Why does SC-1000 reduce free chlorine?

September 24, 2019
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Our scale and metal control product, SC-1000, is powerful and concentrated, but it has one major downside: the purge dose of SC-1000 wipes out free chlorine for a couple of days. But why?

Chelation vs. Sequestration

To understand this issue, we first need to understand what SC-1000 does in water. Many think it is a sequestering agent, but SC-1000 is actually a chelating agent. They are different but solve a similar set of problems in pool chemistry. Here is a not-at-all-scientifically-accurate drawing of the difference:

Sequestration vs. Chelation

sequestering agent draws metals to it, binding them into a clump. When a metal's protons and/or electrons are attached to the sequest, they are unable to be stolen (for oxidation and staining). In the case of calcium, it becomes unable to bind with carbonate alkalinity (CO3--) to form calcium carbonate (CaCO3) scale, dust, clouding, etc.. Hence, these products control scale and metals.

Chelation also inhibits scale and prevents metals from being oxidized–and therefore prevents stains too. Unlike sequestration, however, chelation does not clump metals or calcium ions together. It isolates them individually, forming a strong bond. This is arguably preferable for LSI and calcium management, and less preferred for metal removal. Why? Because chelated metals and calcium tend to pass right through filters, whereas sequestered clusters of metals get caught.

SC-1000 is a chelant, and its bond to metals and calcium is quite strong. Most stain and scale control products in the pool industry are phosphate-based sequestering agents, in some form or another.

Oxidation and Removal of Sequestering Agents

We have plenty of other articles in our blog about oxidation, so we don't need to elaborate too much here. Just remember that oxidation is when an oxidizer steals electrons from a substance, which in turn reduces the oxidizer (chlorine). Oxidation is how non-living contaminants in water are broken down and removed. Unfortunately, most sequestering agents will eventually be oxidized and broken down over time. Though to be fair, sequests do have decent longevity, provided you use a maintenance dose as recommended on the bottle. The point is; eventually, oxidation will remove it from the water.

But there's a faster and easier-to-explain way that sequestering agents are removed from water: phosphate removers. Alas, phosphate-based sequestering products don't stand much of a chance against a phosphate remover like PR-10,000. When the sequest is wiped out, it releases its metals and/or calcium, and suddenly staining and scale are possible again. So in summary, both chlorine and phosphate removers will remove sequestering agents, albeit at different rates.

And then there's SC-1000.

Invulnerable to Chlorine?

SC1000-sizes-JPEG-1As far as we can tell, SC-1000 has exceptional staying power in the water and shows no sign of being removed by chlorine. The most reliable evidence we have of this is the fact that SC-1000 will deplete free chlorine levels to zero (0 ppm) in a matter of minutes when adding the purge dose. You see, on a new pool startup, that's no big deal because there is no chlorine yet, and SC-1000 has time to bind to calcium and metals before chlorination. Our theory is that our product is not vulnerable to chlorine oxidation at all. And until it has linked itself to metals and calcium, chlorine attacks it–and loses.

That being said, once SC-1000 binds to calcium and metals, chlorine seems to coexist with it without issue. Even adding the 3 fl.oz./10,000 gallons/week maintenance dose will not noticeably impact free chlorine levels in the pool. This fact has led us to believe SC-1000 is no longer considered a contaminant by chlorine when it has done its job.

How to prevent chlorine loss with SC-1000

Thanks to some forward-thinking customers who use SC-1000 regularly, we have some clever techniques to mitigate the chlorine loss problem. Primarily, divide the purge dose up over multiple days. Rather than dumping in the full quart per 10,000-gallon purge dose (32 fl.oz./10,000 gallons), maybe add it over 3, 4 or 5 days. Apart from a startup, there are only rare instances where urgency is needed when using SC-1000. If you divide it up, its impact on chlorine is also divided up, and free chlorine can maintain a residual throughout the process.

Another idea is to deliberately not add chlorine for a day or two when purging. Give SC-1000 time to circulate and do its job, then add chlorine. Manually feeding chlorine every day will just waste it if SC-1000 is still trying to work.

Another note is to wait until the water is warm to add it–at least above 60ºF. Almost all water chemistry reactions slow down when the temperature drops and chelation is no different. We do not recommend adding SC-1000 for winterization; instead, we recommend adding it in the springtime. But no rush on getting it in when the water is frigid! Again, we need SC-1000 to activate and do its job before chlorine can coexist with it. If the water is warm, it will activate faster, and chlorine will hold sooner.

Conclusion

Chlorine loss is a consequence we have to live with when using SC-1000, because chlorine will attack it until it has done its job. While we cannot change this reality, we can mitigate it by dividing up the purge dose, adding when the water is warm, and giving it enough time to activate before forcing chlorine into the pool. 

If your pool is already open and being used daily–say, for example, a commercial summer pool–not adding chlorine for a few days is not an option. In this case, divide the purge dose up into small, but consistent, doses. If it takes 7 gallons for your community pool, try adding one gallon a day, and maybe even breaking it down further than that. Maybe half gallon in the morning, and a half at night. This will allow your chlorine residual to stay in the water.

Most of our customers tell us that despite the chlorine loss, SC-1000 is still worth that initial headache. Because once the chlorine levels recover, calcium and metals become much more manageable. If you need help or advice on how to dose your pool, contact us, or request a training below.

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