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- To: Aquatic-Plants at actwin_com
- Subject: Re: CO2-KH-pH-AARRGGHH!!!
- From: Paul Sears <psears at nrn1_NRCan.gc.ca>
- Date: Wed, 3 Jul 2002 09:31:19 -0400 (EDT)
- In-reply-to: <200207030748.g637m6A21216 at acme_actwin.com> from "Aquatic Plants Digest" at Jul 03, 2002 03:48:06 AM
> Date: Tue, 2 Jul 2002 22:44:32 -0500
> From: Michael Batts <Michael at whatisdat_com>
> Subject: Co2-KH-PH-AARRGGHH!!!
> Hmmmm, thought I had this stuff licked. Guess not... I thought that Co2
> lowered PH AND KH.
I never cease to be amazed at the misunderstanding of what is
actually something quite simple. CO2 lowers pH, but does not affect KH
(bicarbonate concentration). The CO2 concentration and KH _together_ set the
pH. Increasing KH raises the pH, increasing the CO2 concentration lowers
the pH. There has been quite a bit of discussion of formulae or tables
that desribe the mathematical relationship between them.
Where is the continuing misinformation coming from?
> My tap PH is 7.6 and Kh is 15. I started with 100 drops
> per minute of Co2. This dropped the PH to around 7 and the KH to around 9.
The KH drop was caused by something else. It doesn't sound as
if the CO2 concentration increased by very much - I make it a factor of
2.4. That is if the KH numbers are good.
> Good, but not good enough. Time to drop some more. Soooooo, I increased the
> Co2 to 120 DPM. Viola! the PH started to drop and settled at 6.8, SPOT ON!!
> (so I thought)... Now the KH has raised to 11!!
I doubt that the KH changed at all. I suspect the measurements.
> Run this through a ANY Co2
> calculator and it is still bad!!! I thought the KH dropped WITH PH???
No, the pH drops if the KH is lowered - you have it backwards.
You can destroy KH with a strong acid (e.g., hydrochloric). This will
also increase the concentration of CO2 (temporarily), which will cause an
immediate drop in pH, followed by a slow swing back, though not quite
to where you started. Cutting the KH by a factor of 2 lowers the pH by
log(2), which is 0.3.
HCO3- + H+ -> CO2 (leaves) + H20
> did this happen? Should I try to drop the KH manually?
What do you mean?
> I should like to
> see it at about 5. What method is best?
Dilute it with RO water - that's what I do.
Paul Sears Ottawa, Canada