pH Rise in SWG Pools

[chem geek]

I was involved in an unfortunately not-so-nice exchange in this thread, but the net result is that I calculated the volume of hydrogen gas bubble production in an SWG and found that even under best-case conditions with perfect kinetics, there simply isn't enough volume of gas generated to remove carbon dioxide (at equilibrium quantities using Henry's Law) at a rate to explain the significant pH rise in SWG pools. Though lowering the TA will reduce the effect of pH rise from carbon dioxide outgassing, in many SWG pools this doesn't help enough and my theory of hydrogen gas bubbles increasing aeration cannot be the primary source of pH rise.

So I looked for other reasons and the most likely candidate is that of chlorine gas outgassing since that is also produced in the SWG and if not fully dissolved into the water it can outgas (once it becomes aqueous chlorine then this combines with water to form hypochlorous acid very quickly and does not outgas very much). This means that there may be other methods that can help slow down the rate of pH rise in SWG pools. For example, pointing the returns downwards may help make the chlorine gas bubbles spend more time in the water and therefore dissolve more completely so less gets outgassed. Using a slower pump speed can help the chlorine spend more time in the pipe, at least in larger pipe and longer pipe runs.

If anyone has an SWG with a significant pH rise (so frequent acid addition) and can experiment with turning down their returns to see if it makes any difference, that would be great. If one has a 2-speed pump and can see if there is any difference running the SWG with the pump at low-speed vs. high-speed, that would also be good.

I copy below some of the relevant computations from that thread just so it's here at TFP.

OUTGASSING VOLUME IN HYDROGEN BUBBLES

As for the volume of hydrogen gas bubble production, a typical SWG for a moderate pool at 20 grams chlorine per hour is ( 20 / (70.9064 g/mole Cl₂) ) = 0.28 moles per hour (of both chlorine and hydrogen gas). Using PV=nRT, Volume = (0.28 moles) * (0.082 l-atm/mole-K) * (298 K) / (1 atm) = 6.8 liters of gas per hour. The normal volume of air that we breathe out is about 0.5 liters so the volume of hydrogen gas that is produced is like 14 breaths per hour (about one every 4 minutes). Though that is not a lot of volume, the bubbles are very small (high surface area to volume ratio) so we can calculate a best-case scenario where carbon dioxide fills the bubble to achieve equilibrium. Henry's Law constant for carbon dioxide is [H₂CO₃]/pCO₂(g) = 0.034 (moles/liter)/atm. At 100 ppm TA (with 30 ppm CYA so 90 ppm carbonate alkalinity and a pH of 7.5) [H₂CO₃] = 1x10^-4 so pCO₂(g) = 1x10^-4/0.034 = 0.0029 or 0.3% of the volume. So that's a removal of carbon dioxide at a rate of 0.28 * 0.3% = 0.00084 moles per hour. A 15,000 gallon pool is about 57,000 liters so that's 1.5x10^-8 moles/liter per hour. So that this isn't very much, but a drop in total carbonates of 2% results in a pH rise of 0.12 so that would take 0.02*1x10^-4 / 1.5x10^-8 = 133 hours of on-time. Nevertheless, we see a direct relationship between the SWG on-time and pH rise so there could be side reactions that produce excess hydroxyl ion (though I am not aware of any that would -- production of oxygen gas would be produce the same quantity of hydrogen ion as its removal at the other plate producing hydrogen gas). Also, the bubbles breaking the surface of the water could influence the rate of surface transfer beyond direct transport in the bubbles themselves. The most likely reason is chlorine outgassing I discuss next.

CHLORINE OUTGASSING

Assuming stable long-term FC so that chlorine consumption plus chlorine gas outgassing equals SWG generation of chlorine gas, then over one week of 2 ppm FC per day generation/consumption/loss (14 ppm FC),

0% chlorine gas outgassing ==> no change in pH
10% chlorine gas outgassing ==> +0.12 rise in pH
20% chlorine gas outgassing ==> +0.28 rise in pH
30% chlorine gas outgassing ==> +0.46 rise in pH

So clearly outgassing of undissolved chlorine gas could explain the bulk of the pH rise in SWG pools (with a smaller portion explained by carbon dioxide outgassing for which lowering the TA helps).

CHLORINE CONVERSION TO HYPOCHLOROUS ACID

Cl₂(aq) + H₂O(l) <---> HOCl(aq) + H⁺(aq) + Cl^-(aq)

The equilibrium constant for the above reaction is K = 10^-3.3 = [HOCl][H⁺][Cl^-] / [Cl₂]. This source gives the equilibrium constant as 4.2x10^-4 whose -log₁₀ is 3.4, close to what I used (my number was adjusted for activities based on ionic strength).

Ignoring the effects of Cyanuric Acid (CYA) which significantly reduce HOCl concentration, 3 ppm FC (which by convention is measured as ppm Cl₂) at pH 7.5 is equivalent to [HOCl] = 2x10^-5 moles/liter (half of the FC is HOCl at pH 7.5 when there is no CYA). A pH of 7.5 is equivalent to [H⁺] = 3.5x10^-8 and a salt pool with 3000 ppm salt (ppm NaCl) is equivalent to [Cl^-] = 5x10^-2. So let's calculate the equilibrium concentration of aqueous chlorine in this case:

[Cl₂] = [HOCl][H⁺][Cl^-] / 10^-3.3 = 2x10^-5 * 3.5x10^-8 * 5x10^-2 / 10^-3.3 = 7x10^-11 moles/liter

The bottom line is that there is very little chlorine (gas in water or aqueous) left -- most of it becomes hypochlorous acid. It's only in more acidic water that chlorine gas doesn't dissolve as readily since the generation of chlorine and the above dissolving reaction makes the water in that half-cell acidic -- but in pools the water is buffered so far more chlorine can dissolve in the water and becomes hypochlorous acid as shown above. Therefore, because this reaction goes essentially to completion, it cannot be ignored yet that is what virtually everyone does when quoting the net result from the SWG. The rate equation for aqueous chlorine to hypochlorous acid is Rate = 28.6*[Cl₂] while the reverse rate is 28000*[H+][Cl-][HOCl]. Using the above numbers, the reverse rate is 9.8x10^-10 M/sec. The forward rate of aqueous chlorine to hypochorous acid has a half-life (conversion time for half of Cl₂) of 0.024 seconds so isn't slow (i.e. 99% of the produced chlorine should be converted to hypochlorous acid in less than 0.2 seconds). I just noticed that the ratio of rate constants is 1x10^-3 which is a little (factor of 2) inconsistent with the equilibrium constant of 4.2x10^-4 so I'm tracking down the source for the rate constants to see why there is this discrepancy. [EDIT] This link gives the forward rate constant as 20.9 sec^-1. [END-EDIT] This link gives a value for the forward rate constant of 15.4 s^-1 so still a 99% conversion in about 0.4 seconds (but the source describes the reaction as slow) while this link reports a rapid hydrolysis where the equivalent rate constant is 3.4x10¹⁴ x 10^(7.5-14) = 1x10⁸. This link gives an equivalent rate constant of 8x10⁸ x 10^(7.5-14) = 250 so more in line with the others. Don't you hate it when sources are so terribly inconsistent? These rates are only for aqueous chlorine to hypochlorous acid, but not for gaseous chlorine to aqueous chlorine. So the rate-limiting step could be diffusion and some of the chlorine gas that is generated may outgas from the pool and not get into the water which would of course account for extra pH rise.

Richard

 

[JasonLion]

I don't know if this applies or not. An engineer at AutoPilot told me that my pool was dissolving nearly all of the hydrogen gas into the water, much more than most pools. My return is lower than average, the flow rate through the return eyeball is higher than average, and the jet is pointed down at about 45 degrees below horizontal. The bubbles in the return move down with the return jet for several feet before becoming lost to view. No bubbles are visible on the surface.

Various tests with the ORP sensor appear to indicate that hydrogen is dissolving in the pool water when the SWG is on and then takes 6 to 24 hours to outgas with the SWG off. The engineer at AutoPilot told me that they had duplicated this result with tanks of hydrogen gas and no SWG involved. (Dissolved hydrogen gas lowers ORP readings substantially.)

My SWG still raises PH at a rate that clearly depends on the TA level. I have been allowing the automatic acid feed to slowly lower TA, and acid use is down significantly now that TA has gone down from 120 to 60 (which took many months over two seasons). Some acid demand remains, but it is much slower than before.

 

[chem geek]

Thanks Jason. I guess I should be clear that there are TWO factors and I'm not saying that carbon dioxide outgassing doesn't still occur. It can still be lessened at a lower TA. It's also still possible that the hydrogen gas bubbles may indirectly increase outgassing of carbon dioxide near the surface, just by physical disruption as the bubbles pop (though not in your case, it sounds like). I wouldn't change my advice about lowering the TA as a method for lowering the rate of pH rise. It's just that it doesn't seem to explain all of the rise since manually dosed pools at a TA of 40 show virtually no pH rise while yours is still showing some though with no visible bubbles escaping we're left wanting for an explanation (perhaps it's still some carbon dioxide outgassing).

So the thing to compare would be your returns pointing downward vs. pointing straight out with the latter having some bubbles breaking through the surface -- all at the same TA level. If the TA rise is higher with the latter, then that could be chlorine gas or just more physical disruption of the surface. That's really strange about the hydrogen gas dissolving since it's not very soluble in water. Of course, there's a little hydrogen and a lot of water that perhaps is not saturated with hydrogen as there isn't much in air. The Henry's Law constant for hydrogen gas is 7.8x10^-4 M/atm while for carbon dioxide it's 3.4x10^-2 M/atm. So for saturation of water such that bubbles wouldn't form and assuming roughly 1 atm for pressure in the pool water I can see that indeed hydrogen gas of reasonable quantity can dissolve in the water. Pools aren't anywhere near the carbon dioxide amount for saturation (they aren't as carbonated as beverages) even though they are over-saturated with respect to air. Interesting...

 

[tphaggerty]

Richard, I read through the exchange. You are to be commended for your civility! I'm not a 100% sure what TxPool is against that makes him so rude - bleach use or SWG use or what?

Anyway, would a 2-speed pump (or multi-speed) enter into this at all based on your theory of CL absorption? From the bits I could gather, you are suggesting that pools in which MORE of the CL gas is absorbed would have LESS rise in PH (since the PH rise is driven by CL gas escaping --not exactly, I know, the PH rise is actually because the escaping CL gas doesn't react in the water and therefore doesn't help drive the PH down)? Is that a correct reading? If so, it would seem that those of us that use low speed most of the time would have less PH rise because the cell is still producing the same amount of CL gas, but it spends more time in the pipes before hitting the pool. OR, maybe NOT - maybe it balances because there is less water for the same amount of CL gas so there is less immediate absorption.

 

[JasonLion]

you are suggesting that pools in which MORE of the CL gas is absorbed would have LESS rise in PH (since the PH rise is driven by CL gas escaping --not exactly, I know, the PH rise is actually because the escaping CL gas doesn't react in the water and therefore doesn't help drive the PH down)?

Both chlorine and hydrogen gas bubbles that escape from the pool provide additional surface area over which CO2 can escape from the pool. CO2 is at concentrations in the water well above equilibrium, but the rate of outgassing is slow. The additional surface of the bubbles, combined with the fact that the initial concentration of CO2 in the bubble is zero, raises the outgassing rate noticeably. The loss of CO2 raises the PH.

(EDIT)The PH changes connected with the chlorine gas nets out to zero if it reacts with the water, but if it doesn't react that changes things.(/EDIT)

At least that is the model we have been working with. The problem is that the rates don't seem to come out quite right. The math is tricky and depends on assumptions about bubble sizes and how much of the chlorine gas reacts with water and how much escapes.

 

[chem geek]

The thing is that even if I assumed perfect kinetic rates to achieve equilibrium between the carbon dioxide in the water going into the hydrogen gas bubbles (up to what Henry's Law constant predicts for equilibrium), it doesn't give enough of a pH rise. The effect may be real, but it's too darn small. I never bothered to calculate it until TxPool brought it up. Essentially, it takes over 100 hours of SWG on-time to have the pH rise by 0.1 to 0.2 units so the effect of the bubbles just doesn't explain the much more rapid rise that is seen in reality. So that theory of mine just isn't right -- at least not the diffusion part of it -- the physical disruption of the water surface might still be a factor though the area with such bubbles is pretty darn small.

I don't want to confuse anyone here. Aeration absolutely works to outgas carbon dioxide. But it takes a significant volume of gas to make it work and the SWG simply doesn't produce very much volume.

tphaggerty, you are correct that my new theory that I'd like to experiment with is that the longer the chlorine gas stays in the water, the more it will dissolve and the lower the pH rise. If chlorine gas doesn't dissolve and instead escapes from the pool (or just hangs out undissolved in the pool, though that would be strange over time) then that will most certainly have the pH rise. The reason isn't the outgassing per se the way it is with carbon dioxide. The reason is that the SWG does make the pH rise upon creation/addition of chlorine to the water, just as adding bleach or chlorinating liquid to a pool makes the pH rise. The difference is what happens to the chlorine that gets into the water. If the chlorine gas becomes hypochlorous acid, then this is an acidic process, and if that hypochlorous acid breaks down in sunlight or oxidizes something, then that is an acidic process. These acidic processes exactly compensate for the initial rise in pH from the chlorine addition (ignoring the "extra lye" in bleach and chlorinating liquid). If the chlorine gas doesn't become hypochlorous acid, then those last acidic steps don't occur so the net result is a pH rise.

In an SWG, the following occurs:

2Cl^- ---> Cl₂(g) + 2e^- ....... at the anode
2H₂O + 2e^- ---> H₂(g) + 2OH^- ...... at the cathode
-----------------------------------------------------------
2Cl^- + 2H₂O ---> Cl₂(g) + H₂(g) + 2OH^-

So if nothing else happened but the above, then the pH would rise substantially due to two hydroxyl ion. What mostly happens, however, is that the chlorine gas dissolves in water as follows:

Cl₂(g) + H₂O ---> HOCl + H⁺ + Cl^-

so combining these two equations yields

Cl^- + 2H₂O --> HOCl + H₂(g) + OH^-

which also has the pH rise, but not as much. The HOCl then breaks down in sunlight or oxidizing an organic which is acidic and eliminates that last hydroxyl in the equation above, but you can see that if the chlorine gas doesn't dissolve in water, then the two acidic steps don't occur so the initial rise in pH from the SWG persists. I've calculated this effect and if just 10-20% of the generated chlorine gas does not dissolve in the water, then that would lead to a substantial pH rise of 0.1 to 0.3 per week assuming 2 ppm FC per day chlorine generation/usage and remember that this is incremental to the pH rise from the outgassing of carbon dioxide that we already know occurs.

As for pump speed, there are two effects and I don't know which one dominates. At a slower pump speed, the water spends more time in the pipe so the chlorine has a longer time to dissolve. You are right that the chlorine gas is at higher concentration in the water with the lower pump speed, but if anything that should just cause it to dissolve faster (assuming the bubble sizes aren't different). When the water comes out of the return, however, the slower pump speed won't push the bubbles as far out so even if the returns are pointed downwards the bubbles may not linger as long in the bulk pool water. I've assumed that the slower speed would be better, but that's something that needs to be determined by experiment, along with pointing the returns downwards.

Richard

 

[dschlic1]

I have a two speed pump and a SWCG. I run the pump at high speed for two hours in the morning and low speed for 7 hours in the afternoon. I have noticed that at low speed I get bubbles in the return. However I have never seen bubles in the return when at high speed. I think that the higher flow rate and larger amount of water lowers the gaseous concentration and allows more of the gasses to dissolve.

On a further notice my pool has a very high acid demand. I use about 32 oz of acid per week. On the other hand it does not appear that the acid demand varies with the setting of the SWCG! While shocking my pool I routinely run the SWCG at 20 to 30% to maintain the FC level. This is with a 24 hour filter run. I am currently run the SWCG at 8% with a 9 hour filter run. No noticable change in acid demand.

 

[chem geek]

What's your Total Alkalinity (TA) level? Do you have any water aeration features such as waterfalls, spillovers, fountains, etc.? In your case, it sounds like the pH rise is mostly from traditional carbon dioxide outgassing. I assume you've had the SWG off for a long enough period of time to see a pH rise and compare that to when it is in use. I don't mean looking at the pH rise over a few hours of it in use since you couldn't tell, but comparing one week with no SWG (presumably manually dosing chlorine) and one week with SWG.

 

[DaveNJ]

Don't know if this helps. High speed 1 hour in morining and evening, low speed for 8 hours. On low speed SWCG on, 1/4 bubbles at first return. High speed no bubbles, but I can tell when its on. Two returns point toward the surface and 2 point down. Spa spills over all day.
FC 3
ph 7.6
TA 90
CH 130
CYA 70
Salt 3100

Since lowering the TA from 130 to 90 and PH 7.6 from 7.4, I use less acid this year. Last year 16oz a week to about 10oz every week and a half.

 

[dschlic1]

My TA was below 60, am now bringing it up some. You are probably correct in that my pH rise is due to CO2 outgassing. However I have tried everything I know to reduce it but the acid demand remains the same.