Chemical test for copper uptake in silicone?


O. vulgaris
This is a general question for the group, especially chemists:

It seems that the issue of copper absorption into silicone (and then leaching back out) comes up all the time in this forum. (Most commonly when people are buying pre-owned aquariums, and worried about previous use of copper medication). It also seems like nobody is entirely sure whether the silicone takes up the copper or not, so we've decided as a group to stay on the safe side and not risk putting an octopus in an aquarium that has ever been exposed to copper.

So here's the question: Couldn't we do a test and resolve this once and for all? I'm not a chemist, but I do work at a university and could probably cook up an experiment if given some guidance. For example, put a few blobs of silicone on some glass samples, soak them in a variety of common copper aquarium medications, clean them off, then test for copper residue. We could do acrylic too, while we're at it. The first part is pretty straightforward, but I have no idea how to test for the copper residue.

In the end, what we care most about is the re-cuprifying (made-up term) of the water. What if I soaked the samples in aquarium-type saltwater for a while and measured the water for trace amounts of copper? I think it would only be convincing if I used an extremely sensitive copper test, and perhaps soaked for a long time.

Any ideas? Suggestions? Finding that there is copper would give an easy answer, but finding no copper would be tougher: What kind of test would convince you that there was really no copper left (to the point that you would trust putting an octopus in such a tank)? For example, what level of precision would the residue test have to be (reading zero) to be low enough?


Staff member
Good ideas and good questions but we don't have anything but strong anecdotal evidence of residual copper and quantity is not known. Both Roy and Jean have had major problems with residual copper (copper contamination known) and animals kept dieing in the lab and public aquarium envirnoments. In Roy's case the tank was abandoned for use with cephs, Jean's tanks were eventually cleared (not silicone) I believe with some kind of resurfacing. Hopefully both will chime in with specifics.

On a high level (discounting the "quantity") you can experiment with a simple test to see if it goes anywhere and then consider refining. My thought would be to attach a glob and a thin layer of both aquarium silicone (this will be vinegar based) and household silicone (ammonia based and why we don't use it) and a piece of acrylic in separate glass bowls of salt water and then throw in a penny (the older the better but even today's coated ones will work). I experimented slightly with the penny and a Poly-Filter (not to be confused with the normal polyester material. This changes color and will show either blue or green for copper. There are also some test kits for copper on the site that will help determine what you would need in the lab for copper detection). I left the penny for a week or more before adding the filter. It immediately detected the copper so I know you can get an initial test this way. Once you detect copper, remove the penny and start experiementing with removal. Leave the silicone attached to the substrate (glass container). Once you no longer see copper on the poly-filter, you can refine detection with chemicals and note the dilution. Once you cannot detect copper, let the water sit and test weekly for reappearance.

This does not fully answer the questions and only positive traces will be meaningful but would be a start in formally observing copper retention in silicone seals. There is also the possibility that it is not so much silicone retention but copper being trapped between the silicone and the glass. This is harder to recreate as there will be organics in that thin space that would not be present in a lab created test.


O. vulgaris
I realize that this is probably old news to everyone here, but I'll write it up anyway...

Test kits:
There are a few standard test kits on the market:

1. API, which appears to detect down to 250 ppb (ug/L) free or chelated copper (also looks difficult to read at the low end).
2. Salifert, which is designed to detect down to 50 ppb dissolved or weakly chelated copper (strongly chelated copper not detected).
3. SeaChem, which is designed to detect down to 10 ppb, all types.

Natural and dangerous concentrations:
I found some online information indicating that the danger zone for copper content is around 50 ppb. They also say that natural seawater has around 30 ppb copper. That's awfully close to the danger zone already!

Another site states that natural levels are 0.25-0.38 ppb (just 100x lower...). This is a good article, which discusses the importance of how the copper appears (bound or not). In natural seawater, it is nearly all bound, and not dangerous. When it is applied in an aquarium as medication, it is unbound and dangerous. So the comparison of copper levels to natural abundances is not really as useful as it might seem.

A third site places the natural level between 1 and 10 ppb, but then shows plots of concentrations around 1-2 ppm (again, a 1000x discrepancy).

The SeaChem test kit instructions put the natural level at about 3 ppb.

First of all, there is much to be confused about regarding the conflicting statements on natural and danger-level concentrations. This needs to be resolved.

For the moment, lets assume that natural seawater contains about 1 ppb, and that concentrations of 50 ppb are dangerous to octopuses (the second number being the critical one, of course). In that case it seems that the SeaChem test would be adequate to do the experiment. I might order a SeaChem kit, but I'm still not entirely convinced that the numbers above are correct, given all the apparent discrepancies.

PLEASE correct my numbers if I've misinterpreted them!



Colossal Squid
We resurfaced the tank with stuff called Rhino lining. The whole aq ia now being demolished so hopefully we can redesign a wole new octi tank :grin:

We had very tiny amounts of copper (we never measured it) but it came from old copper nails in a piece of wooden shipwreck (we think).

We are now very copper shy and won't have it anywhere!


O. vulgaris
No, I haven't pursued it at all. It would be nice to perform the experiment in a clean enough way to publish the results, but I need some advice from a chemist. The only thing that I did was to buy one of those SeaChem copper test kits for my own aquarium, since it looked like the most sensitive. (My water came up clean, with zero detectable copper. I expected as much, but wanted to be absolutely sure.)

Maybe I'll go find someone in the chemistry department at the university here to draw up an experiment. I think it would be a useful result, especially since many different inverts are susceptible to copper poisoning.

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