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Re: solubility of Zinc and Copper (technical)

On Mon, 4 May 1998, Steve Pushak wrote:

> 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?

Maybe the tables are just incomplete.  On the other hand, solubility
products are normally only tabulated for poorly soluble salts.
It may be that they are missing from the table because they are soluble.
Copper sulfates are soluble, and that's the form they're sold in for
treating tanks (and ponds and lakes).  Don't know about zinc sulfate.  I
imagine that it's pretty soluble, as well.

> Does this mean that these anions would not precipitate in the presence
> of bicarbonate under typical aquarium conditions since calcium remains
> dissolved that way?

Nope.  Doesn't mean that.

> I don't think that we should say that zinc and copper are not soluble in
> the way that iron and manganese are. (referring to Neil's comments on
> substrate minerals) These metals, if present, can be toxic to plants and
> animals at high enough concentrations.
> Copper (and probably other metals) are less toxic if chelated such as by
> organic humic complexes from peat. This is one reason why the humic
> material in soil or peat helps to prevent possible toxicity problems
> with certain kinds of soils.

Over the last couple decades there's been something of a revolution in our
understanding of the natural occurance of copper and zinc.  In particular
it turns out that the concentrations of these metals in natural water are
probably much lower than they were once thought to be.  Many earlier
analyses - including virtually all analyses included in our big public
data bases - were collected with methods that may have allowed the samples
to be contaminated with copper or zinc.  The metal originating from the
contaminant sometimes being far greater than the metal dissolved in the
natural environment.

In solution, copper and zinc tend to form complexes with hydroxide,
carbonate and humic acids and possibly other things as well.  The
complexed metals aren't as toxic as the unassociated metal, but its quite
difficult to predict how much of the metal in any given sample is
complexed and how much is in the unassociated, highly toxic form.

Generally, the higher your pH and the more minerals and organic "stuff"
you have in your water, the less likely you are to have problems with
copper or zinc toxicity.  But there's no guarantee that the complexed
metal will remain benign once its ingested by a fish or effected by
the chemical and biological microenvironments that can exist in a
substrate pore spaces.

It's a good idea to minimize the amount of copper or zinc that you
expose your tanks to.  For most of us the risk of toxicity is small but
the consequences are very damaging and difficult to diagnose.

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

What I've read is that there is a *chance* of aluminum toxicity below pH
5.  The chemistry is too complex to say that it definitely will be toxic.
If the aluminum in the substrate is present mostly in clays, then at low
pH those clays may remain stable, or react to form other clay minerals
with the same aluminum content.  In addition to pH, the concentrations of
silica, potassium and sodium may play an important part in keeping
aluminum in non-toxic forms.

I measured the reaction pH of the peat I've been using (briefly, soak the
peat in distilled water, then drain off the water and measure its pH).
The pH was over 5.  In "real" water other buffers may help keep the pH

On a different note...  According the US EPA, boron-sensitive plants can
show toxic effects with only 0.75 mg/l of boron.  Its a good idea if
you want to try dosing with boron to be very careful about how you do it.

Roger Miller