Got ORP?

[prattreef]

Hi Ken. Thanks for stopping by and for your thoughtful reply. I'd like to formally welcome you to MR. We are very much looking forward to your talk next month.

Randy

 

[ellebelle]

Ken, very enlightening (and well explained!) response.

It would be really interesting to look at the ORP reading on softie tanks of various crowding. If it is true that as the soft corals are crowded, they release toxins into the water as a defense mechanism, and if it is true that ORP is some measure of unstable elements (those that gain and lose electrons freely), then ORP should go up when soft corals become crowded and "territorial" - assuming that the toxins released from soft corals have some unstable elements.

I did a quick search of some academic journals to see if I could track down the chemical composition of soft coral toxins. Sadly my organic chemistry days are far behind me which means that my ability to interpretthe scientific data is limited, but I did find that Lobophytum and Sarcophyton specifically contain a toxin composed of 3-carboxy-1-methyl pyridinium, which I believe has a positive charge which should also be electroactive. It could be an indirect means of trying to figure out if ORP is actually measuring what you think it is measuring. It would be difficult to control for other factors at work though.

Just kind of thinking out loud.

 

[meschaefer]

Ken, thanks for taking some time out of your day to take a look at this thread. Generally I want to take the opportunity to thank you, and a handfull of other hobbyists out there, that have the education, training and access to high end lab equipment for taking their valuable time to conduct research for the hobby.

While I appreciate the concerns regarding the make up of DOC , in order to determine how it effects if any on Orp, but I wonder if this is a "2nd level" question . It would seem that this is really a question that doesn't necessarily need to be answered as an entry point into thinking about these things as we can make observations as to whether increased DOC has an overall effect on Orp without necessarily understanding why it has the effect it does. Granted an understanding of what DOC is made up of would be the next line of inquiary and an important one at that (why I refer to it as a "2nd level" question).

I think your point of comparing one tanks Orp with another tanks Orp as being useless is right on point, and one that I should have made clear at the begining. I am more interested in the ability to descern usefull information from long term trends in Orp.

 

[NYPDFrogman]

Thanks Ken!!!

for your input! Look forward to reading more of your input.
great thread Matt!

 

[Ken Feldman]

Hi All

Thanks for your welcome. I realize in reading your responses to my post that I may have led you astray by focusing just on DOC components. In actuality, oxygen (O2) and oxygen-derived species are probably the biggest "players" in determining the ORP reading, as they are likely the most plentiful electroactive species present. In addition, pH plays an important role in modulating the oxidation and reduction potential of specific species (by protonating or deprotonating them, depending upon pH value).

Species such as electroactive metal ions (iron, copper, as examples) and electroactive organic compounds as part of DOC are likely to be much less prevalent. My initial response was actually directed to the specific issue of how/why the organics might influence (or not) ORP measurements; please bear in mind that it is likely that, overall, the organics play a much less significant role than the oxygen-derived species.

I suggested quinones and by implication their reduced partners, hydroquinones, as ORP players as these types of chemical groups are likely to be the most prevalent electroactive compounds amongst the complex pool of organics.

The real key in trying to understand the correlation (or lack thereof) between ORP and TOC is to appreciate just how much of the total TOC consists of electroactive species - that is the question that I don't know the answer to. If it is a lot, then perhaps more of a correlation might be expected; if it is not a major component, then perhaps less of a correlation between ORP and TOC levels might occur.

Soft corals, soft-bodied filter feeders (i.e., tunicates) and sponges (through their symbiont microbiota) are the real chemical factories on the reef. Stoney corals, fish, crustaceans, etc. have not yet been documented as containing anywhere near the chemical arsenal as the soft-bodies species, for evident evolutionary reasons. There is no argument among natural product researchers that the coral reef, and not the rain forest, is the real chemical pharmacopeia of the world. So, it is likely that reef aquaria dominated by these soft-bodied species will have a different TOC makeup that an aquarium that lacks these producers, and this difference might be detectable through ORP measurements. I don't know the answer to this question, but I would like to hear from other aquarists who might have some data on this point.

Ken

 

[meschaefer]

You put forth what I am sure is a very simplfied explanation of what effects Orp within our systems, and I think I am starting to get a much better grasp on the metric.

Within your tests regarding organics, you obviously tested ORP (as it is found within your data). Did you expect to see a closer correlation of Orp to TOC prior to condcuting the tests?... or did the tests bear our your expectations?

 

[Ken Feldman]

Matt

I really didn't have an expectation one way or the other - I was just curious. I try to hold expectations in check so as to avoid biasing the experiment. This point was driven home to me by the surprises that we saw in our skimmer and GAC studies, which I will discuss when I visit MR in Nov. Hope to meet you there!

Ken

 

[meschaefer]

Matt

I really didn't have an expectation one way or the other - I was just curious. I try to hold expectations in check so as to avoid biasing the experiment. This point was driven home to me by the surprises that we saw in our skimmer and GAC studies, which I will discuss when I visit MR in Nov. Hope to meet you there!

Ken

Looking forward to it.

 

[randy holmes-farley]

FWIW, I think that it is fairly well established that the proximate thing that determines ORP in seawater are certain redox active metals, such as manganese. I discuss this in great detail in this article:

ORP and the Reef Aquarium
http://www.reefkeeping.com/issues/2003-12/rhf/feature/index.php

That said, there are many contributors that impact the state of these metals, and organics and O2 are certainly part of the story.

In general, I do not believe that ORP is a useful measure of water quality, especially if it is artificially altered by oxidizers like ozone. But if you come up with some indications otherwise, that would be great to know.

 

[randy holmes-farley]

I just posted a reply I thought went here. I wonder where it ended up?

I'll try again if this one works.

 

[randy holmes-farley]

OK, that worked.

I think it is fairly clear that while O2 and organics play a role in determining ORP, that neither is what is measured in seawater with an ORP electrode. I think it is almost certainly a set of redox active metals that impact ORP electrodes directly, and in seawater it is likely manganese and a few others (e.g., iron). For those that have not seen it, I discuss such issues in both simplified and highly detailed chemical terms here:

ORP and the Reef Aquarium
http://www.reefkeeping.com/issues/2003-12/rhf/feature/index.php

from it:

What Redox Reactions Control the ORP in Seawater and Marine Aquaria?

The nature of the redox reactions that control the ORP in seawater and marine aquaria is very complicated. It is not known exactly which chemical species control the ORP, and it is not an equilibrium situation, so all simple chemical equations will only be an approximation of what is taking place.

Certainly, a big part of ORP is driven by reactions involving oxygen (O2). Oxygen is a fairly strong oxidizing agent, since it can undergo the following reaction:

O2 + 4H+ + 4e- ?? 2H2O

In totally pure fresh water (pH 7), without contact with any atmospheric gases, the ORP is 202 mv at 25?C. If a normal amount of atmospheric oxygen (0.21 atmospheres) is allowed to come to equilibrium with that water, the ORP rises to 607 mv (535 mv at pH 8.2). So obviously the ORP has risen considerably due to the oxygen. [This value of 535 mv is also the same value expected in seawater if this redox reaction dominated.]

However, the effect of the exact concentration of O2 is not very great. At twice the concentration of O2, the ORP only rises to 540 mv at pH 8.2. It also only drops to 531 mv when the amount of O2 is halved (also at pH 8.2).

Why such a small dependence on the O2 concentration? There are actually two answers to that question, depending on what is really being asked.

Why does the ORP not change more when the concentration of oxygen is changed so much? The simple answer is that equilibrium ORP is just not very sensitive to small changes in the concentration of oxygen. After all, ORP only varies over about 1000 mv from the most oxidizing to the most reducing environments found in natural waters. But the oxygen concentration might vary by a factor of 1050 or more.

Keep in mind that ORP is logarithmic in the same sense that pH is logarithmic. If you double the [H+], pH only drops by about 0.3 pH units. In the same way, doubling the [O2] has only a fairly small effect on ORP.

Why does the measured ORP vary so much in aquaria? Does that imply that the concentration of oxygen is varying by huge amounts as ORP rises and falls? Those are very deep questions into the nature of ORP in aquaria. The answer boils down to the fact that ORP is not at equilibrium in aquaria. There are oxidizers (such as O2) and reducers (such as organics) present together. That alone tells us that the system is not at equilibrium. So we cannot assume that any equilibrium relationships between the concentrations of these species and ORP will necessarily hold true.

Since many species can potentially impact ORP in a reef aquarium, all that can be concluded from a change in ORP is that one or more of the redox species has changed concentration. For example, if the ratio of Fe+++ to Fe++ in solution suddenly doubled, then one would expect some rise in ORP. If these species were the only redox active species in solution, then the ORP would rise by 18 mv (the equation to derive this result is shown later).

However, since there may be other redox active species present, these other species will likely blunt, if not totally swamp, the effect from that change in iron. This effect is exactly analogous to adding acid or base to a solution. If it is unbuffered, a large change in pH will be observed. If it is buffered, the change is much smaller. So too with redox. If the iron were alone, a large ORP change (18 mv) would be seen. But with other redox species ready to buffer the ORP, the rise may be much smaller, or even undetectable.

The unfortunate circumstance with ORP, however, is that we do not have a good understanding of the redox active species in seawater and marine aquarium water. Consequently, unlike pH where buffering is readily understood, measured, and theoretically predicted, the effects of oxidizers and reducers on ORP is much harder to fully understand.

What redox active species can contribute most to ORP in marine aquaria? Table 2 lists some possibilities, and the relative importance of each may well vary between aquaria with different concentrations of the various species. Other redox active species in aquaria include arsenic, copper, lead, chromium, mercury, and selenium, among others. One can look up the relative oxidizing and reducing power of all of these under standard conditions to get a rough idea of which will control ORP in seawater and aquaria. However, many of these form complexes with other inorganic and organic materials in seawater, and such complexes can have very different redox properties than the bare ions. Also, how important they are to redox control depends entirely on how much of each is present.

Two of the primary contributors to ORP are going to be oxygen and organics. Since organics comprise a wide array of different species, it has proven impossible to say definitively what controls ORP in seawater. In the end, I expect that the ORP is kinetically controlled by a steady state of oxidation by oxygen and related species with the various organics in the aquarium. Some of the other species listed in Table 2 may also play important redox "buffering" roles.

 

[randy holmes-farley]

Well, I guess the posts are being manually moderated, so I won't proceed to continue for now.

 

[Deanos]

Actually, it's automated. If someone has less than 3 posts historically, they cannot post links. It prevents spammers. Their posts will have to be 'approved' before it shows to the general community.

Welcome, RHF! And thanks for contributing.

 

[meschaefer]

Randy, thank you for your input. I think I posted a link to your article earlier in the thread.

I understand that there are various types of reducers in our aquaria and I understand that many of these reducers have larger effects on ORP than organics. What I think I am missing is the extent to which these other reducers vary within any one aquarium. I know that organics build up in an aquarium without proper maintenance, but to what extent does iron or manganese build up or are removed over time? I know iron will be used by certain macro algae, and is useful in red pigmentation, but I am not so sure about manganese.

It would makes sense to me that it is useless to compare one aquariums ORP with another aquariums ORP, as the two systems could be completely healthy and have wildly divergent ORP measurements. But if your looking at one aquarium over a lengthy period of time can ORP be meaningful? Or do the levels of these other reducers (i.e. other than organics) vary significantly over time to make ORP useless as we just can't be sure what is the cause of the effect?

 

[randy holmes-farley]

but to what extent does iron or manganese build up or are removed over time?

I don't think it is a matter of buildup or removal. IMO those metals are like a thermometer reporting the temperature, not actually causing it. It is the ratio of Fe++ to Fe+++ that impacts ORP, not the absolute amount of them. Same for all other redox active metals.

But if your looking at one aquarium over a lengthy period of time can ORP be meaningful?

Sure. The question is what that meaning is. IMO, most things that you do that raise ORP without adding artificial oxidizers are likely good for a reef tank, as long as they do not cause other harms. More skimming, GAC, removing dead organisms, whatever.

 

[meschaefer]

But if your looking at one aquarium over a lengthy period of time can ORP be meaningful?

Sure. The question is what that meaning is. IMO, most things that you do that raise ORP without adding artificial oxidizers are likely good for a reef tank, as long as they do not cause other harms. More skimming, GAC, removing dead organisms, whatever.

This kind of brings us full circle, and where I keep running into some difficulty. When you indicate:

"most things that you do that raise ORP without adding artificial oxidizers are likely good for a reef tank, as long as they do not cause other harms. More skimming, GAC, removing dead organisms, whatever."

Are you saying that: while these things tend to lower ORP in a reef tank and their removal is a net benefit, we can not be sure that their removal or accumulation will have any real measurable effect on ORP do to the power of other oxidizers and reducers.

otherwise it would seem that a slowly decreasing ORP would signify a problem in the system that needs correction, and that measuring ORP would be effective way to gauge overall system health as your methods of increasing ORP (skimming, GAC, removing dead organisms) have failed or are no longer operating effciently.

 

[randy holmes-farley]

Well, I think there are a lot of unknowns relating to ORP and reef tanks, but I do not think that most organisms know what the ORP actually is. I don't think they can sense it, so they have no way of "caring" about it.

What they do have a way to sense and respond to, positively or negatively, is some subset of chemicals in the water. The actual ORP may reflect some of that assembly of chemicals, but I don't think we have much information on how specific redox active chemicals impact marine organisms.

In people, for example, the reduced form of iron, Fe++ is the form taken up from the intestine. And that is the form of supplements that people take. In a sense, if the ORP in the GI tract were too high, we might have difficulty absorbing inorganic iron. But humans can reduce Fe+++ to Fe++ using enzymes in the GI tract, so that if there is too much Fe+++ and not enough Fe++, we can survive. I expect that marine organisms have similar issues with things like iron and many other trace elements, but exactly what they need, how they get it, and how they might compensate for not having their preferred forms 9or having too much of toxic forms) isn't clear.

But, all of those are just a subset of things that impact ORP. If we go back to organics and O2, we may find that ORP is associated with many factors that are not themselves CAUSED by the ORP. For example, excessive dissolved organics might drive cyanobacteria and might also be associated with low ORP. That doe snot mean the low ORP caused the cyano, but rather they are both caused by the same causative agent (excess organic materials). In that way, ORP measurement may report out a property that is not itself important, but is an indicator of other things.

If that is the case, then we need to make sure that we are interpreting things in the proper way. For example, adding ozone raises ORP, but does not seem to reduce soluble organics appreciably. In that case, we have not reduced the causative agent for cyano, but rather have uncoupled the connection between ORP and organic levels.

Some things you'd do to increase (or decrease) ORP may fall into this noncausative category. Ozone and all other oxidizers, for example, but also vitamin C dropping ORP so drastically. Maybe growing a lot of macroalgae takes away nutrients that cyanobacteria might consume, but also leak certain reduced organics into the water, reducing ORP. In these cases case, you would possibly not want to interpret the ORP change as likely to be reflected in something like cyano growth.

But things you'd do directly to the causative agent that raised (or lowered) ORP, like skimming and GAC, may well be usefully monitored by ORP.

Anyway, that's just a lot of ramblling, but it represents how I think of ORP.

otherwise it would seem that a slowly decreasing ORP would signify a problem in the system that needs correction, and that measuring ORP would be effective way to gauge overall system health as your methods of increasing ORP (skimming, GAC, removing dead organisms) have failed or are no longer operating effciently.

That might be the case, or it might not. For exmaple, as the macroalgae in a refugium expands to larger and larger amounts, reflecting more nutreint export capability, ORP may drop as that larger amount releases some reduced organic species. Is that good or bad?

 

[ShaunW]

I am primarly intersted in using ORP as one of many measurements of overall water qualtiy. To often, you can see that there is something not right in the system (algae build up, cyanobacteria, etc. etc.) but all of the usual suspects check out. You figure that it is probably a build up of DOC (i.e. time for a water change) but it is ultimately just a guess.

The build up of inorganic carbon, carbon not associated with a living cell, is best observed biologically IMO. How often algae builds up on the glass and needs to be cleaned, the amount of skimmate that is removed weekly and the consistency of it, and the presence of cyanobacteria/hair algae are all good indicators of excess nutients that are not biologically associated. Better than any measurement created to date for the aquarium hobby or available to the average reefer.

It seems that knowing the tanks ORP and watching the trend (i.e. is it slowly going down, up, level, sudden spike, etc. etc.), you might be able to measure a build up of organics and take corrective action before there is a problem.

Who out there is measuirng orp?; How are you measurieng it (specific equipment)?; and, What do you do with the information that you get?

IMO by the time you start to witness a trending of ORP higher in a reef tank your already creating a stress response within the living creatures within. Many bacteria are adaptable to fluctuating ORP and very sensitive to the build up of an oxidizing agent such that they contain a compilation of enzymes capable of neutralizing the affects. Catalase is an example when it decomposes hydrogen peroxide. But can corals deal so effectively? I am sure some are better than others.

If the interest in ORP is because of its relationship to decaying organic matter, then IMO it comes back to the Redfield ratio, and the equilbrium that exists between the organic and inorganic within the aquarium. My thoughts on it. Here I am talking about phosphate, but the same applies to inorganic carbon.
http://www.manhattanreefs.com/forum/advanced-reefs/17828-coloring-up-sps-8.html

 

[ShaunW]

Certainly, a big part of ORP is driven by reactions involving oxygen (O2). Oxygen is a fairly strong oxidizing agent, since it can undergo the following reaction:

O2 + 4H+ + 4e- ?? 2H2O

In keeping the thread alive, , and adding some more information to the whole story with the prespective of the biology in mind.

The biological response to increased oxidative ORP levels is called the oxidative stress response. This response is imperitive to life, since every minute cells are stressed from oxidizing agents. Even during cellular respiration, reactive oxidizing species (NO (nitric oxide), superoxide, nitrite, free radicals, etc..) are created just from the process of living, when oxygen acts as a terminal electron acceptor during respiration. So we breathe and make compounds that can kill us. These compounds need to be neutralized otherwise they will cause damage to all components of the cell, including proteins, DNA, RNA, and lipids.

Cells contain within them a reducing environment that they maintain with percise regulation and accuracy. This reduced environment is created by many enzymes (reactive proteins) many of which are involved in metabolism and use the energy from metabolism to keep the reduced state. Additionally co-factors to enzymatic function, NADH for example, are equally important in maintaining the equilibrium (constant reduced state). Therefore all cells are prepared with an arsenal of tools to ward off oxidative damage from internal and external sources. They are also able to respond very quickly, making specific proteins for defense as needed.

So what effect does the entire biological entity's total reducing environment of the aquarium play in the detoxifying of oxidizing agents? :scratchch

 

[Domboski]

Thanks for the headeache Shaun :headache:. Just kidding. Awesome information. This is why I prefer to focus on fish

If I am able to follow this thread correctly, I would guess that ORP alone is not necessarily a good indicator for system issues. There could be spikes and drops in ORP that are part of the natural cycle and fight for equilibrium within the system?