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What is my pH?



My tapwater has very little of anything in it.  For example, it is
about 7.0 - 7.2 pH and the KH is about 2 degrees.  I add CaCO3 to bring
the KH to 5.5.  And I inject CO2 rapidly enough that, after a while,
the pH stablizes at about 6.6 (in the morning or about 6.7-6.8 at end
of lights-on cycle), measured using a freshly calibrated Pinpoint pH
monitor.  What pH should I expect to measure in a cup of water removed
from that tank and sitting out in the open air for six hours?   Assume
no acids other than carbonic acid are present in the tank.

My measurement shows about 7.1 pH.  Can that be right?
Doesn't CaCO3 push pH up higher than that, other things being equal?
Isn't 6 hours long enough for the water to reach equilibrium with the
air?


Scott H.

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