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

[Hans-Werner]

The daily pH swing

So I was on the right track already. I searched the thread for "pH" but didn't find the answer.

I know that pH influences the ORP. Can this be calculated for our tank water?

I like dichotomous explanations: If not only the pH would be responsible for the full ORP swings there would be another paradoxon: While the corals and algae produce oxygen (and the pH rises) the ORP drops.

 

[Lasse]

If not only the pH would be responsible for the full ORP swings there would be another paradoxon: While the corals and algae produce oxygen (and the pH rises) the ORP drops.

@Randy Holmes-Farley Do you know if we get the same or weaker pH - Redox relationship if pH is altered with acids or alkaline compounds in the absence of photosynthesis? Is an interesting question because we do not know if the photosynthesis deliver oxygen radicals or already created O2 molecules out to the water.

Sincerely Lasse

 

[Hans-Werner]

we do not know if the photosynthesis deliver oxygen radicals or already created O2 molecules out to the water

Although it is a little bit less radical, wouldn't normal oxygen, O2, also be an oxidant?

 

[Lasse]

Although it is a little bit less radical, wouldn't normal oxygen, O2, also be an oxidant?

Yes - but if two Oxygen atoms form O2 in the water - is that an oxidation or reduction? One of the O atoms give electrons - one gets electrons. If the forming of O2 takes place inside the organisms - this paradox will not be in the water. Or I´m out in the forest without both a map and a compass

Sincerely Lasse

 

[Hans-Werner]

Yes - but if two Oxygen atoms form O2 in the water - is that an oxidation or reduction? One of the O atoms give electrons - one gets electrons

Yes, Lasse, but the oxygen atoms do not form in the water but in the zooxanthellae during photosynthesis, and H2O is used to reduce CO2 to [CH2O] (C6H12O6) and O2. So the reduction side is the formation of [CH2O] from CO2 and only the product of the oxidation and oxidant O2 is released into the surrounding water.

 

[Lasse]

What´s happens if there is 100% saturation O2 in the surrounding water?

Sincerely Lasse

 

[Hans-Werner]

What´s happens if there is 100% saturation O2 in the surrounding water?

Is this question serious, Lasse? The water gets supersaturated with O2 if it is not skimmed or de-oxygenated in another way.

Especially in freshwater aquaria this may get a problem and cause air bubbles in fish fins, eyes or under the skin and maybe lethal embolism.

Edit: Maybe I did not get the real intention of your question, right?

 

[Lasse]

Nope - you did not get where I was going - the transport system from organism out to the water is a passive diffusion and I have seen reports speculating that the O2 pressure inside the organism form free radicals (single O atoms) that can be the cause of bleaching syndrome (burn tops) I have measured oxygen saturation in heavy loaded coral system and it can be figures over 120 % saturation. In freshwater - it rather common with damage in the tissues of the same reason. What I was thinking is if these O atoms also can leak out into the water.

Edit: In first version I use stomata as the critical point - its wrong because submerged plants do not have stomata

Sincerely Lasse

 

[Hans-Werner]

I have never seen oxygen radicals as a problem of supersaturation but rather a problem of some biochemical processes. Every oxygen respiring or evolving organism must have special enzymes to detoxify oxygen radicals because they are intermediates in oxygen metabolism.

The assimilation of nitrate by zooxanthellae/corals forms oxygen radicals which may cause problems as bleaching ... but yes, maybe also supersaturation ...

 

[Randy Holmes-Farley]

The exact ORP pH relationship cannot be calculated without knowing the exact underlying chemical forms of redox active species that are changing with pH, which we do not know in a reef tank.

I discuss it here:

ORP and the Reef Aquarium - Reefkeeping.com

The theoretical relationship between ORP and pH

One of the complications of ORP is that the measured value can sometimes depend on pH. Whether ORP does depend on pH or not, and to what extent, is determined by the exact redox reactions that are involved in controlling the ORP in that solution. There have been equations proposed that purport to "correct" ORP for changes in pH, giving a new parameter, sometimes called rH. This parameter was proposed in the 1920's by W. M. Clark.7 One form of this correction is shown below:

rH = mV / 29 + (2 x pH)

and sometimes a correction for changes in oxygen concentration is thrown in:

rH = mV / 29 + (2 x pH) + [O2]

where [O2] is the concentration of O2 in ppm. The use of rH, however, presupposes a detailed understanding of the reactions involved, and is simply wrong for general use (as shown below). In a book8 that he published 40 years after his initial publication, Clark stated:

"At this point the author must confess to the introduction of rH. He conceived that there might be occasions when it would be convenient to speak of relative oxidation-reduction intensity without having to specify both potential AND pH...
...Unfortunately both the original intent and the obvious limitations have been overlooked by many who have converted their potentials for SPECIFIC SYSTEMS to rH numbers...
...In brief, rH has become an unmitigated nuisance."

Nevertheless, many people still use rH. Since it is imbedded in many articles relating to aquarists, it is worth understanding where the pH dependence comes from, and why it is not always the same.

As an example of a solution where the redox is not pH dependent, take a solution of Fe++ and Fe+++ in water, with no other redox active species. In that case, the ORP is exactly determined by the relative concentration of the two iron species, and is unchanged with pH.

Fe+++ + e- <--> Fe++

Specifically, the defining equation here is:


The main thing that is clear from this equation is that the ORP is independent of pH, and only depends on the relative concentrations of Fe++ and Fe+++.

The easiest way to think of the lack of pH dependence here is to recognize that neither H+ nor OH- participate in the reaction at all. So changing the pH has no direct impact on the reaction.

For many reactions where oxygen is an important participant, however, that is not the case:

O2 + 4H+ + 4e- <--> 2H2O

In this reaction, H+ does participate. Consequently, the oxidizing power is related to pH. As H+ is raised (by lowering pH), the reaction is driven to the right. One way to think of this is by LeChatlier's Principle where increasing the concentration of one species drives the reaction to the other side. In this case, lowering the pH increases the oxidizing power of the oxygen, and consequently raises the ORP. This result is the basis for the development of rH for many systems.

It is beyond the scope of this article to go into the detailed mathematics behind the pH dependence of ORP measurements, but Pankow does cover such issues in great detail in Aquatic Chemistry Concepts.9 For our purposes, an important result is that the magnitude of the change in ORP with pH depends entirely on the number of H+ involved in the reaction per electron. In the case of the Fe+++/Fe++ situation, this value is zero. For the oxygen/water reaction, the value is 1.0. The standard definition of rH assumes that this ratio is exactly 1.0. Consequently, it may not apply to many redox reactions that take place in aquaria.

Shown below are some typical reactions that also take place in aquaria. First, the oxidation of acetic acid to carbon dioxide, again with one H+ per electron (this reaction typifies many reactions involving neutral organic materials):

2CO2 + 8H+ + 8e- <--> CH3COOH + 2H2O

but if the same reaction proceeds with acetate, the reaction is:

2CO2 + 7H+ + 8e- <--> CH3COO- + 2H2O

and the ratio of H+ to e- is no longer 1.0, but is now 0.875.

For the various reactions of the nitrogen cycle, we have ratios that vary from 1.0 to 1.33:

NO2- + 7H+ + 6e- <--> NH3 + 2H2O

NO2- + 8H+ + 6e- <--> NH4+ + 2H2O

NO3- + 2H+ 2e- <--> NO2- + H2O

N2 + 6H+ + 6e- <--> 2NH3

N2 + 8H+ + 6e- <--> 2NH4+

The iodide/iodate reaction fits the 1.0 ratio:

IO3- + 6H+ + 6e- <--> I- + 3 H2O

Some other redox reactions that have other ratios:

MnO2 + 4H+ + 2e- <--> Mn++ + 2H2O

SO4-- + 10H+ + 8e- <-->H2S + 4H2O

SO4-- + 9H+ + 8e- <--> HS- + 4H2O

So if one really wants to understand how ORP would change with pH, one would have to know what the species are in aquaria that control redox. If it is a mixture of species, then the end result will come back as a complex averaging of the different reactions involved. Unfortunately, the species involved have not been clearly defined for seawater. In aquaria, which vary considerably in the concentrations of many redox active species, the situation is even more complicated.

The empirical relationship between ORP and pH in aquaria

While understanding the details of the theoretical relationship between pH and ORP is complicated, measuring it for a single aquarium is fairly easy. Figure 1 shows simultaneous plots of pH and ORP values over the course of several days in the aquarium of Simon Huntington. Clearly, the measured ORP and the pH are on exactly opposite cycles, as one would expect from a system where reactions involving oxygen are important (and as is shown by rH).

Does the reaction exactly follow the one H+/e- rule? Maybe not exactly. Figure 2 shows a plot of rH as a function of time using Simon's data. If the effects of pH on ORP were exactly removed by calculating rH using:

rH = mV / 29 + (2xpH) + 6.67

then one might expect rH to not have a diurnal cycle. In this figure, the data suggest that there is still a diurnal dependence to rH, possibly due to pH effects. I have seen data from other aquaria as well, and in those cases the same holds: that rH largely compensates for ORP changes with pH, but not perfectly. Since things other than pH (such as O2) may change during the night and day in aquaria, this experiment may be confounded by these other variables.

Simon also ran an additional experiment on his aquarium. He took a water sample, and added either sulfuric acid or sodium hydroxide to it to adjust pH. In this experiment, the other factors that might cycle diurnally in an aquarium are constant. The results are shown in Figures 3 and 4. The fact that the ORP goes almost exactly back to where it was at the start, despite the pH excursions, suggests that the acids and bases are not altering the "base" ORP, but are only impacting ORP through pH.

The ORP moves inversely to pH, as expected (Figure 3). But, the fact that the rH is generally not flat as the pH is changed (Figure 4), but rather tracks with pH changes, suggests that the mathematical conversion used (rH = mV / 29 + (2xpH) + 6.67) is overcorrecting for pH changes. That result in turn implies that the pH dependence of ORP may be less than predicted by the H+/e- ratio of 1.0. Perhaps this result indicates that in Simon's aquarium, some reactions with an H+/e- ratio below one are important in controlling ORP.

Overall, my suggestion for aquarists using ORP measurement devices is to be aware of how pH can influence ORP measurements, but to not overly emphasize specific pH corrections.

 

[edmcnierney]

@Randy Holmes-Farley pointed me here from another thread. I'm just contributing some ORP data to our collective (puzzling) pool. I'm setting up a new tank and treating with 3% H2O2 for a dinoflagellate problem. The ORP probe is only about 2 months old. I've been dosing 1ml/10gal once a day and seeing an immediate sharp drop in ORP when I do so. After 5 or 6 hours ORP returns to the earlier level.

Today I did the same and then put the probe in pure H2O2 from the bottle. It was about 8ml in a glass graduated cylinder, and since the H2O2 was about 15 degrees colder than the tank water I let the cylinder sit in the sump for a while to equilibrate the temperature - it didn't make any difference. I then poured the H2O2 into the sump.

This pic tells the story. ORP had stabilized at around 280. I added H2O2 about 11:30AM and it immediately dropped to about 235 and then started climbing back. At 13:30 I put the probe in pure H2O2 solution, and it popped right up to 400 or so and stayed there. I put the probe back in the sump and poured in the extra H2O2 so the ORP again dropped below where it was before the experiment.

So.... pure H2O2 has an ORP about 120 higher than my tank water, but when added to my tank water it makes the ORP go down immediately. Almost all the drop happens within 6 minutes, which seems awfully fast for it to be due to dieoff (but it does suggest my tank circulation is pretty good!)

 

[Lasse]

New measurements shows a pattern that I suspected before but can´t in an easy way explain. As I have shown before - if I let the oxidator run empty - the dip in ORP when refilling get serve and it takes time for the ORP to recover again. In the chart - timeline to 7/4 shows this. After this I have try to fill up the oxidator close to the time its get empty. from 7/4 and forward. The pattern is a little bit different - the dip is not as deep and the recovery time is shorter. In the graph there is a dip (red circle) that is not caused by refilling - its caused by cleaning of the electrode. Never the less - the graph shows a new pattern. The blue markings shows there the oxydator run empty. The black line yesterday shows that it was refilled before it was empty. The periodical up and down is caused by pH interference.

I will follow this up but IMO - this indicate that the slow dosing of H2O2 does something with the water chemistry as long as it continue and if it stops for a while - there is a "debt" that has to be paid before thing are as usally. I´ll think it also show that a redox electrode (at least mine) need a "recovery" period after cleaning before you can trust it again or at least shows a steady reading. My oxidator is placed around 10 cm from the electrode and "upstream"

Sincerely Lasse

 

[Randy Holmes-Farley]

I will follow this up but IMO - this indicate that the slow dosing of H2O2 does something with the water chemistry as long as it continue and if it stops for a while - there is a "debt" that has to be paid before thing are as usally. I´ll think it also show that a redox electrode (at least mine) need a "recovery" period after cleaning before you can trust it again or at least shows a steady reading. My oxidator is placed around 10 cm from the electrode and "upstream"

Sincerely Lasse

That's certainly possible. There are lots of redox active species and some may need to get used up to see other effects. Many redox reactions are kinetically limited, rather then thermodynamically limited. Thus, not everything that will eventually happen happens at the same rate. O2 reacting with wood, for example, is very thermodynamically driven, but kinetically slow. Something in the water (lets call it metal X) may slowly get oxidized or reduced when the eproxide is stopped, and the new form rapidly reacts with the newly supplied hydrogen peroxide (or maybe even the Cu+ we have hypothesized) until the new form of metal X is gone, and only then do we begin to see the ORP drop and possible accumulation of Cu+ (or whatever the reduced species that causes an apparent ORP drop is).

.

 

[Lasse]

I have some rather interesting graphs to show. I have a 1 liter bottle there I pump in 400 ml water in the morning. when I wake up I load it with 4 cubes cyclops and som frozen artemia. Its a magnetic mixer stirrer involved.

This is distributed out around 10 time during day. Around 21:30 I pump in 400 ml water again and pump it out again (rinse of the bottle) - the same happens during the night once. In the graph - the positive bars (red) is pumping out into the aquarium and the negative bars (black) is filling up the bottle. The interesting thing is how the ORP looks like - it is a saw tanded graph - every dose of food and the first rinse cykel make the ORP to drop. And with the feeding (same amount each time) - very low amplitude in the beginning - higher in the end. And the first rinse cykel give an huge drop - the second no drop. Interesting curve

Sincerely Lasse

 

[Randy Holmes-Farley]

I have some rather interesting graphs to show. I have a 1 liter bottle there I pump in 400 ml water in the morning. when I wake up I load it with 4 cubes cyclops and som frozen artemia. Its a magnetic mixer stirrer involved.

This is distributed out around 10 time during day. Around 21:30 I pump in 400 ml water again and pump it out again (rinse of the bottle) - the same happens during the night once. In the graph - the positive bars (red) is pumping out into the aquarium and the negative bars (black) is filling up the bottle. The interesting thing is how the ORP looks like - it is a saw tanded graph - every dose of food and the first rinse cykel make the ORP to drop. And with the feeding (same amount each time) - very low amplitude in the beginning - higher in the end. And the first rinse cykel give an huge drop - the second no drop. Interesting curve

Sincerely Lasse

That is certainly consistent with a buffering of the ORP in the aquarium being overpowered and not getting back to the starting point between doses.

 

[Lasse]

Some new graphs that will do it more complicated.

I have a small aiptasia problem and try to test if I could eliminate them with a H2O2 direct applied on them. I use a syringer and 20 ml 12 % H2O2. I did it during a period there my oxydator was empty. Blue circle - oxidator empty. Red circles - dosing 20 ml 12 % direct in the DT. When my oxidator not in use - my orp have a daly swing of around 100 mV Between -330 to - 430 mV. When dosing the 20 ml H2O2 in the DT - ORP decline (as expected) but return to the daily average in a day.

The other day - I fill up the Oxydator again - a dip (red circle) as expected but it stabilize the daily swings on a lower level (as been seen before and blue ring)

Its rather obvious that an one time large dose do not affect the ORP levels the same way that a continuous flow of it. I have no explanation for this at all more than the "activation" of the free radicals (H2O2 lose an oxygen atom) may lower the ORP (a reduction process). This should mean that a single dose will soon be activated - but a continuous flow will be continuous "activated" - lower ORP readings

Sincerely Lasse

 

[Lasse]

I have some newer experiences with another system (without animals). We run into problems in a newly started aquarium with high content of NH4 in a epsom salt (MgSO4 7H2O) that was used. I´ll comeback with details later on in another thread. We ended up with a huge, massive blom of heterotopic bacteria and a lot of dissolved organic carbon. For those that are familiar with the parameter BOD7 - it was calculated to around 24 ppm as BOD7 - the biological demand of oxygen during 7 days, It was not a usual BOD7 measuring but a test with an optical oxygen probe in 1 L of water and 17 hours. The BOD7 was calculated from that drop of Oxygen. The nitrification process stall directly - even the first step - the free NH3 was up in the roof. The system use sandfilters and with help of daily back flushes and use of H2O2 around 2 ml H2O2/100L 12% in total but slowly added during 2 days. This did the trick - the bacteria bloom, the DOC disappear - mostly showed in lower daily CO2 addition. The addition of CO2 from bacteria activity was ridiculous - pH got down to around 7 but after this treatment it start to rise again. Alkalinity was steady dropping also - now its better. Both total ammonia (with JBL test) and free NH3 (ammonia alert from seachem) show very high figures . Now - both are zero. BOD 7 is nearly not detectable. NO2 still sky high but slowly decrease. This is the background - now to the result. When I add H2O2 in the situation with very high organic load - I could not see any fast down going ORP values - I was a little surprised. The electrode was rather new - that can be the answer but I´m not sure. Yesterday when we treated with 0.5 ml/100 L 12% H2O2 - we get a sharp decrease

Maybe high organic load will affect the dip - I do not know

Sincerely Lasse