Black Stains Where Granulated Chlorine Falls

Stains on the pool surfaces, pool equipment
or on the swimmers, or off-color swimming pool
water. Discolored but clear pool water.
sailor21
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My Pool: 25' x 8' with 6' Dia Jacuzzi
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Black Stains Where Granulated Chlorine Falls

Postby sailor21 » Tue 06 Oct, 2009 12:41

Back in July, I had a HUGE green algae problem. I super-chlorinated and added a double dose of Copper Based algaecide (Strike Out) twice. After the water cleared up and the DE filter dismantled and cleaned, I now get black stains wherever the powder chlorine touches the pool bottom.

Any help?

It is a plaster pool.
Attachments
DSC00048.JPG
After 3-4 days of major chlorine & algaecide
DSC00048.JPG (41.56 KiB) Viewed 8258 times
New Image.JPG
Before. this is what I found when getting home from vacation.
New Image.JPG (46.54 KiB) Viewed 8252 times


chem geek
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Black Stains Where Granulated Chlorine Falls

Postby chem geek » Tue 06 Oct, 2009 13:30

The stains are most likely from the copper. The granulated chlorine is probably Cal-Hypo so where it is dissolving locally it is high in pH and will precipitate oxidized copper (copper hydroxide, copper oxide, copper carbonate, etc.). This is one reason why one should generally not use a copper-based algicide.

If you lower the pool's pH, the stain may redissolve. Or you can use the ascorbic acid procedure described here for removing the stain and add a metal sequestrant to keep the copper in solution. Longer term, you should dilute/replace your water to remove the copper completely and don't ever use a copper-based algicide again.

A better way of getting rid of algae without side effects is described in Defeating Algae .
Entropy

Black Stains Where Granulated Chlorine Falls

Postby Entropy » Tue 06 Oct, 2009 21:45

You need to post all of the following chemical readings. Chlorine, PH, alkalinity, calcium, cyanuric acid, iron, copper and manganese.

The black is probably Copper(II) oxide (Cupric oxide = CuO). You can use hydrogen to reduce it.

CuO + H2(g ) --> Cu + H2O
sailor21
Pool Enthusiast
Pool Enthusiast
Posts: 17
Joined: Tue 06 Oct, 2009 12:08
My Pool: 25' x 8' with 6' Dia Jacuzzi
Hayward EcoStar Pump
Hayward Aqua Plus system with SWG
Location: San Juan, PR

Black Stains Where Granulated Chlorine Falls

Postby sailor21 » Wed 07 Oct, 2009 06:55

Thanks for the help!

It is a shame, the copper-based agaecide works great, so now I have clear water, black stains when I use granular chlorine and a Naggin Wife! Hehehe!

I will first try reducing the Ph and switching to another algaecide. Which one yo you recommend?

How is the hydrogen method?

Thanks again!
Entropy

Black Stains Where Granulated Chlorine Falls

Postby Entropy » Wed 07 Oct, 2009 17:08

Hydrogen is probably not practical unless you have access to hydrogen gas. You just have to get the hydrogen to the stains. You could use vinyl tubing and maybe add a plastic cup or bowl to the end to get better coverage. Also, hydrogen gas is explosive if it is exposed to an ignition source, so caution is required.

Try ascorbic acid, oxalic acid or citric acid to remove the stains.

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