Redox potential (ORP) is measured in volts (V) or millivolts (mV). The more positive the potential, the greater the tendency for the substance to contract and recover. ORP is a general indicator of water quality and is often used to analyze the state of drinking water, the effects of acid rain, toxic emissions into the atmosphere, etc.
general description
In aqueous solutions, the redox potential is a measure of the tendency to either gain or lose electrons when it undergoes a change by introducing a new substance. A solution with a higher (more positive) reduction potential than a new type of substance will tend to receive electrons from new species (i.e., be reduced by oxidizing them), and a solution with a lower (more negative) reduction potential will tend to loss of electrons for new species (i.e., oxidized by reducing them). Since absolute potentials are difficult to accurately measure, the reduction potentials are determined relative to the reference electrode. The redox potentials of aqueous solutions are determined by measuring the difference in the ORP between the inert sensitive electrode in contact with the solution and a stable reference electrode connected to the solution with a salt bridge.
The most favorable metals
The sensitive electrode acts as a platform for transferring an electron to or from the support half of the cell. It is usually platinum, although gold and graphite can also be used. The reference half-cell consists of a standard ORP. A standard hydrogen electrode is a standard from which all standard redox potentials are determined, and an arbitrary half-cell potential of 0.0 mV has been assigned to it. However, it is fragile and impractical for ordinary laboratory use. Therefore, other more stable reference electrodes, such as silver chloride and saturated calomel (SCE), are commonly used because of their more reliable performance.
Additional factors
Although the measurement of the reduction potential in aqueous solutions is relatively simple, many factors limit its interpretation, for example, the influence of solution temperature and pH, irreversible reactions, slow kinetics of electrodes, nonequilibrium, the presence of multiple redox pairs, poisoning by electrodes, a small exchange of currents and inert redox -pair. Consequently, practical measurements rarely correlate with calculated values. However, measuring the redox potential has proven to be useful as an analytical tool for monitoring changes in the system, and not for determining their absolute value.
Just as the transfer of hydrogen ions between chemical species determines the pH of an aqueous solution, the transfer of electrons between chemical species determines its reduction potential. Like pH, the redox potential is how strongly electrons are transported inside or outside the solution. It does not characterize the number of electrons available for oxidation or reduction, in much the same way as pH does not characterize the buffering ability of a substance.
Standards
In fact, pE, the negative logarithm of electron concentration (-log), which will be directly proportional to the standard redox potential, can be determined in the solution. Sometimes pE is used as a unit of reduction potential instead of Eh, for example, in environmental chemistry. If we normalize the pE of hydrogen as zero, we will have the ratio pE = 16.9 Eh at room temperature. This point of view is useful for understanding the value of the redox potential, although electron transfer, rather than the absolute concentration of free electrons in thermal equilibrium, is how ORP is usually thought of. Theoretically, however, both approaches are equivalent to each other.
Conversely, it is possible to determine the potential corresponding to pH as the potential difference between dissolved and pH neutral water separated by a porous membrane (permeable to hydrogen ions). Such potential differences do occur due to differences in acidity on biological membranes. This potential (where the pH is 0 V) is similar to the redox potential (where the standardized hydrogen solution is set to 0 V), but instead of hydrogen ions, electrons are transported in the redox core. Both pH and ORP are properties of solutions, not elements or chemical compounds as such, and depend on concentrations, temperature and other external factors.
Redox Potential Table
The relative reactivity of various solutions can be compared with the prediction of the direction of electron flow. A higher ORP value means that there is a greater tendency to recover, and a lower value means that there is a greater tendency to oxidize. A person familiar with chemistry will understand this from the tables of redox potentials presented below.
Any system or environment that receives electrons from a normal hydrogen electrode is a half element that is defined as having a positive ORP. Any system that transfers electrons to a hydrogen electrode is defined as having a negative redox potential. ORP is often measured in millivolts (mV). High positive redox potential indicates a medium that favors an oxidation reaction (such as free oxygen). A low negative redox potential (water) indicates a strong reducing environment (such as free metals).
Water feature
Sometimes, when electrolysis is carried out in an aqueous solution, water, not being mixed with another substance, is oxidized or reduced. For example, if an aqueous NaCl solution is electrolyzed, water can be reduced at the cathode to form H2 (g) and OH– ions, and Na + is reduced to Na (s), as occurs in the absence of water. This is the potential of each species present, which will determine which species will be oxidized or reduced.
Absolute potentials
Absolute recovery potentials can be determined if we find the actual potential between the electrode and electrolyte for any reaction. Surface polarization interferes with measurements, but various sources give the estimated potential for a standard hydrogen electrode from 4.4 to 4.6 V (positive electrolyte). So measure the potential of the oxidizing agent / reducing agent.
The equations with half elements can be combined if we turn them to oxidation in such a way as to neutralize electrons in order to obtain an equation without electrons in it.
But that’s not all. Many enzymatic reactions are oxidizing-reducing reactions in which one compound is oxidized and the other is reduced. The body's ability to carry out such reactions depends on the state of the environment in terms of AFP.
Aerobes and Anaerobes
Strictly aerobic microorganisms are usually active at positive values of redox (ORP), while strict anaerobes are usually active at negative values. Redox affects the solubility of nutrients, especially metal ions.
There are organisms that can regulate metabolism in their environment, for example, facultative anaerobes. They can be active at positive values of Eh and at negative values of Eh in the presence of oxygen-containing inorganic compounds such as nitrates and sulfates.
Practical use
In the field of environmental chemistry, the reduction potential is used to determine whether oxidizing or reducing conditions prevail in water or soil, and to predict the state of various chemicals in water (such as dissolved metals).
pE in the range from -12 to 25 are the levels at which water itself is reduced or oxidized, respectively.
In nature
Reduction potentials in natural systems are often associated with the natural stability of water. Aerated surface waters, rivers, lakes, oceans, rainwater and acid mine water usually have oxidizing conditions (positive potentials). In places with restrictions on air supply, such as underground channels of natural origin, swamps and marine sediments, restoration conditions (negative potentials) are the norm. Intermediate values are rare and are usually a temporary condition that is found in systems moving to higher or lower pE values.
In environmental situations, it is customary to have complex nonequilibrium conditions between a large number of types of substances, which means that it is often impossible to make accurate and unambiguous measurements of the recovery potential. However, it is usually possible to obtain an approximate value and determine the conditions as being in the oxidizing or reducing mode.
The following redox components are present in the soil:
- Inorganic redox systems (mainly hydroxy / red compounds of Fe and Mn) and measurement in aqueous extracts.
- Samples of natural soils with all microbial and root components and direct measurements.
Modern research
The oxidative reduction potential (ORP) can be used to monitor the water supply system using a one-off disinfecting potential measure that shows the activity of the disinfectant, rather than the dose used. For example, E. coli, Salmonella, Listeria, and other pathogens have a survival time of less than 30 s when AFP is above 665 mV, compared to> 300 s when it is below 485 mV.
Eh-pH schemes (Pourbaix) are commonly used in mining and geology to assess the stability fields of minerals and dissolved species. Under conditions when the mineral (solid) phase is the most stable form of an element, these diagrams show what a mineral is from a chemical point of view. Like the results of all thermodynamic (equilibrium) estimates, these diagrams should be used with caution. Although the formation of a mineral or its dissolution can be predicted under a number of conditions, the process may be insignificant, since its speed is rather slow. Under these conditions, kinetic estimates are necessary. However, the equilibrium conditions can be used to assess the direction of spontaneous changes and the magnitude of the driving force behind them. The determination of the redox potential in the mining industry is very promising in our time. For other purposes, this procedure is not so useful, but still very important, since it helps to determine many chemical characteristics of any complex compound.