Limits of Copper Concentration

[chem geek]

The main problem with copper is that it will precipitate out of solution, initially coloring the water green but also (possibly over a longer period of time) staining plaster. Though the solubility product of copper hydroxide implies that it should precipitate first, it does not due to soluble ion pairs (including neutral copper hydroxide itself). The data for actual solubility of copper hydroxide (which is blue -- see here) is inconsistent so instead I will focus on the solubility of copper carbonate (which is blue-green -- see here).

At standard conditions of pH 7.5, TA 100, CYA 40, CH 300, TDS 535, Temp 80F, the maximum copper concentration is 2.52 ppm. If one adds a shock level of chlorine to go from 3.5 ppm to 16.0 ppm FC, then the pH will rise to 8.14 and the maximum copper concentration drops to 0.56 ppm. So adding chlorine can make the water turn green and this is almost a sure sign of copper since adding chlorine, if at sufficient levels relative to CYA, usually bleaches any existing algae making it less green while with copper the pool will turn more green. Eventually as the FC level drops, the pH will drop as well and the copper will redissolve and the green will dissipate.

If one starts with a pH of 7.2, TA 93.4, by first adding Muriatic Acid (making the maximum copper limit is 5.3 ppm), then the chlorine addition will only raise the pH to 7.56 with a maximum copper limit of 2.19 ppm. So one way of avoiding green when there is copper while still being able to shock the algae is to lower the pH first. This also makes the chlorine more effective since the disinfecting chlorine concentration in the case above would be 0.243 ppm while first lowering the pH has the disinfecting chlorine concentration be 0.295 ppm.

The easiest way to know if green is from copper or algae is to see if there is any significant drop in FC level overnight. Algae will cause a drop in FC while copper will not. Also, if the water is clear but green and was not cloudy or dull previously, then this is more likely to be copper than algae. Sometimes, water can become cloudy first and then green and still not be algae. This would occur if already at calcium carbonate saturation, but adding more pH Up or Alkalinity Up which makes the water cloudy and then adding a copper-based algaecide which then makes the cloudy water turn green (this thread describes just such a situation).

Richard

 

[JasonLion]

The copper doesn't need to precipitate out of solution in order to turn the water green. Copper(II) chloride can turn the water green while disolved if it gets to that oxidation level, which FC can generally provide.

 

[chem geek]

I thought that copper chloride only turned green at higher concentrations. There was a user who reported an incompatibility with using copper in their salt (SWG) pool and that might explain it though I suspect it was more related to the generally higher pH in SWG pools (and assuming a plaster pool so calcium carbonate was saturated).

 

[Guest]

Realize that this does not apply to the chelated copper (triethanolamine copper complex) that is usually the form of copper in algaecides. However, when the chelating agent breaks down (as they all eventually do) then it becomes applicable. That is why treating with a copper based algaecide does not usually cause these types of problems initally but they appear a few months later, since these forms of chelated copper tend to be stable for a few months.

 

[JasonLion]

There are so many different compounds the copper can be in, I am having some trouble trying to even list them all so I can start comparing the reaction rates. Copper (ii) chloride is only one candidate for the color people describe, which I have heard described as more of an emerald green, while copper (ii) chloride would be more of a blue green. The fact that green water doesn't happen more often suggests there is some additional condition on the reaction, things like the presence of amonia suggest it's self as a candiate. Add in the amonia reactions and things get even more complicated.

Copper (ii) chloride turns into copper with six waters (hexaaquacopper(ii) ion) when dilutted, which is blue in solution. That doesn't seem to actually happen as no one is reporting blue water. So I am not clear if the copper (ii) chloride can actually occur or not, since it would tend to shift to blue as the hexaqua conversion happened. Still, none of the standard text book reactions are done in the presence of HOCl. I am trying to figure out if the HOCl reactions will dominate the the reaction rates and push things towards copper (ii) chloride, which I suspect they will.

Anyway, relating back to your original questions about solubility, some of the copper compounds are more soluble. My impression so far has been that which compound is dominant will vary a little bit depending on some of the other factors (PH, amonia, HOCl, etc), which will affect when the copper percipitates out. Though I must admit that I have yet to work anything out clearly (so it could be very simple despite my difficulty in concluding that).

 

[duraleigh]

Hi, guys,

This is a great thread which, unfortunately, has passed my limited ability. Still interesting to read and I admire ya'lls' curiosity and determination to pursue the answer(s).

I PM'd Richard but would like to open this up to everyone's opinion. It seems to me (suggested by Richard) that the way to get TFP posters to test for copper is simply to test for algae first.

An overnite FC test will determine the presence of algae and, if that proves true, would eliminate the need to test for copper in all but perhaps some very remote circumstances where both are present. Conversely, if the overnite test proves negative for organics and the pool is green, copper is easily the most logical answer.

The above thinking, if true, really helps the average poster from having to chase down a good copper test and would quickly get him/her on track for the correct solution.

 

[JasonLion]

There is a pattern that comes up ocassionally. People say, "I added chlorine and my pool went from clear to transparent green!" If I hear that I think copper. If the water is at all cloudy or murky then it is surely algea. It is only the rare situation where the water is a clear transparent green and didn't turn green when adding chlorine where there is ambiguity. Then they should do the overnight chlorine loss test.

The best I can tell, if they have copper and algae at the same time the algae will consume all of the chlorine and the water won't turn green from copper (because that seems to require free chlorine to happen). The problem will appear to simply be algae and the copper problem won't become obvious until later.

There are really several things going on in this discussion. I am interested in figuring out the chemistry of the water turning green from copper. duraleigh appears most interested in diagnosing copper vs algae. chem geek is working on copper soluability. Fortunatly, the answer to any one of those questions will shed some light on the other two, though they still reamins separate issues.

 

[chem geek]

Realize that this does not apply to the chelated copper (triethanolamine copper complex) that is usually the form of copper in algaecides. However, when the chelating agent breaks down (as they all eventually do) then it becomes applicable. That is why treating with a copper based algaecide does not usually cause these types of problems initally but they appear a few months later, since these forms of chelated copper tend to be stable for a few months.

BioGuard Smart Shock (see this MSDS) appears to be Dichlor plus some metallic copper but mostly copper citrate. It does not appear to have any chelating agent. That would explain why the user with over-saturation of calcium carbonate having a white cloud and then adding this algaecide to the already high pH pool then found the cloud turn green.