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3rd posting: CO2-levels

This is the third posting of the same message... still hoping to get some
reaction (almost giving up on it). With all the expertise on this list, one
should expect to get some answers...

> I am having problems determining my exact CO2-levels. I get different
> results...
> First of all, at the time of the measurement, my tank had a pH of 6.84
> (measured by pH-elektrode calibrated only a few days ago) and a KH of 4.0
> measured by the Tetra KH test kit (instead of the recommended 5 ml of
> I used 10 ml of tank water, so the KH should be accurate within 0.5
> The tank temperature was 25.4 C (measured by a temperature sensor). There
> are no plants and fish in the tank yet because I first want to have all
> parameters right before I introduce any plants. So there's only water and
> gravel (which is completely neutral).
> Now, here goes:
> (1) Judging from the pH, KH and CO2 chart, my CO2-levels are in a range
> between 17 and 19 mg/l. Right?
> (2) However, according to the Tetra CO2 test kit, my CO2-levels are
> 34 and 38 mg/l. (For those who don't already know, the Tetra CO2 test kit
> works as follows: 20 ml of tank water, 5 drops of test reagent 1 and then
> count the number of drops of reagent 2 until there is a slight pinkish
> coloration. To
> determine the actual CO2-level, one has to double the number of drops of
> test reagent 2. In my case resulting in a CO2-concentration of 34 - 38
> mg/l.(17 to 19 drops of reagent 2))
> So my CO2-levels indicated by the tetra test kit are twice as high as
> indicated by the pH, KH and CO2 relationship. What can account for this
> difference between these two testing methods? Which one is right? What are
> the exact CO2-levels in my tank?
> Is it possbile that the pH, KH and CO2 relationship is not valid in my
> because of the presence of other buffers than just carbonates and
> bicarbonates? I don't know which others buffers that would be since I
> nothing to the tank water yet, not even fertilizers... My phosphate-levels
> are approx. 0.2 mg/l according to a Dupla test kit. Is this enough to
> the pH, KH and CO2 relationship unvalid?
> Any thoughts, anyone?
> Davy Cleys