What exactly is redox and how does it pertain to pond owners? Why is it so important?

Q:

I read in your article about redox. What exactly is redox and how does it pertain to pond owners? Why is it so important?

A:

Redox potential is the oxidation-reduction reaction, which is a valuable indicator of the status of the life supporting capacities of a pond ecosystem. Very few pond hobbyists measure redox potential, but for the serious pond owners, they have used this particular technology far years. To understand redox potential we have to simplistically refer to atoms and electrons. Atoms are small packets of matter compose of positively charge protons and neutral neutrons that form a nucleus. Negatively charged electrons surround the nucleus, and distributed, in “shells” at various distances or energy levels from the nucleus. The more protons and neutrons there are, the heavier the element will become. Elements interact with each other in different ways mainly by sharing the electrons in their outer shells, thus joining, and forming various types of molecules that compose compounds with different physical and chemical properties.

The usages of the term “redox” in the language of chemistry is a contraction for “reductionoxidation reaction” and “potential” refers to electrical charge on a molecule that’s been formed in a reduction-oxidation chemical reaction and is dissolved in an aqueous solution. Combining two elements in a reduction-oxidation reaction, one is said to be oxidized and the other is reduced. Therefore any chemical process that includes either a partial or complete transfer of electrons is and oxidation-reduction reactions. Oxidation means the apparent loss of electrons by a molecule or atom and reduction means an apparent gain of electrons. Electrons are not lost or gained in these reactions, the atoms only changed the way they share the electrons of their outer shells.

Whenever elements are oxidized, another element must be reduced. If a molecule is a product of oxidation, it has a positive electrical charge or potential, a cation. The molecule that is a production of reduction has a negative electrical potential, an anion. The redox potential of a pond is a measurement of the relative amount of the positive and negative charges on the oxidized and reduced molecules that are present. Oxygen is one of the most important single oxidizing agents; the presence of free oxygen will always keep the redox potential in the positive range. A negative redox potential only occurs without free oxygen.

However, the accumulation of reduced molecules (the result of the addition of organic matter) will diminish the redox potential, even if oxygen is at the saturation levels. The most widely used method of measuring redox potential is with a platinum indicator electrode immersed in the pond against a reference electrode of calomel or silver chloride. The results are measured and/or expressed in millivolts by using a meter or a redox potential controller. The redox potential of a hobbyists pond should range between 200 and 350 millivolts. Lower levels than 150-200 indicate an unacceptable accumulation of poisonous, reduced organic compounds. Levels of 400-450 millivolts indicate too active oxidizing environment, which can possible damage the delicate aquatic plants and animals’ tissue and cells. Redox potential can have an influence on the growth of algae at various millivolt levels in our ponds.

The higher the oxidative states, the higher the redox potential, this means no pollution to very minute amounts are present in solution, and the water must be of excellent quality. That is exactly what every pond owners is trying to achieve. Another way to express the quality of pond water is by its rH2 number. This is generally considered more accurate than the redox potential itself because it considers more factors. Water that has a neutral oxidation-reduction activity has an rH2 of 21. Water that has more oxidative potential has an rH2 over 21, and when reduction prevails, the rH2 will be lower than 21. No meter measures rH2 directly. It is a number that you have to calculate, by determining both the pH and the redox potential accurately; then by using those numbers given in the formula below.

rH2 = Redox potential / 29 + (2 x the pH).

For instance at a redox potential of 250 mV, and a pH of 7.4, the rH2 index would be 23.4, or definitely in the oxidative area, and not in the reductive area. All this is another tool, and another way of determining at how well your pond is doing. Therefore, it is helpful in describing the water characteristic of a biomass. At this level, you should not have any problems with micro-algae and/or Blue-green algae, but to attain this kind of number, both your nitrates and phosphate levels must be extremely low, and well within the recommendations, which I have given.

Since 80 percent of all fish deaths and diseases are related to poor water quality, knowing the redox potential and/or the rH2 levels of the pond will tell you a lot about the extents of the animal’s environment. This will also alert you in taking and evasive course of action in fixing and/or eradicating the problem that otherwise would be unknown to the hobbyists. As the old saying goes, "And ounce of prevention is worth a pound of cure!”

Professional Oxidation Reduction Potential (ORP) monitor 

Photo by: Doctors Foster and Smith