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CO2 levels and KH

Michael Moncur was puzzled about KH levels, CO2, and pH changes.

pH is a logarithmic scale. This means that 1 pH unit is a 10 fold change in
concentration of protons (H+) and 2 pH units is a 100 fold change. KH and
CO2 concentration on the other hand are linear scales. This is one reason
why the lines for the different pH values on the Krib's chart are not
parallel. You can see, for example, at KH of 6 it takes a very small
increase of CO2 to reduce the pH 0.1 unit from 7.6 to 7.5, while it takes a
much larger increase of CO2 to reduce the pH 0.1 unit from 6.9 to 6.8. The
second change requires a 5 times greater increase in the number of protons
than does the first pH change.

In a tank, carbonates and bicarbonates (which are measured by KH) are in
equilibrium with CO2 and protons. As you increase or decrease either KH or
CO2 the equilibrium shifts resulting in either more or fewer protons (lower
or higher pH). The aount of CO2 that must be added to decrease pH by a given
amount (say 0.1 unit) is greater if (a) The KH is higher, and/or (b) The
starting pH is lower.


Peter G. Aitken