# Redox explination



## dennis (Mar 1, 2004)

One term I have often seen refered to when discussing substrates and nutrient availibility is _*redox*_. Can any of you chemists or plant bio people please explain _*redo*_x, how it works and any ways it affects our aquariums?

Is this related to bacterial activity in the substrate?

Thanks in advance


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## cousinkenni (Jan 24, 2005)

dennis said:


> One term I have often seen refered to when discussing substrates and nutrient availibility is _*redox*_. Can any of you chemists or plant bio people please explain _*redo*_x, how it works and any ways it affects our aquariums?
> 
> Is this related to bacterial activity in the substrate?
> 
> Thanks in advance


Since I enjoy these chemistry threads I will, hopefully, get the ball rolling with the first response  .

Basically RedOx is short for Oxidation-Reduction. Oxidation is the Loss of Electrons and Reduction is the Gain of Electrons (Leo the lion goes GER :razz: ).

I will use Iron as a partial example. I say partial because there is a very complex piece that I will not get into (mineralization).

Reduced iron (ferrous iron Fe2+) is the form that we usually add into our aquariums (seachem products). This ferrous iron is soluble and is the form that is preferred by the plants. When we add it to our aquariums it can react with oxygen that is dissolved in the water. When this reaction occurs, the iron is OXIDIZED to become ferric iron (Fe3+). Basically oxygen steals an electron from the iron. This newly formed oxidized ferric iron (Fe3+) is not soluble so it precipitates from the water column to end up in the substrate.

The substrate is anoxic (lacks oxigen) largly due to the lack of water circulation and decomosition of organic matter. In the substrate, certain bacteria biodegrade organic matter using Fe3+ as the terminal electron acceptor. These bacteria REDUCE the iron back to the ferrous state (Fe3+ -----> Fe2+). The ferrous state is more easliy used by the plants.

So to answer part of the question, yes, it deffinately is related to bacterial activity in the substrate. Now how the substrate itself affects the redox potential I am not sure. Maybe Mr. Barr can enlighten us with an answer. From reading the recent lengthy thread on substrates it seems as though some substrates can adsorb or relase certain elements. Maybe this has something to do with it.

Another factor may be pH of the substrate. For instance, the oxidation of iron (Fe2+ ---> Fe3+) in the presence of oxygen at 21 degrees celcius: at pH 6.0 the reaction takes 100 hrs, at pH 7.0 takes roughly 1hr, at pH 8.0 the same reaction takes roughly 30 seconds.

This RedOx also happens with most other elements........the most common being the nitrogen cycle. NH3 --> NO2 --> NO3 (aerobic oxidations by nitrifying bacteria near plant roots). NO3 --> N2 (anerobic reduction by denitrifying bacteria).

Hope that helps a little, Please anyone feel free to add on to things I may have missed or misrepresented

Ken Takeuchi


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## dennis (Mar 1, 2004)

Ken, thank you for your response. I certainly do understand the basics a bit more now.



> Another factor may be pH of the substrate. For instance, the oxidation of iron (Fe2+ ---> Fe3+) in the presence of oxygen at 21 degrees celcius: at pH 6.0 the reaction takes 100 hrs, at pH 7.0 takes roughly 1hr, at pH 8.0 the same reaction takes roughly 30 seconds.


 Really! I would have thought that the combination of organics and peat/mulm (especiall in pH reducing, water softening, substrates like ADA) would create a fairly acidic substrate. Thinking back though, I don't think I have ever heard anyone mention the pH of their substrate although it was discussed some in this thread.



> The substrate is anoxic (lacks oxigen) largly due to the lack of water circulation and decomosition of organic matter. In the substrate, certain bacteria biodegrade organic matter using Fe3+ as the terminal electron acceptor. These bacteria REDUCE the iron back to the ferrous state (Fe3+ -----> Fe2+). The ferrous state is more easliy used by the plants.


 In our heavily planted tanks, the substrate would not be too anoxic due to the roots and O2 transport by plants to the substrate through their roots. I don't know how much O is added to the substrate by the plants but i would think that the combination of oxygen transfer and the lowered pH of the substrate, by the organics/peat would have an adverse effect on nutrient availibility. My understanding though is that this is not the case, so I must be confused somewhere....

How is RedOx measured in soils/substrates? What conditions are most condusive to beneficial RedOx in our aquariums?

Again, thank you Ken


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## cousinkenni (Jan 24, 2005)

Hi Dennis,

There definatley are ways to measure everyday Redox potential with handheld probe-like meters. You have probably heard of them before they are referred to as pH/ORP meters. The ORP stands for, if I am not mistaken, Oxidation Reduction Potential. However these meters are more geared toward solution/aqueous testing. There are also methods to measure the RedOx potential of soils/substrates but from what I understand it is more involved than just a handheld meter.

To do this correctly you must prevent atmospheric gas contamination. I believe it also involves certain dyes and the use of a spectrophotometer.



> In our heavily planted tanks, the substrate would not be too anoxic due to the roots and O2 transport by plants to the substrate through their roots. I don't know how much O is added to the substrate by the plants but i would think that the combination of oxygen transfer and the lowered pH of the substrate, by the organics/peat would have an adverse effect on nutrient availibility. My understanding though is that this is not the case, so I must be confused somewhere....


Actually you are right on. I am sorry for not being clearer. A lot of the substrate is anoxic, however you are correct, near the roots of the plants oxigen is deffinately present and in a pretty large quantity. Don't get confused.........

A lower pH is a good thing for some nutrients especially iron. The oxidation of iron is a bad (not really) thing in the aquarium. If the pH of the substate such as ADA is lower, than it might prevent oxidations of iron even in the presence of a high oxygen content. Reduced iron is what plants like and if you can have oxygen and reduced iron at the lower ph it may be good. I don't know about other nutrients though.

Sorry, :-s

As for what conditions are good..........Oxygen is not good if you are trying to reduce things. Certain reducing bacteria cannot physically live in the presence of O2. It literally kills them. It is good if you are trying to oxidize things though (such as the oxidation of NH3 ---> NO3). In terms of pH I am not sure what pH is best for other nutrients.

Once again, sorry :-s

Ken T.


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## niko (Jan 28, 2004)

After reading this easy to understand explanations I have two questions:

What would be a redox that would be considered good for a planted tank?

What is the role of organics in the redox?

--Nikolay


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## cousinkenni (Jan 24, 2005)

niko said:


> After reading this easy to understand explanations I have two questions:
> 
> What would be a redox that would be considered good for a planted tank?
> 
> ...


Nikolay,

I am sorry I do not know what a good redox would be. I have never thought about measuring my substrate before. Once again, Mr. Barr, please help me on this one if possible! 

As for what the role of organics is........

I can tell you part of the role but I am pretty sure this isn't the entire role. It deffinately provides a food source for bacteria to live on. If there was no organic matter there wouldn't be as much nutrient cycling (than again there wouldn't be a need either). How much this effects our systems I don't know, but in natural ecosystems the are essential.

Ken T.


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## cousinkenni (Jan 24, 2005)

*New Barr Report*

Hey Dennis and Niko,

The new Barr Report for May is out and Tom talks a little more about the RedOx Potential and organic carbon fraction. He seems to go a little more into depth than I did.

If you guys subscribe to the BarrReport then check it out. If not, hopefully Tom will answer a few questions posed here. If he doesn't maybe I will sum up what he says in the article for you.

Ken Takeuchi


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## plantbrain (Jan 23, 2004)

Well, don't expect a 12 page review article here.
Organic carbon is oxidized by bacteria through decomposition in the presence of an electron acceptor. So O2 is the acceptor(like the one we use when we breath), the carbon is the donor.

CH4 is the reduced form of carbon and the CO2 is the oxidized form.

Okay, now when you add lots of soil(lots of organic matter-OM) the bacteria go wild and use up all the O2(unless more is added) so fast that the O2 levels fall to near zero. This causes more reduction(less oxidation) to occur.

Since the flow in/out of the substrate is very slow, the result is that the redox is lowered (or a higher Eh potential- more negative).

Anytime the there is OM, the bacteria will go after it fast.
This will reduce the substrate.

I suggest adding peat since it semi/non labile but allows for some reduction, just not a lot for too long. About 1 month later, the carbon is all oxidized but by then, mulm has started to accumulate.

Good ranges of redox are about 100-200mv and up.
Bleow this, you get sulfur reduction and H2S formation. You added too much OM in that case.

Depends on the type of substrate particles also.

Measurements are done by slowly and incrementally sinking a redox or a DO probe into the substrate or adding several at different depths and then taking the reading.

Plants easily control the redox around their roots to optimize the nutrients they need(say Fe or Mn).

They have their limits though and loading too much mulm over time will sour a substrate.

I suggest a vacuuming to remove excess OM build up once a year or fluffing the sections up in 1/4 to 1/2 tank sections yearly.

Regards, 
Tom Barr

www.BarrReport.com


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## cousinkenni (Jan 24, 2005)

Thanks Tom


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