Calcium Hardness ?

[Entropy]

Copper staining usually occurs when the copper ion level is above 0.3 ppm though this is dependent on pH. 0.3 ppm copper is 4.7x10-6 moles/liter concentration. So at this concentration, the half-reaction has an oxidation potential of -0.18V.

I'm not sure that I see why the copper ion concentration is relevant. The ions are not being oxidized; the solid copper is being oxidized. The solid copper is exposed to the water, which contains the oxidizers, but the copper is not in solution.

You're talking about oxidation and about staining, which are two different things. First, the solid copper is oxidized to Cu(+2) ions, and then it combines with anions, such as oxides or hydroxides to form stains. Copper stains are not caused by oxidation; the copper is already oxidized.

The copper can exist in the water as ions until the water becomes saturated with a particular copper product, such as copper (carbonate, borate, cyanurate, phosphate, oxide, hydroxide etc.)

Iron can be further oxidized from iron II to iron III. However, I don't think that copper undergoes further oxidation once it is oxidized from solid copper to copper (+2) ions.

[chem geek]

I wasn't saying that the oxidation of copper directly causes staining nor that copper ions get oxidized (they don't). What I was saying (or meant to say, since I wasn't clear) was that oxidation of solid copper produces copper ions in the water and if these rise too much in concentration from such continued copper corrosion (especially if the pH is higher), then staining can occur.

The poster was wondering how solid copper could corrode with acid since the thermodynamics indicated that it wouldn't. I was just pointing out that 1) he was right if there were no oxidizers in the water and 2) that even dissolved oxygen in the water is strong enough to corrode copper and 3) chlorine is a strong oxidizer so would increase corrosion. IF such copper corrosion (oxidation) occurred, then this would increase the level of copper ions in the water and then staining would become possible if such levels got too high.

[Andrew88]

Richard, do you know the chemical formula of the black stains? Also, do you know the chemical equations that describe the formation of the black stains?

[Richard The Third.]

Richard, do you know the chemical formula of the black stains? Also, do you know the chemical equations that describe the formation of the black stains?

Nope.
But here is the formula for an idiot:
Sneek in a bathroom and look in the mirror.

[chem geek]

I'm not positive specifically about the black stains, though copper oxide is typically black. Copper oxide can form directly from the metal as follows:

2Cu(s) + O2 ---> 2CuO(s)

However, when forming from copper ions, it probably goes through either copper hydroxide or copper carbonate solids first as follows:

Cu2+ + 2OH- ---> Cu(OH)2(s) which is blue or blue-green
Cu2+ + CO32- ---> CuCO3(s) which is light green

The normal green patina on copper exposed to air is likely a mixture of the above. These solids can form copper oxide, though in moist air this usually occurs slowly (perhaps in pool water it can occur more quickly):

Cu(OH)2(s) ---> CuO(s) + H2O
CuCO3(s) ---> CuO(s) + CO2

It is also possible to get copper cyanurate stains which are amethyst (purple) in color:

3Cu2+ + 2C3N3O33- ---> Cu3(C3N3O3)2

though that usually is only seen if the CYA level is quite high (> 100 ppm).

Basically, higher pH and higher copper ion levels don't mix and can precipitate (stain) in a variety of ways depending on the TA and CYA levels.

[Andrew88]

Nope.
But here is the formula for an idiot:
Sneek in a bathroom and look in the mirror.

What is your problem?

[Guest]

To avoid foaming, you can raise it to 100-120 ppm. It doesn't have to be very high unless you have exposed grout in tile, plaster, etc. A vinyl pool does not need a higher calcium level. There is controversy over whether it is needed to prevent metal corrosion, but pH is the primary factor for metal corrosion (i.e. don't let the pH get very low -- keep it near 7.5 and not below 7.0 for any extended length of time).

So my 18 feet above ground pool does not need the calcium hardness raised?

[chem geek]

If you do not have plaster or grout exposed to the water, then you do not need calcium in the water.

[chem geek]

Again, if you do not have a plaster pool or grout exposed to the water, then you do not need extra calcium. So if you have a vinyl pool you're fine. If you have a plaster pool you should increase your CH, probably to around 300 ppm.