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Re: CO2 tests vs. looking it up in a table

I an earlier letter I cautioned about the need for precise pH values when
determining CO2 by some methods.  In reply, James wrote:
> Roger, doesn't this same caution apply to the use of the CO2 - pH - KH
> table, which a lot of people here seem to depend upon? In looking at the
> table (posted by George Booth and archived at
> http://www.thekrib.com/Plants/CO2/co2-booth-faq.html), the pH measurement is
> incremented in units of 0.2 and the KH in units of 0.5.

It does, but there is a difference in degree.  The first time I used the
CO2 charts I also checked to see what the range of possible answers was
from my test.  I measured a KH of 7 degrees; KH could have been anything
from 6 to 7, and I measured pH at 7.4, but I used a cheap kit with 0.4 pH
units between color references, so I took my possible pH range to be 7.2
to 7.6.  My CO2 concentration could have been anywhere from less than 5
ppm to about 13 ppm.  I could have used better test kits to get a smaller
range of possible results.

If I used the log(CO2) method from my reply to Sylvia with a better pH kit
(one with readings at 0.2 pH units), and measured the tank pH at 7.4 to
7.6 and the aerated pH at 8.8 to 9.0, then the possible range of CO2
concentrations would be 8 to 20 ppm.

The method that Sylvia cited anticipated the possibility that the pH would
be strongly controlled by something other than the CO2-bicarbonate buffer,
so that a big change in CO2 could cause only a small change in pH.  In
that case the entire range from 0.6 to 60 ppm CO2 could fall in a pH range
between color patches on an inexpensive pH test kit, making the method
useless without a pH meter.

If you can use the KH-pH-CO2 charts or tables then they will give you the
estimate with the smallest problems caused by precision in the
measurement.  the log(CO2) method is always very sensitive to precision,
and the method that Sylvia cited may or may not be very sensitive,
depending on the degree of buffering.

Roger Miller