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[APD] Re: pH at which CaCO3 dissolves

Roger Miller writes: 
"pH < 3.44 - log([Ca++][HCO3-])
would dissolve CaCO3. Expressing the calcium concentration as ppm hardness
and the bicarbonate concentration as ppm alkalinity the equation is
pH < 13.44-log(alkalinity*hardness)."

I certainly think this is getting closer to accurate, but [Ca++] is a molar
concentration, while ppm hardness is based on weight. You would have to
divide by the molecular weight in order to make this equation work.

In a practical sense the point is moot anyway. As Roger said, the real
question is how fast the calcium will dissolve. Different crystalline
structions of CaCO3 will dissolve at different rates. CaCO3 even dissolves
in sea water, but it happens so slowly that redeposition (i.e. slow regrowth
of CaCO3 crystals) keeps things like reefs and shells from disappearing,
although they do change in composition and structure over time.

- Jim Seidman

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