Is copper EDTA safe for my tank

[lakshwadeep]

But so is copper (II) chloride, right?

Yes. As I have said before, it's not clear which copper species is actually used. My mention of copper(I) chloride was only to give an example of why a metal would need to be attached to EDTA to avoid it precipitating out in a solution of chloride. It could also be that some redox reactions would occur between the different metals, but that will take some time to calculate. Basically, there must be a reason EDTA was used for not only copper but other metals in the bottle and yet not all the metals.

 

We've done that in the past to keep it between 0.17-0.22 ppm. This is the "safe" range for fish but will stop the life cycle of Cryptocaryon. If there's any calcium carbonate in the tank it will take a ton of copper just to get it above zero, but once you do the additions are fairly straightforward.

 

I didn't mean to imply there is no possibility of dissolved copper ions, which is why I wrote that copper hydroxide/carbonate compounds are "problems".

 

Here's an overview of the percent contributions of different copper species/compounds in seawater at a salinity of 35 ppt, taken from figure 30.13 of An Introduction to Marine Biogeochemistry by Susan Libes (p. 624). The units are % total metal, and the scale of the graph was logarithmic, making the exact numbers used somewhat difficult to identify.

 

 

Cu(OH)₂ ~80%

Cu-Humic* ~10%

CuCO₃ ~5%

Cu²⁺ ~.7%

 

*humic acids

 

 

Dunno what to make of that list, lithium hydroxide and sodium hydroxide are plenty soluble. And calcium hydroxide is of course soluble enough to make kalkwasser...

 

Note in that list that sodium hydroxide is a "sodium salt" mentioned in part 1 and lithium was not mentioned explicitly but should have a similar activity as sodium and potassium since they are both in group I of the periodic table. I apologize for linking to an overgeneralized solubility list. Here's a more specific list:

• 1. Salts containing Group I elements are soluble (Li+, Na+, K+, Cs+, Rb+). Exceptions to this rule are rare. Salts containing the ammonium ion (NH4+) are also soluble.
  
• 2. Salts containing nitrate ion (NO3-) are generally soluble.
  
• 3. Salts containing Cl -, Br -, I - are generally soluble. Important exceptions to this rule are halide salts of Ag+, Pb2+, and (Hg2)2+. Thus, AgCl, PbBr2, and Hg2Cl2 are all insoluble.
  
• 4. Most silver salts are insoluble. AgNO3 and Ag(C2H3O2) are common soluble salts of silver; virtually anything else is insoluble.
  
• 5. Most sulfate salts are soluble. Important exceptions to this rule include BaSO4, PbSO4, Ag2SO4 and SrSO4 .
  
• 6. Most hydroxide salts are only slightly soluble. Hydroxide salts of Group I elements are soluble. Hydroxide salts of Group II elements (Ca, Sr, and Ba) are slightly soluble. Hydroxide salts of transition metals and Al3+ are insoluble. Thus, Fe(OH)3, Al(OH)3, Co(OH)2 are not soluble.
  
• 7. Most sulfides of transition metals are highly insoluble. Thus, CdS, FeS, ZnS, Ag2S are all insoluble. Arsenic, antimony, bismuth, and lead sulfides are also insoluble.
  
• 8. Carbonates are frequently insoluble. Group II carbonates (Ca, Sr, and Ba) are insoluble. Some other insoluble carbonates include FeCO3 and PbCO3.
  
• 9. Chromates are frequently insoluble. Examples: PbCrO4, BaCrO4
  
• 10. Phosphates are frequently insoluble. Examples: Ca3(PO4)2, Ag3PO4
  
• 11. Fluorides are frequently insoluble. Examples: BaF2, MgF2 PbF2.
  

http://www.csudh.edu/oliver/chemdata/solrules.htm

[markandstaci]

Also note that "insoluble" reactions does not really mean that the solid/salt is completely insoluble. For pretty much every reaction there has to be equilibrium between a non-dissolved and a dissolved phase, and the concentrations of dissolved ions are related by an equilibrium constant. For extremely insoluble salts, the free ion concentrations can be on the order of 10^-8 to 10^-12 moles/L. For slightly less insoluble salts, free ion concentrations can be on the order of 10^-3 to 10^-7 moles/L. But for some ions, very low concentrations can still have meaningful effects in the aquarium. The EDTA in the Kent Marine solution is mostly likely used to prevent rapid bio-availability of free ions like Cu²⁺ to corals while still allowing low level concentrations that are probably required for biological purposes. Additionally, keeping mostly insoluble metal ions bound to a bigger, more stable group of ions (it's called a ligand) helps keep them in solution by preventing less soluble anionic interactions (like with CO₃^2- or S^2-) from occurring - i.e. it keeps salts from precipitating out of solution.

 

Here's some references and stuff:

http://en.wikipedia.org/wiki/Equilibrium_constant

http://www.jesuitnola.org/upload/clark/refs/aqueous.htm

http://en.wikipedia.org/wiki/Chelating_agent