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Re: Zinc and Copper solubility

> Subject: solubility of Zinc and Copper (technical), {but pretty rough :) PLS}
> My old chemistry text (Mortimer) gave the solubility product of some
> carbonates as follows:
> CaCO3 4.7 * 10**-9
> CuCO3 2.5 * 10**-10
> ZnCO3 2   * 10**-10
> CaSO4 2.4 * 10**-5
> No solubility products were given for the sulphate salts. Does that mean
> that the sulphate salts of those metals are fairly soluble?

	They are.  Remember the bright blue CuSO4 solutions?
> Does this mean that these anions would not precipitate in the presence
> of bicarbonate under typical aquarium conditions since calcium remains
> dissolved that way?

	All these cations have 2+ charge.  The ionic radius of Ca++ is
0.94 Angstroms, and those of Cu++ and Zn++ about 0.69 A.  The solubilities 
of salts are dependent on the sizes and charges of the ions involved (and
other things). The Cu and Zn salts behave similarly for that reason, and
the Ca salts show _some_ similarities.  The bicarbonates of Cu and Zn may
well be reasonably soluble, simply because these ions really do not pack
well with _two_ HCO3- ions in a crystal - packing with _one_ CO3-- ion is
O.K..  Remaining in the solution is more energetically favoured for the 
bicarbonates (probably!).
	To say that this is an oversimplification is quite an understatement!

> I don't think that we should say that zinc and copper are not soluble in
> the way that iron and manganese are.

	Talking about solubilities of low-concentration ions in a system
full of organic material and lots of other stuff will be a bit dicey.

> Copper (and probably other metals) are less toxic if chelated such as by
> organic humic complexes from peat.

	The chemical potential (effective concentration) is reduced.

> Sphagnum peat has low enough pH to dissolve aluminum yet we do not see
> aluminum toxicity with peat/clay mixtures which contain certain forms of
> aluminum. Is it because those aluminum compounds are not chemically
> active? 

	I don't think pH is the issue here.  Aluminium is attacked by water
if the surface oxide layer is disrupted.  Peat itself cannot have a pH.
Do you mean that water in contact with peat has a low pH?  How low?

	Clays contain aluminosilcates.  The Al ions in these are very firmly
liked into the Al/Si/O structures.  The corrosion of Al and the dissolution
of Al from clay will be pretty different.

> Al(OH)3 has solubility of 5 * 10**-33

	This number is the solibility _product_ of Al(OH)3, that is:

	[Al+++][OH-]^3   (all concentrations in moles/litre)

	At pH 7, [OH-] is 10^-7, so [Al+++] is _at_ _most_ 5*10^-12 molar
	At pH 5, [OH-] is 10^-9, so [Al+++] is _at_ _most_ 5*10^-6 molar

	The atomic weight of aluminium is 27, so these concentrations
are 0.135 * 10^-6 ppm and 0.135 ppm, respectively.  The maximum possible
aluminium concentration in the tank water is a _very_ strong function
of pH of that water.  The pH in the substrate is not relevant to that.

> From: "Roger S. Miller" <rgrmill at rt66_com>
> Subject: Re: solubility of Zinc and Copper (technical)
> On the other hand, solubility
> products are normally only tabulated for poorly soluble salts.


> What I've read is that there is a *chance* of aluminum toxicity below pH
> 5.

	This is consistent with the calculations above.

>  The chemistry is too complex to say that it definitely will be toxic.

	And how!

Paul Sears        Ottawa, Canada