This simple version:
ORP and the Reef Aquarium by Randy Holmes-Farley
http://reefkeeping.com/issues/2003-12/rhf/feature/index.php
From Randy's article:
"What about pH? pH can impact the ORP readings in aquaria. Often, ORP goes down as pH rises. A typical aquarium ORP reading will change on the order of 59 mv/pH unit. The easiest way to understand this is to simply think of pH as a measure of hydrogen ions (H+) in solution, and to think of H+ as being on the side of the oxidizers. In reality, H+ doesn't usually oxidize things itself (though it can), but more typically it can hype up other oxidizers, like oxygen, making them much more potent. So during the course of a 24-hour day in a reef aquarium, ORP will vary as pH and O2 also vary."
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The more technical version from Randy's same article:
"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"
