The cathode can be a generic piece of sheet metal, non galvanized. Or
copper, or better yet, titanium, but I doubt you have any of that on hand

> By the way, I think I can find a relatively cheap source for solid graphite: fishing rods / golf clubs (OK maybe not the latter).

- Not the right form. Those are carbon fiber, which may or may not be
graphite as such. The usual form is a slab of industrial graphite. A
single crystal slab would probably be even better but would be just a bit
more expensive...if even produced.

> What type of amperage are we talking here? I am assuming that you are
> talking about DC, yes? This is, perhaps, the most promising solution
> for reasons I state below.

Ya, probably 10 amps. You want more surface area and circulation than you
do current. The problem you'll have is running out of copper atoms to
remove from a given volume of solution around the electrodes.

> No, there is no constant source. This winter, our outdoor fire pit
> (copper bowl) got blown into the pool.

Hmm, seems to me that couldn't possibly provide much copper, unless the
whole thing disappeared... I might be underestimating the strength of colors, though. (Potassium permanganate for instance makes solutions so strong you can see parts per million in solution!)

> I didn't realize it until very
> recently. I am positive that this is the source as all the pool plumbing
> is PVC. And it's true that the pH of the water was slightly acidic over
> the winter (about 6.8 or so).

Eh, I don't know that that's enough. But I don't have a pool, either.

> I am guessing that since I brought the pH into nominal for
> pools and then dumped in the chlorine the copper precipitated out of the
> water and attached itself firmly to my pool liner.

That could certainly do it. I've ran experiments where copper compounds
precipitated on the plastic container wall. Glass is much better for the
most part. Not that that helps you any.

> I googled EDTA and it turns out that the main component of "Metal Out",
> mentioned previously, is EDTA with a twist, MSDS lists its chemical name
> as: Ethylenediaminetraacetric Acid Tetresodium Salt. Salt being the
> difference.

Ah, that'll do fine, too. You also see calcium disodium EDTA a lot. I
should've mentioned, all that matters is the EDTA molecule is there and not
attached to anything stronger than it attaches to the things you want to
remove (copper or iron in this case). It does form a salt with sodium and
calcium, but it doesn't complex with them nearly as strongly as with iron or
lead.

> Could this possibly cause the chemical in "Metal Out" to be
> weaker than other sources? The problem here is that even though this
> works, it's temporary as indicated in my earlier email. As I understand
> it, my pool filter (diatomaceous Earth) filters particles no smaller that
> 5 microns when clean and presumably when EDTA binds with the copper in
> solution the particles are smaller than I can filter. Can you shed light
> on that?

I would think the stuff would make the ions precipitate, but if there are
few ions in solution, I suppose it would be kind of hard for the molecules
to clump together and form large particles.

> Yes, this is my experience. Unfortunately it precipitates right onto the
> liner of my pool and no amount of scrubbing takes it off.

You might be able to rub it off with a mild hydrochloric acid solution. Use
lightly around grout, if any. Don't really know how you could do that
without draining the pool, though.

> Thanks again for taking the time to help me solve a problem pool owners
> have been dealing with (unsuccessfully -- as in not permanent) forever.

You're welcome.

Tim