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Re: HCl dosing

> From: "Len Kutz" <len at kutzfamily_net>
> Subject: HCL dosing
> I recently purchased some 20% HCL and have been dosing a small amount every
> night trying to lower the ph of my tank.  It is a 75g tank with tap water
> that is about 8.25 ph, 100+ ppm GH and 120+ ppm KH.  For the past 4 nights
> I've added 15-20 drops of HCL to the sump.  Every night I measure the ph
> before putting in the HCL and about 2 hours afterwards.  Every evening
> before putting in the HCL the ph has been up to or very close to 8.25, but
> after adding the HCL the ph drops to about 8.  I'm concerned about dropping
> the ph to quickly, but I didn't expect the ph to keep going back up so much.
> Am I doing something wrong or is it the buffering of the hard water causing
> this?  Should I put in more HCL?  Any suggestions would be appreciated.

	When you add the HCl, you get the reaction:

	H+  +  HCO3-  ->   H2O   +   CO2

	This reduces the KH and increases the CO2 concentration.  The
CO2 concentration change can be quite substantial even for a small KH
change.  The pH drop is much more because of the CO2 change than the
KH change.  The extra CO2 doesn't stay around though - the concentration
returns to what it was before the HCl addition, and the pH rises again,
leaving you with just the effect of the KH change (small).  Keep on adding
the HCl and you will eventually reduce the KH enough to get the pH you want.
The alternatives are to accept the higher pH or use RO water for some of
the water entering the tank.

Paul Sears        Ottawa, Canada