Re: Non-carbonate buffer experiment

[Bill Warner <lww at ictech_com>]

>Date: Fri, 11 Apr 1997 20:47:53 -0500
>From: Chunx <joannaj at why_net>
>Subject: Non-carbonate buffer experiment
>
>[...]
>Since a given source of CO2 will continue to produce the same
>concentration of dissolved CO2 in water no matter how the water
>is buffered, both tanks should still have 14ppm of CO2.

Hmm.  I'm not convinced that this is true.  The pH in your 2 tanks is
different, and this will affect the distribution of CO2/bicarbonate/carbonate.

>Even if
>we assume that tank A has a carbonate hardness in the range of
>3-5dH (not including the non-carb buffer) the chart says that 
>the pH should range from 6.8 to 7.0 (but the actual pH=7.2).
>
>Maybe the existence of a non-carbonate buffer does a little more
>than alter our KH test kit readings?

For one thing, your statement above, that "the pH should range from 6.8 to
7.0", assumes that the pH is controlled by the carbonate/bicarbonate/CO2
equilibrium.  In the presence of other buffers this assumption is not
valid.  Under these conditions the pH is now controlled by the more complex
set of competing equilibria which you have established.

Having said that, please keep in mind that a system like this will never
actually reach equilibrium.  Depending on how constant your rates of CO2
addition/loss are, you may reach a steady state condition but this is
technically not the same as equilibrium.  Equilibrium will only be
established if the system is free of external perturbations, which this
system obviously is not.  Therefore, the application of equilibrium
calculations, and hence the CO2 charts, to a system like this should be
considered only an approximation.  In particular, since the reaction

	CO2 + H2O --> H2CO3

is kinetically slow, it is *possible* for the steady state concentration of
CO2 to be significantly different than would be predicted by simple
equilibrium calculations.

IMO, trying to determine the exact CO2 concentration in an aquarium is a
waste of time anyway.  As long as one is satisfied that plant growth is
sufficient, pH is stable, and the fish are living long and healthy lives,
what difference does it make what the actual concentration is?

Bill

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