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Re: CO2 measurements

> From: busko at stsci_edu (Ivo Busko)
> Subject: Re: C02 Measurements (long answer)
> In summary, conditions in the aquarium have been pretty stable for several
> months. I throw some peat in, and bang ! the pH drops and the kH doesn't
> change. So if I blindly apply the Ph/Kh/CO2 relationship, I would conclude
> that CO2 jumped from 40 ppm or so to well above 100 ppm. This obviously
> didn't happen, so I conclude then that peat throwed the Ph/Kh/CO2 relationship
> out of whack. I also could conclude that this particular peat wasn't good in
> decreasing alkalinity but very good in releasing acids, thus the observed
> pH drop which has no relationship at all with added CO2. 

	If the peat adds a (weak) acid (and it probably does), then
we get the reaction:

		HCO3-  +  HA (weak acid)  ->  CO2 (leaves) + H2O  + A-

	This reduces the HCO3- concentration, but does nothing at all
to the _measured_  KH, since the titration cannot distinguish between
HCO3- and A-.  The relationship between pH, CO2 and HCO3- still applies,
but there is no easy way to measure the HCO3- concentration.  If you
want to reduce the "alkalinity", you must add a _strong_ acid (e.g.,
hydrochloric), the anion of which (Cl-) will not be measured by the "KH"

	What do we mean here by "KH", anyway?  I understood it to mean
the (bi)carbonate present, but it seems that people have of late been
using it to mean _all_ the weak acid anions, i.e., "alkalinity".

Paul Sears        Ottawa, Canada