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Re: [APD] CO2 problems (??)

There might be some rickety tables out there ;-)  
There have been a number of formulas offered over the years that compute roughly the same CO2 values under roughly the same parameters, even though the formulas contain some very diff constants.
I like the formula that was handed down after all the great discussion by Paul Sears, and George Booth and Roger Miller, and Jeff Ludwig, etc.. This version is actually adjusted for a temperature of about 78-80 degrees F:
CO2 (in ppm) = 15.7*KH*10^(pKa(CO2/HCO3- system) - pH)

generally                   =15.696*KH*10^(pKa-pH)
@ 78-80F                  =15.596*KH*10^(6.34-pH)
@ KH 9, pH 7            =15.596*9*10^(6.34-7.0)
                                =30.9 ppm
KH 9, pH 7.0 => about 31 ppm
I like this version because it directly incorporates the values for KH, pKa of CO2 carbonate relationship, and pH, with only one assumed constant.
The value for pKa changes with temperature although the changes are very small. For example, at 70 degrees, pH7.0 and KH 9, the CO2 would be about 33 ppm. Of course the diff between 31 and 33 is less than the compound margin of error in doing home KH and pH measurements. ;-)
If I've put things badly, there was a nice summing up of the points by Paul in this post:
* * * * * * * * * * * 
The Northeast Council of Aquarium Societies (NEC) celebrates its 50th year of service at its 31st annual convention, April 7-9, 2006 -- including *An Evening with Aquarium Legends*.
Read the speaker list, then download the registration form, and get set to join the fun at what promises to be the year's biggest tropical fish convention.
And while you're there, attend the AGA's annual meeting Friday afternoon (2-4pm). This year's speaker is Claus Christensen, Tropica's Managing Director. 

----- Original Message ----
From: Billionzz at aol_com
To: aquatic-plants at actwin_com
Sent: Thursday, March 2, 2006 1:05:16 AM
Subject: Re: [APD] CO2 problems (??)

>>>By the way, correct me if I am wrong but KH 9 and pH 7.0 yields  about 27 
ppm of CO2.  Not 170.

I use the formula:  3*KH*(10^7-pH)

That means 3*9*(10^0) = 3*9*1 = 27 ppm.

Or am I  missing something?<<<

With a KH of 9 and a pH of 7 the CO2 would be 27.

Here is the formula    3*KH*10^(7-pH)


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