Redox potential is definitely a new concept to the hobby. Many articles have been written about it, and more confusion has resulted because of the many ways in which redox potential is explained. Chemically speaking, redox potential, also referred to as oxidation reduction potential, abbreviated to ORP, is a most difficult concept to explain. Because the hobbyist does not really need to understand the chemical meaning of redox potential, but only how it can be used and interpreted with respect to his, or her, tank, its explanation can, in my opinion, be kept fairly simple and easy to understand by every hobbyist interested in it.

Very simply, the redox potential of the water in the tank is a measure, usually expressed in a 3 number millivolt value, of the oxidation potential of that water. Organic material in aquariums soon starts to decay and requires oxygen in the process. The greater the amount of decay taking place, the more oxygen is used. The more oxidative power the water has, or has left, the better it will be able to deal with organic material in the tank, the material that pollutes the water that we want to reduce to a minimum for the benefit of all animals in the aquarium. The more efficiently this takes place the better off the tank will be.

Highly oxidative media demonstrate high millivolt readings. Case in point oxygen: ozone, chlorine, fluorine, and so on. All these compounds, two of which are of interest to us, oxygen and ozone, have a very large ability to oxidize organic material in the tank.

If media that have high oxidative powers also have high redox potential values, it follows for the purpose of our explanation, that if the tank's water has a high redox potential, it also has a high oxidative power.

Because the oxidative power of a medium gets "used" as it chemically goes to work on organic material in the tank, such power will be reduced, and the redox potential of the medium will be lower, some of its power having been employed in the process of oxidizing the pollutants present in the water.

Putting all of the above together we can say the following: Water of high purity must exhibit a rather high redox potential value, because if it was not pure, its oxidative power would have been used up, lowered that is, in the process of cleaning up the water and dealing with the organic material that was present in it. Decay lowers the number, so in water with low numbers, decay is taking place. We do not want such water in our tanks. We want water where no decay is taking place, or very little. Consequently we are looking for our tank to demonstrate a rather elevated redox potential value. More on numbers later.

Given that high numbers = high oxidative power, pure water must have a high number as well because if it was not pure the high number would have been reduced as a result of the lower quality (the decay that is taking place). So we know that the redox potential of the water in our tank should be high. How high? We will get into that a little later, as just indicated.

Since we keep animals that come from natural reefs, many a diver/hobbyist/author has taken measurements of the redox potential around natural reefs. Because the redox potential of any medium, in our case salt water, is influenced by many factors, there never is a unique number that can be found around the reef, applied and suggested. The redox potential around reefs varies considerably during any 24 hour period.

Why? Because of light, temperature, metabolism, the number of animals, the pH of the water, the level of dissolved oxygen, and so on. Many factors influence ORP, and its value can change for any number of reasons.

How is redox potential measured? Several instruments are offered for sale in the hobby that will indicate the ORP level in either 2 or 3 digits. Some of these can act as controllers at the same time, because they have a relay built-in that can switch another device on and off. This concept is of great interest because we can use the relay trigger to add compounds to the water that will clean it up if its redox potential falls below a certain number. Again, more on this later.

Besides the instrument, whether a plain meter or a controller, one needs a sensing device, called an electrode or probe, that can send a signal to the meter or controller, which can then in turn display a value: the redox potential.

Two decisions the hobbyist interested in measuring redox potential has to make, after deciding whether he, or she, wants a meter or a controller, are:

• Which type of meter or controller to buy.
• What type of redox electrode to acquire.

These are important decisions because the equipment we are talking about is not cheap, and once you decide to spend several hundreds of dollars, you want to be sure that you have spent them on the right piece of equipment. Good meters and good controllers cost quite a bit of money, and so do good electrodes.

As a general rule, several types of meters and controllers exist, and so do several types of electrodes. All are excellent for the purpose they are intended for. They are not bad units. There are just buyers who buy the wrong unit for the application that is theirs.

Because redox potential is such a new concept, and because some resellers in the hobby do not fully understand the differences between various units either, several meters and some controllers are offered for sale that are not accurate enough for use in the hobby around reef aquariums.

Two general types of units exist: industrial quality and laboratory quality. Industrial units allow for general range measurements and offer a fair amount of accuracy, whereas laboratory quality units are obviously very accurate. And since the degree of accuracy determines the price...

Meters and controllers also come in two general finishes: bench types with a full enclosure, and panel mount units, with partial enclosure. Bench types are plug-in ready, and you can use them immediately, whereas panel mount units usually aren't. You may have to do quite a bit of wiring yourself. They often also have fully exposed and unprotected wiring in the back of the unit, which is not recommended underneath an aquarium because of the high humidity (over 90%) and the salt which will corrode the unit.

Industrial units are on average not accurate enough for our purpose, but have the benefit of being inexpensive. Companies such as Ex-tech, Markson's, Cole Parmer, and others, sell such units starting at prices in the $200 to $300 range without electrode. Many of these units are, in fact, now sold in the hobby, albeit at much higher prices. Often they are panel mount types. All resellers do is get a unit that you can buy yourself, place their name on it and mark it up substantially. If industrial is the way you decide to go, buy the units direct from the companies mentioned, and save.

Laboratory units are much more expensive, and are far more accurate than what the hobbyist really needs. Meters and controllers from companies such as Hach, Beckmann, WTW, and Fisher, can cost well over $1000 without electrode. But if money is no object, and/or you want a top of the line unit, this is the way to go.

These differences in type and finish pose a problem insomuch as to offer a unit that is affordable, most resellers offer industrial units, either as they come, or with some wiring added to it in the case of panel mount meters/controllers. Hobbyists buying such units end up with industrial type meters or controllers that are, in my opinion, not quite good enough to use on your reef tank.

How much less accurate are these units? Large differences exist, but often the error is expressed as a % of scale. Ask the question before buying. A 2% of scale error in meters/controllers that read from -1000mv to +1000mv is equal to a possible 40 millivolt difference. Similarly a 1.5% of scale works out to 30 mv, and 1% of scale is still 20 mv in such meters.

The difference between a reading of 400 mv and 440 mv is significant in a reef, especially when the number read by the controller is used to trigger the on and off cycle of an ozonizer. Buy a unit with low error rating if you can. They are offered for sale too.

Because only two companies offer controllers specifically made for the hobby, they should be your first preference, because they are made taking the requirements of the reef tank into account. Next, consider getting a bottom of the scale laboratory unit, they are still more accurate than industrial types, and only then an industrial type, using it while you keep its inherent drawbacks in mind.

The problem of selecting the right setup does not end here. Great variations exist also in the types of electrodes that can be used to measure redox potential. This is similar to what was described about pH electrodes. The far greatest problem, I have found, is that most electrodes are not suited for continuous immersion in salt water. It takes a special electrode to resist the harsh environment that a reef tank is, with all the salt, the organic material in the water, and ozone being used etc.

Of course, when buying an electrode, not necessarily from the same people you buy the meter or controller from, make sure it can take that rough treatment. Again, only 2 companies offer such electrodes besides the catalog places already referred to. When dealing with the latter, talk to their technical department first to determine which one you should order. Expect to spend in excess of $150.00 for the type of electrode you need. Get an ORP electrode cleaning solution and a standard reference solution as well (for example a 465 mv).

Depending on where exactly you place the meter or controller, you may need an electrode extension cable as well. Cost: approximately $25-30.00 for a good one with "silver" connections and cable that does not distort your reading even if 15 feet long.

The resolution of the unit you buy should be at least 3 digits, meaning it should read in hundreds of millivolt. The readout should indicate at least 3 numbers, not two.

Some meters/controllers are sold with LCD's (liquid crystal readout). Think twice before getting them. LCD's do not hold up well at all in a moist environment. Buy units with LED's (light emitting).

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