Fun little experiment with Iron and Chlorine

[jesse-99]

Recently I fixed my pool's auto-fill so I no longer have any iron being added to the pool (How to test for Iron without sending out water to testing lab or pool store! (pics included)) , but I was curious if I still had some iron in the pool water from the last 3 months that was still lingering around. I've drained a bit of pool water lately (maybe 6,000 of my 36,000 gallons?) to get rid of some of the sequestered iron and some CH as well.

Normally I keep FC around 4-6 or so, and decided to try to make my pool turn nuclear green with some added chlorine. I like to call it nuclear green because the water is still super clear and it just reminds me of some kind of weird nuclear waste water.

Now on to the fun stuff...

I can go from super-clear blue to super-clear green by bumping up my FC from 4-6 to 14. It only takes a couple of hours (maybe 4, IIRC?) for the water to change that drastically. Keep in mind, I'm not doing this with 1 pump, I had 3 separate pumps running (1 at 2750rpm, 1 at 1050rpm, and 1 at 1550rpm) to keep water circulating really well during this test.


IMPORTANT NOTE: The main pool and the overflow basin are 2 different shades/colors in each picture because I started my experiment/testing the overflow basin about an HOUR ahead of the main pool. (I wasn't going to proceed with the main pool if I didn't see the results I -expected- with the smaller overflow basin). So every pic you see here, the overflow basin will have a "head start", hence the difference in colors between the two bodies of water.

In short, I just used a very small amount of AA to neutralize -some- of the FC and ran 3 pumps to circulate and move the water.

This is the the water color in the beginning, once FC hit about 14.5.



1 Hour Later after adding in a bit of AA...



1 Hour Later with circulation (no more AA being added)...


1 Hour Later with circulation (no more AA being added)...


1 Hour 30 Minutes Later with circulation (no more AA being added)...


2.5 Hours later with circulation (no more AA being added)...


Basically about 7 hours total of water circulation, and an FC drop from 14 down to about 5. So this shows that I still have some iron in my pool.

In a span of about 12 hours, I went from clear-blue, to clear-green, to clear-blue again. Anyways, just thought it might be fun to share with others.! (iron sucks, heh)

 

[ajw22]

Thank you. You just demonstrated that emerald green clear water is cause by iron and not copper. This has been a bit of a debate at times.

 

[jesse-99]

Aw man, you don't like my nuclear green made-up color? haha. Emerald green is probably a lot more accurate, good call.

 

[mgtfp]

I never thought I'd give a to a green pool. But that deserved one.

Thanks for sharing that one

 

[SoDel]

Very cool! A brain cell in my head way in the back seems to barely remember that maybe pH also has an effect on that reaction. Like lower pH will slow or even stop the “greening”, but once pH rose and it went green it was not reversible? Any experts can help add to the fun?

 

[jesse-99]

I keep my pH at 7.1-7.2 consistently (I have an MA doser), FWIW. Not really sure if pH would affect what I did above... Someone else would need to weight in on that as well.

 

[duraleigh]

pH has a direct affect on the process. If your pH is low enough the iron will still be in your pool water but it will not come out of solution so the pool stays blue.

In fact, it is the high pH of the added chlorine that causes the iron to precipitate into yellowish/brownish visible particles. Combined with blue water, it turns the pool green.

Left alone, the high pH from the chlorine drops back (usually over a few days) and the pool water will return to blue.

 

[jesse-99]

@duraleigh To confirm, my pH is/was at about 7.1 when I did that test (and continual MA dosing) so it was not high and in fact low compared to where most people run their pH (I MA dose with a Stenner pump to keep mine between 7.1 and 7.2) . I can say with 100% certainty that my pH was 7.1 right before starting this test (when water was blue), and when it went green (and before neutralizing the chlorine with AA), the pH was no higher than 7.25 based on the notes I took. (and I contributed the slight rise in pH because I was running my water features non-stop to make sure I was circulating water everywhere)

So when you say, "If your pH is low enough ..." that's where I'm not quite following you, cause it's not like my pH suddenly shot up to to 7.8 or 8.0 when doing this test. It stayed consistently low/steady in the pool throughout the process. (though I realize I'm measuring pH at a given point/spot within the pool, not measuring exactly where the chlorine dispersed and pH might be slightly higher.)

To to talk sheer numbers, all it took to go from FC6 to FC14 was 2 gallons of 12.5% chlorine. 1 gallon first, then one hour later another gallon.

(for reference, in the "5 gallon bucket tests" I've shared on TFP (How to test for Iron without sending out water to testing lab or pool store! (pics included)) , that pH -is- definitely higher at 7.8, since the water is straight from my well which tests out at 7.8).

Thanks for any clarification. I think I'm just getting stuck on you saying "high" pH cause I'm the opposite of that, yet it still went emerald green.

 

[duraleigh]

cause it's not like my pH suddenly shot up to to 7.8 or 8.0

Actually, it sorta' did.....even higher.

The pH of chlorine as it enters your pool is around 13.7 (or thereabouts)

It is the process of converting chlorine to HOCl that spikes the pH locally. Others can describe it better than I but on a microscopic level, your pH precipitates the iron into visible particles and it is only after that process stops that the iron returns to solution.

 

[jesse-99]

So even though my overall pH level never went above 7.25 during this process (and multiple tests), the exacts spots the chlorine entered the pool was around 13 and triggered those iron molecules to go from solution to suspension. That's just kinda hard to understand I guess. So are saying its NOT the fact that my FC shot up from 6 to 14 that triggered the color change? or moreso saying that, yes the FC going from 6 to 14 is what triggered the color change but it's BECAUSE the pH shot up to 13 in the areas the chlorine entered the pool? I dunno if what you're saying makes sense to me because I have 2 SWGs, and those are obviously spiking pH during the salt to chlorine conversion, but my pool water doesn't go green from coming out of the returns? Ah well, the science behind it right now is beyond my knowledge I think. All I know is I raised FC from 6 to 14 and the the pool went emerald green and neutralizing the chlorine, turned the pool blue again. (I need a bigger brain).

 

[mgtfp]

So even though my overall pH level never went above 7.25 during this process (and multiple tests), the exacts spots the chlorine entered the pool was around 13 and triggered those iron molecules to go from solution to suspension. That's just kinda hard to understand I guess. So are saying its NOT the fact that my FC shot up from 6 to 14 that triggered the color change? or moreso saying that, yes the FC going from 6 to 14 is what triggered the color change but it's BECAUSE the pH shot up to 13 in the areas the chlorine entered the pool? I dunno if what you're saying makes sense to me because I have 2 SWGs, and those are obviously spiking pH during the salt to chlorine conversion, but my pool water doesn't go green from coming out of the returns? Ah well, the science behind it right now is beyond my knowledge I think. All I know is I raised FC from 6 to 14 and the the pool went emerald green and neutralizing the chlorine, turned the pool blue again. (I need a bigger brain).

It is kind of both. But the higher the FC, the lower pH needs to be to prevent metals from falling out of solution. You must have a fair bit of iron in the water to get such a "beautiful" effect while keeping pH low.

Just keep in mind that the pH-rise from adding chlorine will get compensated when the chlorine gets used up again, as pointed out by Dave. So, when compensation the pH-rise from adding bleach when getting up to SLAM by adding MA, then you'll effectively end up with a net pH reduction. As long as TA is not too low, that will usually get compensated pretty quickly by CO2 outgassing. But there are extrem cases like ammonia in a pool. If you fight the pH-rise while adding bleach by adding acid, you will likely crash your pH eventually.

The detailed chemistry is as follows. I'm just quoting back to the relevant passages in Chem Geek's water chemistry thread.

Adding bleach is a slightly basic process:

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.

Similar with an SWG:

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.

The following "use" of chlorine (turning it into chloride) is a basic process that compensated the initial pH-rise:

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.

Put together:

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).

And with an SWG:

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).

 

[mgtfp]

As long as TA is not too low, that will usually get compensated pretty quickly by CO2 outgassing. But there are extrem cases like ammonia in a pool. If you fight the pH-rise while adding bleach by adding acid, you will likely crash your pH eventually.

Just to elaborate on that a bit more: If you compensated the initial pH-rise from adding bleach straight away by adding MA, then you turn the net-pH-neutral effect of adding bleach into a net-acidic process, similar to chlorinating with Dichlor, Trichlor or direct Chlorine gas injection.

The main thing to keep in mind with acidic chlorination processed is that it brings TA down, i.e. requires keeping TA up by regular baking soda additions. As long as TA is high enough, pH will not crash. Not just because of the buffering effect (the actual "alkalinity" effect), but also because of CO2 outgassing that will eventually compensate the net-pH drop from using net-acidic chlorine sources.

But once TA is too low, pH will crash. If you get a red TA test straight away before adding R-0009 drops, your pH is at or below 4.5. The TA indicator is simply a pH-indicator that changes colour around pH 4.5.

Going into a SLAM and correcting each initial pH-rise from bleach by adding acid (for example because the pool shop told you that bleach is bad because it raises pH), then TA will eventually get down to zero and pH will crash.

That's where you need to be careful with an automated acid-dosing system during a SLAM. You can keep it on, but need to keep a very careful eye on pH (with a calibrated pH-meter to avoid FC interference with the test) and TA.

That's one of the reasons why our SLAM guideline says: "Reduce pH before the SLAM and then ignore pH".

Someone finding TFP with the goal to clear a green pool needs to be able to act quickly without a deep introduction into pool water chemistry.

 

[jesse-99]

Thanks for all this info, though I admit it may be a bit over my head (I'm trying! ha). FWIW, the pH testing I did, even when my FC was up at 14, was with my Apera pH60 digital meter, I wasn't using the drop tests and comparator block for pH. As for dosing MA, I'm dosing (roughly) about .3 gallons per day (over the course of about 16 hours while my filter pump is running). I guess I just wanted to be clear that my pH readings didn't really change much (just very slightly) when going from FC of 6 to FC of 14.5, and then back to FC of 6 again, nor did I dump any additional MA in my pool, other than what my doser does every single day. So maybe there was a pH rise and then compensated as I used AA to neutralize the FC, but I sure didn't read or see any noteworthy rise with my pH testing during that process.

I think I'm struggling to understand if you're saying that when someone SLAMs, their pH is (temporarily) going up from (for example) their 7.5 starting point to over 8.0 or higher? and then compensated for as the chlorine breaks down again and is a net-zero back to 7.5 when SLAM is complete and reading returned to normal? I guess I just didn't see that with my testing... (though I never reached SLAM levels).

 

[jesse-99]

Just to elaborate on that a bit more: If you compensated the initial pH-rise from adding bleach straight away by adding MA, then you turn the net-pH-neutral effect of adding bleach into a net-acidic process, similar to chlorinating with Dichlor, Trichlor or direct Chlorine gas injection.

One more thing to clarify, I didn't compensate by adding MA at all during this testing either. Other than to add my "standard" .3 gallons of MA that day (through my Stenner pump dosing) which ONLY services to maintain my pH level every day to counteract the natural pH rise due to my TA -and- more importantly the aeration that occurs in my pool every day (I have a ton). But I do get what you're saying here, that if I did try to adjust, I'd have come out with lower pH than expected, once the chlorine was neutralized or used up.

 

[mgtfp]

I think I'm struggling to understand if you're saying that when someone SLAMs, their pH is (temporarily) going up from (for example) their 7.5 starting point to over 8.0 or higher? and then compensated for as the chlorine breaks down again and is a net-zero back to 7.5 when SLAM is complete and reading returned to normal? I guess I just didn't see that with my testing... (though I never reached SLAM levels).

The effect is small while going a bit up and down at target levels. It gets more pronounced when starting with a green (from algae, not iron ) pool with no chlorine, and go from 0 to SLAM. Here some calculation examples:

As an example, if you were to add 3 gallons of 6% bleach to a 15,000 gallon pool that has 30 ppm CYA, then the following shows the change in various water chemistry parameters after adding the bleach and then after it is used up:

FC ..... 0 ... 12.3 ..... 0
pH ... 7.5 .... 8.1 ... 7.5
TA ... 100 ... 109 ... 100
DC ..... 0 .... 0.24 ..... 0

If you first lower the pH with 24 fluid ounces of Muriatic Acid, you get the following sequence

FC ..... 0 ..... 0 ... 12.3 ..... 0
pH ... 7.5 ... 7.2 ... 7.6 ... 7.2
TA ... 100 ... 94 ... 103 ... 94
DC ..... 0 ..... 0 ... 0.30 ..... 0

The "DC" is disinfecting chlorine (i.e. hypochlorous acid) so you can see that first lowering the pH does result in more disinfecting chlorine, but not by as much as one might think with the traditional chlorine graphs. The reason is that CYA "buffers" chlorine (specifically, hypochlorous acid) so resists changes in its concentration with changes in pH. So normally when shocking a pool at reasonable CYA levels one doesn't worry about the pH jump, but if shocking at high levels for mustard/yellow algae or at high CYA levels, then lowering the pH first makes more sense. Normal shock level is a disinfecting chlorine (DC) of around 0.30

Now if you needed to lower your TA level anyway, then adding acid before shocking does two things at once as you'll end up with lower TA as you could see above (and, if desired, you could then let aeration make the pH rise without any change in TA).

Repeating the same analysis at a higher CYA level of 80 ppm and using 8 gallons of 6% bleach I get the following:

FC ..... 0 ... 32.9 ..... 0
pH ... 7.5 .... 8.8 ... 7.5
TA ... 100 ... 123 ... 100
DC ..... 0 .... 0.21 ..... 0

If you first lower the pH with 30 fluid ounces of Muriatic Acid (it takes more since the CYA buffer is slightly stronger and I still assumed the same 100 for TA), you get the following sequence

FC ..... 0 ..... 0 ... 32.9 ..... 0
pH ... 7.5 ... 7.2 ... 8.5 ... 7.2
TA ... 100 ... 92 ... 115 ... 92
DC ..... 0 ..... 0 ... 0.25 ..... 0

If you add a larger amount of acid, 60 ounces, to get the pH down to 7.0, then you have the following

FC ..... 0 ..... 0 ... 32.9 ..... 0
pH ... 7.5 ... 7.0 ... 7.8 ... 7.0
TA ... 100 ... 84 ... 108 ... 84
DC ..... 0 ..... 0 ... 0.30 ..... 0

So significantly lowering the pH before shocking makes sense when you have to add a lot of chlorine to get to a high FC, but it's not as necessary at lower normal shock levels.

Richard

 

[jesse-99]

Okay SIDEBAR question I just thought of. When my pool was on it's way to going emerald green during this test, I immediately added some polyfill to my skimmers, figuring, "Hey why not catch some of the iron when its going from solution into suspension?" even though I knew I was going to quickly neutralize the chlorine and that polyfill wouldn't have all that much time to work at catching things anyways. Oddly enough, the polyfill literally caught -nothing- at all, (and I have caught plenty of iron with polyfill in the past!). I'm wondering, is it possible the polyfill didn't catch anything because my pH was just so low (along with the combination of sequestering agents)? I'm guessing my sequestering agents were pretty much neutralized by the chlorine addition (2 gallons of 12.5%), so I don't think that was the reason the polyfill didn't catch anything. I'm wondering hypothetically, for example, if my pH had been 7.8 when I started this experiment (instead of 7.1), that I'd have been able to catch some of the iron in the polyfill?. (I should mention, that I did leave the pool in it's emerald green state overnight with the filter pump running over night as well because I was figuring I could catch some suspended iron overnight before I started neutralizing the chlorine when I woke up that morning, but as mentioned, I didn't catch anything when I checked that morning).

edit -- to rephrase, maybe the pH was just too low to "stain" my polyfill. ?

 

[jesse-99]

The effect is small while going a bit up and down at target levels. It gets more pronounced when starting with a green (from algae, not iron ) pool with no chlorine, and go from 0 to SLAM. Here some calculation examples:

Now that is some awesome reading!!!! Richard's stuff is just golden... ("That's gold, Jerry! Gold!") Thanks for finding that. I re-read your previous messages a couple of times now (like 4, lol) and it's all starting to sink in better.

 

[jesse-99]

(sometimes I feel like the knowledge I gain from reading the science and technical pieces relating to pool water chemistry available here at TFP (offered up by TFP's amazing members) is like.... a crack addition. I need my "TFP fix" to keep educating myself)

 

[mgtfp]

Okay SIDEBAR question I just thought of. When my pool was on it's way to going emerald green during this test, I immediately added some polyfill to my skimmers, figuring, "Hey why not catch some of the iron when its going from solution into suspension?" even though I knew I was going to quickly neutralize the chlorine and that polyfill wouldn't have all that much time to work at catching things anyways. Oddly enough, the polyfill literally caught -nothing- at all, (and I have caught plenty of iron with polyfill in the past!). I'm wondering, is it possible the polyfill didn't catch anything because my pH was just so low (along with the combination of sequestering agents)? I'm guessing my sequestering agents were pretty much neutralized by the chlorine addition (2 gallons of 12.5%), so I don't think that was the reason the polyfill didn't catch anything. I'm wondering hypothetically, for example, if my pH had been 7.8 when I started this experiment (instead of 7.1), that I'd have been able to catch some of the iron in the polyfill?. (I should mention, that I did leave the pool in it's emerald green state overnight with the filter pump running over night as well because I was figuring I could catch some suspended iron overnight before I started neutralizing the chlorine when I woke up that morning, but as mentioned, I didn't catch anything when I checked that morning).

edit -- to rephrase, maybe the pH was just too low to "stain" my polyfill. ?

Not sure, honestly. Maybe it does need a bit higher pH, maybe your iron concentration is just not high enough anymore.

 

[jesse-99]

Not sure, honestly. Maybe it does need a bit higher pH, maybe your iron concentration is just not high enough anymore.

Yeah the concentration of it is another factor too - maybe the ppm level of the iron is just not at staining level anymore, but still enough to oxidize with high chlorine and make a pretty emerald green. I guess I could re-do the experiment and start at 7.8 pH this time to see if it captures in polyfill, but... mehhh.... I'd have to be really bored that day to try it, ha.