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CaCO3 supplementation in R/O water


So long as we're discussing the topic, I thought I'd shoot my
question out there...

I live in Austin TX, and one thing that I *don't* need to go out
to the store and buy is Calcium Carbonate.  The rocks around here
are mainly Dolomite, aka Limestone, aka CaCO3 (mostly).

Originally I figured that I should reconstitute my tapwater with
R/O-right and crushed limestone -- using the crushed limestone
to both replenish the Calcium levels for the snails and to
add CO3, for buffering.

Ah, but Calcium Carbonate was trickier than I thought.  Just adding
a couple tablespoons to my water raised up the KH to about 3, but
lots of it remained undisolved.  I took a liter sample of R/O water
and dumped in a tablespoon of crushed CaCO3.  In that sample, the
CO3 concentration went to 130 ppm, but still, much was undisolved.
According to my girlfriend's chemistry textbook, CaCO3 is largely
insoluble in water, but CaCl2 is.  I have some HCl, so I figured I'd
put a bit in the mix to facilitate:

  2HCl + CaCO3 --> CO2 + H2O + CaCl2

(That is, my pea brain decided that lowering the Ph would make the
CaCO3 disolve more readily.)

I believe I was right, in that suddenly, my Aq. Pharm. water hardness
test kit went off the scale.  (It reads more than 500ppm now.)   That
could be just because of my testkit not coping with the acidity of the
water.  I put in 1ml of 37%HCl (pool stuff) to the 1l, thinking that
would be so dilute as to be ignorable.  Wrong.  The Ph went from >8 to
<6 (the limits of my testkit).

Another interesting bit was that the water, which was cloudy before I
added the HCl, became much clearer.  I'm assuming that a good part of
the "disolved" CaCO3 was actually just held in suspension.  Adding the
HCl just disolved what was in suspension.

But enough of this foolishness.  Anybody know roughly how much crushed
limestone to use and why?  Am I correct in assuming that I should use
*both* CaCO3 and NaHCO3 to achieve the desired KH?  How many grams of
CaCO3 / NaCO3 are there in a tablespoon?  And is there a simple method
oir someplace on-line where you can get english/metric conversions like
that for other stuff?

					- Steve

------
Steve Benz (steveb at tall-tree_com)   |   Tall Tree Software Co.
http://www.tall-tree.com            |   Ph/Fax: 512-453-4909