Black Stains Where Granulated Chlorine Falls

[sailor21]

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.

[chem geek]

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]

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]

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]

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.