re: pH swings

---------- Forwarded message ----------
From: psears at emr1_NRCan.gc.ca (Paul Sears)
Message-Id: <9702241403.AA09682@emr1.emr.ca>
Subject: Re: pH swings
To: Aquatic-Plants at ActWin_com
Date: Mon, 24 Feb 1997 09:03:31 -0500 (EST)

> From: STDIXON <stdixon at bechtel_com>
> Subject: pH Swings
> 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).  

	You have halved the CO2 concentration, so the pH should rise by
log(2), which is 0.3  -  O.K.

> 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.

	The CO2 concentration has fallen by a factor of 4/5, so the pH 
should rise by log(5/4), which is 0.097  -  O.K. again.

> 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.

	10^0.2 = 1.58, so from 15 ppm, the drop would be to 15/1.58 = 9.5,
i.e., a fall of about 5.5 ppm.

> 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. 
> Right?


Paul Sears        Ottawa, Canada

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