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Re: Poor man's ion exchange?

> From: "David Ozenne" <dozenne at 10fold_com>
> Subject: Poor man's ion exchange?
> 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.

	There would only be any point in doing this if the hardness
was permanent.  That is, GH > KH.

>  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?

	Lab suppliers should have it.
> 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?  

	Yes. The point has been made before, by Dave Huebert and myself.

>How fast would this process be?

	Well, if we wanted 1 KH, we would have to add 2 * 0.179 millimolar
OH- (because 1KH is HCO3- corresponding to 1GH Ca++, and we need two
HCO3- for each Ca++).  We need to add then, 2 * 0.179 mM CO2, and the
molecular weight of CO2 is 44, so that means 2 * 0.179 * 44 ppm CO2.
This is 15.7 ppm CO2, which I suspect would take quite a while (days?)
if there was no CO2 system on the tank, and at least hours if there was
a CO2 system.  This is why dissolution of CaCO3 takes a long time.
Adding CaCO3 is a lot safer, because it _is_ just about insoluble and
won't cause pH swings, which KOH, Ca(OH)2 or NaOH would.

	KH is the total of OH-, HCO3- and CO3--.  Carbonic acid is
unique in that it can come and go.  At aquarium pH, the negative ion
is just about all HCO3-, but the interchange is always going on.

Paul Sears        Ottawa, Canada