The redox potential is a measuring value, which gives information on the reduction and oxidation characteristics of water samples. Reductive compounds decrease the oxygen contents of the water. These are all organic compounds, e.g. proteins, faeces, feed or blood. These agents react fast to toxic compounds like ammonia or nitrite and they start off decay processes. Reductive agents decrease the redoxpotential. The water quality gets worse. Oxidative agents are, e.g. oxygen or more intense, ozone. They are very important for any water, as of course any organism needs oxygen for its respiration but furthermore these agents are able to neutralize or to temper the negative action of the reductive compounds. As with any oxidation or reduction process electrons are released or absorbed, a voltage is caused which is measured in mV.
As already mentioned, ozone is a very good degermination or sterilisation agent, and the redoxpotential is an indicator of the degree of degermination. While at a redoxpotential of 200 mV 100 % germs exist, the formation of germs is reduced by increasing the redoxpotential from 200 mV to 300 mV, e.g. by 90 % to only 10 % of the formation of germs at the beginning. If the redoxpotential is increased to a value of 400 mV only 1 % of the original germs exist. This definitely shows that redoxpotentials exceeding 400 mV are not necessary for aquarium systems. Absolute sterility is achieved at a much higher redoxpotential of 700 mV. But this is not recommendable for aquarium-systems and can not be achieved with common aquarium ozonizers. Especially for fresh water systems with water plants or for salt water systems with invertebrates a lower redoxpotential of about 300 mV is favourable.
This special field is very difficult and each species of algae shows different reactions to ozone concerning redox changes. But it can be said that an increase of the redoxpotential supports the growth of green and higher algae and suppresses brown and red algae.
All reduction and oxidation processes produce an input or output of electrons. Due to this chemical reaction in the water a voltage is produced which is measured by the redox probe almost without using energy. During the generation of the redox tension electrones flow from the measuring probe to the redox system (ozone - organic substances for example). As a result of this separation of electric charges, a tension is formed on the surface of the metal sensor, which counteracts a further electron transport. A balance develops as the electro-chemical energy (voltage) and the chemical energy (oxidation or reduction energy) neutralize each other. The reference electrode forms a constant comparitive or shunt potential against the metal sensor. The separation at the contact point of the electrolyte/measuring solution (water) takes place via a ceramic capillary connection, the diaphragm.
The redox probe consists of a glass tube with a plate of platinum or a gold pin at the lower end. At one side of the glass there is a ceramic diaphragm inserted. Inside this diaphragm there is a flow of ions in accordance with the redoxpotential of the water into the measuring chain and reverse. The electrode should remain in the water. The first time it will take approximately 30 mins. until a reliable measuring value is indicated.
1. Installation in a water stream
The redox probe should be installed in a water stream or inside a tube. In calm water there is the risk that due to the poor flow around the probe wrong redox values are measured.
2. Avoid direct light
The electrode should be installed in a dark place. Strong light can encourage the growth of algae or a layer of bacteria. Both prevent a good exchange of water in the border layer of the probe. Furthermore, the flow of ions could be reduced or interrupted. As a result the probe may memorize retarded values, and the measuring unit might show values that were measured some time ago.
3. Avoid soilings
Soilings can lead to various impairments. Soiling of the probe usually has a reductive effect so that a too low redoxpotential can be simulated. Soiling of the metal sensor or of the diaphragm prevent the ion-exchange and lead to faulty measuring results