[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Poor man's ion exchange?
- To: Aquatic-Plants at actwin_com
- Subject: Re: Poor man's ion exchange?
- From: Paul Sears <psears at nrn1_NRCan.gc.ca>
- Date: Tue, 16 Mar 1999 10:56:36 -0500 (EST)
- In-Reply-To: <199903160848.DAA25248 at acme_actwin.com> from "Aquatic Plants Digest" at Mar 16, 99 03:48:02 am
> 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.
> 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