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Re: CO2 myths

Paul,  I am so glad you wrote that post.  There had been a raging 
controversy on a message board concerning the relationship 
between CO2, KH and pH.  I don't think any of us really understood 
it.  I know I contributed my share of confusion.  

When I first read your post stating that KH does not interfere with 
the availability of CO2, I spluttered and gasped--the usual reaction 
when one's view of reality is challenged.  I think the hard part to 
grasp (for me, anyway) is the exponential nature of pH changes.  
I'm still struggling with the concept of pH decreasing by 1 unit for 
each 10x increase in CO2, but I have the annoying feeling that you 
are right.  Have to think on it some more.

So you say that KH has no influence on the availability of CO2.  In 
other words, pH can be reduced by CO2 increase, or increased by 
KH increase, but KH and CO2 do not influence one another 
directly.  In other words, there is no buffering going on?  Can you 
elaborate?  What happened to the buffering concept?  What am I 
missing? (And thank you so much for allowing us to blame the 
textbooks rather than ourselves!)

Cathy Hartland, living in the state of Confusion

> From: Paul Sears <psears at nrn1_NRCan.gc.ca>
> Subject: Re: CO2 having no effect
> 	A couple of replies to the original posting on this stated that
> having a high KH can stop the CO2 from reducing the pH.
> 	This has come up before, and it is not true.
> 	A higher KH will cause the pH to be higher for a given CO2 
> concentration, but the pH _change_ is the same for a given CO2 change,
> whatever the KH is.  Look at the tables, or, better, the equations discussed
> recently.  The relationships are logarithmic, so by "change", I mean the
> factor involved, not the number of ppm CO2 increase (or KH increase).
> 	An increase of a factor of 10 in the CO2 concentration will
> decrease the pH by 1.  An increase of a factor of 10 in the KH (bicarbonate)
> will increase the pH by 1.  
> 	If the CO2 is increased by a factor of "x", the pH will drop by log(x).
> 	Also, the higher KH will not stop the CO2 being available to plants.
> The CO2 concentration in the water depends on the CO2 system and the rest
> of the tank setup - the KH has no influence on it.  The misconception can
> be blamed on a number of aquarium textbooks.
> - -- 
> Paul Sears        Ottawa, Canada