Re: CO2 determination and Tannin/Humic Acid levels

Hi Everyone,

Thanks to Paul and Dave for their clarifications on Alkalinity. I agree
about the weakness of most aquarium trade books on the subject of water
chemistry. Its a good thing that most plants and fish are so adaptable and
can live in a wide range of conditions. But this list is read by people who
wish to optimize their aquarium's ability to maintain aquatic plants and we
should have better resource material. Maybe I'll write a FAQ once I get a
really good handle on this. I'm glad I never got rid of my university
chemistry texts.

I have noticed a number of posts on the reliability of CO2 test kits and
how their results may vary from published tables, such as the one in The
Optimum Aquarium and the one that George Booth recently posted. I contacted
LaMotte and asked them about the discrepancies and also about interferance
from tannins and humic acids in the water of many aquariums. The answer
might surprise some people:

_______ E-Mail from LaMotte Technical Services ______________________

The test method for carbon dioxide is an acid/base titration: meaning that
base (sodium hydroxide) is added to the sample until it neutralizes all of
the acid in the sample and brings the pH of the sample up to 8.3 (shown by
the color change of phenolphthalein).  Carbon dioxide in water is an acid,
it is titrated by the base.  But, any other acid in the sample will also be
titrated by the base, including the humic acid in your sample.  Also, any
bases already in the sample will affect the titration.  Unfortunately,
is no way to accurately calculate out these interferences.

For this reason the carbon dioxide titration test is considered to be a
field test method, only.

The nomographic method is more accurate, if the pH and alkalinity are
measured accurately.

I am not sure what levels of tannin are in you water.  So I am not positive
that our tannin test kit will work for you or not; it does not read below
1ppm.  But, it is possible that you have levels higher than that, due to
nature of your set up.  If so, the kit should be just fine.  Most of our
distributors do no stock the tannin kit, since it is not widely used.  You
can, however, buy it directly from LaMotte for $54.35.  You can do this by
calling 800-344-3100, asking for Customer Service, and requesting a 7831
tannin kit.

So it would seem from this that when trying to determine the CO2
concentration of our tanks, it would be better to rely on accurate
measurement of pH and Alkalinity, and use the CO2 test kits as a fall back
only. Wish I had known this before I laid out the money for the Lamotte CO2
Test Kit (we all live and learn I guess). Does anyone know how accurate
those battery operated pH "pens" are?

Now, for the tannin/humic acid question. In trying to recreate conditions
found in blackwater streams, be they from South America or South-East Asia,
a lot of people (myself included) add either peat based extracts or use
commercial "Blackwater Extract". My bottle of Sera Morena Blackwater
Developer lists it's contents as Humic acid 0.50%; Peat extract 2.50%;
Tanine 0.12%; Sodium alguinate 1.25%; Formaldehyde 0.12%.

Would it be worthwhile (from an anal-retentive analytical perspective of
course) to monitor the tannin/humic acid levels in an aquarium? In The
Optimum Aquarium, Dupla states that humic acids act as carriers for a
number of nutrients and trace elements. From this I assume that they are
acting as catalysts for the uptake of these nutrients. There must be an
optimum level for these compounds, one where the catalytic function can be
carried out without deleterious side effects to either the plants or the

Can anyone point me to a reference which would show the natural levels of
tannins/humic acids in tropical waters? Several of my plant books talk
about blackwater streams and I know how the tannins get into the water in
the first place (leaching from fallen tree leaves), but nobody seems to
give any indication of the concentration of these compounds in the water
column. If we at least knew their natural concentrations we might have a
starting point for experimentation within our aquaria.

James Purchase
Toronto, Ontario 
(Where it's COLD!!! and WINDY!!!, but the frost patterns on my windows are
pretty - from here on in, those of you in Vancouver and Seattle can laugh
as we shiver and sneeze.)
jpp at inforamp_net