Alkalinity, pH, CO2 triangle

> From: James Purchase <jpp at inforamp_net>
> Date: Wed, 1 Jan 1997 23:57:05 -0500
> But I am wondering about the CO2-pH-Alkalinity triangle. 

<groan> What a way to start the New Year.  :-)

> I have not yet set up the equipment for CO2 addition to the tank, due
> mainly to the holidays. I have installed the Dupla Visual CO2 test and
> it has remained Green (indicating that the CO2 level is O.K.) since I
> put it in the tank several days ago.

This Dupla test responds to the pH of the water vapor entering the
chamber.  IMHO, it's not very accuracte.

> How accurate is the Dupla CO2 Chart? Given my pH of 7.4 ppm and the
> Alkalinity level of 3.76 KH, I would expect to see only 4.5 ppm CO2

It's accurate.  For an easier to use chart, draw your own with the
data I posted before Christmas. 

A CO2 level of 4.5 ppm is consistent with a non-injected tank,
especially one with lot's of bacterial startup activity.

> Are my readings bogus (I was VERY carefull when I ran the tests and
> the LaMotte kits are supposed to be accurate) or could something else
> be causing the readings I am getting (like am I pushing the pH kit
> beyond it's limit - I know that they aren't as accurate at their
> limits)?

Nope, they sound good.  

If you're flush with money, you might spring for a Tetra KH test kit
(about $8).  I've found the "KH" reading from that kit to be more
consistent with with the charts and pH and CO2 measurements.  

> Or am I being anal, and should I just relax and let the tank settle
> into it's own rythym?

If you're concerned with the chart indicating 4.5 ppm and the CO2 test
kit indicating 7 ppm, yes, you're being anal.  <g>

Consider the measurement errors possible with the pH kit and the CO2
kit.  The pH might be +/- 0.05 and the CO2 test kit might be +/- 2
ppm.  Also consider that you have to interpolate the values on the
Dupla table and those values are non-linear.  Also consider that the
tables are based on "carbonate hardness" and the Lamotte kit is
measuring "total alkalinity".  Carbonate hardness is just one
component of total alkalinity; this would cause the chart CO2 value to
read high.

Given the variables, I would say your measurements are very