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Author: Subject: Mechanics of Phosphate Removal of HOCl in Swimming Pools
AJKOER
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[*] posted on 12-9-2017 at 12:10
Mechanics of Phosphate Removal of HOCl in Swimming Pools


A source claims in a piece on the internet "Phosphates affect chlorine efficiency", see http://orendatech.com/phosphates-affect-chlorine-efficiency/ . The argument starts with the disassociation reaction for the significantly more powerful disinfectant hypochlorous acid:

HOCl ⇌ H+ + OCl−

The author goes on to state:

"The point is, phosphates affect chlorine because the hydrogen in the equation (H+) is drawn away from HOCl. The result is the weaker form of chlorine."

With the latter weaker form of chlorine refering to the less powerful disinfecting hypochlorite ion.

My question, left unanswered, is how does phosphates help remove the H+ ion?

As the target pH level in a pool is between 7.4 to 7.8, with a target of 7.6 (see https://dengarden.com/swimming-pools/Cloudy-Swimming-Pool-Wa... ), I doubt that the action of a phosphate buffer is the issue, as the range of a phosphate buffer is pH 5.8 to 8.0.

My argument of what is actually depleting the H+ ion is centered on the presence of oxygen (and/or HOCl acting as an oxygen source also) together with transition metals. In the current case of a swimming pool containing ferrous bicarbonate, the latter may interact with oxygen (or HOCl) in an electrochemical cell, with the further conversion of the ferric product back to ferrous induced by sunlight. As such, a possible reaction sequence could be first with a possible electrochemical cell with a net reaction:

Fe(ll) + O2 + H+ = Fe(lll) + •HO2 (see, for example, http://pubs.acs.org/doi/abs/10.1021/ja01600a004 )

which consumes H+. However, with the pKa values for .HO2 at 4.8, the likely species present are H+ and superoxide, .O2- anion at pH above 5, which occurs in swimming pools. Half cell reaction with ferrous would be:

Fe(ll) --> Fe(lll) + e-
e- + n H2O = e-(aq)

Followed by either:
e-(aq) + H+ --> •H
•H + O2 --> •HO2 = H+ + O2•- (for pH over 4.8)

or:
e-(aq) + O2 --> O2•-
H+ + O2•- = •HO2 (only for pH under 4.8, not applicable here ) = H+ + O2•-

Other side reactions:

O2•- + HOCl = •Cl + OH- + O2 (or, Cl- + •OH + O2 , see https://www.sciencemadness.org/whisper/viewthread.php?tid=68...)
•H +•H = H2 (g)

Further, in the presence of strong sunlight, a possible photo-fenton reaction:

Fe(lll) + hv --> Fe(ll)

which recycles the iron.

Overall, it appears that the presence of solvated electrons can lead to the removal of H+ (thereby converting some HOCl to OCl-) or the direct destruction of hypochlorous acid by any formed superoxide anion radical.

It has been noted that some anions (and cations) can impact the ionization potential (IPs) and electron affinities (EAs) of aqueous solutions, including phosphates. See, for example, discussion at http://advances.sciencemag.org/content/3/6/e1603210/tab-pdf where the author notes in water, which he describes as essentially a wide band gap insulator, that simple anions/cations can perform the same role as defects in insulators and semiconductors.

As such, my best guess is that it is the increased presence of organic phosphates to promote solvated electron transport, and the reactions noted above. A supporting source states (see abstract at http://m.pnas.org/content/101/51/17588.full ) to quote:

"the lowest ionization channel for the other three mononucleotides takes place from the phosphate group."

My chief contention is that such organic phosphates concentration (which promotes solvated electrons) may arise from the presence of algae. This is supported by numerous references on the web citing a phosphate algae growth factor effect associated with HOCl loss. To quote a typical example:

"There’s been plenty of talk about removing phosphates from pool water. In truth, phosphates do not consume chlorine and do not combine with chlorine. Rather, phosphates promote the growth of algae (the green, slippery stuff in a pool) which then consumes chlorine as it tries to destroy the algae. The net result is that you can be left with less than the desirable amount of chlorine in your green pool."
Link: http://www.concordpools.com/help/245-top-10-things-that-will...

If others have alternate explanations or paths, lets discuss.

[Edited on 13-9-2017 by AJKOER]
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Melgar
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[*] posted on 13-9-2017 at 02:57


It could just be as simple as the fact that common water-soluble phosphates, like trisodium phosphate, are fairly basic salts. However, I think the issue is that phosphates are water softening agents that remove calcium from solution. I know that it's common to add calcium chloride to pools to raise the water hardness. Water that is too soft can actually corrode metal parts in your pool, and this can be mitigated a lot by making sure there's sufficient calcium ions.



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AJKOER
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[*] posted on 13-9-2017 at 08:40


So soft water promotes metal corrosion. The latter metal ions may foster fenton-type reaction involving HOCl reducing it to chloride. For example with ferrous:

Fe(ll) + HOCl → Fe(lll) + •OH + Cl- (see, for example, p. 40, ebook, "Free Radicals in Biology and Medicine" by Barry Halliwell and John M. C. Gutteridge )

The hydroxyl radical could then react with organics and the product with oxygen producing the superoxide anion. As noted above, the latter breaks down hypochlorous acid. Sample reaction sequence:

CH3CH2OH + •OH → H2O + •CH3CHOH

•CH3CHOH + O2 → CH3CHOH + H+ + O2•-

O2•- + HOCl → Cl- + •OH + O2

Note, the hydroxyl radical and oxygen are regenerated to continue the chain with organics and, in the case of rapidly growing algae, are in abundance. Finally, in the presence of strong sunlight, a photo-fenton reaction recycling iron (or whatever is the active transition metal species present):

Fe(lll) + hv → Fe(ll)
-----------------------------

Thanks Melgar for your feedback as I feel it has led to a stronger alternative model candidate due to the chain and recycling features consuming primarily HOCl.

[Edited on 13-9-2017 by AJKOER]
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[*] posted on 13-9-2017 at 16:53


Don't forget about interactions with the pool itself, plaster, tile and grout. The hardness is adjusted to keep it from balancing itself by stealing or depositing Ca from above sources. If you take the common view of water maintenance from a pool owner, it's to kill algae/bugs/fungi. From a maintenance or repair standpoint to protect stone and pipes. There are not presented many complete sources of pool maintenance b/c you'd need to be wealthy to stay on top of all the pitfalls with out it going green on ya.

I have an area on my pool that is prone to dump out Ca on walls and areas where it prefers to steal from. One gets frosted with rounded nodules of small scale features, and the rest gets really coarse feeling as only high points are left.

Point is there is a LOT to bear in mind while pondering the nature of a simple pool. And btw, most tecs that serviced my pool previously were idiots. They didn't care about manufacturers suggestions, but knew what would keep it clear. And FLIP OUT if you so much as criticized them for using too much ... well,.. anything. Even if citing state funded research and maintenance schedules for public pools. So they aren't welcome at my house :)




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AJKOER
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[*] posted on 14-9-2017 at 03:42


Just came across a very curious way claiming to reduce scale, moderate pH and increase available chlorine in a pool, namely magnetic field treatments (see http://gmxinternational.com/applications/swimming_pools/thes... ). I will let others (up on this science) comment. On a more practical note, even if the technology works, there could be legal disclosure issues arising from visitors using the pool. As such, I would recommend posting a sign that your pool water is magnetically treated, to mitigate the threat of baseless, but nevertheless costly, lawsuits.

On a more serious chemistry note, please read some discussion details on how magnetic fields may potentially impact chemical reactions per comments posted at https://chemistry.stackexchange.com/questions/24507/can-magn... . An important point concerns radical based reactions, which per my prior reaction mechanics for sunlight swimming pools, as noted in my comments above, are likely extensive and may be potentially impacted. This could particularly alter pool chemistry especially surrounding the fate of HOCl. An immediate example that comes to mind concerning the alteration of spins and a decrease in the reverse recombination reaction to Cl2 is:

Cl2 + hv → •Cl + •Cl
•Cl + •Cl = Cl2

resulting in more chlorine radicals. The latter reacts with water, for example, slowly producing hydroxyl radicals:

•Cl + H2O → HCl + •OH. (see Table l in "Photochemistry of HCl and Other Minor Constituents in the Atmosphere of Venus" by Prinn, R.G.)

among other reactions.
----------------------------------

There is also an interaction between the pool and its users (other than complaining about magnetic waves). I have the pleasure of the use of a large pool in a house in the southern USA that was adjacent to a nature preserve. The pool was poorly maintained and even after it was cleaned up, the pool guy didn't have the correct level of chlorine. I known this because weeks after using the pool, I noticed wild animals from the nearby nature preserve drinking the pool water. As I suspect wild animals to have a host of bacterial, viral, and parasitical infections, having those mouths in my pool is likely unwise. This would be even more so for swimmers exposing all their oracles (mouths, ears, ....) to these potent invaders.

Somehow this problem wasn't in the pool handbook, but I do have a potential solution, add CuSO4, normally suggested for algae control, as copper ions are deadly in micro doses to small animals (caution: this includes fish and small pets like dogs and cats).

On another occasion, I have the use of a pool which had a water fall feature. I thought this was good as aeration has many benefits. To my dismay, the pool man was turning this feature off. I confronted him once and told him the benefits of the waterfall on getting oxygen into the pool. His reply was that the waterfall may it harder for him to clean up leaves that have sunken to the bottom. He continued to still turn off the waterfall.

I also recall reading a thesis that involved the interaction of the organics (apparently, mostly sweat, at least I hope so) expelled by swimmers and its possible effect on pool chemistry. No surprise here.

[Edited on 14-9-2017 by AJKOER]
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