Re: KH, CO2, and pH

[Dave Millman <dave at tactics_com>]

At 01:44 PM 3/29/2003, Bill wrote:
>Hi, Dave,
>
>In APD 99, in a most informative response to a question, you said, in part:
>
>     "pH is a function of KH and CO2: Increasing KH raises pH, and increasing
>CO2
>      lowers pH."
>
>Is this really 100% correct?  Isn't pH a measure of the free hydrogen ions at
>a point in time?  Sure, the addition of CO2 will cause some of them to
>combine with the O2,
>but in a tank that doesn't have CO2 injection and is in equilibrium CO2-wise,
>with a constant KH, the pH can vary all over the place.

OK, I forgot all the chemistry I learned in engineering school, but here's 
what I know:

1. Injecting CO2 always lowers the pH. The best discussion of the chemistry 
is at:
http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm

2. CO2 level is not static, and the rate of change is not constant. 
Biological processes and surface agitation both impact CO2 level, and can 
do so at variable rates throughout the day. pH in tall planted tanks can 
skyrocket during the day, as plants consume the CO2 to near zero and 
diffusion from the atmosphere through the small surface area can't keep up. 
In my current tank, when I dramatically reduced surface agitation two 
months ago, pH dropped to below 6 within 24 hours (oops!). With the reduced 
surface agitation, I've been able to reduce my bubble rate from 100 per 
minute down to 35 per minute to achieve the same CO2 concentration.

3. In the case you describe, a planted tank with no CO2 injection, pH can 
be expected to rise and fall a point or more over a 24 hour period, again 
depending on surface agitation.

Looking at your note, I think the key issue to note is that equilibrium is 
not achieved instantaneously. Without any biological activity at all, water 
achieves CO2 equilibrium with the atmosphere over a period measured in 
hours. With plants consuming CO2 during the day and producing it at night, 
pH would be expected to swing quite a bit.

Here's a write-up I did for the planted tank study group of our fish club 
(http://www.svas.info):

http://groups.google.com/groups?threadm=3E42D019.C73A5358%40spamyahoo.com