Poor man's ion exchange?

["David Ozenne" <dozenne at 10fold_com>]

Paul described how boiling water can reduce GH and KH by precipitating
out CaCO3.  This got me thinking that you could perform ion exchange 
water softening by adding NaHCO3 and then boiling the CaCO3 out.  Of
course, we generally don't want ion exchange in our hobby, because the
sodium is undesirable.  So what about potassium?  Anyone know if 
KHCO3 is readily available?

BTW, I was looking at the equilibrium equations and it seems to me
that adding NaOH would have the same long term effect on your water
(after re-equilibrating with atmospheric CO2) as adding NaHCO3.  Taking
the following 3 reactions:

  H20              <-> H+ + OH-
  CO2 + H2O  <-> H2CO3
  H2CO3         <-> H+ + HCO3-

Some simple manipulations indicate the following net reaction:

  OH- + CO2   <-> HCO3-

Since the CO2 is at an external equilibrium with the atmosphere, this
seems to indicate that OH- and HCO3- can be freely interconverted.
Can any chemist types confirm?  How fast would this process be?

David Ozenne
Berkeley, CA

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