pH/KH/CO2 relationship in Amano's book

• To: Aquatic-Plants at actwin_com
• Subject: pH/KH/CO2 relationship in Amano's book
• From: George Booth <booth at hpmtlgb1_lvld.hp.com>
• Date: Mon, 11 Sep 1995 08:46:57 -0600
• In-Reply-To: Your message of "Sun, 10 Sep 1995 03:39:01 EDT." <199509100739.DAA07269 at looney_actwin.com>

> From: Roger Crowther <lysander at crl_com>
> Date: Sat, 9 Sep 1995 12:39:54 -0700 (PDT)
> 
> I believe that George's table is reliable, so does anyone have any ideas
> why Amano's values disagree so strikingly with the table?  Are CO2
> concentrations difficult to measure? 

If you use a good CO2 test kit, accuracy is around 1-3 ppm.  If you
measure pH and KH and use a table, the accuracy depends on the
accuracy of the two test kits and the granularity of the table.  It's
possible to get a CO2 values such as:

... if the KH kit is accurate to +/- 0.5 dKH and the pH kit is +/- 0.2
units, the range of CO2 values is quite large.  Let's say you measure
3 dKH +/- 0.5 and the pH is 7.0 +/- 0.2.  This would indicate CO2
ranging from

    KH = 2.5, pH = 7.2  -> CO2 =  5 ppm
    KH = 3.0  pH = 7.0  -> CO2 =  9 ppm
    KH = 3.5  pH = 6.8  -> CO2 = 16 ppm

Also, the table depends on measuring KH.  Most test kits actually 
measure total alkalinity, which can be affected by other factors 
in the water. 

George

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