Redox potential, abbreviated as ORP (abbreviation of English Oxidation-Reduction Potential) or Eh. ORP has been used as a comprehensive indicator of the environmental conditions of media (including soil, natural water, culture media, etc.) for a long time. It characterizes the relative degree of oxidative or reductive properties of media.
The unit of ORP is mv. It consists of ORP composite electrode and mv meter. The ORP electrode is an electrode that can absorb or release electrons on the surface of its sensitive layer. The sensitive layer is an inert metal, usually made of platinum and gold. The reference electrode is the same silver/silver chloride electrode as the pH electrode. The Redox electrode is a precious metal electrode. It is used for potential measurement, but at the same time it cannot participate in the chemical reaction process, which means that it has to withstand chemical shocks. Therefore, only precious metals such as platinum, gold or silver can be used here. As the reference electrode, the Ag/AgCl reference system is used in the same way as the pH measurement. Inserting a platinum needle Redox electrode into the chlorine-containing solution forms a phase boundary layer between the platinum needle surface and the water surface, which is called "Helmholtze electric double layer". This phase boundary layer is equivalent to a capacitor, one end of which is connected to a platinum needle, and the other end is connected to a reference electrode like a pH measurement. This capacitor will be charged due to the electrochemical potential difference between the platinum needle and the solution. The potential of the solution depends on the sum of the log concentration ratio Log COX/CRED and the potential difference of all ions in the water. At the same time, platinum will also be oxidized, and depending on the concentration of the oxidant, a platinum oxide layer with a thickness of 3 to 4 atomic layers will be formed on the surface. This oxide layer conducts electrons on the one hand, that is, hinders the Redox measurement process. However, this oxide layer also establishes an oxidation memory, which will cause measurement delay when the chlorine content decreases. The more dilute the test solution, the longer this delay process. Under high Redox buffer conditions, this process can be ignored. This effect can also be explained by the examples of the two jars mentioned earlier. One jar is filled with water and the other is empty. If the diameter of the connecting pipe is smaller, the process of balancing the water levels of the two tanks is slower, otherwise it is faster. The roughness of the electrode surface also brings the above-mentioned measurement inertia. This is because the pits and grooves on the rough surface also have a storage effect, which makes the ion exchange process worse. The surface of the Redox electrode should be kept as smooth as possible. Since the "Helmholtze electric double layer" acts like a capacitor, a charging current flows when the potential changes, until it reaches electrochemical equilibrium. If the measurement amplifier does not use the zero-current method to measure the potential of the composite layer, it will not reach electrochemical balance. At this time, the measured value will continue to drift, and under certain conditions, the electrode surface may also undergo chemical changes.
In the water body of nature, there are a variety of ions and dissolved oxygen of varying prices. When some industrial sewage is discharged into the water, the water contains a large amount of ions and organic substances. Due to the different properties of the ions, redox reactions occur in the water body and tend to Balance, so in the natural water body is not a single redox system, but a mixed redox system. The measuring electrode also reflects a mixed potential, which has a large experimental error. In addition, the pH of the solution also has an effect on the ORP value. Therefore, it is meaningless to emphasize the absolute potential of the solution during the actual measurement. We can say that the ORP value of the solution indicates a reduced or oxidized state of the solution near a certain value point, or it indicates a certain property of the solution (such as the degree of hygiene, etc.), but this value will be quite different, you It cannot be quantitatively determined, and these are two concepts of accuracy in pH testing. In addition, the temperature coefficient that affects the ORP value is also a variable and cannot be corrected. Therefore, ORP meters generally do not have a temperature compensation function.
The ORP value (redox potential) is an important indicator in water quality. Although it cannot independently reflect the quality of water quality, it can integrate other water quality indicators to reflect the ecological environment in the aquarium system.
In water, each substance has its own redox properties. Simply, we can understand that: on the microscopic level, each different substance has a certain oxidation-reduction ability. These substances with different redox properties can influence each other, and ultimately constitute a certain macro-redox property. The so-called redox potential is used to reflect the macro-oxidation-reduction of all substances in the aqueous solution. The higher the redox potential, the stronger the oxidizability, and the lower the potential, the weaker the oxidizability. A positive potential indicates that the solution exhibits a certain degree of oxidation, while a negative indicates that the solution exhibits reduction.
Industrial wastewater treatment
The redox system used in water treatment is mainly the reduction of chromic acid and the oxidation of cyanide. If sodium disulfide or sulfur dioxide is added to the wastewater, the hexavalent chromium ion can be changed into trivalent chromium ion. If added chlorine or sodium hypochlorite can be used to oxidize cyanide, followed by the hydrolysis of cyanogen chloride to form cyanate. This chemical reaction process is called a redox reaction system. The redox potential is a measure of electronic activity, which is very similar to the method of measuring hydrogen ion activity.
Disinfection and application of water
The redox electrode can measure the disinfection effect on swimming pool water, mineral water and tap water. Because the bactericidal effect of coliform bacteria in water is affected by the redox potential, the redox potential is a reliable indicator of water quality. If the redox potential value of the pool water and mineral water is equal to or higher than 650mv, it means that the bacteria content is acceptable.
Changes in soil ORP
Observe the dynamic changes of ORP in the soil, etc.
For example, after the paddy soil was irrigated to plant rice, the redox status of the soil changed drastically. There is a kind of paddy soil from the cultivation layer, which is generally maintained at 450-650mV before irrigation. After irrigation, ORP decreased rapidly, and during the period of strong organic matter decomposition, ORP decreased to minus 200mV to 100mV. When applying a large amount of fresh green manure, it could even drop to minus 300mV. It will rise again in the future, generally maintaining at 0-200mV. Before the rice was harvested, the soil dried down and the ORP rose back to above 450 mV (from Yu Tianren et al., Physical Chemistry of Paddy Soil).
Applications in other fields
Various sectors of the national economy such as marine exploration, bioengineering, environmental protection, and brewing industry have been widely used.
Although the platinum electrode is the most ideal for measuring ORP, the chemical adsorption of oxygen on the surface is likely to occur in the oxidizing solution, which makes the electrode measurement response slow; in the reducing solution, there are similar problems due to the adsorption of hydrogen by the platinum electrode. Therefore, when measuring ORP, attention should be paid to the pretreatment method of ORP platinum electrode.
1. Mechanical pretreatment
Grinding and polishing are used to remove the dust of the oxide film on the electrode, so that the surface of the ORP platinum electrode is smooth and bright, so as to achieve the purpose of not attracting oxygen and hydrogen. Alumina powder (AlO3), ceria (CeO2) or diamond powder is generally used for grinding and polishing, and cleaned with an ultrasonic cleaner. Care should be taken during operation to avoid scratching or rough wear on the electrode surface.
2. Chemical pretreatment
3. Electrolytic pretreatment
Electrolytic anode polarization method and cathode polarization method are used to regularly remove trace oxygen or hydrogen adsorbed on the ORP platinum electrode, so as to ensure the rapid response performance of the ORP electrode measurement.
4. Distilled water cleaning pretreatment method
In some ORP measurement occasions, special pretreatment is not required. The platinum electrode can be cleaned with distilled water to meet the requirements. Pay attention to shaking the electrode during cleaning. If necessary, gently brush with a fine brush to remove contaminants on the surface of the platinum electrode.
1. The redox electrode can be used on any pH/mv meter.
2. No calibration is required when using the ORP meter, it can be used directly. Only when there is doubt about the quality or test result of the ORP electrode, can the ORP standard solution be used to check whether the potential is between 200-275mv to judge the quality of the ORP electrode or instrument . Redox electrode instruction manual Redox electrode instruction manual Redox electrode instruction manual
3. ORP measuring electrode (platinum or gold), its surface should be bright, rough or contaminated surface will affect the electrode potential (mv). The following methods can be used for cleaning and activation.
(1) For inorganic contamination, the electrode can be immersed in 0.1mol/L dilute hydrochloric acid for 30 minutes, washed with pure water, and then immersed in 3.5MOL/L potassium chloride solution for 6 hours before use.
(2) For organic oil and oil film pollution, the platinum or gold surface can be cleaned with detergent, then washed with pure water, and then immersed in 3.5MOL/L potassium chloride solution for 6 hours before use.
(3) The surface of platinum is seriously contaminated to form an oxide film. Toothpaste can be used to polish the surface of platinum or gold, then washed with pure water, and then immersed in 3.5MOL/L potassium chloride solution for 6 hours before use.
Should ORP standard solution be used for calibration in ORP determination?
The ORP meter can be used directly without calibration. Only when there is doubt about the quality of ORP electrode or test result, the electrode potential can be checked with ORP standard solution to judge the quality of ORP electrode or instrument. When the requirements are strict, the new platinum electrode should be checked with ORP standard solution.
The ORP measuring electrode can be made from a variety of metals, such as nickel, copper, silver, iridium, platinum, gold, etc. It is composed of an ionic lattice structure, electrons can move inside the lattice, and they will also generate potential differences due to the presence of the same kind of ions . The standard potential values of 6 metals are listed. The ORP value of platinum and gold is higher, and the measurement sensitivity is higher. Compared with other ORP electrodes, the redox potential of platinum and gold precious metals is extremely low in the ion equilibrium activity, so It has almost no effect on the measurement of ORP; platinum can form a purified surface, and the surface is prone to generate an oxygen-containing surface layer, which increases the standard potential of the electrode; this oxide/hydroxide layer is mainly composed of PtQ or Pt(OH) 2 composition, only when the critical ORP is determined above, the chemical adsorption of oxygen starts, and the thickness of the surface protective layer increases with the increase of potential. In most cases, it only reaches the thickness of a single molecular layer. It can be seen that when platinum Eh>1200mv, platinum ion activity>1M, platinum electrode is an ideal sensor for ORP measurement, in addition, gold electrode measurement can also be used.
(1) Influence mechanism
In all electrochemical processes, electrons are transferred from the electrode material to the sample solution. Similarly, electrons in the solution are also transferred to the electrode. Suppose the current flowing from the electrode to the solution is i+, and the current flowing from the solution to the electrode is i-. In equilibrium, the two are equal in size. The modulus is called the exchange current density io: io=i+=i-=A/cm2 (8) The exchange current density depends largely on the electrode material, redox system and its concentration in the sample solution. In order to correctly compare the relationship between the AC current density, a solution with an electrode area of 1 cm2 and a molecular concentration of 1 g is selected, and the exchange current density ioo under standard conditions is obtained in this way, with a value ranging from 10 to 10-25 A/ cm2. Figure 2 illustrates the importance of exchange current density. When the ORP is correct, it has a high exchange current density i. At this time, i+ and i- are both high. The measured ORP value has good repeatability and fast response; when the ORP is incorrect, only the lower exchange current density io , At this time, i+ and i- are both small, the measured ORP value has poor repeatability, and the response is slow. It can be seen that the size of the io pair affects the ORP measurement value.
(2) Influence of electrode materials
The most important factor affecting the exchange current density is the electrode material, so the electrodes for ORP measurement require special materials.
(a) Metal electrode material When the metal material is Ag, Cu, Ni, Fe, etc., the metal reacts to the presence of its own ions, so the ioo value is often very low, so that electrodes made of such materials do not actually measure ORP The role.
(b) Platinum and gold
The single-molecule "oxide" layer produced by the oxygen adsorbed on the platinum surface is conductive, does not affect the sensitivity of the electrode ORP measurement, and maintains a higher value of the electrode potential, even when the redox potential of the sample solution has decreased. However, when measuring in a lower ORP solution, the response is slow; the platinum electrode with a rough surface can absorb more oxygen than the platinum electrode with a smooth surface, and the response is more sluggish, so it is better to use a platinum electrode with a smooth surface or polished light. The surface oxygen absorption of gold is far lower than platinum, so some occasions are more suitable for gold. Gold can form cyanide and halide in concentrated salt solutions such as urban sewage and industrial wastewater, and the presence of oxygen in the solution, gold will also be corroded quickly, but platinum has much stronger corrosion resistance than gold. In addition, the exchange current density of platinum is greater than that of gold, so for natural water ORP determination, platinum is better than gold electrode, platinum also has a higher catalytic ability, so that the measurement solution can quickly establish a balance and obtain a more accurate measurement. For example, a platinum/hydrogen electrode coated with sponge platinum can be used for high-precision ORP detection at ordinary room temperature. In general ORP measurement, a gold electrode is used for a strong oxidizing solution, and a platinum electrode is used for an oxidizing solution (containing fluoride), natural water (jianghehutang), and other solutions.
A large number of ORP measurements have proved that in dilute solutions or redox systems with low ioo values, the dissolved oxygen concentration will affect its ORP value. Figure 3 illustrates the effect of free oxygen on the ORP value of the redox system. When the OR system and the electrode are in equilibrium, the anode current i+ and the cathode current i- are equal in opposite directions, and the combined current i=0. If i+ or i-(i≠0) is applied to the OR system, the polarographic changes of the OR system with a high i value will change sharply up and down, as shown in curve A in Figure 3; while the polarographic changes of the OR system with a low i value will be gentle, such as the curve B. If the solution contains both OR systems A and B, and there is no reaction between the two systems, the synthetic potentials i1 and i2 measured by the electrode, the partial current i1=i2, are in opposite directions. It can be seen from Fig. 3 that the synthetic potential Eres is roughly consistent with the equilibrium potential EA of the OR system with a large io value. When the solution is diluted and the exchange current density of the redox buffer decreases, curve A becomes a flatter curve C, so that the new synthetic potential Eres* deviates more from the equilibrium potential EA, and this effect is caused by the free Oxygen-caused by the concentration of dissolved oxygen, the higher the concentration, the greater the impact, so the En value measured in the dilute solution of the OR system is often incorrect. The way to overcome its deficiencies is to fill the OR liquid with low io value with an inert gas such as N2 and Ar to make it saturated before the OR measurement. However, this measurement method has a great error when there are compounds with high volatility affecting ORP; when the OR solution contains dissolved CO2, the inert gas charged easily drives CO2 out, which changes the pH value of the solution and will Large measurement error.
When the sample solution contains mercury chloride (HgCl2), hydrogen sulfide (H2S) and other substances, it is easy to "poison" the ORP electrode and cause the exchange current density io to drop sharply, making the measurement invalid. Table 4 lists the effects of some substances on the poisoning of the ORP electrode. It can be seen that H2S has the greatest effect on the poisoning of the ORP electrode. The longer the time, the deeper the poisoning. In addition to the substances listed in the table, other substances will also reduce the io value, especially when measuring the ORP of the system with low exchange current density, which affects its normal measurement. At this time, the ORP electrode needs to be cleaned regularly, or automatic cleaning technology is used. In order to ensure the correct continuous measurement of ORP.
The filtration system, to remove denitrification, is actually an oxidizing biochemical filtration device. For organic matter, microorganisms break long carbon chains (or open various carbon rings) through oxidation, and then undergo complex biochemical processes to eventually oxidize various forms of organic carbon into carbon dioxide; at the same time, these oxidations also Nitrogen, phosphorus, sulfur and other substances are disconnected from the corresponding carbon bonds to form corresponding inorganic substances. For inorganic substances, microorganisms oxidize low-valent inorganic substances into high-valent substances through oxidation. This is the essence of oxidative biochemical filtration (here we only care about those substances that are oxidized and decomposed by microorganisms, not those that are absorbed and assimilated by microorganisms). It can be seen that while biochemical filtration, the substances in the water are constantly oxidized. The process of biochemical oxidation is accompanied by the continuous generation of oxidation products, so from a macro perspective, the redox potential is continuously increased. Therefore, from this perspective, the higher the redox potential, the more thoroughly the pollutants in the water are filtered.
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