Mps

[RaviePoolGirl]

I am curious what MPS really is and how it actually works in water chemistry. When does it get used? How often it should get used? Is there a way to measure it in the water? Do I add based on a level, or just add a static amount routinely?

Is Potassium peroxymonosulfate MPS?

Thanks

[chem geek]

Yes, MPS is Potassium Monopersulfate but the monopersulfate part of the name has synonyms like peroxymonosulfate or peroxysulfate or the trade name from Dupont for the product called Oxone. When sold as a product, it is really a triple salt of MPS with potassium sulfate and potassium bisulfate. The chemical formula of this triple salt is 2KHSO₅•KHSO₄•K₂SO₄ (see Dupont Oxone).

The monopersulfate is an oxidizer where the extra oxygen is in effect in the 0 oxidation state and is sometimes called "active oxygen". This is similar to the extra oxygen in hydrogen peroxide and to organic peroxy acids.

While MPS is a somewhat strong oxidizer, it is a selective one so oxidizes some of the same compounds that chlorine does, such as creatinine, while it does not quickly oxidize others such as ammonia or monochloramine that chlorine more quickly oxidizes. So its use will generally form fewer chlorinated disinfection by-products because it will oxidize some of the bather waste instead of having chlorine do so.

MPS will interfere with chlorine test kits generally registering as Combined Chlorine in DPD tests and as Free Chlorine in FAS-DPD tests (unless the FC is 0 in which case it shows up as CC). There is a Taylor K-2041 MPS interference remover that can be used in Taylor test kits if you are using MPS and chlorine.

You normally do not need to use it if you use chlorine properly and regularly. If you do use it, then it's a supplement to using chlorine. Roughly speaking, with no ozonator, every person-hour of soaking requires around 3-1/2 teaspoons of Dichlor or 3-1/2 fluid ounces of 8.25% bleach or 7 teaspoons of non-chlorine shock (43% MPS) to oxidize bather waste. You do not maintain a constant level -- you do that for chlorine (in between soaks) but for MPS you just use it to help oxidize bather waste after a soak.

There is another use for MPS which is as an actual disinfectant when in the presence of silver ions as in the Nature² system. In that case, you use MPS alone (with the silver ions from Nature²) though in practice to keep the water clear one usually needs to shock with chlorine every week or two.

[RaviePoolGirl]

Thanks for the details Chem Geek! Curious to know if that every person-hour soaking is a set amount of chlorine or MPS regardless of tub gallons.. This is independent of tub volume? Meaning it doesn't matter if I have a 200 gallon tub or a 500 gallon tub?

[chem geek]

Yes, that rule-of-thumb is independent of the spa size because it is the amount of chlorine needed to oxidize bather waste, not the amount needed for a certain FC level (say, for disinfection). It's just a rule-of-thumb and the real amount to add is whatever it takes such that you end up with a small residual (1-2 ppm FC) 24 hours later.