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Redox Potential


  1. Light frequency / distribution
    by (Uwe Behle) (Mon, 30 Aug 1993)

Light frequency / distribution

by (Uwe Behle)
Date: Mon, 30 Aug 1993

George Booth (booth-at-hplvec.LVLD.HP.COM) wrote:

:     Redox potential (or at least is connected to it). A high redox potential 
:     is known to be better for algae than a low one. Let's say it increases  
:     the odds.
: Hmmm, again, my experience is counter to this.  The tank that has some 
: algae also has lower redox "readings" (which may not be correct - see
: ORP comments below). 
Hueckstett made experiments with salt water aquaria. The way he lowered the
Redox potential was to increase the oxygen demand of the tank. He discoverd
that if you let the Redox potential increase you see the following sequence:

    brown algae - red algae - green algae - blue algae.

He discovered that this is reversible i.e. if you decrease the Redox  the
blue algae goes and green algae appears. Finally you have brown algae again.
I think to a certain degree this is applicable to fresh water as well.

Now the plants come in. At a Redox potential which leaves most of the
nutrients in their usable stage the plants are in a better position of
out-competing algae. 

You can achieve this at higher redox levels by constantly adding the trace-
elements (Duplaplant 24 e.g.). 

: Well, I guess I could try this also.  Is it fair to ask what observations
: you made that indicated the plants received some benefit?  I'm afraid 
: I would be biased to *not* see any difference if it was subtle. 
It has been 4 weeks now that I am doing this and I think that the morning
after the dark phase the new plant leaves look like they grew twice as much
as during a "light" day. Try it, perhaps you can confirm this.

: You have suggested that the higher oxygen level will oxidize trace 
: elements too quickly.  This seems logical but we always have good iron
: levels.  Do you know if Fe test kits measure both Fe++ and Fe+++?  I 
: can check our kits to see if they mention this.  Perhaps Dupla has 
: taken fast oxidation into account when they set the dosage for 
: Duplaplant-24 drops.  Will oxidation remove Fe from the EDTA chelator?

I think both Fe++ and Fe3+++ are measured, however the "rust" usually is in
the filter substrate and not very soluble, so you probably measure Fe++.

EDTA shields Fe++ for a certain time from getting oxidized. It probably de-
pends on the Redox potential how long it takes to be rendered useless.

:     He recommends a probe which is accurate to 0.1mg/l (and also too expensive
:     for most hobbyists).
: That's for sure ...

I have seen one for DM 700,- !!!!

:     Another point, George: Krause also says in his book that having a Redox
:     probe in the tank permanently will show you lottery numbers for the Redox
:     voltage. He recommends to clean the probe EVERY time you make a measurement:
: This doesn't bode well for the reefkeepers who use an ORP controller to
: run ozone (which is probably passe now anyway). 
:     1) use a non-abrasive detergent to thoroughly clean the probe.
: "Bon Ami" cleanser is recommended - we use that.
:     2) put the probe into 15% HCl for 5 minutes.
:     3) rinse with distilled water
:     4) put the probe into 5% FeSO4 solution for 2 minutes
:     5) rinse the probe with distilled water.
: We don't do that ...
:     6) do your measurement. Most important: do it in moving water (non stagnant)
:        this excludes taking a sample from the water and measuring the sample.
:        Also wait at least 15 minutes before you take a reading and watch that the
:        value stays constant.
: Hmph.  It usually takes overnight before a reading is stable.  This is with
: a Sandpoint II pH/ORP controller.  Based on the cost, I hope it's a 
: quality unit.  What is there about ORP or the ORP probe that prevents
: long term use?  Algae build up on the tip?

I guess algae and bacteria build up at the tip is the point. Once they are
there the concentration of their waste products is much higher than the
concentration of the Redox-determining substances in the water.

My probe takes a long time to settle (30 minutes) also. I usually stop if
the change is less than 5mV/hour. 
Here is a test for the quality of your ORP-amplifier: unplug the probe
for 30 minutes. Then plug it back in and read the ORP value. If it is
much higher than before and drops back down within minutes the ORP amplifier
has a too low input resistance.

: Is there another way of determining ORP besides electronically?  I would 
: guess not, since it's an voltage potential difference.  Of course, so 
: is pH ... 

There a colour indicators, but I don't know how acurate they are.
: Where?  Betta mudpuddles?  :-)
Cum'on George, I just read your posting about the natural habitat of discus
fish. I am sure the water there is not saturated with oxygen.

: How do you *reduce* the oxygen level in an aquarium?  I guess you could 
: increase the BOD (biological oxyen demand) but that would mean leaving
: a lot of stuff decomposing or increasing the fish loading.  Am I missing
: something here?  This seems completely counter to maintaining good water
: quality.

I am wondering about this too. I guess my solution is to use a reductive
filter that uses up the oxygen. Of course this would have to be checked care-

:     Krause shows pictures of thriving plant tanks with only 3 - 5 mg/l oxygen.
: Again, what is the purpose of this recommendation?  To avoid oxidizing
: trace elements?  If this is all it is, it seems kind of a narrow focus. 

The purpose is to get rid of the misconception that high oxygen levels are
the best thing that can happen to you. Don't worry too much about this.
There are other more important factors to a healthy good looking aquarium.
Of course one aspect would be that you don't have to throw in as much trace-
element fertilizer and clean 98% of it out during the next filter change.

I agree that we have been focusing on some - maybe not very significant - 
side-aspects of aquarium plants.


NAME    Uwe Behle, HP Boeblingen Instruments Division
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