Test if it is possible to explain the know ORP reduction when adding H2O2 into a saltwater

Lasse

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It has been a lot of discussion and speculation why ORP drops directly after H2O2 have been added. In my aquarium - around 320 L - i always get a drop when I fill up my oxidator - it is always some uncatalyzed H2O2 that goes directly out into the water column when I fill up the container

It happens around 1 minute after the fill up (påfyllnad)

1627854173454.png

Possible reason for this had been suggested to be 1) The H2O2 breaks up cells 2) H2O2 can be both a reducer and oxidizer 3) false reading because of local reaction between the metallic part of the redox probe. 4) reactions in the saltwater.

My idea is to first start with 10 L RO water and 10 L tap water. Let them stand a while so it will be a steady reading. One redox electrode in each bucket. A small pump circulating each bucket. Sampling the ORP every 5 minutes in the beginning. After some days I will add 10 ml 12% H2O2 to each bucket and see whats happen.

If nothing drastic happens in the bucket with RO water - interaction of the measurements tool is unlikely. If something happens - we can´t exclude interaction off with the redox electrode (platina in contact with water)

If nothing happens in the tap water bucket - an indication that the ion strength is not enough to cause this. The tap water TDS value will be measured in both before the adding.

After this I will test with one bucket of fresh mixed saltwater and one containing 4 years old saltwater. This in order to get a hint if organics play a major roll in the ORP dip or not.

I will play around with a potato too - in all 4 buckets

I will post updates

Sincerely Lasse
 
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DrZoidburg

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"Oxygen in hydroxide is in the -2 oxidation state; for it to directly act as an oxidant would require going to some equivalent of hydrogen peroxide (-1 oxidation state) and that is uphill energetically. However, there are some reactions where hydroxide can indirectly promote oxidation of an organic compound."(google) Depending on ph it is in can make it a reducer or oxidizer. In salt water favors reducing. This maybe why you see orp go down.
 

Dan_P

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It has been a lot of discussion and speculation why ORP drops directly after H2O2 have been added. In my aquarium - around 320 L - i always get a drop when I fill up my oxidator - it is always some uncatalyzed H2O2 that goes directly out into the water column when I fill up the container

It happens around 1 minute after the fill up (påfyllnad)

1627854173454.png

Possible reason for this had been suggested to be 1) The H2O2 breaks up cells 2) H2O2 can be both a reducer and oxidizer 3) false reading because of local reaction between the metallic part of the redox probe. 4) reactions in the saltwater.

My idea is to first start with 10 L RO water and 10 L tap water. Let them stand a while so it will be a steady reading. One redox electrode in each bucket. A small pump circulating each bucket. Sampling the ORP every 5 minutes in the beginning. After some days I will add 10 ml 12% H2O2 to each bucket and see whats happen.

If nothing drastic happens in the bucket with RO water - interaction of the measurements tool is unlikely. If something happens - we can´t exclude interaction off with the redox electrode (platina in contact with water)

If nothing happens in the tap water bucket - an indication that the ion strength is not enough to cause this. The tap water TDS value will be measured in both before the adding.

After this I will test with one bucket of fresh mixed saltwater and one containing 4 years old saltwater. This in order to get a hint if organics play a major roll in the ORP dip or not.

I will play around with a potato too - in all 4 buckets

I will post updates

Sincerely Lasse

Lasse, Looking forwards to your test results. Here are some additional ideas for the investigation.

Possible reason for this had been suggested to be

1) The H2O2 breaks up cells
Compare the ORP reading of a sample aquarium water mixed with H2O2 to a sample aquarium water filtered through 0.22 micron filter before mixin with H2O2.

2) H2O2 can be both a reducer and oxidizer
For this idea, I would again study a sample of filtered aquarium water or freshly prepared saltwater and measure the ORP response for a wide range of H2O2 concentrations, making up a fresh solution for each concentration. Also, monitor the pH.

If you are very industrious, measure the H2O2 concentration after the ORP stabilizes.

For bonus points, monitor the ORP, pH and H2O2 concentration after each dose of a series of doses to the same saltwater sample.

3) false reading because of local reaction between the metallic part of the redox probe.

No ideas

4) reactions in the saltwater.

I don’t have candidate reactions in mind, but this idea did make me wonder whether salinity affected the ORP response. Is the ORP response different at 10 ppt vs 35 ppt?
 

taricha

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Nice, Lasse. If you get something that looks like an answer, I will try to replicate.

I wonder if your bucket material is completely non-reactive to the peroxide, or hopefully at least are made of material that would exist in most systems anyway.

Also, I love the fact that I'm still not quite sure if the potatoes are some Swedish idiom or if you are literally using potato to introduce organic material to each bucket.
 
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Lasse

Lasse

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First a test with expected outcome. I know since before that if I add H2O2 directly in the DT - I will have a drop in ORP value - and that´s what happens this time too. My peroxide works in my DT as it has before.

1627927650665.png
My water in the two buckets (tap water = kranvatten and RO water = RO vatten) have a steady ORP reading between 160 and 200 mV

1627928031690.png

I decide to add 5 ml H2O2 in each bucket but in small portions - in 1 ml portions.

I´ll be back

Sincerely Lasse
 

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An interesting read on ORP to supplement this discussion. I've been noticing how steady ORP is rising as my tank cycles and came across this article while wondering more about what ORP indicates. So this topic/experiment is timely. Can't wait to read more.

http://reefkeeping.com/issues/2003-12/rhf/feature/index.htm
 
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Lasse

Lasse

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Now - the tap water.

I add first 1 ml H2O2 and after around 38 minutes I hade 1 ml each 10 minutes. The ORP rise after the first 1 ml 12% H2O2 but after that - no change in the other 4 occasions of H2O2 adding (red marking). Adding 2 x 1 ml Triton iron did not cause any dramatical effects (blue marking) - neither one small potato changes anything (black marking)

1628007036128.png
Neither did a new addition of peroxide this morning change anything (green marking

1628007838046.png

It is neraly the same for RO water - even if the first rise seems to be in two steps.

1628008751958.png

pH in tap water 8.30 - in RO water 8.30

Took away the potatoes and ad half a teaspoon of sodium bicarbonate to each bucket

Result later on

Sincerely Lasse
 
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Lasse

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The postulates and what the test indicate


1) The H2O2 breaks up cells.

Probably not the reason - no reaction when adding a potato - well known enzymatic catalyst for H2O2 break down

2) H2O2 can be both a reducer and oxidizer

still no evidence for or against. But it was in a high pH where H2O2 should be a reducer

3) false reading because of local reaction between the metallic part of the redox probe.

Busted IMO

4) reactions in the saltwater.

Still not tested (new and old saltwater)

Sincerely Lasse
 
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Lasse

Lasse

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Took away the potatoes and ad half a teaspoon of sodium bicarbonate to each bucket
Here its come First tap water. Red is sample taking for pH, Blue addition of bicarbonate and black addition of 1 ml H2O2 (12%). pH before and after the addition of bicarbonate - the same 8.3

1628066189298.png


Total graph.

Additions: Red = peroxide; blue = iron; black = potato: yellow = sampling; green = bicarbonate

1628066720577.png


The same for RO water

Additions: Red = peroxide; blue = iron; black = potato: yellow = sampling; green = bicarbonate

1628067241566.png



1628067504740.png


There is a huge difference when I add bicarbonate . The ion weak RO water show a larger drop. One possibility for the ORP drop we see in our saltwater aquarium that have not been mentioned before is that the high ion strength in saltwater cause it. This experiment says the opposite!.

Generally an experiment should make things clearer - but as usual - it did not:p

I had plan to stop this experiment at this point but I will add 3 ml H2O2 again and see if something happen when now when the ORP is back on the same level as before the experiment.

I´ll be back

Sincerely Lasse
 
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taricha

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Generally an experiment should make things clearer - but as usual - it did not:p

Since we now know bicarbonate moves ORP (I'm guessing via pH effect) I'd like to have seen the tap and RO water settle for some time with bicarbonate added before h2o2 went in.
 
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Lasse

Lasse

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Since we now know bicarbonate moves ORP (I'm guessing via pH effect)
pH was the same in both tests - before and after 8.3 bicarbonate addition. No pH effect. I also dose 3 ml H2O2 this morning - not showed yet. But nothing happens.

Sincerely Lasse
 
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Lasse

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Nothing happens - not even after addition of 1 g potato RO Water; blue= peroxide addition, black = potato addition

1628087383692.png

Tap water; blue= peroxide addition, black = potato addition

1628087545730.png

Sincerely Lasse
 
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