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Re: bicarbonate use (was new plant set-up)

On Tue, 21 Sep 1999, Bob Dixon wrote:

> Roger Miller writes:
> > With 4/watts per gallon lighting and no added CO2 probably the plants will
> >  use bicarbonate for carbon and force the pH up.
> Okay, Roger.  Explain this , please.  how can reducing alkalinity raise pH?

Did I say the alkalinity was reduced?

As I understand it plants may use a couple different methods of harvesting
carbon from bicarbonate.  In the one process that seems most widespread
(or at least most widely studied) the plants produce separate alkaline and
acidic zones on the surfaces.

At the acid zone they export H+ and get the reaction:

H+ + HCO3 - -> H2O + CO2

and they import the CO2.

At the alkaline zone they export OH- and get the reaction:

OH- + HCO3- -> H2O + CO3--

The net reaction is

2HCO3- -> CO2 + CO3-- + H2O

There is no decrease in the alkalinity, because there are two equivalents
of HCO3- on the left side of the reaction, and two equivalents of CO3-- on
the right.

This only happens after CO2 is quite low (and pH is correspondingly high).
The reaction has no direct effect on either pH or alkalinity, but the
CO3-- that is formed in the reaction usually does have an effect.  It can
go through either of two reactions:

CO3-- + H+ -> HCO3-  (which decreases alkalinity and increases pH)


CO3-- + Ca++ -> CaCO3 (which decreases alkalinity and hardness)

The first of these reactions happens when the CO3-- leaves the alkaline
zone next to the plant.  The second usually occurs in the alkaline zone
and produces carbonate deposits on the plant.  In seawater this reaction
is a major reefbuilding process.  We also see it in freshwater, where the
substrate in some (mostly polluted) ponds and springs can consist largely
of organic sludge and an ash-like precipitate of CaCO3.

Roger Miller