TA and CO2 outgassing follow up

[Matt]

This discussion originally started in this thread. Moved to The Deep End because of it's technical content. JasonLion

Plain wrong here. TA for unstabilized chlorine like bleach or a SWG should be 70-90 ppm. For stabilzied chlorine it should be higher 100-120 ppm.

While anything acidic will burn off some carbonate alkalinity, people have different preferences for their TA. The key point is that you need to keep your pH in the right range, and the TA acts as a buffer to facilitate this. Find what works for you and go with it. There are as many opinions on exactly what your TA should be as their are pool owners.

 

[JasonLion]

There are as many opinions on exactly what your TA should be as their are pool owners.

Technically, there are more opinions than there are pool owners. I have several different opinions all on my own

But that misses the point waterbear was making. There are good reasons for adjusting TA lower with a SWG and higher when using trichlor tablets. TA does not only act as a PH buffer. TA levels in swimming pools are out of equilibrium and cause CO2 outgassing which will shift the PH up towards 8.4 over fairly long time scales, depending on the amount of aeration in the pool. SWGs create aeration from hydrogen bubbles created at the cell plates, which causes CO2 outgassing which raises PH. There are dozens of people on this forum with a SWG who have lowered TA to the levels waterbear suggests, or even lower in several cases, and experienced greatly reduced PH drift and no significant new problems.

Everyone is welcome to have any opinion they want about the ideal TA level. But if you want to look at the evidence from people who have actually tried it different ways, they consistently have better results when TA lower with a SWG, as waterbear suggested.

 

[Guest]

Plain wrong here. TA for unstabilized chlorine like bleach or a SWG should be 70-90 ppm. For stabilzied chlorine it should be higher 100-120 ppm.

While anything acidic will burn off some carbonate alkalinity, people have different preferences for their TA. The key point is that you need to keep your pH in the right range, and the TA acts as a buffer to facilitate this. Find what works for you and go with it. There are as many opinions on exactly what your TA should be as their are pool owners.

APSP guidelines (as taught in CPO courses and adopted by many State Health Departments) for TA with unstabilized chlorine are 80-100 ppm (and even this turns out to be a bit high in actual practice) and 100-120 ppm for stabilized chlorines. The reason for this is because the the higher the TA the more the tendency for the bicarbonate/carbonic acid buffer to move the pH toward 8.2. The stabilized chlorines are acidic and the reaction of chlorine sanitizing and being converted to chloride ions is also acidic so this will have a tendency to lower ph and convert bicarbonates to carbonic acid which then outgas as carbon dioxide.
Unstabilized chlorines are alkaline and because of the acidic nature of the reaction of FC converting to chloride ions the net effect of the unstabilized chlorines (including the sodium hypochlorite produced by a SWG) is close to pH neutral so there is much less impact on pH. Therefore having less carbonation in the water (lower TA) will result in less outgassing of CO2 and therefore less acid demand and more stable pH. SWG's have a special situation because they also produce hydrogen gas in the cell along with chlorine and this actually aerates the water and speeds the outgassing of CO2. This is the reason that SWGs have a greater acid demand than a manually chlorinated pool, all other things being equal. Running a an even lower TA (60-80 ppm) and also adding borates to about 50 ppm to introduce a secondary boric acid/borate buffer system can help reduce acid demand and increase ph stability in a salt pool even more.
This is not an opinion but is based on the chemistry that actually occurs in the pool, which is why it had been adopted in the APSP/NSPI guidelines.

 

[Matt]

I'm pretty much agreeing with you... different TA's can be desirable for different situations. HOWEVER, the key idea is that TA, in and of itself, is not DIRECTLY relevant to pool sanitation (it is INDIRECTLY very important though, obviously). It's just a buffer, and to the extent that a given TA helps you maintain the right pH, then it's a good TA level for your situation. If you had no TA, but were able to maintain sanitizer and pH levels (not suggesting this is a good plan, it's a hypothetical), then the pool would be perfectly safe to swim in. It would, of course, be very hard to manage pH w/o any buffering capacity in your pool. My point is simply that following reference ranges isn't always the best way to go, and sometimes it's more about figuring out how your particular pool acts and finding the ideal TA number based on experience. I can't tell you how many different reference ranges I've read for suggested TA. My pool liner's book, my ecomatic, my pump, my taylor kit, the test strips I bought at walmart, a computer program I bought for managing a pool, my local pool store's printout, and multiple sites on the web have given different ranges... sometimes very different ranges. None of them suggest different ranges based on the type of chlorine that is being used.

Also, SWG's increase pH because the net reaction directly produces base (see chem_geek's post on it), not just from outgassing. The net reaction is:
Cl- + 2H2O -> OH- + H2 + HOCl. HOCl is a pretty weak acid, so OCl- is a pretty strong base, as the strength of an acid and its conjugate base are inversely related. OH- is obviously a strong base. Cl- is a very weak base because the highly electronegative Cl atom can easily stabilize the negative charge, and as its conjugate acid, HCl, is very strong. Thus, on a molar basis, SWG's produce much more base than acid from the reaction, and hence increase pH.

 

[Guest]

Also, SWG's increase pH because the net reaction directly produces base (see chem_geek's post on it), not just from outgassing. The net reaction is:
Cl- + 2H2O -> OH- + H2 + HOCl. HOCl is a pretty weak acid, so OCl- is a pretty strong base, as the strength of an acid and its conjugate base are inversely related. OH- is obviously a strong base. Cl- is a very weak base because the highly electronegative Cl atom can easily stabilize the negative charge, and as its conjugate acid, HCl, is very strong. Thus, on a molar basis, SWG's produce much more base than acid from the reaction, and hence increase pH.

Actually, Richard (chemgeek) and I have actually discussed this at length and he will agree that the pH rise is primarily from outgassing of CO2.

 

[Matt]

Seems counterintuitive to me, but I have no clear quantitative idea of the magnitude of each effect. Did Richard do some tests on it?

What is the reaction you believe is occurring for the outgassing?

 

[Guest]

It does seem counterintuitive. Richard and I have discussed that also. Read this post http://www.troublefreepool.com/viewtopic.php?t=4979 for CO2 outgassing.
It is simplified because I 'dumb down' the chemistry so the average joe can have a better chance of understanding it but you should pick up what I am talking about.
Are you familiar at all with CO2 injection for pH control in pools? If you are then what is going on should be very clear.
With a SWG you have constant aeration because of the production of hydrogen at one plate and chlorine at the other plate. Ozone generators produce a pH rise for the same reason. They drive the CO2 out of the water much in the same way shaking a bottle of club soda drives out the CO2 or using an airstone in an aquarium drives out the CO2. Be aware that by introducing the bicarbonate buffer we are purposely overcarbonating the pool and it is NOT at equalibrium.

 

[chem geek]

Seems counterintuitive to me, but I have no clear quantitative idea of the magnitude of each effect. Did Richard do some tests on it?

What is the reaction you believe is occurring for the outgassing?

Matt, see this post. You are forgetting about the reactions of chlorine getting used up, mostly from breakdown from sunlight as well as oxidizing ammonia (and urea). This latter process is acidic and exactly compensates for the initial rise in pH when hypochlorite sources of chlorine are initially added (including the reaction from an SWG). The post goes into that in detail and the result is pH neutral except for a very minor increase in pH due to excess lye in hypochlorite sources (but this is VERY small and essentially negligible).

The outgassing reaction is simply a phase transfer of dissolved carbon dioxide in the water, CO₂(aq) to carbon dioxide in the air, CO₂(g). Of course, the removal of the carbon dioxide in the water shifts all the equilibrium reactions (most notably bicarbonate becoming carbonic acid and then dissolved carbon dioxide) with the net result being a rise in pH with no change in TA. This outgassing is due to the fact that pools are over-carbonated. At a pH of 7.5, the "natural" equilibrium TA of a pool would be around 10 ppm (if CYA were 0), so the far higher TA is an out-of-equilibrium situation and outgassing is always occurring though it is very dependent on aeration, surface area with the air, etc. (so a pool cover can virtually stop this process).

Richard

 

[frustratedpoolmom]

Re: Helping out the neighbor with his pool chemistry

My neighbor has a in ground pool and is going to take a shot at doing the maintenance himself. I own the taylor 2006 test kit and I told him I would test his water. I own a in ground pool but it uses a SWG. My neighbors pool uses the pucks in the float.
What kind of numbers should I be looking for? I have a good handle on the FC levels for SWG pools vs Manually dosed pools but what about the CYA, TA, CH for a manual dosed pool?
Here is a breakdown of my neighbors pool to be tested.
10,000 gal, Diamond brite, tricolor pucks in a float, Sunny South Florida pool temp at 91F, Long hot summer days.
Thanks

Aren't you glad you asked... :lol:

Guys, take it to the Deep End, your gonna scare the OP.... LOL

 

[duraleigh]

FPM's post is quite timely. Folks come to this forum and follow the prescribed methods of BBB because of their simplicity.

Making the responses far more complex than they need to be may be interesting to some but seldom to the OP.

The OP question could have been answered by.....

FC - 3-6ppm
pH - 7.2 - 7.8
Alk - 80 - 120
CH - 200 - 400ppm
CYA - 40 - 60ppm

or, alternately.....

Recommended levels are found in the stickies under "ABC's of Pool Water Chemistry"

This forum depends on and needs the input of the chemists to keep us rednecks from getting out of control and start wearing tin foil hats to ward off algae. (man, it was HOT in that hat here in Raleigh yesterday but it seemed to work )

However, the responses should be aimed at the understanding level of the OP which is usually pretty easy to pick up. :lol:

 

[whowouldfigga]

Ok I am going to buy the first round of beers!!!

 

[JasonLion]

HOWEVER, the key idea is that TA, in and of itself, is not DIRECTLY relevant to pool sanitation (it is INDIRECTLY very important though, obviously). It's just a buffer, and to the extent that a given TA helps you maintain the right pH, then it's a good TA level for your situation.

The section I put in bold is the key point here. The whole point of TA is to help you maintain the PH at appropriate levels. Setting TA at zero is not a practical way to maintain PH at appropriate levels. Likewise setting TA at 500 is also not a practical way to maintain PH. With some information about the situation, is there a SWG, do they use trichlor, etc it is possible to know what an appropriate TA is to help maintain PH most effectively.

My point is simply that following reference ranges isn't always the best way to go, and sometimes it's more about figuring out how your particular pool acts and finding the ideal TA number based on experience. I can't tell you how many different reference ranges I've read for suggested TA. My pool liner's book, my ecomatic, my pump, my taylor kit, the test strips I bought at walmart, a computer program I bought for managing a pool, my local pool store's printout, and multiple sites on the web have given different ranges... sometimes very different ranges. None of them suggest different ranges based on the type of chlorine that is being used.

In the majority of cases I have more experience with TA than the pool owner does, and more experience than the authors of many of those books/guidelines have, to what appropriate TA levels are. I have made a special point of exploring different possible TA levels for use with a SWG and waterbear has even more experience with this than I do. Along with others here at TFP we have worked with many people to find TA levels that produce the best long term results for SWG owners across a wide range of pools. Nearly everyone else who makes TA recommendations is quoting some older source, the vast majority of which were written to address pools that use trichlor tablets, which is a totally different situation than a SWG.

The TA levels we recommend work better for pools with a SWG than other levels do. You don't need to take our word for it, ask people around here who have adjusted their TA following our recommendations. Chem geek and waterbear can explain the chemistry behind it far better than I can. But I can tell you that many many people have had excellent results with the levels we recommend and no serious problems. The same can not be said of the levels suggested in all of those other reference materials you refer to.

 

[smuggs]

Jason, et al - (or should I say "chemistry gods"?)

Would you say that pools with requlary used water features have similar CO2 outgassing levels as SWG pools?

 

[Matt]

Richard,

Are you essentially saying that the only reason we have any carbonates in our pool above 10ppm is that the rate constant of the reaction to form CO2 is so slow? Is it just a kinetics issue and not determined by the change in gibbs free energy of the entire reaction?

Matt

Jason,

I agree with you about TA recommendations for novice users. I guess my posts there were more targeted towards Waterbear, not the OP.

Matt

 

[JasonLion]

smuggs , it depends greatly on the water feature and how much you run it. Some of the laminar flow waterfalls have fairly low aeration and some of the negative edge pools have huge amounts of aeration. Many water features are only on perhaps an hour a day, others run 24/7. For adjusting TA purposes, a good measure of aeration is how quickly your PH rises from 7.2 to 7.8. The faster PH rises the more useful it will be to lower TA.

 

[Matt]

I found on my own, not through theory but just through experimentation, that 70-80 works the best for me in my pool with a SWG. My pH constantly trends upward, and I prefer to keep it lowish (I go through muriatic acid like Ralph Wiggum goes through paste). I'm always lowering it to 7.2ish, and then waiting for it to creep back up. If I let it go, it'll hit 9+.

 

[Guest]

This forum depends on and needs the input of the chemists to keep us rednecks from getting out of control and start wearing tin foil hats to ward off algae. (man, it was HOT in that hat here in Raleigh yesterday but it seemed to work )

Dave, your not selling those tinfoiil algae ban hats again, are you!

 

[Guest]

I found on my own, not through theory but just through experimentation, that 70-80 works the best for me in my pool with a SWG. My pH constantly trends upward, and I prefer to keep it lowish (I go through muriatic acid like Ralph Wiggum goes through paste). I'm always lowering it to 7.2ish, and then waiting for it to creep back up. If I let it go, it'll hit 9+.

The lower you place your pH the faster CO2 will outgas (basic chemical equilibrium) because you will have more of the buffer in the carbonic acid phase and less as bicarbonate ion. Your pH will rise more slowly if you only lower it to about 7.6 which is why we recommend this. You posted in one of your threads that you like to keep your pH lower because there is more HOCL and less OCl- at lower pH but this is only true in an unstabilized pool. Once cyanurates are added to the mix the effect of pH on chlorine's activlity is greatly minimized so this becomes a moot point in practical application. Also keeping the CYA at the upper limits recommended will also reduce the on time of the cell and there will be less total aeration so there will be less outgassing also. Finally adding borates to 50 ppm will introduce a secondary buffer system that tends to move pH downward so this, combined with the bicarbonate buffer tends to stabilize pH around 7.6-7.7 for a longer time than without the borate buffer. When the pH climbs to about 7.8 or higher lower it back to 7.6-7.6 .
You have experience with one pool, your own. We have had, through our collective experience here, at PF, and those of us in the industry like myself, Poolsean from Autopilot systems and Strannik from Autochlor Australia (if I left out anyone just know that you are included also) experience with literally hundreds and hundreds of pools.

 

[Titanium]

Matt,

My pH constantly trends upward, and I prefer to keep it lowish (I go through muriatic acid like Ralph Wiggum goes through paste). I'm always lowering it to 7.2ish

No wonder you are going through muriatic acid like water. Why are you trying to lower to a pH of 7.2. You would be much better aiming for a level of 7.6.

Titanium

P.S. Which reminds me that I have to go add muriatic acid myself this morning. And does anyone else have trouble distinguishing the pink colors of 7.8 vs 8.2? Is there a drops-based pH test? I HATE comparing colors. I NEVER use the OTO test for chlorine - I would rather buy more DPD/FAS powder and reagent than try and compare those shades of yellow.

 

[Matt]

I believe everything you're saying, but I still prefer the lower pH for a number of reasons. First, I am assured that as it creeps up, if I miss a day, it will have further to creep before going very alkaline. I realize that my methodology DOES increase my acid demand long term... but I don't mind really, as acid here is dirt cheap at 4 bucks a gallon and easy enough to add. Also, lower pH pools are often more clear (at least I've experienced this with my pools, and have read others experienced this too).

Interestingly, my TA stays pretty constant. I've added less than one box of baking soda in 2 years now, and my TA has never gone out of the 70-80 range since I first brought it down (my brilliant pool building company started me off with over 200ppm TA... and my water is only about 40ppm).

I'm curious how CYA affects the equilibrium between hypochlorous acid and hypochorite. It should be pH dependent, obviously, but I can't see any other way that CYA would affect it. What is the mechanism by which this occurs?

I like to run relatively low CYA levels for a few reasons. First, CYA is a suspected carcinogen. Second, it has diminishing returns over a certain point. Finally, it binds up too much free chlorine and reduces sanitization power of the chlorine present. I do use some (about 30ppm) but I don't want to go much higher. My SWG runs anywhere from 10% to 30%, so I'm already running it pretty low. Living in the northeast, the sun isn't too bad here either. We don't get that really powerful direct sunlight that the you guys get down in Florida. Also, high levels of CYA can turn into algae food.

Finally, like Titanium, I also hate distinguishing the higher pH's by color. Below 7.8 I have no problem, but 7.8+ all look the same to me. Some diff between 7.8 and 8.0 I guess, but if I hit the lower end of the range I'm more confident.

I keep the spa at more like 7.4-7.6. First of all, it's bromine, which isn't as pH dependent for sanitization. I still have to add acid a lot because the bleach I use each night to bring up the bromine levels increase pH and I have to compensate for that. Most nights I add about 2 tbsp to the spa.