acid. The other plate produces hydrogen gas from hydrogen ions and though that is very basic/alkaline, the dissolving of chlorine gas into water at the other plate is very acidic. The net result is the same rise in pH one would have from adding bleach. When the hypochlorous acid gets broken down or consumed (back to chloride ion) this is an acidic process so the net, net is pH neutral. this post for more detailed equations (SWG is near the end of the post). The net formula shown there could also be written as follows (combining the weak acid, HOCl with the OH-):

H2O + Cl- --> OCl- + H2(g)

The OCl- is the same thing that comes from bleach (which is NaOCl). So really the SWG produces hypochlorite ion plus hydrogen gas. Though the above is slightly basic/alkaline, the usage of the hypochlorous acid is acidic as shown in the post referenced above.

The rise in pH that is found with most SWG pools is due to the hydrogen gas bubbles that cause aeration. Aeration with higher TA (and low pH) leads to a pH rise. So the easiest solution is to have the TA lower and also to keep the pH a little higher.

Richard (chem geek)
 
Hm. I thought the reaction was this:


2NaCl + 2H2O ==> Cl2 +2NaOH + H2


Now I'm not going to pretend that I wrote that! :wink: I had done some electrochemistry research around the time I purchased my SWC, and to me, the above made sense. Also, in the engineering company I work for, I've seen examples of using electrolysis on seawater that's used for cooling so it generates chlorine as a result. Downstream of the cell there's *always* an acid injection system. I'm not saying that you're wrong or anything like that, it's just that it runs counter to my experience so far. Just trying to understand something new here! :D

Michael
 
nater said:
I went from using about 1/2 gallon of Muriatic acid every 5-6 days to needing almost none. I kept pH in the 7.6-7.8 range. Between rain and everything else, I used less than 1 gallon from late Aug to now.

I started using the solar cover a lot just after that, which will reduce off gasing and keep pH in check, so there may be some confounding actions going on, but I was very pleased with the results.
Click to expand...

Ok. I like to keep my pH lower than that, usually 7.2 - 7.4. Higher pH reduces the effectiveness of chlorine, since it is more hypochlorite, rather than the hypochlorous acid that really does the work.

Be careul with the constant usage of that solar cover. Offgassing is a good and necessary thing for a pool. That's how the scuzz that gets broken down by chlorine leaves the pool...by offgassing. It's one of the reason indoor pools are so hard to maintain, there isn't enough air circulation (usually) to carry off the gasses formed during chemical decomposition. In other words: if you aren't using it to heat the water, I'd leave it off.

Michael
 
Michal,
If you have CYA in your water then the effect of pH on the amount of HOCl and OCl- in the water becomes very small and can basically be disregarded since the reaction between the the chlorine and the cynauric acid becomes the primary one. If the pool has NO CYA then pH plays a major roll in the effectiveness of the chlorine's sanitizing ability. Keeping the pH at 7.6 instead of lower will decrease the outgassing of CO2 and help minimizing acid useage to maintain pH.
Also, adding 50 ppm of borax to the water will significantly reduce santiizer demand and you CAN dial down your SWG output!
All you have to do is try these suggestions to see if they work for you!
Evan
 

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Michael,

You forgot the reaction of chlorine gas with water so the net is as follows starting with your equation:

2NaCl + 2H2O ==> Cl2(g) +2NaOH + H2(g)
Cl2(g) + H2O ==> HOCl + HCl
--------------------------------------------------
2NaCl + 3H2O ==> 2NaOH + HCl + HOCl + H2(g)

One NaOH with the HCl combines to produce NaCl salt (dissolved in the water, of course) while the H+ and OH- ions produce water so we have:

2NaCl + 2H2O ==> 2Na+ + OH- + Cl- + HOCl + H2(g)

and if you eliminate the Na+ on both sides and one of the Cl- you get:

Cl- + 2H2O ==> HOCl + OH- + H2(g)

Now HOCl is a weak acid so the reaction can also be written as follows (combining the H+ from HOCl with the OH-):

Cl- + H2O ==> OCl- + H2(g)

The OCl- hypochlorite ion is identical to what is introduced into the water from bleach or chlorinating liquid (ignoring the sodium ion and the extra salt). So the above reaction is slightly basic/alkaline.

The usage of chlorine is slightly acidic and EXACTLY counteracts the above. Please refer to this link I gave in my earlier post as it shows various reaction possibilities for the usage of chlorine all with the same net result in terms of chlorine usage being slightly acidic.

Richard
 
Just comparing pH and TA without discussing fill water is really comparing apples & oranges.
Every pool has water evaporation and fill water addition. In different parts of the country the pH
could be from 5.0 to 9.0, a huge difference. TA could be 0 to 250 or more.

In areas where the fill water has low pH this will cancel out, in some cases, the increase in pH from
the Salt Generator or added chlorine. Other areas(mine) have a higher pH of fill water and higher
TA so additional acid addition will be needed. How much is dictated by the chemistry of the fill water.

I use a combination of CO2 and acid to control both pH and TA. CO2 will control pH only.

Yes, aeration will raise the pH because you liberate the CO2 from the water.
If you live in an area with low pH fill water a fountain works great to control the
pH, but in an area with high pH or TA fill water it works against you.

Just because one technique works in one area does not mean it will work in another.

Cliff s
 
chem geek said:
Michael,

You forgot the reaction of chlorine gas with water so the net is as follows starting with your equation:
Click to expand...

Not really my equation. ;) I'm not smart enough to figure out the balances, hehe. I just found it on the net, but apparently wasn't getting the whole story. Your explanation does make perfect sense tho.

chem geek said:
Please refer to this link
Click to expand...

Which would be great if PF worked, but it hasn't been up (surprise!) since I joined the discussion. On another tangent, I think we should link back to PF as little as possible, since it's a crapshoot if it's going to actually be up at the time. If you are lucky enough to see it up, I'd appreciate it if you could paste it here; i'll keep checking too.

Thanks for taking the time to explain all of this, I really appreciate it! I know just enough chemistry to be dangerous. :D

Michael
 
waterbear said:
Michal,
If you have CYA in your water then the effect of pH on the amount of HOCl and OCl- in the water becomes very small and can basically be disregarded since the reaction between the the chlorine and the cynauric acid becomes the primary one.
Click to expand...

Ok, *but* my pool seems to be happier at a lower pH. *shrug* Although it could be that I'm imagining things, haha. I do have a paper that outlines killtimes with cya at varying pH levels, and there is a difference. I haven't looked at it in a long time, but I'll see if I can find it and put the scans up on my ftp site so you all can look at them. I think Richard will *really* want to look at them, but then on the other hand, he may already have them! :D

waterbear said:
Also, adding 50 ppm of borax to the water will significantly reduce santiizer demand and you CAN dial down your SWG output!
All you have to do is try these suggestions to see if they work for you!
Evan
Click to expand...

Yup. I remember LOOOOOONG ago we had a borax discussion that ended up with you finding some test strips and me buying them. I bumped it up to 50 and never looked back. Algae really dropped off after that. HOWEVER, it's important to remember that borax is an algae inhibitor, NOT a sanitizer. You still need to keep appropriate levels of Cl in the pool for it to be sanitary. (you know this already, but I'm repeating for the other folks out there who might not!)

Michael
 
"Be careul with the constant usage of that solar cover. Offgassing is a good and necessary thing for a pool. That's how the scuzz that gets broken down by chlorine leaves the pool...by offgassing."

I initial worried about that, however I've never measured any CC, even with the cover on 24/7, warm weather, etc. Once the water hits a temp the wife is happy with, the cover stays off. We've actually had a Spring here this year, warm during the day, cool enough at night to kill the pool temp. Not complaining, just using the cover more than last year.

I did have the cover off from last Thursday through Sunday night. I had three jets pointed to the surface and the fountain going and my pH went from 7.3ish to 7.5ish (I looked long and hard at the colors to come to that scientific evaluation) I've used absolutely no acid since April 6th. TA is 80, CYA is 60, and FC is 3-6. Borates are at least 50 ppm. Gotta love the Borax! I'd to see pH level out at 7.4-7.6. I'll report again after this long weekend of pool use.

BTW, glad to see this site so active! Great effort by everyone :)
 
Michael,

I have moved the technical information I wrote and linked to from Pool Water Chemistry in the China Shop at the Pool Forum over to this thread. The specific post referring to the equations for chlorine chemistry and that shows that SWG output is (when working properly) pH neutral when taking into account chlorine usage is at this post while the change of effectiveness of disinfecting chlorine (HOCl) via pH in the presence of CYA is at this post.

Hopefully, people will be able to access this info on this forum worldwide.

As for nater's observation regarding keeping a pool covered, my pool is always covered with an opaque electric safety cover except when in use about 3-4 times per week (open for 2-3 hours each time). Even when it's been kept closed longer, I've never had a problem with CCs (always < 0.2 ppm). Probably because the bather load is low and we do keep it exposed to sunlight while in use and for a short time thereafter.

Thanks,
Richard
 
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