phosphonic acid

[SalemCastles]

In a HTH opening kit was a bottle of phosphonic acid. It's said to prevent metal stains and "improve" the feel of the water. I have a vinyl pool and no stains but do have a Gas water heater. Is there any reason to use it? If not what other uses are there for phosphonic acid?

Glenn

[PoolDoc]

The general rule is: never use a pool chemical you don't need.

Phosphonic acid -- HEDP -- can be used to lift copper stains in the early stages. But it's effect is temporary, because it breaks down to phosphates (algae food!). I think Chem_Geek can give you some more detailed technical info.

But, the bottom line is: don't use it, if you don't need it. If you DO needed (high copper), you need to fix the cause of the problem (dissolving copper OR adding copper algicides). Then remove the copper from your pool and stop using phosphonates.

If you want to improve the 'feel' of your water, add borax.

Ben

[chem geek]

First off, I completely agree with Ben that you don't want to add something to the pool that you do not need. Since you have no metal stains and assuming you don't have high levels of metal ions (iron or copper) in your pool, then there is no need to use such a product. It will do nothing for the "feel" of the water. As Ben described, if you want your water to shimmer more and possibly "feel" better, use 50 ppm Borates.

Now if you have to use a metal sequestrant because you've got metals in the water, then HEDP is a good one to use because 1) it binds to copper and iron quite well and 2) it is reasonably resistant to breakdown from chlorine. It will break down, but relatively slowly compared to other non-phosphate based metal sequestrants like EDTA that have a noticeable increased chlorine demand when used. Of course, metal sequestrants just make metal ions more soluble to prevent them from staining, they don't actually remove the metal from the water. Ben has some techniques for that by forcing their precipitation into the filter and there is an expensive and not-yet-proven product called CuLator that may physically remove the metal, but you don't have this problem in your pool.

I wouldn't worry so much about increasing the phosphate level in the water anyway since the appropriately maintained Free Chlorine (FC) level relative to the Cyanuric Acid (CYA) level will kill algae faster than it can grow even at higher phosphate levels. I had over 3000 ppb phosphates in my pool and there are quite a few other pools reported with even higher phosphate levels that don't get algae because the proper FC/CYA ratio is maintained. Now, that said, it is true that such pools are very "reactive" in the sense that IF you let the chlorine level drop then algae can grow fairly quickly. However, ultimately, algae growth gets limited by sunlight and temperature regardless of nutrient (phosphates, nitrates) level. Algae can only double in population every 3-8 hours under the most ideal of conditions so visible "blooms" always have nascent non-visible algae growth in the days before such a bloom.

But getting back to YOUR situation, don't use this product -- it's not needed and will only increase chlorine demand (a little) and phosphates (over time) for no good reason.

[SalemCastles]

Thanks...how would you safely get rid of it other than taking to the hazardous chemical depot?

[chem geek]

You mean the metal sequestrant product? I'd just follow the instructions on the label for disposal. If there are no instructions for disposal, then just put it into the trash. The MSDS for the phosphonate products I could find just say "Dispose of in accordance with all applicable local, state and federal regulations. Empty containers retain vapor or product residue and can be dangerous." On product labels, I couldn't find any disposal information.

HOWEVER, you might just need a metal sequestrant someday if for whatever reason you do get metal ions (copper or iron) into your water so you could just keep it around in a storage shed, just in case. Up to you.

Other environmental information is as follows -- it's far less toxic to aquatic organisms than chlorine, for example.
ENVIRONMENTAL TOXICITY:
Invertebrates: 48 h, EC50 Water flea (Daphnia magna) 527 mg/l
Fish: 96 h, LC50 Rainbow trout (Oncorhynchus mykiss) 368 mg/l
96 h, LC50 Bluegill sunfish (Lepomis macrochirus) 868 mg/l
Algae: 96 h, EC50 Algae (Selenastrum capricornutum) 3 mg/l
Algal growth inhibition is due to ability of this product to complex materials not to toxicity per se.
ENVIRONMENTAL FATE
BIODEGRADATION: Zahn-Wellens Dissolved Organic Carbon removed 33 % 28 d
Modified OECD Screening theoretical CO2 evolution 2 % 70 d
Modified SCAS Dissolved Organic Carbon removed 90 %
Closed Bottle BOD30/COD 5 %
Degrades after acclimatization
BIOCONCENTRATION FACTOR (BCF): Not expected to bioaccumulate.

[PoolDoc]

Chem_Geek gave you the 'right' answer.

Here's a simpler one that's not quite so compliant with all the 'rules': HEDP is not very toxic, but is pretty acidic if concentrated. Still, if you just flush it, and then flush again after, there shouldn't be any problems.

But, I'd keep it. Just store it out of sunlight and out of severe heat, so the bottle won't deteriorate.

Ben

[rcy100]

What a viscious circle the pool stores have going.

Every opening/closing kit that I've seen where I live has - Oxidizer, 40% algaecide and "stain and scale" remover (which is the phosphonic acid). So you use this stuff at least twice a year (my pool store recommended algaecide and the stain and scale remover ONCE A MONTH), then they test for phosphates (which are increasing because of the phosphonic acid) and sell you phosphate remover.

[waterbear]

What a viscious circle the pool stores have going.

Every opening/closing kit that I've seen where I live has - Oxidizer, 40% algaecide and "stain and scale" remover (which is the phosphonic acid). So you use this stuff at least twice a year (my pool store recommended algaecide and the stain and scale remover ONCE A MONTH), then they test for phosphates (which are increasing because of the phosphonic acid) and sell you phosphate remover.

NOW you are catching on!:cool::cool::cool: