To: aquatic-plants at actwin_com
Subject: pH Swings
From: STDIXON <stdixon at bechtel_com>
Date: Fri, 21 Feb 1997 11:10:06 -0500 (EST)
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Paul Sears wrote:
> 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.
I think I understand what you've said Paul, but let me try an example to be
sure: Situation A: Assume a pH of 7.0, dKH of 2 and CO2 level of 6 ppm.
If the plants pull out 3 ppm CO2 during the day, the pH will rise to a
little over 7.3 (interpolating from the KH/pH/CO2 chart). Situation B:
Assume a pH of 7.0, dKH of 5 and CO2 level of 15 ppm. If the plants are
pulling out 3 ppm CO2, the pH will rise only to about 7.1.
I recently went from Situation A to Situation B by first cranking up my CO2
reactor to 15 ppm and higher. Per the chart, the pH fell to 6.6 and lower,
so I added bicarbonate to get back to pH 7.0. I noticed that my daily pH
swings were significantly reduced. (My CO2 consumption has probably
increased to 6 ppms or so, because the pH rise is actually about .2. The
plants are growing wonderfully!) The reduction in pH swing is because the
CO2 consumption is a smaller percentage of the higher CO2 levels and NOT
because of the increase in KH. I can see this from the chart because if I
had left the pH at 6.6 (with dKH 2 and CO2 15 ppm), a 3 ppm CO2 usage would
have increased the pH by .l to 6.7--the same amount as in Situation B.
Regards, Steve Dixon