Re: redox potential

Calvin Chin <cfc at pacific_net.sg> wrote:
> There has been talk about Redox potential and MV readings. what exactly is
> Redox potential? (my high school chemistry was really bad) What does the MV
> reading on an MV meter mean?

Redox potential is a relative measure of the concentrations of
oxidants (e.g., O2, H2O2, HOCl, O3, NO3-) and reductants (e.g.,
most organic compounds, sugars, H2S, NH3), weighted by their oxidizing
power.  Redox potential is properly a measure only of a system at
equilibrium, which excludes it from being valid in aquaria.
The readings you get from the meter depend on the rates at which
particular chemical compounds present in the aquarium react with the
electrode surface.  These rates are highly variable.  I suspect that
marine organisms give off organic compounds that react rapidly with the
electrode and that these reactions dominate the observed reading.
A reading of 500 millivolts sounds about right for an electrode responding
mainly to O2 and H2O (a weak reductant!), considering that the reference
electrode is probably a silver/silver chloride electrode.

> Readings of 300 plus was considered good for marine reef
> tanks. I get readings of 480 plus in the night and it goes up to some 500
> plus in the day. Is this normal?
> Why is it that we get much higher MV readings in Freshwater tanks than in
> reef tanks?
> Calvin Chin

I hope that helps.
Curtis Hoganson
Dept. of Chemistry, Michigan State University
East Lansing, MI 48824   517-355-9715 ext 260