ORP and the Reef Aquarium - avb4you.info
potential [ORP]) to indicate wastewater quality online in aeration tanks treating medium . The relationship between ORP, COD, and DO along the length of the . Jun 4, ORP (Oxidation Reduction Potential) is a popular water quality parameter that is Through known relationships, results can be converted to other that the user does not need to adjust the calibration value for temperature. May 10, ORP Meter for Dissolved Oxygen The Reef Chemistry Forum. Nope, they have no relationship what so ever However, the ORP meter can.
For this reason, the electrode exhibits a "memory effect" which strongly affects the measurements of the following samples. Several cleaning methods were used to overcome these problems [ 1916 - 18 ]. Among these methods, and after a variety of essays, we have selected the following cleaning procedure before each measurement: So, the potential vs time curves and the EH and rH values obtained were highly reproducible.
Once the electrode is immersed into the water, the recording of ORP starts. This recording was made in a continuous mode, maintaining the electrode into the water until the end of the experiment. The ORP varied with the measurement time in the general shape shown in Figure 1that is, reaching a limiting value at long times. This general behaviour was found for all synthetically prepared waters, irrespective their composition. Thus, EH is the "equilibrium oxidation-reduction potential" and corresponded to ORP when the equilibrium in the interface is reached.
It must be remarked that the measurement corresponded in all cases to a unique oxygen content and the stabilisation time was reached when the EH values changed less than 5 mV being this quantity the accuracy in EHin this case after seconds.
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As can be seen, higher oxygenations implied higher EH and rH values. Conversely as occurs for a natural aquatic medium, the lower oxygenation implied the higher pH-values. This fact can be explained by taking into account the method used to obtain the actual oxygen concentration: Potential vs time curves from middle salinity prepared waters. Likewise, the higher the concentration of the ions, the higher the distortion. This can be explained by taking into account the high adsorption of oxihydroxides of Mn and Fe generated during the experiments that harmed the diffusion processes associated to the kinetics of the measurements of ORP.
Moreover, the number of possible oxihydroxides generated for Mn is higher than those expected for Fe [ 20 - 22 ], explaining the above sequence. On the other hand, the more oxygenated samples showed always the highest rH values and the more positive or less negative EH values, with independence of salinity and concentrations of ions table This could be associated to the reversibility of the process as occurs in nature.
The range of variation of EH values in the samples containing Fe or Mn ranged more widely than in the rest of the samples. This can be explained by the presence of organic matter that show a strong tendency to adsorption on the Pt electrode surface. Table-3 shows the evolution of the water characteristics along the treatment for drinking water production.
Characteristics of water under treatment for drinking water production maximum and minimum values The evolution of EH and rH in waters under treatment follows the sequence: The oxidized water presented the higher values due to the dosage of some oxidants O3 and Cl2 specially in the oxidation-prechlorination step. The residual oxidant was lost by both oxidation and atmospheric diffusion, implying the decrease of the above values in filtered waters with respect to oxidized waters.
The raw waters showed the lowest EH and rH values of each group due to the oxidant power of O3 and Cl2, much higher than the air oxidant power. However, the raw waters showed always ORP values corresponding to oxic waters [ 1919 ]. Waters under treatment for drinking water production: A EH; B rH. Oxidation processes used in water treatment originate the removal of oxidizable species dissolved iron and manganese, organic matter, etc.
The efficiency of this process must be associated to both the specific dose of oxidant employed and the increase of water ORP. In this study, the dosages used for both O3 and Cl2 were relatively low. Moreover, the raw waters untreated showed a reasonably good oxidation state and low levels of Fe, Mn and organic matters. For this reason, it was difficult to prove an experimental relation between high removing of substances easily oxidized, present in water, and high increases of EH and rH in oxidized waters against raw waters.
According to these results, the ORP value is not an adequate parameter for controlling the treatment process in production of water intended for human consumption: Disinfection of water is made with the same reactants as oxidation, the oxidant playing two roles: In the present study it has been observed the complete elimination of the total coliforms number present in raw waters for increases of EH and rH values in oxidized water against raw water equal to 0.
This can be due to the low microbiological amount in our waters against the higher used in other studies. Moreover, such previous studies considered only laboratory experiments and not practical industrial cases as that studied here.
Understanding pH and ORP
Nevertheless, approximately after seconds the equilibrium was established. Table-4 shows the evolution of waste water characteristics along the biological treatment: Particularly, the EH and rH values followed the sequence: This can be explained by taking into account that the treated water experienced a process of aeration prior to the biological treatment being relatively well oxygenated and thus relatively oxic.
Settled water did not experienced aeration, its very low O2 level was consumed and, in consequence, its reduction state was increased with respect to the wastewater.
Finally, waste water was originally anoxic as is typical in these kind of waters. Nevertheless, the EH and rH values measured in this study were lower than those found in the literature [ 124, 9 ] for recently produced waste waters. This can be due probably to the consumption of O2 in these waters along the sewage network pipes that carry waste water to treatment plant.
Waste waters under biological treatment: This has not an evident explanation but it can be assumed that under these conditions other redox species different from O2 can influence the redox state. Among such species, the equilibrium "sulphate-sulphide" could be very outstanding in this case, as well as all the reduction processes used by the waste waters anaerobic microorganisms [ 482526 ].
Domestic waste waters contain high amounts of residual organic substances and are poor in dissolved O2. An analogous reaction is the reaction of acids and bases. Nearly all acids and bases in an aquarium will rapidly reach equilibrium, and that equilibrium is very well represented by a single value, the pH. Likewise for redox reactions, a steady state of electron pushing and pulling is reached, and that state can be represented by ORP. The analogy breaks down, however, because not all oxidizers and reducers are capable of reacting with each other or the ORP probe in a short period of time.
Given enough time, for example, oxygen as the oxidizer will react with ethanol the reducer to form a variety of products, ultimately ending in carbon dioxide and water. That reaction is very slow, however, and might not happen at all over the lifetime of a reef aquarium. So, there is a subset of oxidizers and reducers that are actually capable of reacting with each other, and moreover for interpretation of ORP, in impacting an ORP electrode.
ORP level ozone, ORP levels, ozone, oxidation reduction potential
Consequently, a single ORP value measured for a given aqueous solution may not correctly describe the relationship between any given pair of redox species in the solution.
These reactions are often shown as half reactions, where one half loses electrons and the other half gains electrons: The reactions shown above are the simplest sort of electrochemical reactions, involving only two species. However, most of the reactions important to aquarists are much more complicated and involve several species. For example, the half reactions involving oxygen: 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: If a normal amount of atmospheric oxygen 0.
So obviously the ORP has risen considerably due to the oxygen. It also only drops to mv when the amount of O2 is halved also at pH 8. 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 mv from the most oxidizing to the most reducing environments found in natural waters. But the oxygen concentration might vary by a factor of or more. Keep in mind that ORP is logarithmic in the same sense that pH is logarithmic. 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. 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. Some oxidizers and reducers in marine aquaria.
In other words, the chemicals themselves pull and push the electrons to and from a suitable probe, and the resulting voltage is a direct measure of the redox properties of the solution. ORP can be measured in other ways, such as with redox sensitive dyes, but that is rarely done by aquarists.
The electrode that actually does the ORP sensing is usually an "inert" metal, such as platinum or gold. However, one cannot simply put a single electrode into a solution and expect to get anything useful because the voltage needs to be compared to something else. That is, voltage is always the electrical potential difference between two different points, not an absolute measure at a single point. So one needs a reference electrode that provides a constant "ground" with which to compare the electrical potential in solution.
One such reference electrode can be provided by the standard hydrogen electrode. The standard hydrogen electrode involves the following reaction: This reference electrode is arbitrarily defined as having a zero voltage. To make a measurement with a platinum redox electrode, one then measures the voltage difference between the platinum redox electrode put into your solution of choice, and this reference electrode put into its reference solution plus an electrical connection between the two, usually provided by a salt bridge.
The value measured in this fashion is often called EH. Unfortunately, the standard hydrogen electrode is cumbersome to use, and only specialized labs typically use them.
Luckily, much simpler reference electrodes have been developed that are very easy to use. These electrodes are typically included in ORP electrodes, even if the manufacturer does not specifically say so.
Consequently, all ORP readings taken by aquarists and all values quoted in this article unless otherwise stated are using this reference electrode. Inside of such an electrode is a silver wire coated with silver chloride and surrounded by a solution saturated with potassium chloride. The reaction setting the potential for this reference electrode is: Using a saturated potassium chloride KCl solution keeps the chloride concentration steady at a given temperaturemaking this a good choice as a reference electrode.
One then only needs a tiny electrical connection to the solution being measured to complete the circuit, and allow measurement of ORP using a platinum electrode. If they did, and the potential difference between them was measured, there would be no voltage difference. 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 's by W. 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: 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 In brief, rH has become an unmitigated nuisance.
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. In that case, the ORP is exactly determined by the relative concentration of the two iron species, and is unchanged with pH.
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: Consequently, the oxidizing power is related to pH.
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. The standard definition of rH assumes that this ratio is exactly 1.
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. For the various reactions of the nitrogen cycle, we have ratios that vary from 1. If it is a mixture of species, then the end result will come back as a complex averaging of the different reactions involved.