[APD] RE: how low can you go?

["Thomas Barr" <tcbiii at earthlink_net>]

> >At what pH does CaCO3 dissolve? pH=8.8
>
> Scott and Augustine, it isn't that simple.  Augustine, your first step
assumes
> that the molarity of calcium and carbonate are equal, which is rarely the
> case.  I'm not sure what you are doing in the second step.  If CaCO3
dissolved
> at pH 8.8 then it would dissolve in sea water (pH ~8.4).  The reefs,
shells
> and banks would not exist.

Freshwater clams, mussles and snail shells can exist at a pH of 4.7 and
virtually no Ca or HCO3. 
So as the Limbo Gods ask" How low can you go?"
I can show folks this at the plant fest. 

Many have a protective layer to prevent dissolution. They can pump OH's out
and rasie the pH mighty high and deposit the CaCO3 at about a pH of 10.1
just like all CaCO3 secreating aquatic organisms. These critters can
control the rate of desposition, but it does have limits and it's not a
static inorganic system.
 
FW algae do this, many marine species of algae too. Algae can pull a great
deal of CaCO3 out of the water(and co precipitate PO4 along with Ca). I'm
amazed at the amount of Ca and KH removal marine species use up. Many folks
with refugiums think it's their Corals when most of the CaCO3 is removed
from their refugium with the Caulerpa etc growing in there. 

Even if the tank water has a pH of say 7.0, at the leaf's surface on the
microscale, the plant's adaxial side is pumping out OH's while stripping
the HCO3 for the CO2(=>direct bicarb users, Hyrilla uses an indirect
method), removing the buffering at this small scale and adding OH's causing
the pH to go way up and desposit CaCO3(if there is any CO3/HCO3 left, so
this does have a limit). Similar processes exist with algae and organisms
with a CaCO3 shell.
It is interesting to note there is no CaCO3 on the deep ocean floor 4km
depth etc.  
It's all been dissolved. 
 
Regards, 
Tom Barr


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