Redox potential

Redox potential is a measure of the activity of electrons. It is related to pH and oxygen content. It is analogous to pH since pH measures the activity of protons and the redox potential measures that of electrons.

Summary

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  • 1 It is calculated
  • 2 Sulfur cycle
  • 3 Redox potential as an ecological condition
  • 4 Photosynthesis
  • 5 Decomposition
  • 6 Source

Is calculated

Eh = 1,234 – 0.058 pH + 0.0145 log (10) Po, where Po is the partial pressure of oxygen expressed in atmospheres .

In waters, if oxygen is in equilibrium with atmospheric oxygen and the pH is 7, the value is + 0.86 mv at 0 ºC and + 0.80 mv at 25 ºC. In fresh and marine waters it rarely drops below +0.3 mv except when there is great oxygen shortage.

In the profile of the marine sediment three zones distinguishable by color can be seen: the oxidized zone of yellow color and whose redox potential is more than 200 mv, the discontinuity of gray redox with potential between 0 and 200 mv and the reduced zone with negative potential. Oxygen, carbon dioxide, and nitrate are replaced by hydrogen sulfide , methane, and ammonia in the reduced sediments. The ferric ion passes into the ferrous ion in the redox discontinuity and in the reduced zone.

There are limits in which life cannot develop in the redox potential. But there are also intermediate zones in which organisms use one redox pair or another.

In waterlogged soils there are problems for the diffusion of oxygen , hydrogen sulfide and methane are produced in reducing environments such as marshes at great depths.

In oxidizing potentials, matter tends to rust and decompose, and in reducers it tends to decrease. The substance with the most negative reduction potential is the one that oxidizes. Therefore, it is important to know the normal values ​​of the redox potentials. There are various aquatic environments because this: the acidic water mine is highly oxidizing, the atmospheric water , the river water and sea also, at an intermediate point are marsh water and underground water , and as more reducing are water from flooded soils and water in contact with reducing marine sediments .

In shallow areas is where the greatest variation of the reduction potential is. The seawater is reducing at a pH of 8 and groundwater have concentrations of carbonates which deflect the pH.

Oxygen diffuses badly in completely still waters, but in ecosystems there is usually wind and there is oxygen that can come from photosynthetic plants . During the day and due to photosynthesis , in the layers closest to the surface, there is a loss of oxygen to the atmosphere as oxygen saturation is exceeded, in the depths oxygen decreases due to bacteria, animals and organic matter in sedimentation; at nighta limited passage of oxygen from the atmosphere into the water occurs and is consumed by respiration and bacteria. Thus there is a situation in which the water is stratified. When turbulent water is present, an accelerated passage of oxygen into the water occurs and a mixture is produced that evens out the vertical oxygen gradient.

In aerobic respiration , the last acceptor of electrons is oxygen that produces water, in an anaerobic medium anaerobic oxidation occurs in which nitrates, sulfates are reduced …

Sulfur cycle

The primary production by the plants (biomass) is first carried out by consuming sulfates. The following steps are performed by living things and specialized microorganisms. Decomposition occurs by heterotrophic microorganisms that gives hydrogen sulfide , and the excretion of animals that produces sulfates. Colorless, green, and purple sulphobacteria convert hydrogen sulfide to sulfur and sulfates. The transition from sulphide to sulfate occurs by oxidizing bacteria and vice versa occurs by anaerobic sulfur-reducing bacteria. All this occurs in biologically available pools in rapid circulation of water and shallow sediments. There are also very slow flow pools in deep sediments where disulfide is produced fromiron that forms anaerobic black mud.

Redox potential as an ecological condition

In the sediment at the bottom of aquatic ecosystems we find a rusty upper zone and as the oxygen supply is exhausted at depth a reduced zone appears. The potential is positive in the oxidized zone and negative in the reduced or sulfur zone.

In the reduced area, microbial activity recovers nutrients upwards in the form of gases. Most of the animals in the benthos are in the oxygenated zone, such as polychaetes , lamelibranchs , copepods , flatworms , ciliates and nematodes .

The oxidized zone can be thin and if the water above the bottom is emptied of oxygen the reduced zone extends upwards, as occurs in the Black Sea . In the transition zone there are chemo-synthetic and photosynthetic bacteria in the event of light .

In the reduced area, only anaerobic bacteria such as sulfate-reducing and methane bacteria , anaerobic protozoa that feed on bacteria and some nematodes .

Photosynthesis

In a layered water without oxygen , with hydrogen sulfide and with a very low potential value, in the upper illuminated area there are photosynthetic bacteria that work through photosystem I. Photosystem I cannot use water as electron donor, the reduction of cytochrome and that of the NADP lead to the reduction of carbon dioxide. Bacteria that use hydrogen as electron donor (free or organic matter)) and do not decompose water, they only have a photosystem similar to I. In photosystem II oxidation of water occurs, the release of oxygen through the corresponding cytochromes and NADP leads to the reduction of carbon dioxide .

Decomposition

When there is no oxygen, the oxidation of organic matter is done at the expense of the reduction of other compounds: from sulfate to sulfite , from nitrate to [nitrite]]. If there are no compounds to reduce, organic matter accumulates in the sediment.

 

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