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Re: Alkalinity and Phosphates

Paul wrote:

> The end point(around pH 8.2) where you get the first persisting pink
> color is arbitrary.  The choice of this end point does not mean that
> there is no free CO2 when the pH is higher.  No matter what the pH,
> there is always some free CO2 as long as you have some of the other
> forms of CO2 as a result of the equilibria between the various forms
> of CO2 in water: free CO2, carbonic acid (H2CO3) bicarbonate (HCO3-),
> and carbonate (CO3--).  The phenolphthalein end point is chosen
> because it is in the vicinity of the CO2 concentration where free CO2
> becomes significant for aquatic plants.


The test is a standard acidity test with the results stated in ppm CO2.  The
titration endpoint isn't arbitrary and the reasoning for putting it where it 
is certainly has nothing to do with aquatic plants.

At a pH of 8.2 or 8.3 the total dissolved inorganic carbon is 98% 
bicarbonate.  The remainder is split between CO2 and CO3--, both of which 
are usually unmeasurably small.  To the extent that the test misses some of 
the CO2 it also picks up some of the bicarb to offset the difference.  If the 
endpoint were set lower then the test would pick up less of the CO2.  If the 
endpoint were set higher the test would read more of the bicarbonate as if it 
were CO2.

Roger Miller