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Re: Aquatic Plants Digest V2 #788

Ed wrote:

Date: Sun, 22 Jun 1997 00:19:34 -0500
From: Ed Hengel <hengel at computer_net>
Subject: redox potential

Can anyone please explain what redox potential is.



Ed are you ready for a chemistry:
Redox stands for Oxidation-reduction reactions. In water chemsitry most
reactions involve ionic solutions nad thus a transfer of electrons. The
loss of electrons or increase in oxidation number is called Oxidation.
The gaining of elctrons or the lowering of oxidation numbers is called
reduction. The two occur together, in other words one must donate and
must accept for these reactions to occur.
Since moving electrons are actually can be measured as electricity you
can read this tiny amount using a Oxidation-reduction probe (ORP).

What does this mean to aquariums? Well as far as plants are concerned,
water reduces carbon dioxide in the photosynthtic reaction to
carbohydrates, and the carbon dioxide oxides the water to form oxygen.
Metabolism is the opposite, oxygen oxidizes glucose to carbon dioxide,
and glucose reduces oxygen to form water.
Hydrogen normally has an oxidation number of +1 and oxygen normally is
-2. Oxygen is generally thought of as the main oxider, but by true
definition it can be any electron donor.

What are you measuring?
A molecule that is a product of oxidation has a positive electrical
charge or potential. A molecule resulting from reduction has a negative
potential. If all reactions produced the exact same number of oxidized
and reduced molecules, the potential would be 0 millivolts. If your
water is mostly reduced compounds it would have a negative value. If
mostly oxidized- a positive value. Since oxygen is the main oxidizing
agent in water the higher the positive value the higher the amount of
free oxygen for oxidizing compounds. Negative values can only occur in
the absence of free oxygen.(Hopefully not in your tank).
Redox potential goes down as pH and temperature rise so it will
fluctuate throughtout the day.
So why not have screamin' high Redox values?
Very high redox potentials indicate increasing amounts of oxidizers,
including oxygen. These can be very damaging to living tissue in both
plants and animals. Like all other water chemistry factors, fish and
plants have evolved to live within a specific range. Higher does not
mean better when you approach the limits of proper ranges for aquatic
Well, there it is hope this answers your question????
Bill Ruff