Gas bubbles from Salt Water Chlorine Generator

[Vegas Mike]

Have seen statements on TFP that the gas bubbles generated by the SWCG are hydrogen and chlorine. I believe chlorine reacts too quickly with water for that to be true. Although chlorine gas is stated as only slightly water soluble, that is irrelevant because it rapidly reacts with moisture to form hypochlorous acid (the part we want as a sanitizer) and hydrochloric acid AKA muriatic acid. (Fun story – I interned at a titanium metal production plant, and got to run with the rest of the crew to shelter when we had a fugitive chlorine gas event, which created a cloud of hydrochloric acid.)

I think the gasses seen downstream of an SWCG cell are a mix of oxygen and hydrogen, and delightfully explosive based on my experiment. Gas bubbles emerge from the first return port in my pool when the SWCG is running. Was able to capture the gas with an inverted plastic cylinder, then extract some using a 60ml plastic syringe. When a flame is held to the syringe tip and the plunger depressed, get a satisfying pop and the plunger goes airborne (don’t point this at anyone, or yourself, obviously). So got to be hydrogen there, and an oxidizer. Yes, you could claim it was chlorine, but that means whenever anyone electrolyzes water they get chlorine instead of oxygen. Have to have some electrolyte present to get sufficient conduction, usually common salt. The volumes displaced in all the experiment photos show twice the volume for the hydrogen cathode over the oxygen anode, matching H2O.

Now, is another possibility my Pentair IC40 SWCG cell is not running properly, and generating oxygen from malfunction? Current chemistry: pH 7.5, TA 80ppm, CYA 70ppm, salt 3400ppm, FC 4.6ppm, water 90F, calcium 975ppm. Thoughts?

 

[proavia]

A SWG produces chlorine gas and hydrogen gas.
The chlorine gas dissolves in the pool water and the hydrogen gas escapes as the bubbles you see.

In my pool, I only occasionally see the hydrogen bubbles when the cell is on. Maybe they are extra small or my eyesight isn't as good as it once was. No matter, my SWG is working just fine - based on my test results.

To check your SWG output, you can do an overnight gain test. Test your pool water FC after sunset, turn the SWG and pump on all night, check the FC just before sunrise.

Also, keep your FC close to the upper end of the target range for your CYA. FC/CYA Levels

What is your pump run times and SWG percentage?

 

[pjt]

FC 4.6ppm

You need more FC. Also, use the 10 ml sample so each drop is 0.5 FC. Saves time and reagent.

calcium 975ppm

You're looking at a drain and refill when the weather allows. Look into getting a water softener for your pool fill line.

 

[Vegas Mike]

Guess I'll need to construct an electrolysis experiment using my pool water to prove to myself the bubbles are H2 and O2. Obviously, chlorine is also generated, but I assert it reacts almost instantly and does not appear downstream. For my syringe to explode as it did in my demo, must have both hydrogen and an oxidizer present.

My Intellichem system includes an ORP sensor. I adjust the ORP setpoint to maintain SWG on-time to reach my desired FC. As others at TFP have noted, ORP is not really FC, but I find it works OK in the control system if I adjust the setpoint from time to time. The SWG can run anytime during the 11 hours daily pump operation. Last numbers are 43.1%. From your CYA / FC reference (thanks!) I am currently just below appropriate FC.

Yup, calcium getting high. Water in Vegas costs $15/1000 gallons, so I'll probably ride this out one more season. Have to time this just right, because the threshold for really expensive water shifts with the seasons. I lose 20k gallons/year to evaporation, and tap water is over 200ppm calcium.

Have a friend here planning to use a softener for make-up water. Need to calculate how fast the salt will increase in the pool. (A softener exchanges Ca, Mg for sodium.) Have to keep the salt between 2800 and 5000ppm.

 

[JamesW]

For my syringe to explode as it did in my demo, must have both hydrogen and an oxidizer present.

Oxygen from the air is mixing with the hydrogen unless you are somehow preventing the air from getting to the hydrogen as you light it.

 

[proavia]

@JamesW and @JoyfulNoise can explain - in detail - what gases are produced by the SWG cell.
Chances are the oxygen is from the air outside the tip of the hydroden filled syringe.

With a properly functioning water softener, no salt is added to the softened water.
If salt were added to the softened water, the water inside the house would taste salty,

The salty brine solution is used to regenerate the softener's resin beads. And excess brine solution is purged during the rapid rinse cycle.

Softened water will have a little bit of sodium in it - but not salt.

Muriatic acid and liquid chlorine will increase the salt level a bit.

EDIT - @JamesW beat me to it with his comment of oxygen from the air.

 

[an1vrsy]

Have a friend here planning to use a softener for make-up water. Need to calculate how fast the salt will increase in the pool. (A softener exchanges Ca, Mg for sodium.) Have to keep the salt between 2800 and 5000ppm.

I have my whole house water softener plumbed to my pool fill line. The salt content from my fill softened water is negligible.

 

[Vegas Mike]

In Las Vegas, we have over 200ppm calcium and magnesium in tap water. A softener substitutes two atoms of sodium for every atom of calcium or magnesium. So the sodium output would be over 400ppm. (No, you can't taste the salt -- far below your taste threshold. Please look up how an ion-exchange water softener works on Wikipedia.)

Also, Vegas is very hot and dry with peak July Evapotranspiration of about 14 inches / month, meaning my (uncovered) pool level would fall by that amount without makeup water. As a result, my pool evaporates 20k gallons per year. I actually measure daily water addition via flow meter. This equates to an increase in calcium of about 350ppm per year, which seems a bit lower than what I have actually measured, but close. If softened water were used, I would see about 700ppm sodium increase per year. I have to start out with at least 3000ppm sodium, so would exceed the maximum 5000ppm in just under three years. Without the softener, I'd be at 1250ppm calcium in that same time. And I do realize the SWG uses the chlorine atom, but the water will remain electrically balanced, and you'll get the same number of chlorine atoms as sodium.

So does not appear softened water results in any water savings because the pool still needs refilled after the same time. But, maybe less calcium scale on the pool tile before refill is necessary? The real issue is concentration by evaporation. I have a kidney-shaped pool, and have not figured out how to cover without lots of work each time I swim.

 

[PoolStored]

Also, Vegas is very hot and dry with peak July Evapotranspiration of about 14 inches / month, meaning my (uncovered) pool level would fall by that amount without makeup water. As a result, my pool evaporates 20k gallons per year. I actually measure daily water addition via flow meter. This equates to an increase in calcium of about 350ppm per year, which seems a bit lower than what I have actually measured, but close. If softened water were used, I would see about 700ppm sodium increase per year. I have to start out with at least 3000ppm sodium, so would exceed the maximum 5000ppm in just under three years. Without the softener, I'd be at 1250ppm calcium in that same time. And I do realize the SWG uses the chlorine atom, but the water will remain electrically balanced, and you'll get the same number of chlorine atoms as sodium.

So does not appear softened water results in any water savings because the pool still needs refilled after the same time. But, maybe less calcium scale on the pool tile before refill is necessary? The real issue is concentration by evaporation. I have a kidney-shaped pool, and have not figured out how to cover without lots of work each time I swim.

Salinity is a measurement of the chloride in the water, not sodium.

Many of our members install them with excellent results and no impact on salinity. Even our chemistry expert @JoyfulNoise

Water softener installation!!

So this morning, before the mercury hit 100F, the autofill line was dug up - Yikes...a little too close to the gas line for my comfort :shock: The water softener is a double tank system - the top tank is a GAC (granular activated carbon) prefilter (chlorine/chloramine/VOC eliminator) and...

www.troublefreepool.com

 

[proavia]

Using softened water (from a properly operating water softener) for the autofill makeup water will not increase the salt level in the pool. And my CH will stay relatively constant for years by using softened water for my autofill..

I'm in PHX. My evaporation rate is about the same as yours. Unsoftened tap water (used for initial fill) has a CH of 200-250, based on time of year and water source at my tap.

Any increase in salt level in my pool is from muriatic acid additions along with occasional liquid chlorine.

While you need about 3000 ppm of sodium chloride for a SWG, adding sodium will no change the sodium chloride content. With little precip, it's better to start off at the low end of the needed salt range as we have little rain to dilute the pool water and muriatic acid / liquid chlorine additions will increase the salt level slightly.

Once @JoyfulNoise is back from his trip, he can fill you in on the technical details and chemistry of using softened water for autofill makeup water.

 

[JoyfulNoise]

In a water softener, the ion exchange resin holds sodium ions in its molecular structure. The resin can also hold on to calcium ions as well and there is almost no chemical energy difference between either configuration. So the exchange interaction can be simplified to a concentration controlled reaction - depending on which ion has a greater concentration in solution, that is the ion that will be captured by the resin and the other ion will be released. Because of charge differences between calcium and sodium, two sodium ions will be released for every one calcium ion absorbed by the resin.

Saltwater is generated during the regeneration cycle from rock salt (NaCl) and water and it is used to created a highly concentrated brine. The resin is washed with this brine in order to force all of the calcium out of the resin and into the regeneration water and replacing the calcium ions with sodium. After regeneration, the resin is thoroughly rinsed and backwashed to remove all traces of the brine. Therefore, no brine or salt should ever enter the potable water system. If it does, then the softener is malfunctioning and the seals in the control valve are probably leaking.

No chloride ion makes it into the pool from the softener except for whatever is naturally occurring in your water supply. EPA rules typically suggest that municipal suppliers keep their chloride ion concentration below 50ppm for safe operation of the treatment plant equipment.

Na ion ≈ 23 g/mol
Ca ion ≈ 40 g/mol
CaCO3 ≈ 100 g/mol (unless otherwise specified most municipal water supplies express calcium hardness in terms of equivalent calcium carbonate concentration)

200 ppm CaCO3 is typical for western water supplies. So the actual calcium ion concentration is roughly -

200 x (40/100) ≈ 80ppm Ca ion

You get two sodium ions exchanged into the water for every calcium ion captured, so -

2X 80 x (23/40) ≈ 92ppm Na ion

Sodium ions are not considered part of a waters “hardness” because they are highly soluble even at excessively high concentration so they almost never scale out of solution. There is no exchange of chloride (Cl-) by the softener and so there will be no increase in chloride ion concentration in the pool when using a water softener. Therefore there will be no need to adjust the SWG.

As for other concerns - if your SWG is actually producing oxygen gas then it is not operating properly. The entire point of using a catalytic coating on the titanium SWG plates is to lower the over potential needed for electrolysis to occur and for the formation of chlorine to be favored over that of oxygen gas. The reactions produced at the SWG cell should be chlorine formation, hydrogen gas formation, and hydroxide anion formation. Oxygen is not part of the process and only occurs when the electrodes are being driven well beyond the potential needed to form chlorine.

Also, while it is true that chlorine is highly soluble in water and that it almost instantly forms hypochlorous acid at the anode, some gas formation can occur and get mixed in with the hydrogen bubbles that are generated. Once a bubble forms, the process of diffusion is very slow and there is ample time in a short pipe configuration for the chlorine gas to escape. This is in fact exactly how chlorine gas is generated by electrolysis of brine in a membrane separated process. Chlorine gas bubbles out of solution and is captured, dried/processed, and pressured into cylinders.

 

[Vegas Mike]

Thank you all for being so generous with your time. Especially JoyfulNoise – apparently, I have forgotten much of my chemistry training. I now see that using softened water will reduce the total pool water use because pool refills would be needed much less frequently than my current three-year schedule (and I really wait too long). Have to think how to calculate how often using softened water.

Regarding my SWG, assuming it really is generating oxygen, does this indicate I need to replace the cell soon? The cell is over four years old, but only runs 6 months of the year because I don’t heat in winter. Or is there possibly a configuration error in the Pentair IntelliChem that would overdrive the cell? Equipment installed by a pool contractor.

 

[proavia]

If you use soft water for your autofill/refill water, your CH increase will be minimal.
The CH will increase a little if the autofill is running while the water softener is regenerating (in a singe resin tank softener system). Based on CH only, you should be able to go years without needing a drain/refill before CH gets too high.

I think what you are seeing is hydrogen bubbles.
I remember in high school chemistry we gathered hydrogen in a test tube and then plunged a glowing ember into the test tube. The result was a quick flash and a high pitched whistling noise. Of course, high school was MANY years ago, so it could have been something other than hydrogen in the test tube.

The IntelliChem pH side isn't as reliable as one would think. And its interface with your EasyTouch may be affecting the cell shutting down. That cell should continue producing chlorine down to 60 degree pool water - maybe lower. Your cell should be able to produce chlorine a lot longer than 6 months of the year in Vegas. Last winter in Phoenix, my Circupool cell ran almost all winter. I only added 1 gallon of liquid chlorine all winter.

 

[an1vrsy]

I now see that using softened water will reduce the total pool water use because pool refills would be needed much less frequently than my current three-year schedule (and I really wait too long). Have to think how to calculate how often using softened water.

The chemistry discussion above is way over my head. I live here in Las Vegas and I use our whole house water softener plumbed to our manual pool fill. I followed recommendations here on TFP when I built the pool.

This past March was our 3 year anniversary since our initial fill with municipal water (LVVWD). Since that initial fill I’ve used softened water with occasional hose water. Three and a quarter years later our CH 360. Salinity is 3200. This was our real world experience.

 

[JoyfulNoise]

Once the coating on the cell goes (it wears off) you are left with a bare titanium plate. It doesn’t conduct very well and develops a very stable passive oxide coating. This causes the voltage of the cell to rise if the power supply doesn’t clamp it. The higher potential could conceivably generate both oxygen and chlorine gas but the bigger effect you will see is scale formation. Typically when a cell wears out, the returns start “spitting snowflakes” and you’ll find little piles of white scale at the bottom of your pool.

Pentair IC’s don’t make it easy to inspect cells as they are completely closed units. So you really don’t have good visual diagnostics. Pentair also doesn’t really provide the user with operating data (even though it’s available from the onboard electronics) so you are flying a bit blind. You can run the system diagnostic by pressing and holding the MORE button for 3 sec. That will tell you roughly how long the cell has been operated for. If you’re close to the 10,000 hour mark, then the cell is definitely reaching end-of-life.