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pH drop with increase of CO2

Chuck Gadd wrote:
>Ok, except for the case of zero KH, when adding CO2, won't the shift
>be the same for high versus low KH? Using that famous (infamous?) CO2
>chart, it says that the shift will be the same.

Chuck is right.
Some time ago I deduced the formula below from the charts at the Krib
and other places. As it is deduced from the charts it may not be 100%
correct, but it is quite close.

  CO2 = KH * 10^(7.5 - pH)

    CO2 is carbon dioxide in mg/l (ppm)
    KH is carbonate hardness in German degrees
    pH is acidity in good old pH units
    log is the 10-based logarithm

You can juggle this formula to get

  pH = 7.5 - log(CO2/KH)

You can further juggle this to get

  ph_change = log( CO2_at_time1 / CO2_at_time2 )

If you for example increase your CO2 level 10 times you will get a
drop of 1.0 pH. This is independent of the KH!

Roger Miller at 
http://www.thekrib.com/Plants/CO2/kh-ph-co2-chart.html uses similar
arguments to show how you can use (a juggled form of) the formula 
to calculate CO2 contents without knowing KH, by only measuring pH
(precisely) before and after aerating a (large) water sample.