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Alkalinity and copper
---------- Forwarded message ----------
Date: Fri, 4 Apr 1997 13:05:19 -0500 (EST)
From: "Roger S. Miller" <rgrmill at mack_rt66.com>
To: Aquatic-Plants at actwin_com
Subject: alkalinity and copper
First about alkalinity. Organic acids do add to alkalinity and invalidate
the interpretation that alkalinity is equal to bicarbonate concentration.
Cases of significant interference are fairly rare in unpolluted water.
Several years ago I read an article where the potential interference was
considered significant and the authors played with the issue a little.
They found that they could determine whether organic acids contributed to
the measured alkalinity by measuring the pH as they performed the
alkalinity titration. The titration is done by gradually adding a strong
acid to the sample until the pH falls to 4.5. If organic contributions to
alkalinity are important the pH tends to decline gradually as acid is
added to the sample. If the alkalinity is due largely to bicarbonate,
then the pH remains nearly constant as acid is added, then - over the
course of just a few drops - it falls to a pH near or below 4.5.
The method requires little more than an acid, an eyedropper and a pH
meter. It can be used to find out if you have organic acids fouling up
your CO2 determinations, but it doesn't correct the numbers and it might
be difficult to do with nice, clean water.
And now to copper...
I was surprised the find that anyone would intentionally add copper to a
plant fertilizer. The following is taken from "Quality criteria for
water, 1986" EPA 440/5-86-001.
"Acute toxicity data [for copper] are available for species in 41 genera
of freshwater animals. At a hardness of 50 mg/L the genera range in
sensitivity from 17.74 ug/L for _Ptychocheilus_ to 10,240 ug/L for
_Acroneuria_. Data for eight species indicate that acute toxicity
decreases as hardness increases. Additional data for several species
indicate that toxicity also decreases with increases in alkalinity and
total organic carbon."
"Chronic values are available for 15 freshwater species and range from
3.873 ug/L for brook trout to 60.36 ug/L for northern pike. Fish and
invertebrate species seem to be about equally sensitive to the chronic
toxicity of copper."
"Toxicity tests have been conducted on copper with a wide range of
freshwater plants and the sensitivities are similar to those of animals."
Note that the values sited are in _micro_grams per liter, not in ppm or
milligrams per liter. If you start talking about copper at mg/l or ppm
levels then you are talking about copper concentrations that are 1 or 2
orders of magnitude higher than the concentrations where chronic toxic
effects occur in both plants and animals.
Copper is an essential nutrient, but it's needed at levels that are
incredibly small and normally present in plants at levels that can't
normally be measured. Plants and algae do adsorb it when its available
and can concentrate it to toxic levels - toxic for the plant and
potentially toxic for anything eating the plant or algae (oh no! my poor
SAEs!, my shrimp!). Pamela Stokes (Copper accumulations in freshwater
biota _in_ Copper in the Environment, Jerome O. Nriagu, ed., 1979) offered
an excellent summary of copper bioconcentration in algae and macrophytes.
Copper is not common in unpolluted surface water. Edward Boyle (Copper in
natural waters, also in volume by Nriagu) reports copper analysis from 17
locations in the Amazon Basin, and none of the concentratons exceed 40
nm/L (about 2.5 _micro_grams/liter).
So how does anyone get by with adding copper to an aquarium plant
fertilizer? Much of the copper in solution combines with hydroxide, with
bicarbonate or with complex organic compounds that appear to be less toxic
(see, for instance Bioaccumulation and toxicity of copper as affected by
interactions between humic acid and water hardness by Robert Winner in
Water Resources, v. 19, 1985). The actual toxicity of copper in your tank
will depend on the stability of the original chelate (assuming that it is
chelated), on the pH and alkalinity of your aquarium, and on the presence
of other organic compounds in the water. And of course, on the
sensitivity of the plants and animals you keep. These are all factors
that vary from tank to tank, so copper levels that are relatively safe in
one tank could potentially destroy another. The worst case would probably
be when the water is comprised mostly of RO product, it is changed
frequently and few if any hardening chemicals added, where the pH is
maintained below 7 and nutrients are added through regular addition of a
copper-containing trace element mix.
Why take the chance?
Roger Miller