Become a TFP Supporter Pool Math Forum Rules Pool School
Results 1 to 20 of 20

Thread: SWG PH neutral?

  1. Back To Top    #1

    In the Industry

    Join Date
    Jul 2008
    Posts
    144

    SWG PH neutral?

    Split off of this topic. JasonLion

    dschlic1, if I have my calculations correct, here is the process involved in the creation of chlorine from the SWCG:
    4NaCl + 4H2O <> 4NaOH + 2Cl2 + 2H2 <> 2HOCl + 2NaOH + 2H2 + 2NaCl
    4Sodium chloride + 4water <> 4Sodium hydroxide + 2chlorine gas + 2hydrogen gas <> 2hypochlorous acid + 2sodium hydroxide + 2hydrogen gas + 2salt.

    You can also get some HCl (Hydrochloric acid) and OCl (the hypochlorite ion). As you'll notice, sodium hydroxide (aka lye or caustic soda) is produced in about the same amount as the hypochlorous acid. Sodium hydroxide is a very strong base, and, as such, it raises the pH.

    The plaster can cause the pH to rise for a year or more, although it should be decreasing in effect now.

    You should set out something to catch a sample of your rainwater so that you can test it and know what it is contributing to the water. You should also test your fill water to know its contribution.

    Since you need to add acid and cyanuric acid on a regular basis, I would recommend an off-line chlorinator. Whenever you need to add cyanuric, you could just turn off your SWCG and put a few Trichlor tabs in the feeder and use that for a while. This will decrease your pH and add cyanuric acid.

    I know that I will probably get some disagreement on using the Trichlor feeder, and that's fine. And, it is not the right idea for everyone. But, I think that it can be a good addition to a SWCG in some cases. Only you can determine if it would be beneficial to you. They are easy to install and use. Just make sure to put the feeder output tube past the cell. You would also have to be sure that it did not conflict with the acid pump.

  2. Back To Top    #2
    JasonLion's Avatar
    Join Date
    May 2007
    Location
    Silver Spring, MD
    Posts
    37,879

    Re: Sources of acid demand

    The SWG is almost exactly PH neutral once you take into account the consumption of the chlorine produced.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

  3. Back To Top    #3
    Guest

    Re: Sources of acid demand

    Jason is correct, that is only half the equation. You left out the reduction of the hypochlorous acid and hypochlorite ions back to chloride ions.

  4. Back To Top    #4

    Join Date
    Mar 2007
    Location
    San Rafael, CA USA
    Posts
    12,082

    Re: Sources of acid demand

    PoolOwnerNumber9,

    See this post that describes chemically why using a hypochlorite source of chlorine or having an SWG is pH neutral when accounting for chlorine usage (which is acidic). The net result from an SWG including the usage of chlorine is the following:

    When Chlorine is Generated by an SWG and Breaks Down In Sunlight:
    2H2O --> 2H2(g) + O2(g)
    Water --> Hydrogen Gas + Oxygen Gas

    When Chlorine is Generated by an SWG and Oxidizes Ammonia:
    2NH3 --> 3H2(g) + N2(g)
    Ammonia --> Hydrogen Gas + Nitrogen Gas

    The pH rise in SWG pools can come from two factors and we're still sorting out how much weight to give to each. Some or most of the pH rise is due to the outgassing of carbon dioxide which occurs faster at higher TA and lower pH. We know this is a significant factor since lowering the TA level helps reduce the rate of pH rise though technical calculations of bubble volume from the SWG are not sufficient to explain this by itself (though even in non-SWG pools, the pH will tend to rise since pools are intentionally over-carbonated so the SWG just contributes to an existing effect).

    Another factor may be the outgassing of undissolved chlorine gas. So pools with short distances between the SWG and the pool or where the returns aren't pointed downward *may* experience more of this effect, but this has yet to be proven. Theoretically, it doesn't take a lot of undissolved chlorine gas to have the pH rise. If 20% of the chlorine gas did not dissolve and instead outgassed, then in a typical SWG pool at 2 ppm FC per day this would contribute a 0.13 pH rise per week (in addition to whatever rise would occur from carbon dioxide outgassing).

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

  5. Back To Top    #5

    In the Industry

    Join Date
    Jul 2008
    Posts
    144

    Re: Sources of acid demand

    More notes about the chemistry.

    4NaCl + 4H2O <> 4NaOH + 2Cl2 + 2H2 <> 2HOCl + 2NaOH + 2H2 + 2NaCl
    4Sodium chloride + 4water <> 4Sodium hydroxide + 2chlorine gas + 2hydrogen gas <> 2hypochlorous acid + 2sodium hydroxide + 2hydrogen gas + 2salt.

    On the right side of the equation you are producing 2 Hypochlorous acid HOCL (aka Hydrogen hypochlorite). Although it is an acid, it is a weak acid; therefore it only partly offsets the 2 sodium hydroxide being produced.

    You can identify an acid by its dissociation components. pH is a measure of the activity of dissolved hydrogen ions. Due to the way that the mathematical formula for pH is written, it is an inverse relationship. Therefore, the higher the hydrogen ion concentration, the lower the pH. Hypochlorous acid dissociates into the hydrogen ion and the hypochlorite ion. Because it contributes hydrogen ions, it is an acid.

    When you add muriatic acid HCL (aka hydrogen chloride or hydrochloric acid) it dissociates into hydrogen ions and chloride ions. The hydrogen ions lower the pH. Here is why it neutralizes the sodium hydroxide. HCl + NaOH < > H2O + NaCl. As you can see the hydrochloric acid combines with the sodium hydroxide to produce water and salt.

    As the hydrogen gas is in the H2 form it does not contribute hydrogen ions to the water. Therefore, that hydrogen is leaving as gas. The loss of hydrogen contributes to the rise in pH.

    Therefore the overall net effect is that the pH rises. Most SWCG pools have pHs that tend to rise.

    Also sodium hypochlorite and calcium hypochlorite are both very basic and cause the pH to rise.


    When Chlorine is Generated by an SWG and Breaks Down In Sunlight:
    2H2O --> H2(g) + O2(g)
    Water --> Hydrogen Gas + Oxygen Gas
    I'm not sure I understand this part of your post. You mention chlorine but show water. Could you explain it more? Thanks.

  6. Back To Top    #6

    Join Date
    Mar 2007
    Location
    San Rafael, CA USA
    Posts
    12,082

    Re: Sources of acid demand

    Look at this post that I linked to in my post as it explains what goes on in more detail. I didn't copy that info and instead just referred to it and wrote out the NET equation.

    I use only 12.5% chlorinating liquid in my pool and the pH is stable. I have an opaque electric safety cover so my chlorine usage is only around 1 ppm FC per day (the pool is used most days of the week). If the chlorine did not get used up, then it's addition would cause the pH to rise by 0.5 in one week. Instead, I see a slow rise of about 0.1 in one month. This is because, as I said before and is described in the link, the usage of chlorine is acidic. The sources you are apparently quoting don't account for that and in fact hardly any do. I can actually measure the pH rise when I add a larger amount of chlorine and then drop when the chlorine level drops. For reasons I do not understand, the industry only looks at the first half of what goes on -- the chlorine addition -- and doesn't look at the second half -- the chlorine consumption -- in spite of the obvious fact that consumption must be occurring since the FC level does not continue to rise over time.

    The way I got to the net equation for chlorine addition and then breakdown from sunlight is simply the following where I double the quantities in the SWG chlorine production equation so it balances with the breakdown equation (plus I have the equation for water since the base from the SWG cancels with the acid from the chlorine breakdown). You'll notice that the first equation is the same as what you wrote except that the sodium isn't shown (since it's irrelevant).

    4H2O + 2Cl- --> 2HOCl + 2OH- + 2H2(g)
    2HOCl --> O2(g) + 2H+ + 2Cl-
    2H+ + 2OH- --> 2H2O
    ---------------------------------
    2H2O --> 2H2(g) + O2(g)

    (I missed a "2" in the hydrogen gas in my previous post and have now corrected that)

    Though you are absolutely correct that chlorine production in the SWG produces a weak acid (hypochlorous acid) with a strong base (hydroxyl ion) for a net pH rise, the breakdown of chlorine from sunlight is acidic where a weak acid (hypochlorous acid) produces a strong acid (hydrochloric acid). The net effect (as shown above) is pH neutral. Essentially, the SWG plus sunlight simply result in the hydrolysis of water producing oxygen and hydrogen gasses, though this is done (over time) via a chloride-to-chlorine-to-chloride intermediate. Please look at the link I gave and if you still have specific questions from that then post them.

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

  7. Back To Top    #7

    In the Industry

    Join Date
    Jul 2008
    Posts
    144

    Re: SWG PH neutral?

    Richard, this all gets quite complicated, and I'm sure that I don't understand it as well as you do. Here is my understanding (please correct as appropriate, thanks)

    The equation shows that H2 (molecular hydrogen gas) is produced. The H2 can be produced in two ways.

    1) H2O <> H2 + O. The O can then combine with a variety of other atoms or molecules, including H3O+ (hydronium ion) as shown here:

    O + H3O+ <> H2O + OH . This reduces the effective hydrogen ion concentration and increases the hydroxide ion concentration.

    2) H3O+<> H2 + OH. This also decreases the effective hydrogen ion concentration and increases the OH (hydroxide or hydroxyl ion) concentration

    The bottom line is that the hydrogen gas is lost and the reaction decreases the effective hydrogen ion concentration and increases the hydroxide ion concentration. That's the basic definition of a pH rise.

    The oxygen can also oxidize ammonia.
    The oxidation of ammonia is given by this equation
    4NH4+ + 2OH + 8O2 <> 6H3O+ + 4NO3-
    4 ammonium + 2 hydroxide + 8 Oxygen2 <> 6 hydronium + 4 nitrate ions.

    Oxygen can also form O2 (molecular oxygen gas) and O3 (Ozone).

    Chlorine gas has a very low pH; therefore any chlorine gas that does not dissolve will result in a higher pH than if it all dissolved.

    I suppose that if the Carbonate CO32− or the Bicarbonate HCO3 was converted into carbonic acid and then into CO2 (carbon dioxide) and it outgassed then it could raise the pH; but I'm not so sure that is actually happening. Is that the theory? Is the aeration causing that much carbon dioxide to outgas? How does aeration cause the carbonates or bicarbonates to become carbonic acid?

    NaOCl (Sodium hypochlorite) when poured into the pool will cause available hydrogen ions to replace the sodium creating HOCl (Hypochlorous acid). This will reduce the available hydrogen ions causing the pH to rise. When the HOCl dissociates it gives back the hydrogen ion thus lowering the pH back to where it was. So the net effect is neutral. However, the liquid chlorine also contains sodium hydroxide which causes some pH rise.

    The reason a lower pH gives more available HOCl than a higher pH is that there are more hydrogen ions available to combine with the hypochlorite ions.

    The sources you are apparently quoting don't account for that and in fact hardly any do.
    Note: What I posted was written mainly by me and is not a wholesale cut and paste of anyone else's work. It is based on my general understanding of what is happening.

  8. Back To Top    #8

    In the Industry

    Join Date
    Jul 2008
    Posts
    144

    Re: SWG PH neutral?

    OK, I think I have the carbon dioxide part figured out. The some of hydrogen is combining with the carbonates like this:

    H2 + CO3 <> H2CO3 <> CO2 + H2O

    to form carbonic acid, which is then outgassed as carbon dioxide. However, as this causes a net loss of carbonates, it causes an overall reduction of pH instead of a rise! The hydrogen that combines with the carbonates is saved by converting carbonate into carbonic acid, therefore the entire process ending with the outgassing of the carbon dioxide is actually causing the pH to drop, not rise!

    Since only part of the hydrogen is combining with the carbonates, there is still a net loss of hydrogen, and therefore a net rise in pH. The longer the hydrogen has to combine with carbonates, the lower the pH rise will be. That’s why shorter runs to the pool cause higher pH increases, less time for the hydrogen to be saved.

  9. Back To Top    #9
    Guest

    Re: SWG PH neutral?

    Perhaps this will help you
    viewtopic.php?f=67&t=4979

  10. Back To Top    #10

    In the Industry

    Join Date
    Jul 2008
    Posts
    144

    Re: SWG PH neutral?

    WaterBear, thanks, that link was very helpful. I guess I need to reevaluate the equations. It's all quite complicated, especially what's happening in the cell, and what exactly causes the pH rise. The electrolysis can create some counterintuitive reactions due to the energy being introduced into the equations. Three questions for anyone who knows, please:

    1) How much hydrogen gas is produced?
    2) What happens to it?
    3) What are the chemical equations related to the hydrogen gas?

  11. Back To Top    #11

    Join Date
    Mar 2007
    Location
    San Rafael, CA USA
    Posts
    12,082

    Re: SWG PH neutral?

    My comments in bold in your quoted post below.

    Quote Originally Posted by PoolOwnerNumber9
    Richard, this all gets quite complicated, and I'm sure that I don't understand it as well as you do. Here is my understanding (please correct as appropriate, thanks)

    Virtually everyone in this industry believes the same misconceptions that continue to get taught, at least initially. When I first read about these things, I initially believed them as well until I either read something countering it at The Pool Forum or figured something out that contradicted what was written or read about inconsistencies with what actually happens in pools (and spas). The important thing is to ask questions and try and work together to figure out a theory that is consistent with observation (and scientific principles) to the greatest extent possible.

    The equation shows that H2 (molecular hydrogen gas) is produced. The H2 can be produced in two ways.

    1) H2O <> H2 + O. The O can then combine with a variety of other atoms or molecules, including H3O+ (hydronium ion) as shown here:

    O + H3O+ <> H2O + OH . This reduces the effective hydrogen ion concentration and increases the hydroxide ion concentration.

    2) H3O+<> H2 + OH. This also decreases the effective hydrogen ion concentration and increases the OH (hydroxide or hydroxyl ion) concentration

    The above is not correct. There is no oxygen radical formed when hydrogen is produced. The reaction for the creation of hydrogen at the cathode (negative plate) is the following as I showed in the link I gave earlier:

    2H2O + 2e- --> H2(g) + 2OH-

    You are forgetting that in electrolysis which occurs in the SWG that there is a transfer of electrons. Now in practice you could have one transfer at a time producing unstable intermediates such as a hydrogen radical as follows:

    H2O + e- --> H• + OH-

    or more likely the following which is equivalent:

    H+ + e- --> H•

    or the following which represents how hydrogen ion is usually associated with water:

    H3O+ + e- --> H• + H2O

    and then two of these hydrogen radicals combine to produce hydrogen gas as follows:

    2H• --> H2(g)

    for the net result that I showed.


    The bottom line is that the hydrogen gas is lost and the reaction decreases the effective hydrogen ion concentration and increases the hydroxide ion concentration. That's the basic definition of a pH rise.

    The above is true but only accounts for the production of chlorine in the SWG. It does not account for what happens when that chlorine then gets used up by breakdown in sunlight or oxidation of ammonia. It also doesn't account for what is happening at the other plate since the production of chlorine gas that then dissolves in water is acidic, though the overall net effect is a pH rise from production in the SWG (but again, this ignores the consumption of chlorine that is acidic).

    The oxygen can also oxidize ammonia.
    The oxidation of ammonia is given by this equation
    4NH4+ + 2OH + 8O2 <> 6H3O+ + 4NO3-
    4 ammonium + 2 hydroxide + 8 Oxygen2 <> 6 hydronium + 4 nitrate ions.

    The above does not occur to any great extent. If it did, dilute ammonia solutions in water would decompose and this does not happen quickly.

    Oxygen can also form O2 (molecular oxygen gas) and O3 (Ozone).

    I'm not sure what you mean by the above when you say "oxygen" can form from oxygen gas or ozone. If you mean that oxygen in the atmosphere can become dissolved oxygen in the water, that is true, but again it doesn't react quickly with ammonia. Also, the amount of ozone is essentially negligible unless you have an ozonator.

    Chlorine gas has a very low pH; therefore any chlorine gas that does not dissolve will result in a higher pH than if it all dissolved.

    If what you mean by this is that any chlorine gas that escapes and does not dissolve in the water will result in a net pH rise, then yes that is true and is one of the two things I said can explain a rise in pH.

    I suppose that if the Carbonate CO32− or the Bicarbonate HCO3 was converted into carbonic acid and then into CO2 (carbon dioxide) and it outgassed then it could raise the pH; but I'm not so sure that is actually happening. Is that the theory? Is the aeration causing that much carbon dioxide to outgas? How does aeration cause the carbonates or bicarbonates to become carbonic acid?

    Yes, this isn't just theory, but proven in practice and seen in studies. Pools are intentionally over-carbonated. The equilibrium amount of TA in water at a pH of 7.5 exposed to normal amounts of carbon dioxide in the air would be about 9 ppm (with no CYA in the water). The reason the TA is higher in pools is to provide a pH buffer, but this makes the pools out-of-equilibrium with the air so carbon dioxide will outgas from the pool into the air. This does not happen quickly, but it does happen faster at higher TA levels and at lower pH (because both of these will increase the amount of dissolved carbon dioxide or carbonic acid relative to bicarbonate and carbonate) and it happens faster when there is better air/water mixing such as with aeration or churning of the water surface (this increases the kinetics of net carbon dioxide transfer from water to the air). In fact, you can read more about the procedure for lowering TA and how it works chemically here that depends on this very effect.

    Also, we have seen in pool after pool how a high TA level will lead to a much faster rise in pH (or slower drop, if using an acidic source of chlorine) than a low TA level.

    As for how carbon dioxide outgassing shifts the equilibrium, note the following equations relating these carbonate species:

    CO2(aq) <--> CO2(g) ..... going to the right is outgassing of carbon dioxide
    H2CO3 <--> CO2(aq) + H2O ..... going to the right is from carbonic acid to dissolved carbon dioxide
    HCO3- + H+ <--> H2CO3 ..... going to the right is from bicarbonate ion to carbonic acid
    CO32- + H+ <--> HCO3- ..... going to the right is from carbonate ion to bicarbonate ion

    As the first reaction goes from left to right for outgassing, the reactions below go from left to right as well since the concentration of the species on the left of one reaction is getting reduced so the reaction below it moves to the right to produce more to keep things in balance. You can see that going from left to right in the above reactions consumes hydrogen ions so that the pH rises. Technically, the TA doesn't change because the hydrogen ion counts negatively towards TA and the bicarbonate or carbonate are paired up with the hydrogen ion (i.e. both drop by the same amount for each equation).


    NaOCl (Sodium hypochlorite) when poured into the pool will cause available hydrogen ions to replace the sodium creating HOCl (Hypochlorous acid). This will reduce the available hydrogen ions causing the pH to rise. When the HOCl dissociates it gives back the hydrogen ion thus lowering the pH back to where it was. So the net effect is neutral. However, the liquid chlorine also contains sodium hydroxide which causes some pH rise.

    This is correct though at a pH near 7.5 only about half of the hypochlorite ions become hypochlorous acid. Hypochlorite sources of chlorine are essentially pH neutral except for the small amount of "extra lye". The pH rise in my pool at a rate of 0.1 per month is due to a combination of this "extra lye" which theoretically would have the pH rise by 0.1 every 2 months plus some amount of carbon dioxide outgassing. My use of a pool cover significantly lowers the outgassing compared to a pool without a pool cover.

    Note that you talk about HOCl dissociating to give back the hydrogen ion and thus lowering the pH, but this doesn't just happen with hypochlorite sources of chlorine, but ANY source of chlorine including that from an SWG.


    The reason a lower pH gives more available HOCl than a higher pH is that there are more hydrogen ions available to combine with the hypochlorite ions.

    Though this is technically true, when CYA is present it acts as an HOCl buffer so HOCl does not vary as much with pH as the traditional industry graphs show (while hypochlorite ion, OCl- actually varies more). This post shows the traditional industry graph which is false unless there is no CYA present and the true graph when CYA is present.

    The sources you are apparently quoting don't account for that and in fact hardly any do.
    Note: What I posted was written mainly by me and is not a wholesale cut and paste of anyone else's work. It is based on my general understanding of what is happening.

    OK.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

  12. Back To Top    #12

    In the Industry

    Join Date
    Jul 2008
    Posts
    144

    Re: SWG PH neutral?

    Richard, thank you for your assistance. Is the following correct?
    ____________________________________________________________ ________

    The reduction reaction for the creation of hydrogen at the negative cathode is:

    2H2O + 2e- --> H2(g) + 2OH-

    This can also be described as: 2H+(aq) + 2e- --> H2(g)
    ____________________________________________________________ ________

    The oxidation reactions at the positive anode are:

    2Cl- --> Cl2(g) + 2e-

    And

    (Oxidation) 4OH-(aq) --> O2(g) + 2 H2O(l) + 4e-

    Or 2H2O(l) --> O2(g) + 4H+ + 4e-

    ____________________________________________________________ ________

    The net effect of the Electrolysis of the water is shown here:

    2H2O(l) --> 2H2(g) + O2(g)

    ____________________________________________________________ ________

    So 2H2O + 2Cl- --> Cl2 + 2H2(g) + O2(g)

    ____________________________________________________________ ________

    The chlorine gas combines with water like this:

    Cl2 + H2O --> HOCl + H+ + Cl-

    Or this: Cl2 + OH- --> OCl + H+ + Cl-

    ____________________________________________________________ ________

    With the effective net of this: 3H2O + 2Cl- --> HOCL + H+ + Cl- + 2H2(g) + O2(g)

    ____________________________________________________________ ________

    But, what happens to the H2(g) and the O2(g)? Are they just lost as gas? Or, do they react with anything?

  13. Back To Top    #13

    Join Date
    Mar 2007
    Location
    San Rafael, CA USA
    Posts
    12,082

    Re: SWG PH neutral?

    My comments are in bold below.
    Quote Originally Posted by PoolOwnerNumber9
    Richard, thank you for your assistance. Is the following correct?
    ____________________________________________________________ ________
    The reduction reaction for the creation of hydrogen at the negative cathode is:
    2H2O + 2e- --> H2(g) + 2OH-
    This can also be described as: 2H+(aq) + 2e- --> H2(g)
    Yes, this is correct.
    ____________________________________________________________ ________
    The oxidation reactions at the positive anode are:
    2Cl- --> Cl2(g) + 2e-
    And
    (Oxidation) 4OH-(aq) --> O2(g) + 2 H2O(l) + 4e-
    Or 2H2O(l) --> O2(g) + 4H+ + 4e-
    The chlorine reaction occurs, but the hydrolysis of water to produce oxygen does not occur if you have sufficient salt in the pool. Well, I should say that it doesn't occur very much. The reason you have the salt in the pool is to provide enough conductivity to have efficient electrolysis and to make the chlorine reaction more predominant than the water to oxygen reaction. The materials used for the anode also help make the chlorine reaction more likely.
    ____________________________________________________________ ________
    The net effect of the Electrolysis of the water is shown here:
    2H2O(l) --> 2H2(g) + O2(g)
    This is correct for hydrolysis of water except that this isn't happening to any great extent in the SWG cell since chlorine is predominantly produced, not oxygen.
    ____________________________________________________________ ________
    So 2H2O + 2Cl- --> Cl2 + 2H2(g) + O2(g)
    Because the production of chlorine is predominant, the net reaction in the SWG is really the following:
    2H2O + 2e- --> H2(g) + 2OH-
    2Cl- --> Cl2(g) + 2e-
    --------------------------------------------
    2H2O + 2Cl- --> H2(g) + 2OH- + Cl2(g)

    ____________________________________________________________ ________
    The chlorine gas combines with water like this:
    Cl2 + H2O --> HOCl + H+ + Cl-
    Or this: Cl2 + OH- --> OCl- + H+ + Cl-
    This is correct except I added a negative charge to the hypochlorite ion above
    ____________________________________________________________ ________
    With the effective net of this: 3H2O + 2Cl- --> HOCL + H+ + Cl- + 2H2(g) + O2(g)
    This is incorrect because the chlorine reaction is predominant and little oxygen is produced in the SWG so the real net reaction is as follows (and you normally remove extra species that show up on both sides of a chemical equation):
    2H2O + 2Cl- --> H2(g) + 2OH- + Cl2(g)
    Cl2 + H2O --> HOCl + H+ + Cl-
    H+ + OH- --> H2O
    ---------------------------------------
    2H2O + Cl- --> H2(g) + OH- + HOCl
    or equivalently
    H2O + Cl- --> H2(g) + OCl-

    ____________________________________________________________ ________
    But, what happens to the H2(g) and the O2(g)? Are they just lost as gas? Or, do they react with anything?
    The H2(g) is outgassed from the pool and in fact the buildup of some of it that dissolves in water affects ORP measurements. The O2(g) as I said above is not produced very much since chlorine is produced instead, but any that is produced would also outgas though could somewhat over-saturate the water with oxygen until it ultimately outgassed.

    Notice that the last reaction above is basically the same as adding a hypochlorite source of chlorine to the water (ignoring the hydrogen gas that just outgasses). This is why I say that the production of chlorine in an SWG is essentially like adding bleach in terms of pH. Also remember that the above is the production of chlorine and not its consumption later on by sunlight nor oxidation of ammonia or organics.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

  14. Back To Top    #14

    In the Industry

    Join Date
    Jul 2008
    Posts
    144

    Re: SWG PH neutral?

    OK, thank you, Richard. This seems to be getting clearer and more understandable. Is the following correct?

    The chloride anion has a lower standard electrode potential than the hydroxide anion which causes the chloride to be oxidized preferentially over the hydroxide. The OH- (hydroxide) remains primarily hydroxide instead of being oxidized into oxygen.

    When the chloride ion gives up an electron (oxidation) it becomes a neutrally charged chlorine atom and forms a covalent bond with another chlorine atom to form CL2(g). Covalent bonding is different than ionic bonding. Covalent means that the atoms are bonded by sharing electrons.

    In this reaction (Cl2 + H2O --> HOCl + H+ + Cl-) one chlorine atom is gaining an electron and one is losing an electron, so that one becomes a positively charged cation and one becomes a negatively charged anion.

    The hydrogen ion has a higher standard electrode potential than the sodium ion; therefore it accepts the electrons (it is reduced) instead of the sodium, and the sodium remains an ion, which combines with the hydroxide to become sodium hydroxide.

  15. Back To Top    #15
    Guest

    Re: SWG PH neutral?

    Quote Originally Posted by PoolOwnerNumber9
    and the sodium remains an ion, which combines with the hydroxide to become sodium hydroxide.
    In solution sodium hydroxide (or any salt) does not exit except as the ionic species, in this case Na+ and OH-

  16. Back To Top    #16

    Join Date
    Mar 2007
    Location
    San Rafael, CA USA
    Posts
    12,082

    Re: SWG PH neutral?

    My responses in bold below.

    Quote Originally Posted by PoolOwnerNumber9
    OK, thank you, Richard. This seems to be getting clearer and more understandable. Is the following correct?

    The chloride anion has a lower standard electrode potential than the hydroxide anion which causes the chloride to be oxidized preferentially over the hydroxide. The OH- (hydroxide) remains primarily hydroxide instead of being oxidized into oxygen.

    Actually, it's more complicated than that. If one looks just at the standard potentials as follows:

    2Cl- --> Cl2(g) + 2e- .......... Eo = -1.35827V
    2H2O --> O2 + 4H+ + 4e- .......... Eo = -1.229V

    and even adjusting these equations for actual concentrations and pH, it appears that it takes more voltage to produce chlorine gas so oxygen gas should be preferentially generated. What the above equations do not show is the overvoltage that is necessary and this makes the production of chlorine gas more likely at the chloride concentrations in SWG pools. The overvoltage is mostly from the activation energy of the reaction so determines the kinetics. The production of oxygen may be more thermodynamically favored, but the production of chlorine is favored kinetically and that's what really matters in this case.


    When the chloride ion gives up an electron (oxidation) it becomes a neutrally charged chlorine atom and forms a covalent bond with another chlorine atom to form CL2(g). Covalent bonding is different than ionic bonding. Covalent means that the atoms are bonded by sharing electrons.

    In this reaction (Cl2 + H2O --> HOCl + H+ + Cl-) one chlorine atom is gaining an electron and one is losing an electron, so that one becomes a positively charged cation and one becomes a negatively charged anion.

    True. The neutrally charged chlorine atom is sometimes called a free radical and the reaction is similar to what I showed earlier with hydrogen (so it would be 2Cl• --> Cl2). For practical purposes, both chlorine gas and hydrogen gas are produced from two electrons and the fact that this occurs in two steps isn't important.

    The hydrogen ion has a higher standard electrode potential than the sodium ion; therefore it accepts the electrons (it is reduced) instead of the sodium, and the sodium remains an ion, which combines with the hydroxide to become sodium hydroxide.

    True except for the last part that waterbear already corrected. Sodium remains as an ion in water.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

  17. Back To Top    #17

    In the Industry

    Join Date
    Jul 2008
    Posts
    144

    Re: SWG PH neutral?

    In the HOCl (hypochlorous acid or hydrogen hypochlorite) molecule the Hydrogen ion has a plus 1 charge, the Oxygen ion has a negative 2 charge, and the Chloride ion has a plus one charge. Since the chloride ion usually has a negative 1 charge, it has the power to oxidize (Each ion can accept 2e- electrons).

    HOCl can oxidize by this equation here:

    HOCl + 2e- --> OH- + Cl- (This process would be basic unless the molecule being oxidized contained a lot of hydrogen like NH3 ammonia where the nitrogen would be oxidized and the hydrogen ions would be liberated) as shown here:

    2NH3 + 3HOCl --> N2(g) + 6H+ + 3OH- + 3Cl- = N2(g) + 3H2O + 3H+ +3Cl-

    Chlorine gas can also oxidize ammonia directly as shown here:

    3Cl2 + 2NH3 --> N2(g) + 6H+ + 6Cl-

    Although, this does not happen much, if at all, due to how fast the chlorine gas dissolves in the water.

    So, what are the most common molecules that the HOCl (hypochlorous acid) has to oxidize besides ammonia?

    If the other molecules do not contain a lot of hydrogen, then the oxidation process would be basic. I assume that most compounds that will be oxidized will contain some hydrogen.

  18. Back To Top    #18

    Join Date
    Mar 2007
    Location
    San Rafael, CA USA
    Posts
    12,082

    Re: SWG PH neutral?

    Quote Originally Posted by PoolOwnerNumber9
    So, what are the most common molecules that the HOCl (hypochlorous acid) has to oxidize besides ammonia?

    If the other molecules do not contain a lot of hydrogen, then the oxidation process would be basic. I assume that most compounds that will be oxidized will contain some hydrogen.
    What you wrote in your post above what I quoted was correct.

    As for your question, the greatest loss of chlorine in outdoor residential pools is breakdown from the UV in sunlight which produces oxygen as follows:

    2HOCl ---> O2(g) + 2H+ + 2Cl-
    2OCl- ---> O2(g) + 2Cl-

    with the latter reaction of breakdown of hypochlorite occurring over 6 times faster than that of hypochlorous acid which makes this very pH dependent since the concentration of hypochlorite ion varies a lot with pH (much higher in concentration at higher pH). CYA is a hypochlorous acid buffer so hypochlorous acid concentration doesn't vary as much with pH (which is why hypochlorite ion varies more since they are in equilibrium).

    When there is some bather load, the next greatest loss of chlorine is in oxidizing the ammonia and urea that is in sweat and urine. This WHO document on document page 62 (PDF page 85) in Table 4.1 shows how sweat and urine contain a lot more urea than ammonia. Urea is also oxidized by chlorine though it apparently takes longer than the breakpoint reaction with ammonia. Though the reaction of chlorine with ammonia is well understood with the Jafvert & Valentine (1992) model, the oxidation of urea by chlorine is not well understood. As noted in this article, Chip Blatchley of Purdue has a grant from NSPF to look at disinfection by-products in pools and part of this work is to develop a model for the chlorine oxidation of urea. I am very much looking forward to the results of that work.

    Chlorine will also combine with other compounds, some of which it further oxidizes and some of which stay as Combined Chlorine for longer periods of time. There are many types of reactions of chlorine with organic compounds. One is not an oxidation reaction, but a substitution reaction of a chlorine with a hydrogen that I can designate generically as follows (where "R" is the rest of the organic molecule) which forms a Combined Chlorine (CC):

    .... R .............................. R
    .... | ............................... |
    R-N-H + HOCl ---> R-N-Cl + H2O

    I show a nitrogen above since chlorine will not substitute with a hydrogen attached to a fully saturated carbon in large quantities (though byproducts such as chloroform can form from more complex derivatives and even slowly from methane itself though the mechanism in pools is from less volatile compounds). Another possible reaction is chlorine opening up a double bond to attach as follows:

    R-C=C-R + HOCl ---> R-C(-OH)-C(-Cl)-R

    and formation of an epoxide may also occur as follows where the oxygen is connected to the two carbons (I can't draw this here, but you can see examples of epoxides here) and in this case this is a reaction of chlorine oxidizing the organic.

    R-C=C-R + HOCl ---> R-C-(-O-)-C-R + H+ + Cl-

    You are correct in saying that the oxidation of organics releases hydrogen so is an acidic process. This is almost always the case as organics typically do have many hydrogen attached to carbon. With a complete oxidation, carbon dioxide is produced from carbon in organics while nitrogen gas is produced from nitrogen in organics (or in ammonia) and any hydrogens in the organic become hydrogen ions or water (depending on what happens with the oxygen in chlorine). Though the oxidation of methane is not typical in pools, it is instructive as shown below:

    CH4 + 4HOCl ---> CO2 + 2H2O + 4H+ + 4Cl-

    Each HOCl accepts 2 electrons so going from methane to carbon dioxide requires giving up 8 electrons. The Carbon is usually considered to have an oxidation state of +4 so having hydrogen attached has each of them be -1 so essentially each one gives up 2 electrons which are taken up by the chlorine to form chloride ion.

    The reality is that complete oxidation of most organics does not occur in pools. Saturated hydrocarbons (all single bonds between carbons) do not generally get oxidized, for example. So instead what happens is that the substitution reactions or the creation of epoxides or addition of oxygen in other ways creates molecules that are more polar (have separated charges) and these then become more soluble in water. So while some organic compounds do break down into smaller pieces, whatever remains is mostly made more soluble. Pool water builds up such soluble products while any that are volatile will outgas, but except for simpler compounds such as ammonia and to some extent urea, most organics do not fully oxidize to carbon dioxide [EDIT] unless helped by enzymes. [END-EDIT]

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

  19. Back To Top    #19

    Join Date
    Apr 2007
    Location
    Wyoming, Michigan
    Posts
    93

    Re: SWG PH neutral?

    Quote Originally Posted by chem geek
    Pool water builds up such soluble products while any that are volatile will outgas, but except for simpler compounds such as ammonia and to some extent urea, most organics do not fully oxidize to carbon dioxide.
    A little off topic, but is this a problem? Will they get filtered out, or should one consider water replacement every so often?

  20. Back To Top    #20

    Join Date
    Mar 2007
    Location
    San Rafael, CA USA
    Posts
    12,082

    Re: SWG PH neutral?

    The incomplete oxidation of organics is not usually a problem in residential pools. For those organics that remain insoluble (such as fully saturated oils), they will either form a film on the surface that can be removed by scum balls or similar skimming techniques or if only partially soluble they will be suspended in the water and can potentially be coagulated with a clarifier and caught in the filter or caught in a flocculant to drop to the floor and then get vacuumed to waste.

    For those organics that become or remain soluble after oxidation, they stay soluble and flow right through the filter, but don't cause any problems (they are similar to salts in that sense) unless the water becomes so saturated with them that the water starts to get cloudy. This latter situation is rare unless bather loads are frequently high and in that case only water replacement can solve it [EDIT] OR enzymes can be used to more fully oxidize such compounds. [END-EDIT] This is one reason why public/commercial pools often replace their water at a rate of 7 gallons per bather (it should be a unit more like bather-hour, but you get the idea).

    Most of what gets put into the pool either gets oxidized completely, such as ammonia and most likely urea, or produces end-products that are completely soluble or are volatile and outgas such as most skin oils, amino acids, suntan lotions, and general organic matter (leaves, etc.). Of course, larger objects take a long time to oxidize and dissolve so generally get caught in a pool cleaner or skimmer basket. [EDIT] The insoluble oils that get pulled into the circulation system can get caught in the filter if they consolidate or are otherwise more attracted to the filter material (and to each other) than to water. When I clean my filter I presume that's mostly what the somewhat gooey gunk is. [END-EDIT]

    [EDIT] Also, as PoolOwnerNumber9 noted here, enzyme products can accelerate the oxidation of organics such as those in the first case I noted above that would normally not get handled (quickly enough) by chlorine alone. I do not have direct experience with these products and have not read enough cases of their use to know which sorts of oils they would breakdown more quickly and which would still remain unoxidized. Note that enzymes are catalysts so accelerate specific chemical reactions such as the oxidation of organics by chlorine and dissolved oxygen -- they do not perform the oxidation by themselves. This means that there may be an increase in chlorine demand until the organics are completely oxidized. The enzymes themselves will slowly break down by oxidation from chlorine (they are, after all, organics themselves) which is why one does not use enzymes when the active chlorine level is high such as shock levels. [END-EDIT]

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •