[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Aquatic Plants Digest V3 #613

Hello,

While agreeing with what Paul said about zwitterion buffers, I would add
that an amino acid buffer will not last in an aquarium for very long --
the bugs will "eat it" -- even if it is an "unnatural" alkyl or aryl
amino acid, generally resistant to biodegradation.

From the seller's viewpoint that presents an ideal situation: Every day
or so you must add buffer, because what you added yesterday is already
partly decomposed. Speaking of a gold mine!

Best,

George

> Subject: Re: Seachem's buffers
> 
> >From: "A. Inniss" <andrewi at u_washington.edu>
> >Subject: Re: Seachem's buffers
> >
> >     Paul, I believe Seachem's Acid and Alkaline buffers are
> >zwitterionic.  Would you be so kind as to give a basic description of what
> >this buffer system is and how it works?  Also, how might it interact with
> >other buffering systems, such as bi/carbonate?

        A zwitterion is a molecule with opposite charges in two places.
The sort we have here is probably a molecule with a weak acid end and
a weak base end.  If we take as an example an amino acid:

        H2N-xxxx-COOH

        there are two acid/base buffer systems here:

        H+  +  H2N-xx..  <->  (H3N-xx..)+  The "+" is on the N.

        H+  +  ..xx-COO-  <-> ..xx-COOH

        The zwitterion is the one with a charge at each end, as if the
H+
from the COOH migrated to the NH2.

        If I wanted to sell an "acid buffer" and an "alkaline buffer"
based
on a zwitterion, I would use the sodium (or potassium) salt:

        H2N-xxxx-COO- Na+  for the alkaline one, and the chloride (or
sulphate)

salt as the acid one:

        Cl- (H3N-xxxx-COOH)+   (the "+" on the N again)

and tell people to mix them as required.  If I wanted to _use_ such a
system,
I would use a combination of the amino acid itself and either a strong
acid or a strong base, as required.  That way, I would end up with less
sodium and chloride ions in there, with the same buffering capacity.

        When there are several different buffer systems present in a
solution, the pH can be calculated by finding a solution to a set of
simultaneous equations involving the amounts of things there, the 
equilibrium constants for all the acid-base systems there (including
water itself) and the requirement that charges balance.  This can be
a (solvable) mess, but one usally tries to operate in such a way that
the main
buffer swamps the rest.  If you want the zwitterionic one to swamp
the bicarbonate one, and the water is hard, you could end up putting
a lot of this stuff in there.

        At the end of this, you wouldn't have any idea what the KH or
CO2 concentrations were, and they would be impossible to measure with
our usual methods.

        I really wouldn't bother.  Use HCO3-/CO2.

- -- 
Paul Sears        Ottawa, Canada