re: CO2 swings

---------- Forwarded message ----------
From: psears at emr1_NRCan.gc.ca (Paul Sears)
Subject: Re: CO2 swings
To: Aquatic-Plants at ActWin_com
Date: Thu, 20 Feb 1997 08:36:18 -0500 (EST)

> From: STDIXON <stdixon at bechtel_com>
> Subject: CO2 Swings
> I have a similar situation: I start with dKH 3-4 water with a 7.5 pH and a
> low CO2 level.  My CO2 reactor will crank up the CO2 level to 15 mg./l and
> higher, so if I lower the pH of the change water to 7.0 (with acid) and
> then add the water, the pH will drop even further when the CO2 level goes
> up (forcing one to add bicarbonate to raise the pH back).  Before I figured
> out the KH/pH/CO2 relationship, I used to do this for a couple hours on
> Saturday mornings--trying to hold the pH around 6.8-7.0 as I changed the
> water.  I don't bother with this anymore.

	Good.  The acid was destroying KH, and the bicarbonate putting
it back.  There wasn't a lot of point in that.

> I've raised
> the KH a little in my tank and believe this has reduced daily pH swings
> based on plant usage of CO2.  

	I doubt it.  For the same CO2 concentrations, the higher KH gives
you the same change in pH, but at a higher pH.

> (It is at least clear from the KH/pH/CO2
> chart that a 6.8-7.0 pH swing requires a much greater change in CO2 levels
> as the KH increases.

	A change of 0.2 in the pH means that the CO2 concentration has
changed by a _factor_ of 1.58.  (log(1.58) = 0.2).  If you increase the
KH, you will have to increase the CO2 concentrations if you want to
keep the pH where it was.  If you can do this, you will reduce the 
effect of a change of x ppm in the CO2 concentration, but only because
it is a smaller fraction of the CO2 concentration.  It is the higher
CO2 concentration that has done it. 

Paul Sears        Ottawa, Canada

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