This thread is all about satisfying my curiosity and by no means should indicate that I am looking for something else.

Those of us who have been on TFP for a while understand that there are really only three EPA approved sanitizers that are suitable for widespread use in swimming pools, these being chlorine, bromine, and baquacil. This got me to thinking about why that is and what else has been tried in the past.

Now, I am no chemist, but I know that the first two, chlorine and bromine, are both halogens and both can be used for bleaching purposes. But what about iodine and fluorine though? Both of these elements can be classed as halogens. Iodine has long been used in the medical and pharmaceutical industry due to its antimicrobial properties. Additionally, iodine has historically had a place in making non-potable water into potable water. I'm not so sure about fluorine though. Aside from what I remember about from chemistry in college, I think that the reactivity of fluorine would make it a not-so-good choice for sanitizing water.

What else has been tried in the past for water sanitation and what would preclude the use of iodine-based or even fluorine-based products in doing the job?

A huge number of things have been tried, most notably copper, silver, zinc, ozone, UV, iodine, biological filters, hydrogen peroxide, chloramine, radiation, magnets, and prayers to name a few. The problems with the various choices fall into a couple of categories. Many choices are too slow. They kill everything off eventually, but leave too long a window of opportunity where person to person transmission can occur. Other substances pose risks to swimmers. Some are too expensive, or dissipate too quickly.

Well, I'm not sure it's fair to lump magnets into that category of failures.

My magnets have worked fine with just a small side effect........They pull the tin foil hat off the top of my head and then I can no longer receive the alien transmissions through the fillings in my teeth.

Thanks for moving this thread to the proper place. I forgot about "The Deep End".

Yes, I am familiar with the minerals and I completely understand the shortfalls associated with those. I slo see the limitations of radiation as the effect of it is limited only to that given volume of water that is currently being irradiated and that it isn't doing anything for the volume of pool water receiving a bolus of urine from the 5 year old on the other side of the pool.

So, iodine for example...why isn't it used? Jason mentions it on the list of those tried. What was the failure point? I have my suspicions which include price, toxicity, and convenience. My suspicion is that chlorine is the greatest because it may be the cheapest, it may be the easiest to use, it may be the safest, and it may just work better than all of the rest. But I don't know.

Oh, and duraleigh, tin foil hats are so "cold war" era. My suggestion is to keep your magnets and upgrade to an aluminum foil hat. The magnets won't affect it, it is much lighter, cheaper, and you can throw it in with your empty cans come recycling day.

The chemistry of chlorine and bromine are such that they are very effective sanitizers, and they are oxidizers to boot so you get a 2-fer. Look at the chemical structure of hypochlorous acid and compare that with water and you will see that they are fairly similar. Hypochlorous acid is a neutral molecule that looks a lot like water so is able to get into cells fairly easily. It's a reasonably strong, though selective, oxidizer. It mostly attaches to reduced nitrogen sites in molecules and there's plenty of those in most critical organic compounds in cells, including amino acids, proteins, DNA, etc. So it can pretty much wreck havoc and thus not be something where resistance would be developed since, unlike antibiotics, it does not have only one method of action. And, as noted, it can oxidize bather waste (mostly urea and ammonia from sweat and urine).

Bromine (hypobromous acid) is somewhat similar to chlorine and most of the above applies to bromine as well, though bromine is a weaker oxidizer than chlorine, but doesn't get moderated by CYA so on balance may be roughly comparable in disinfection.

The main problem with the halogen-based sanitizers is that they can create disinfection by-products, some of which increase the risk to health at higher concentrations. Since these come from organic precursors, using supplemental oxidation (ozone, UV, non-chlorine shock) in higher bather-load situations can help reduce this problem.

The only other compound that has good properties of disinfection and oxidation and doesn't produce as many disinfection by-products is chlorine dioxide, but it is unstable at higher concentrations so usually needs to be generated on-site and that can be tricky to avoid producing too much chlorate. It also breaks down in sunlight, but does not combine with CYA.

Iodine is basically an effective sanitizer, but it does not control algae and it does not effectively oxidize organic matter.

As Jason noted above, metal ions do not kill pathogens quickly enough to be used (by themselves) in commercial/public pools since they may not be able to prevent person-to-person transmission. When the FC is around 10% of the CYA level, most common heterotrophic bacteria are killed by chlorine in under one minute (for a 99% kill). It takes silver ion around 10-20 minutes and copper ion around 40 minutes for an equivalent kill rate. Also, metal ions are not effective (at pool concentrations) at inactivating viruses. For example, this paper shows that copper ions do a 90% inactivation of Herpes Simplex Virus in 30 minutes at 100-200 ppm, but that is far, far higher in concentration than found in pools (copper is usually < 0.3 ppm in pools). This paper shows that silver ions have virtually no effect on vacciniavirus, adenovirus, VSV, poliovirus, HVJ, but that with herpes simplex virus there is a 5-log kill in 60 minutes (roughly a 90% kill in about 5 minutes), but at over 3200 ppb compared to the usual limit of 20 ppb to prevent silver staining.

UV, ozone, "active oxygen" and other systems that work via circulation do not provide a residual disinfectant in the bulk pool water so they are primarily used as supplemental oxidizers.

Thanks for that explanation. Just what I was looking for.

I found nothing in chem geek's (excellent) survey of sanitizers/oxidizers that was new to me, but it took several years and lots of reading to enable this familiarity. Those short paragraphs are all what most members need to understand this subject. Someday I hope it will find its way into Pool School.

Not related to pools here, but this makes me wonder about the effectiveness of the Ag ion cycle in our front loading washer. By selecting this choice, it adds 30 minutes to a wash cycle to allow the Ag ions enough time to exert their effect. What I don't know is what the concentration of silver is in the water at that time. Hmmm.