vacuuming the purple residue

[rlsmith]

Also, is okay to vacuum this stuff up.

[Guest]

If it is a purple dust i would vacuum to waste usually a purple dust is high CYA combining with copper...The only way to get rid if high CYA is water replacement i would also add metal control after doing some really good draining

[chem geek]

That's the first I've heard of purple dust possibly being from a combination of CYA and copper. This link and some other sources talk about it so thanks for bringing this up. This helps solve yet another mystery we sometimes see on these pool forums. I just love solving these mysteries. Thanks again for the tip.

Another mystery, totally unrelated, is when some pools keep dropping in pH or move to a low pH even though there is no addition of any acidic chemical (i.e. no Trichlor or Dichlor -- only hypochlorite sources of bleach -- no non-chlorine shock, etc.). Since pools are intentionally over-carbonated and the outgassing of carbon dioxide makes the pH rise, I would think that pools using only hypochlorite sources of chlorine would be either stable in pH or rise in pH -- not lower in pH. The usage of chlorine is acidic, but exactly compensates for the rise in pH from the hypochlorite sources (well, there's a small amount of extra lye, but it's pretty small). Any idea what might cause a drop in pH in a very small number of pools as I described? The fill water was not acidic and there was no rain. The water balance had a normal saturation index so there wasn't scaling going on (which would make the pH rise).

Richard

[mr_clean]

CYA at high levels cause purple dust/ring, does not have to have high copper level to cause reaction.

[chem geek]

I have seen this study here where very high CYA levels (above 150 ppm) may lead to plaster degradation, but no mention of purple dust even at 500 ppm. If purple dust/ring can form from CYA alone, it must take longer than the 5 weeks of the study or it may indeed need something else such as copper. A low level of 0.1 ppm copper would only be 5.7 grams in 15,000 gallons so wouldn't expect "dust" of any significance. 1 ppm copper and 57 grams, maybe (this weight is of the copper alone -- the copper cyanurate is about 3 times heavier than copper for the same molar quantity).

Richard