Maintaining Ph when using a salt chlorine generator

[mikeyliny]

Using a salt chlorine generator results in the generation of chlorine as well as sodium hydroxide (a strong base) that increases PH. Therefore, if I increase the chlorine generation % of my device, I will also be increasing PH.

2NaCl + 2H2O → 2NaOH + Cl2 + H2
OR
Sodium Chloride + Water = Sodium Hydroxide + Dichlorine + Tritium

I'm using a Hayward AquaRite 900 on a 17100 gallon pool and would like to figure out the ideal settings and process to maintain both Chlorine and Ph at reasonable levels.

I realize there are many variables involved, so being precise will be difficult and I am in the process of gathering data via trial and error.

For example, I would like to come up with something like:
To maintain your Chlorine between 3 - 6 and your PH between 7.2 - 7.6
1. Set Desired OutPut at 30% daily for 6 days
2. Day 7 set to superchlorinate
3. Repeat

To begin to gather this information, I plan on doing measurements of independent variable1 = % Desired Output; independentVariable2 = Alkalinity, with dependent variables of Chlorine & PH.

Has anyone had experience doing something like this before or have an idea of what the proper balance would be, best total alkalinity, % desiredOutput or other variables such as CYA, when using a salt chlorine generator?

 

[mknauss]

The creation of chlorine by a swcg is pH neutral.

 

[Donldson]

For example, I would like to come up with something like:
To maintain your Chlorine between 3 - 6 and your PH between 7.2 - 7.6
1. Set Desired OutPut at 30% daily for 6 days
2. Day 7 set to superchlorinate
3. Repeat

Why would you do all of that? Follow the FC/CYA Levels, find the setting that maintains the proper FC and you're done with that.

As far as pH, what TA are you maintaining?

 

[Bperry]

Using a salt chlorine generator results in the generation of chlorine as well as sodium hydroxide (a strong base) that increases PH. Therefore, if I increase the chlorine generation % of my device, I will also be increasing PH.

2NaCl + 2H2O → 2NaOH + Cl2 + H2
OR
Sodium Chloride + Water = Sodium Hydroxide + Dichlorine + Tritium

I'm using a Hayward AquaRite 900 on a 17100 gallon pool and would like to figure out the ideal settings and process to maintain both Chlorine and Ph at reasonable levels.

I realize there are many variables involved, so being precise will be difficult and I am in the process of gathering data via trial and error.

For example, I would like to come up with something like:
To maintain your Chlorine between 3 - 6 and your PH between 7.2 - 7.6
1. Set Desired OutPut at 30% daily for 6 days
2. Day 7 set to superchlorinate
3. Repeat

To begin to gather this information, I plan on doing measurements of independent variable1 = % Desired Output; independentVariable2 = Alkalinity, with dependent variables of Chlorine & PH.

Has anyone had experience doing something like this before or have an idea of what the proper balance would be, best total alkalinity, % desiredOutput or other variables such as CYA, when using a salt chlorine generator?

Welcome to TFP!

There are two ways to answer depending on whether your a person who just wants to the pool to be easy to maintain or if you’re someone just interested in experimenting with chemistry. Neither option is bad.

The simple pool maintenance procedure is:
1. Set SWCG output to generate chlorine level appropriate for CYA level using this chart. Chlorine / CYA Chart

2. Reduce the TA to 50-60

3. Check pH once a week and once it gets to 8.0, reduce to 7.6 using muriatic acid.

Here’s the “experimental fun” answer:
In the real world, coming up with a dosing process without any kind of monitoring simply isn’t possible. There is just too much daily variation in chlorine consumption and pH balance. The use of a SWCG is pH neutral “overall”. The tendency of pH to rise tends to come from switching out from previous use of trichlor which is an acid along with very high TA which is required to balance out the use of the acidic trichlor. If you lower the TA to 50-60 the pH will tend to stay pretty stable, mine only requiring lowering once every couple weeks. But every pool has different characteristics around aeration which drives up pH so it’s really hard to figure out a formula because by the time you figure one out, the weather changes and you have to start over.

 

[SoDel]

Using a salt chlorine generator results in the generation of chlorine as well as sodium hydroxide (a strong base) that increases PH. Therefore, if I increase the chlorine generation % of my device, I will also be increasing PH.

2NaCl + 2H2O → 2NaOH + Cl2 + H2
OR
Sodium Chloride + Water = Sodium Hydroxide + Dichlorine + Tritium

I'm using a Hayward AquaRite 900 on a 17100 gallon pool and would like to figure out the ideal settings and process to maintain both Chlorine and Ph at reasonable levels.

I realize there are many variables involved, so being precise will be difficult and I am in the process of gathering data via trial and error.

For example, I would like to come up with something like:
To maintain your Chlorine between 3 - 6 and your PH between 7.2 - 7.6
1. Set Desired OutPut at 30% daily for 6 days
2. Day 7 set to superchlorinate
3. Repeat

To begin to gather this information, I plan on doing measurements of independent variable1 = % Desired Output; independentVariable2 = Alkalinity, with dependent variables of Chlorine & PH.

Has anyone had experience doing something like this before or have an idea of what the proper balance would be, best total alkalinity, % desiredOutput or other variables such as CYA, when using a salt chlorine generator?

Apparently we think alike (so we both must be nuts!) and I get why you’re doing this. Because why not lol.

First off, your chemistry skills look above par but you need to take the reaction “all the way down.” The hydroxide isn’t done. You have a few more lines to add. As was pointed out, the net chemistry is pH neutral.

But even that doesn’t end the story. There’s a fair amount of churn in the SWG cell and on down the return plumbing and you will lose CO2, hence pH rise.

Yes, it has been done before (the figuring it all out at least), the wheel they made here is pretty darned round, and the TFP numbers will give you a good starting point without having to reinvent it. What Are My Ideal Pool Levels?

The basic concept is to reduce the amount of CO2 needed in the water to maintain CO2 equilibrium (or as close as one can practically get) with the partial pressure of CO2 in the air at a reasonable pH. You do this by lowering TA. To compensate, you raise CH to maintain calcium saturation. Use the app PoolMath to save a lot of time & trouble calculating your calcium saturation index.

For chlorine, figure out the recommended level and track chlorine in (again, PoolMath has a function that makes it easy to calculate how much Cl(g) the SWG makes per day based on your SWG run time), versus chlorine loss. Your chlorine loss varies with a number of variables, like sun, rain, bathers, plant detritus, etc., so it needs to be tested every day (at least until you get a solid handle on how your pool sort of reacts to various environmental conditions).

So speaking of testing, what test kit do you use? I went with the Taylor K-2006c-salt. It tests everything you need. Further reading, Swimming Pool Test Kits Compared

 

[mgtfp]

Sodium Chloride + Water = Sodium Hydroxide + Dichlorine + Tritium

That must be a very expensive pool, filled with heavy water

 

[mgtfp]

Therefore, if I increase the chlorine generation % of my device, I will also be increasing PH.

When increasing the FC level, e.g. going from target to slam, will increase pH. When you're back down to target, pH will be back to where it was before, plus increase by outgassing in the meantime. The complete chlorination cycle is pH neutral.

Here is the complete chemistry (second half SWG);

Equations for Chlorine Chemistry

I'll do this in words and in symbols. Adding bleach is a basic process; it is the using up of bleach (chlorine) that is an acidic process so the net result is almost neutral. When I said "chlorine usage" I didn't mean your using chlorine (i.e. adding it) -- I meant when chlorine gets used up by "doing its thing" or "breaking down". Sorry for the confusion I caused.

Adding Chlorine
NaOCl + H₂O --> Na⁺ + HOCl + OH^- (+ extra base Na⁺ + OH^-)
HOCl --> H⁺ + OCl^-
Sodium Hypochlorite (liquid chlorine or bleach) combines with water to produce sodium ions (part of regular table salt) plus disinfecting chlorine plus hydroxyl ion. The hydroxyl ion makes this a basic reaction that raises pH, but because the disinfecting chlorine is a weak acid this overall reaction raises the pH by less than a strong base would. Note that there is a small amount of extra base in the form of Sodium Hydroxide (lye or caustic soda) that comes with Sodium Hypochlorite and is there to help preserve it, but this amount is rather small.

Using Up Of Chlorine
Breakdown of Chlorine by Sunlight (UV)
2HOCl --> O₂(g) + 2H⁺ + 2Cl^-
2OCl^- --> O₂(g) + 2Cl^-
Chlorine breaks down in the presence of ultraviolet radiation, such as found in sunlight, and forms oxygen gas and chloride ion (and hydrogen ion, if starting with HOCl hypochlorite). Because a hydrogen ion is produced, this is an acidic process, but since disinfecting chlorine is a weak acid, only some of it breaks down in a way that lowers pH as shown above (i.e. only HOCl produces H⁺; OCl^- does not). During the process of chlorine breakdown by sunlight, there are hydroxyl (OH•), oxygen anion (O^-•) and chlorine (Cl•) radicals that are also produced as short-lived intermediates (technical details in this post). This can help oxidize organics in the pool.

Net Chlorine To Breakpoint (Ammonia "Oxidation")
2NH₃ + 3HOCl --> N₂(g) + 3H⁺ + 3Cl^- + 3H₂O
OCl^- + H⁺ --> HOCl
The disinfecting form of chlorine (HOCl) combines with ammonia through a series of reactions (that I have not shown) with the net result being the production of nitrogen gas (which is why it is important to keep your cover off and have good circulation when shocking) plus hydrogen ion and chloride ion. Though by itself this would be a strong acid reaction, there is also OCl- present that will combine with hydrogen ion to form more HOCl since the ratio of HOCl to OCl- will remain constant (and is about 50/50 at pH 7.5). So the net reaction is acidic, but not strongly so. Further technical details are in this post.

Overall combination of adding chlorine and having it used up
The net reactions are as follows if you combine the ones I showed above.
2NaOCl --> 2Na⁺ + 2Cl^- + O₂(g)
3NaOCl + 2NH₃ --> 3Na⁺ + 3Cl^- + N₂(g) + 3H₂O
So the overall net reaction of adding sodium hypochlorite to your pool and having it used up in its most typical ways is simply to produce salt (yes, sodium chloride or table salt, dissolved in water, of course) and either oxygen or nitrogen gas (and water).

Other things that could happen
If you do not have enough chlorine in your pool relative to your bather load (ammonia demand), then the chlorine may not completely oxidize ammonia and instead you will get chloramines (first, monochloramine). This reaction is basic. However, sunlight may break down monochloramine which will result in the rest of the breakpoint process which overall is acidic (so it's the same as I showed above overall).

It is also possible for chlorine to combine with organic compounds to form chlorinated organics that are hard to breakdown. When people talk about the health problems with chlorine, it is usually about some of these chlorinated organics (Disinfection By-Products, DBPs) known as Tri-Halo-Methanes(THMs) such as chloroform. Also, some chloramines such as nitrogen trichloride (NCl₃) not only smell bad, but can cause health problems (especially in indoor pools with poor air circulation). In an outdoor pool exposed to sunlight and with a good residual of chlorine you typically don't get these "bad" compounds. If you are really concerned and have money to burn, you can use a constant maintenance level of non-chlorine shock (monopersulfate, MPS) to oxidize organics before chlorine gets a chance, but this is probably overkill for an outdoor pool (though may be a good idea for an indoor pool).

Salt (SWG) Pool
In a salt water pool you produce chlorine through the following reactions:

At the anode (positive plate):
2Cl^- --> Cl₂(g) + 2e^-

At the cathode (negative plate):
2H₂O + 2e^- --> H₂(g) + 2OH^-

which nets out to the following where the chlorine gas dissolves in water:

2H₂O + 2Cl^- --> Cl₂(g) + H₂(g) + 2OH^-
Cl₂(g) + H₂O --> HOCl + H⁺ + Cl^-
H⁺ + OH^- --> H₂O
----------------------------------------------
2H₂O + Cl^- --> HOCl + OH^- + H₂(g)

or equivalently

H₂O + Cl^- --> OCl^- + H₂(g)

Note that the products of HOCl and OH^- are exactly the same as you get when you add liquid chlorine or bleach (ignoring sodium ion). This process is partly basic, but not strongly so due to the HOCl weak acid. So the overall net result in a salt pool is simply the production of oxygen or nitrogen gasses. The disinfecting chlorine that was created from chloride ion gets converted back to chloride ion as it is "used up".

[EDIT]
The net reactions in an SWG pool for chlorine addition from the SWG and then breakdown from sunlight and oxidation of ammonia are as follows:

2H₂O --> O₂(g) + 2H₂(g)
2NH₃ --> N₂(g) + 3H₂(g)

The chlorine is not "seen" in the above net reactions because the chloride that became chlorine goes back to being chloride again. The oxygen gas comes from water when chlorine gas dissolved in it (i.e. from hypochlorite ion or hypochlorous acid) while the nitrogen gas comes from the ammonia (the oxygen or hydroxyl in the chlorine reverts back into water in this case, using the hydrogen from the ammonia to do so).
[END-EDIT]

If you have a salt pool and don't use CYA (this isn't normal) then you could also outgas chlorine in the same way that CO₂ is outgassed. This is more likely if you are aerating the water (e.g. have water features, slides, fountains, jets pointed up, lots of splashing, ...). This process is strongly basic and greatly increases the pH (HOCl + Cl^- --> Cl₂(g) + OH^-). The reason this would tend to only happen in a salt pool without CYA is that a high concentration of both chloride ion (Cl^-) and disinfecting chlorine (HOCl) are needed and it occurs more readily at lower pH. [EDIT] If chlorine gas produced by the SWG did not fully dissolve and instead was outgassed, then this would result in a net pH rise and could be one factor for the pH rise seen in SWG pools (the other factor being carbon dioxide outgassing from slightly increased aeration from the SWG, but that is not enough to fully explain most pH rise in SWG pools by itself). [END-EDIT]

I know, I know...more than you wanted to know. I hope it helps and that you made it this far...

Richard

 

[SoDel]

Tritium is H3. I have some in my wrist watch. I figured he didn’t have a centrifuge to separate Deuterium but I’d believe a breeder reactor to get some Tritium.

[edit — I stand corrected — Tritiated water is also referred to as heavy water. Well I learned something so that’s a plus + ]

 

[jark87]

For example, I would like to come up with something like:
To maintain your Chlorine between 3 - 6 and your PH between 7.2 - 7.6
1. Set Desired OutPut at 30% daily for 6 days
2. Day 7 set to superchlorinate
3. Repeat

That’s pretty interesting. If there was a way to truly eliminate pH rise, I’d give it a try if I could. My controls don’t include a way to put SWCG production on a schedule. I can only change % and it produces at that rate unless I change it manually, so I couldn’t implement the suggestion. If it meant having to remember to superchlorinate every 7th day, I’d just stick with my current routine.

I’ve contended with pH rise for 15 years, but followed the recommendation of keeping TA in the 50-60 range and that has helped. I still have to give the pool a dose of MA each week, but the dosage has been reduced. It takes me less than 5 minutes and has become so routine that I know when I need to add it without even testing. Just part of my normal pool maintenance.

 

[mikeyliny]

Apparently we think alike (so we both must be nuts!) and I get why you’re doing this. Because why not lol.

First off, your chemistry skills look above par but you need to take the reaction “all the way down.” The hydroxide isn’t done. You have a few more lines to add. As was pointed out, the net chemistry is pH neutral.

But even that doesn’t end the story. There’s a fair amount of churn in the SWG cell and on down the return plumbing and you will lose CO2, hence pH rise.

Right now my CYA is low, which I did because I thought I could add to it after getting ideal levels. I havent achieved because of a lack of vigilence I believe, but also working on the many variables that make pool maintenance 'fun' or 'annoying.'

My observation, is that with relatively low CYA levels and TA around 50 when you turn on superchlorinate for 20 hours, and cover your pool, a chlorine level of close to 0 and pH < 7 becomes > or= to 10 & pH > 8. I think "neutrality" can be observed if your chlorinator does less chlorination and if a buffer like CYA is higher or perhaps those with more experience don't use superchlorinate unless the water is green.

Yes, it has been done before (the figuring it all out at least), the wheel they made here is pretty darned round, and the TFP numbers will give you a good starting point without having to reinvent it. What Are My Ideal Pool Levels?

The basic concept is to reduce the amount of CO2 needed in the water to maintain CO2 equilibrium (or as close as one can practically get) with the partial pressure of CO2 in the air at a reasonable pH. You do this by lowering TA. To compensate, you raise CH to maintain calcium saturation. Use the app PoolMath to save a lot of time & trouble calculating your calcium saturation index.

For chlorine, figure out the recommended level and track chlorine in (again, PoolMath has a function that makes it easy to calculate how much Cl(g) the SWG makes per day based on your SWG run time), versus chlorine loss. Your chlorine loss varies with a number of variables, like sun, rain, bathers, plant detritus, etc., so it needs to be tested every day (at least until you get a solid handle on how your pool sort of reacts to various environmental conditions).

So speaking of testing, what test kit do you use? I went with the Taylor K-2006c-salt. It tests everything you need. Further reading, Swimming Pool Test Kits Compared

I use both Taylor2006c also and I use Acquacheck 7 test strips from HACH. Overall, I would say the teststrips correlate well and work much faster. But I use both.

 

[mikeyliny]

Why would you do all of that? Follow the FC/CYA Levels, find the setting that maintains the proper FC and you're done with that.

As far as pH, what TA are you maintaining?

around 50-70

 

[mikeyliny]

When increasing the FC level, e.g. going from target to slam, will increase pH. When you're back down too target, pH will be back to where it was before, plus increase by outgassing in the meantime. The complete chlorination cycle is pH neutral.

Here is the complete chemistry (second half SWG);

thanks for the chemistry info, you never realize until later in life when those chem courses in high school, sometimes college become important, such as when having a pool. Meanwhile, at what pH would it be best to add CYA? does it make sense to add after titrating pH to within a specific range?

 

[JamesW]

Anode 4Cl^- -> 2Cl₂.

Cathode 4H₂O -> 2H₂ + 4OH^-.

2Cl₂ +2H₂O -> 3H⁺ + HOCl + OCl^-.

HOCl + OCl^- + uv light -> O₂ + H⁺ + 2Cl^-.

Following the process, we can see that there are 4H⁺ and 4OH^- created, which nets out to pH neutral.

The chlorine gas generated is very acidic and creates 3 hydrogen ions for every 4 hydroxide ions created.

As the hypochlorous acid is broken down by UV, 1 more hydrogen ion is created for a net neutral result.

Assuming that chlorine gain and loss are equal, there's no pH rise.

 

[Bperry]

Right now my CYA is low, which I did because I thought I could add to it after getting ideal levels. I havent achieved because of a lack of vigilence I believe, but also working on the many variables that make pool maintenance 'fun' or 'annoying.'

My observation, is that with relatively low CYA levels and TA around 50 when you turn on superchlorinate for 20 hours, and cover your pool, a chlorine level of close to 0 and pH < 7 becomes > or= to 10 & pH > 8. I think "neutrality" can be observed if your chlorinator does less chlorination and if a buffer like CYA is higher or perhaps those with more experience don't use superchlorinate unless the water is green.




I use both Taylor2006c also and I use Acquacheck 7 test strips from HACH. Overall, I would say the teststrips correlate well and work much faster. But I use both.

There’s no reason to ever use the superchlorinate feature if you just maintain the correct levels in the first place.

The test strips have a very wide range of “acceptable”. How do you know what the actual level is?

 

[kul]

Whelp. My head exploded after seeing chemistry equations. As someone who got a hard earned C (or was it a D?) in chemistry, I’d say uhm can’t we just use poolmath.

Edit: oh wait. This is the deep end. For the smarties. Please continue.

 

[mgtfp]

Meanwhile, at what pH would it be best to add CYA? does it make sense to add after titrating pH to within a specific range?

It's usually not too critical. As long as TA is not too low, pH will rise back up quickly after adding CYA.

 

[Orion7319]

If there was a way to truly eliminate pH rise, I’d give it a try if I could.

I think I figured that one out partially by accident. I always just run my TA at 60. I have a SWG that I run 24/7 and adjust it for how much chlorine I’m loosing a day, I also run it a bit hot. I don’t have any water features and keep the return in my deep end pointed down so it’s not doing much in the way of aeration (I quit worrying about my skimmer because it doesn’t work well enough to keep up and I have since bought a skimmer robot). I last added muriatic acid on May 9th to bring my PH down to 7.6 from 7.8. TA didn’t change, PH went back up to 7.8 six days later. Realized it’s silly to worry about trying to bring my PH down to 7.6 from 7.8. Quit adding acid to the pool. Over the course of a month TA remained at 60, PH slowly fell to 7.6 on its own. I’m assuming rain is bringing it down at this point.

 

[jark87]

I always just run my TA at 60.

Yes, I try to do the same and it helps. My CSI was a little out of range last year and I raised my TA, but raised it a little higher than I wanted to. I’ve been at 70 for months and the little MA I have to add to control pH hasn’t lowered it noticeably yet. I’ll get there eventually.