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Re: Poor man's ion exchange? (was APD V3 #909

Hello David,

It's a neat thought exercise -- but no dice! (I won't go into the
chemistry of it -- its too boring!) Also, heating water is very energy
demanding process. In a home setting, one can not recycle the energy, so
it gets to be very expensive.

There is, however, a neat reaction, along your thought lines, that
actually works: Counterintuitively, one has to add calcium hydroxide
[Ca(OH)2] to the water to be softened (i.e. calcium
carbonate/bicarbonate removed). This process is actually used in
municipal water works.

The stumbling block is finding the *amount* of Ca(OH)2 to be added. Add
too much -- you are reintroducing hardness and a high pH. Add too little
-- you are not removing enough hardness. So dosing, is the key to this
process. Not many aquarists have the necessary facilities to determine
the dose. Also, the water has to be protected from atmospheric CO2 while
CaCO3 is precipitating, but that's a relatively simple task.

I have never tried it on a small scale. But a 5-gal carboy would be just
about the right size for an experiment. One would also need either an
analytical balance, or a Ca(OH)2 solution of known concentration.



> 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