[APD] Re: pH at which CaCO3 dissolves

[Roger Miller <roger at spinn_net>]

Jim Seidman wrote:

> 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.

Actually, *multiply* by the molecular weight, otherwise you're right, Jim.  
Fortunately, I did convert to ppm hardness by weight.  The conversion is 
imbedded in the second to last line of my letter. It goes like this:

[Ca+2] (in gram-moles/l)*100(grams CaCO3/gram-mole of Ca+2)
    = Ca-hardness (in grams CaCO3/l)

Ca-hardness (grams CaCO3/l)*1000 = Ca-hardness (in mg/l or ppm)

The whole conversion is
 [Ca+2]*100000 = Ca-hardness in ppm.

or as I said in the letter:

molarity Ca++ = Ca-hardness in ppm divided by 100000.

There are some kinetic data available on CaCO3 dissolution, but putting it to 
use is a complicated process.


Roger Miller
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