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Re: testing for CO2 in water

On Sat, 5 Feb 2000, Sylvia wrote: 
> I would appreciate if others already successfully injecting CO2 could 
> evaluate the following formula and see if it is, indeed, accurate with 
> respect to pH levels in their own tank water. This could be immensely helpful 
> to those reading and having difficulty determining bubble rate/CO2 level/pH 
> and finding results from charts and CO2 tests questionable.
> This is posted at the Krib. Thanks for your help.
> Sylvia
>  <A HREF="http://www.thekrib.com/Plants/CO2/co2-meter.html">A Cheap CO2 Meter<
> /A> 

The method is not generally accurate.  It assumes that there's a straight
line relationship between CO2 content and pH; there isn't.  Things might
not be too bad if there is only a small pH difference between the high pH
you get on the aerated water and the low pH you get from the breathed on
water.  If the difference is more than a pH unit or so, then the error can
get pretty big.  I think that for common conditions this method could
easily overestimate CO2 levels by a factor of two.  And that doesn't even
consider the possible problems that come up if the CO2 content of the
aerated water isn't 0.6 ppm or the CO2 content of the breathed on water
isn't 60 ppm.

If you do have the case where there is a small difference between the two
extremes (implying a large buffer capacity), then you will need to use a
pH meter to get numbers that are accurate enough to use.

If you're willing to depend on the pH and CO2 content of aerated water,
then you might try something else.  Take a sample of your tank water and
measure its pH (call it pH(t)), then aerate the sample so it's pH rises
and remains constant.  At that pH (call it pH(a)) it should contain about
0.5 ppm CO2 - in equilibrium with air.  Now put those values into this

log(CO2) = pH(a) - pH(t) - 0.3

This is based on the assumption that the bicarbonate content of the water
doesn't change significantly when the sample is aerated.  It would be
possible to make a simple graph of this formula comparable to the
pH/KH/CO2 charts.

I tried this in one of my tanks but it didn't work because the pH of the
aerated sample rose above the range of my test kit.  So I made up this

A sample of the tank water has a pH of 7.5.  That's pH(t).  After the
sample was aerated its pH rose to 8.9.  That's pH(a).  Putting those
values into the equation above, I get:

log(CO2) = 8.9 - 0.3  - 7.5 = 1.1

using a calculator, CO2 is 10^(1.1), or about 13 ppm.

The biggest problem with using this equation is that your pH numbers need
to be precise; if any pH measurement is off by 0.1 unit up or down, the
resulting CO2 estimate is off by 20% to 25%. The estimate that the aerated
water will contain 0.5 ppm CO2 is also a source of problems.  I don't
think there's a problem with my assumption that the bicarbonate content
doesn't change significantly when the sample is aerated, but heck, maybe
there is.

You can't use this method to split hairs, but you should be able to use it
in the presence of other buffers.

Roger Miller