Stain Treatment - Muriatic vs. Ascorbic

[douglee25]

I recently bought this house that had an inground pool (~20,000 gallons) and hot tub (~2500 gallons). I spoke to the old pool company who serviced the pool for several years and he mentioned that he painted the spa about a year ago. The spa always had a slight yellow color to it which I attributed to the paint. The pool has always been clear/blue.

Fast forward to this past weekend... we had a party and about 10 people+ were in the tub. I noticed when I was testing the water 2 days ago that the spa was much much clearer. The closer I looked, the more I saw the yellowing was gone. There still is some remaining stains on the side, etc. It appears as though I had iron staining the whole time. Last night I poured some stain control in and isolated the spa and ran the filter over night. The stains appears to be lightened some. I think this confirms iron staining.

My question is, can one use muriatic acid vs. ascorbic acid in the spa and just circulate and brush? Wouldn't this lift the stain as opposed to ascorbic acid?

Doug

 

[bk406]

Those are 2 different types of acid. They are not interchangeable for what you want to do. Although the muriatic acid could reduce the iron, it's not advisable.

Muriatic acid is actually hydrochloric acid, or HCl. It's known as a strong acid. That is, the "H", which is a proton, more completely disassociates from the "Cl" part. The pH is very low since the concentration of hydrogen ion is very high due to the high disassociation..

Ascorbic acid is known as a weak acid, and is also classified as an organic acid. The pH of ascorbic acid is higher than that of HCl. The proton that disassociates is harder to pull off than the proton on HCl, so the pH is not as harsh. Both acids added to the water will lower pH. But, ascorbic acid added in the quantities needed to reduce the oxidized iron (rust stains) will not be nearly as harsh as adding a strong, inorganic acid.

 

[chem geek]

It's not the difference in acidity between the two acids that is the issue here since one can lower the pH using any acid one wants. What makes ascorbic acid (AA) different from a regular acid is that it is also a reducing agent. It can reduce ferric iron to ferrous iron and the latter is more readily captured by metal sequestrants and stays dissolved better in water. Iron metal stains are mostly oxide-hydroxide combinations with ferric ion. The lower pH combined with the reducing agent power of AA help dissolve the metal stains and convert the ferric ion to ferrous ion.

The fact that AA is a reducing agent is why it will consume all of the Free Chlorine (FC) in the pool and why you lower the FC first before adding AA. Chlorine will oxidize AA or equivalently AA will reduce chlorine.

 

[douglee25]

Thanks for the help people. I could actually rub the spot with my hand and it would slowly wipe away. I think because my spa is manageable, I can just scrub it with a magic eraser and be done with it. I'll just use some sort of metal out from this point on.

Doug

 

[bugger23]

where do i get absorbic acid ?

 

[JasonLion]

You can find ascorbic acid, some times labeled at vitamin C, in some health food store, at some Trader Joes grocery stores, and various places on the Internet, including eBay and The Chemistry Store. Prices vary quite a bit, so it is worth doing some shopping around.