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Re: KH falling

Mr. Paul Sears wrote:


 There seems to be a fairly common misconception about the effect
on KH of plants that can use HCO3-.  If a plant takes its CO2 from
it will leave OH-.  In practice, that means that the HCO3- concentration
will fall, and the CO3-- and OH- concentrations will rise.  The total
negative charge on the three remains the same.

 That total is what you measure when you measure KH, so the use
of HCO3- by plants does not by itself cause KH to fall at all.  For that
to happen, the CO3-- concentration must rise sufficiently that CaCO3
precipitates.  This does happen, reducing GH and KH equally, but only
if there is enough calcium in solution (biogenic decalcification).  One
sees the calcium carbonate deposit on the plant.

 If your KH is provided by baking soda (NaHCO3), there will be no
carbonate precipitation, and the KH cannot fall.  The addition of a
acid is required to reduce KH under those circumstances.

Paul Sears        Ottawa, Canada


I take it that this means that calcium will not precipitate if there is
even a little disolved CO2 present. I also take this to mean that if CO2
is added again that it will just redisolve the calcium and the KH will
just return to where it was?