which raises pH more... Spillover or chlorinator? WHY?

[CRUSH-IT!]

I know that both raise the pH.

Spillover. This aerates the water creating a large surface area for gas exchange which facilitates the off-gassing of CO2 shifting the equilibrium of carbonate -carbonic acid - CO2 Causing the pH to rise and lowering TA.... right?

SWG electrolytically concentrates the chloride to form bubbles of gaseous Cl2 which dissolves in the water thus chlorinating it. How does it lower pH? Do the chlorine gas bubbles create a large surface area for gas exchange leading to off gassing of CO2 similar to above? or is it another mechanism?

As a practical matter, I would like to spend less time chasing my pH. I add muriatic one to two times a week currently and my salt cell is nonfunctioning. When I replace it I am concerned this may get even more burdensome. The real question for me is if I reduce the amount of time i run the spilover will it really make a difference or is the pH issue 90% chlorinator so the spillover is inconsequential. We have a pretty good spillover from the spa... almost 2' wide.

Thanks in advance for your insights.

Chris

 

[mknauss]

Spillover, by a very large margin.

 

[CRUSH-IT!]

Ouch. Well that is very helpful. I thought it might be just the opposite. I can run the spillover less easy enough.

Thanks

 

[JamesW]

Causing the pH to rise and lowering TA

Offgassing CO2 raises the pH but it has no effect on the TA.

The SWG is mostly pH neutral.

 

[CRUSH-IT!]

If it does not effect TA then where does the C come from?

 

[JamesW]

If it does not effect TA then where does the C come from?

I don't understand the question.

 

[CRUSH-IT!]

The Carbon in the Carbon dioxide is coming from the Carbonate. Each CO2 that goes to the atmosphere removes one Carbonate ion

 

[JamesW]

HCO3 --> OH + CO2

Bicarbonate becomes hydroxide and carbon dioxide. You lose one molecule of bicarbonate but you gain one molecule of hydroxide.

One molecule of hydroxide is equivalent to one molecule of bicarbonate as far as TA is concerned.

 

[JoyfulNoise]

The Carbon in the Carbon dioxide is coming from the Carbonate. Each CO2 that goes to the atmosphere removes one Carbonate ion

(Similar to @JamesW explanation ... you can either look at it as adding hydroxide or removing hydrogen ion, they are equivalent).

Bicarbonate (HCO3-) consumes a hydrogen ion (H+) to form carbonic acid (H2CO3) which, at normal pH, dissociates into aqueous CO2 and water (H2O). Pool water is oversaturated with CO2 at normal pH values and therefore aqueous CO2 always has a driving force to offgas. The consumption of the hydrogen ion raises the TA in the same amount that the loss of the bicarbonate decreases the TA. Therefore, there is no measurable change in TA. TA is defined as

TA = [HCO3⁻] + 2[CO3²⁻] + [H2CY^-] + [B(OH)4⁻] + [OH⁻] − [H⁺]

Total Alkalinity = Bicarbonate + 2 * Carbonates + Cyanurate + Borate + Hydroxide - Hydrogen

Carbonate concentrations are typically quite small at pool pH and so the dominant dissolve inorganic carbon species is bicarbonate. Borates may or may not be present. Cyanurates are present as well. Both borates and cyanurates are pH dependent in their speciation and so their total contribution is much smaller than the actual measured concentration of cyanuric acid or boron.

 

[Dirk]

I can recommend the setup that is working for me: Pentair automation running my IntelliChlor, into which I plugged in an IntellipH (automatic acid injection system). With that setup up, you'd drastically reduce pool maintenance and could run your spillover all you want. That system runs my solar heater and vacuum, too. There is some settings adjustment to do as the seasons change, and the occasional refill of the acid tank (once every couple of month or so), but that's about all I do for most of the year. My SWG and IpH stop working when the water gets below 50°, so for a couple winter months I have to add chlorine and acid by hand, once a week. If your water never gets that cold, then you could make use of automation year-round.

Then the issues of which would raise pH more, or how long you "get to" run your spillover, would become moot.

I will be adding a solution to my system to be able to run the acid injection automation year-round, so eventually I'll only be manually adding chlorine for those few months.

 

[mgtfp]

SWG electrolytically concentrates the chloride to form bubbles of gaseous Cl2 which dissolves in the water thus chlorinating it. How does it lower pH? Do the chlorine gas bubbles create a large surface area for gas exchange leading to off gassing of CO2 similar to above? or is it another mechanism?

The SWG cell creates chlorine gas at one electrode and hydrogen gas at the other electrode. The chlorine gas dissolves quickly into the water forming the desired hypochlorous acid (HOCl). The electrolytic process itself is basic as it creates NaOH and raises pH. But since the following use of chlorine (in the form of HOCl) as a sanitizer and oxidizer and via UV decay is an acidic process, the whole cycle of chlorine generation and use of chlorine is a pH-neutral cycle.

What remains are the hydrogen gas bubbles, which travel through the pipes and leave the water at the returns, so they remain in the water for a bit. Since these hydrogen bubbles initially contain no CO₂ gas at all, CO₂ that's dissolved in the water (the higher the TA, the higher the content will be) will be encouraged to out-gas into these bubbles, which will raise pH. The question is, how dominant this process is.

When you read through Chem Geek's sticky threads in the deep end, you will notice that in the earlier days this out-gassing into hydrogen bubbles was deemed responsible for the pH-drift that some owners of SWG-pools are experiencing. But later on Chem Geek did some calculations and realised that this process is not sufficient to explain the extent of the observed drifts.

Chem Geek concluded that another process must be responsible for the drift that occurs in some (not all) SWG pools. The most likely candidate is out-gassing of undissolved chlorine gas. Remember when I said that the whole chlorination cycle is pH-neutral? Well, when you interrupt the cycle, it's not a cycle anymore, and therefore not pH-neutral. If chlorine gas gets produced in the cell, pH will rise in the process. Should this chlorine gas now not get dissolved in the water (but out-gas straight away into the air) and will therefore not create HOCl which will eventually do its job as a sanitizer/oxidizer or decay being hit by UV, then the initial pH-rise will not get compensated, i.e. the pH will effectively rise.

Ideally, all of the created chlorine eventually gets dissolved in the water, and the above process should not be significant. But if the generated chlorine gas doesn't have enough time to get dissolved while in the return pipe to the pool, it can bubble out at the return eye-balls. This can happen e.g. when the return line is too short, or the pipe diameter too small so that the water is forced to move faster, or if the pump speed is too high.

This chlorine out-gassing process is usually not the dominant process. CO₂ outgassing into air bubbles created by aeration due to turbulence at spillovers, waterfalls, return eye-balls or swimmers is in most cases by far the dominant cause of pH-rise.

An indication that chlorine out-gassing plays a role, would be that the rate of the observed pH-rise is not depending on TA and that the pH-rise doesn't get significantly smaller once pH gets into the 7.8 to 8 range - if TA is sufficiently small, the CO₂ that's dissolved in the water reaches equilibrium with CO₂ in the air, and out-gassing stops.

 

[CRUSH-IT!]

Thank-you very much