ORP Oxygen reduction potential.

Discussion in 'General Discussions and Advice' started by Tridan, 21 Dec 2007.

  1. Tridan

    Tridan

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    hi hennie thought i would get some info from you about this orp.

    now from what i know ,( not much) please add your coments about orp.

    "ORP stands for Oxidation-Reduction Potential. In practical terms, it is a measurement to oxidize contaminants. It's as simple as that." quote rhtubs.

    The above quote is non reef related,( not from a reef keeping site) so my question is, is the same what is said above reef related or not.

    Sorry for the stupid question but i have not found to many articles related to this subject.


    ok this is the best info on orp i can find but some of those scientific names/words just have no meaning to me.
    http://en.wikipedia.org/wiki/Redox

    So please help in a language i can understand.

    Regards and seasons greetings Hennie.
     
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  3. Reef Maniac

    Reef Maniac MASA Contributor

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    Oh, so you're throwing me into the deep end, are you ...

    OK, lets see... firstly, please note that I'm not a chemist, so everything I say here is open to "audit and recheck" - chemical gurus, please confirm if what I say makes sense.

    Chemically, oxidation-reduction reactions are reactions in water where electrons are transferred between reactants. One can view this as similar, but inverse, to that of neutralisation reactions (acid/base reactions), where hydrogen atoms (H+) are transferred. The neutralisation reaction controls the pH of the water, with more H+ in the water resulting in a lower pH, and less H+ causing a higher pH.

    In the oxidation-reduction reaction, a substance is oxidised when it loses electrons, and is reduced when it gains electrons (but when one reactant loses electrons another reactant must gain them as the electrons are transferred, and one can never have just one half of the reaction, hence oxidation-reduction, or redox for short...). In nature, some substances (such as chlorine, oxygen and ozone) readily take on more electrons from other substances, and are thus called oxidising agents. One of the most widely known oxidising reactions is the rusting of iron. Oxygen (in the presence of water) "steals" electrons from the iron when it reacts to form iron oxide, thus oxidising the iron.

    ORP (Oxidation-reduction potential) is a measurement of the equilibrium established between all oxidized and reduced species in solution (i.e. a measure of how oxidising or reducing the water is), just the same as pH is a measure of how acidic or alkaline (basic) the water is. Because oxidation-reduction reactions result in a movement of electrons (thus electricity...), ORP is a direct potentiometric measurement, in millivolt (mV).


    Now, what does all this have to do with our tanks?

    Well, in simplistic terms, if any living cell loses or gains too many electrons (i.e. is oxidised or reduced to much), it dies. As with pH, there is a narrow range of ORP within which our animals can live (200mV - 500mV maximum, with the recommended range for marine organisms being 300mV - 450mV). If the ORP is too high, living cells will be killed (one can thus disinfect water by adding an oxidiser such as chlorine or ozone), and if the ORP is too low, the cells will be killed as well.

    Decomposing organic waste (reducers...) are broken down (oxidised) chemically and by bacterial action in our tanks, and in this process oxidisers (such as oxygen and ozone) are "used up", resulting in a drop in ORP if not replaced. Conversely, if more oxidisers are introduced into the water than are used, the ORP will increase. Unfortunately, ORP is also influenced by pH, temperature, and the type of oxidising and reducing agents in the water, thus making it impossible for us to use an ORP reading as a direct measurement of a tank's health.

    In my (limited) experience, one can, however, use a history of ORP measurements as an indication of one's tank's health, by comparing trends (not single readings !!!). If, for example, the ORP drop substantially below the established trend (for the same pH and temperature...), there is a good chance that something has died and it's decomposition is using up more oxidisers than normal.

    I monitor my tank's temperature, pH and ORP readings with industrial monitors, and automatically record the readings on my computer every 5 minutes. This way, I can graph the readings in Excel, thus allowing me to easily compare current and past trends. Here is a screen shot of the last few days...

    [​IMG]


    As can be seen, there was quite a deviation from the norm on the 19th to 20th. During this time, I was working on the electricity supply to the tank, and all the power was off for a few hours (the dip to "zero") - thus no lights, and no water movement (except for some hand stirring every now and then...). After this period, I restored power to the pumps, but the lights remained off for a much longer time. The lack of lights caused the pH to remain low (7.9), but the low pH caused the ORP to remain high, even though there would have been a drop in the oxygen level.

    I hope this was of some help

    Hennie
     
    Last edited: 23 Dec 2007
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  4. Alfie

    Alfie

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    Informative Hennie Thanks. So woul you say in laymans terms that it is the waters ability to clean itself
     
  5. Mekaeel

    Mekaeel Moderator MASA Contributor

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    excellent Hennie!always wondered how and ORP is.thank you
     
  6. Tridan

    Tridan Thread Starter

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    Hennie , thank you i now understand this a whole lot better.
    Your post was extremely informative and in terms i can understand.
    Sorry for dropping you in the deep end.

    I appreciate your response.

    Oh and this is the first of many i have to ask, so get geared up. lol
     
  7. Reef Maniac

    Reef Maniac MASA Contributor

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    No, not really... more like a general wellness indicator.

    In terms of our tanks, even at a healthy high ORP (say 400 - 425), the water cannot "self clean" if there are no bacteria in the system. At much higher ORP levels the organics will be oxidised (broken down) into mostly CO2 and water, but 'organics' will then include the fish, corals and algae, and that would not be such a good thing :whistling:

    Hennie
     
    Last edited: 11 May 2008
  8. Alfie

    Alfie

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    Thanks Hennie
     
  9. Reef Maniac

    Reef Maniac MASA Contributor

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  10. Alfie

    Alfie

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    Hennie, maybe a silly question here but my one tank's Mv rises to as much as 430. I have swapped probes to check and both read the same. What would be the cause and should I be worried about it?
     
  11. Reef Maniac

    Reef Maniac MASA Contributor

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    There are so many things which could either increase or decrease ORP, it's really not possible for me to say with any amount of accuracy, but you could consider the following:
    • is the skimmer large in comparison to the tank size and bio load
    • additives
    • strong lighting and a high amount of algae growth
    • calibration of the probe
    • age of probe
    • addition of ozone
    • stray electrical current in the tank
    • the tank is just doing exceptionally well
    I have found that leaving the probe in the tank where light can shine on it results in a rather rapid growth of (I presume) diatom algae on the probe - any algae growth on or near the probe would result in a localised increase of oxygen, which would increase the ORP.

    The probe needs to be cleaned (soaked in a special cleaning fluid for a few hours) and then calibrated regularly (every month or two...), and the reading will be abnormally high for a few days after calibration.

    The most likely cause, IMHO, would be stray electrical current. Switch off ALL the electrical equipment in/on the tank (lights, heaters, pumps, everything...) and observe the ORP for at least 30 minutes - if there is any drastic change in reading you've found the cause. Switch the equipment back on one at a time, and observe which equipment causes the change. You could also remove the probe from the tank and place it in a glass beaker with freshly removed tank water, if you don't want to power down the tank.

    Not with an ORP of 430 - that's still well within the safe upper limit of 450mV. As stated earlier, ORP only becomes dangerous as you approach 500mV.

    Hennie
     
    Last edited: 11 May 2008
  12. Alfie

    Alfie

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    Thanks Hennie, Definately no stray electrical current. I was working barefoot in the tank this morning. Will check out the rest.
     
  13. Reef Maniac

    Reef Maniac MASA Contributor

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    Alfie, don't be to sure of that - even a few volts can affect the probe... Check as previously suggested.

    Hennie
     
  14. Alfie

    Alfie

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    Ok Hennie will do, thanks
     
  15. Alfie

    Alfie

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    Hennie just to let you know it was stray electric current. Thanks for the advice.
     
  16. Reef Maniac

    Reef Maniac MASA Contributor

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    My pleasure :)

    So, what is the actual ORP reading?

    Hennie
     
  17. Alfie

    Alfie

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    It has dropped from 430 to 402.
     
  18. Reef Maniac

    Reef Maniac MASA Contributor

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    That's just about perfect :thumbup:

    Hennie
     
  19. Alfie

    Alfie

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    Hennie, yes I do feel more comfortable with the levels now. Thanks again
     
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