Extraction is a unit operation that uses the different solubility of the components in the system to separate the mixture. Using the principle of similar compatibility, there are two ways of extraction: Liquid-liquid extraction, which uses a selected solvent to separate a certain group of components in the liquid mixture. In addition, the solvent must be immiscible with the liquid mixture to be extracted, have selective dissolving ability, and must have good thermal and chemical stability, as well as low toxicity and corrosiveness. For example, benzene is used to separate phenols in coal tar; organic solvents are used to separate olefins in petroleum fractions; Br2 in water is extracted with CCl4. Solid-liquid extraction, also called leaching, separates components in a solid mixture with solvents, such as water leaching Take sugars from sugar beets; extract soybean oil from soybeans with alcohol to increase oil yield; extract active ingredients from traditional Chinese medicine with water to make a liquid extract called 'leaching' or 'leaching.' Although extraction is often used in chemical experiments, its operation process does not cause changes in the chemical composition (or chemical reaction) of the extracted substances, so the extraction operation is a physical process. Extraction is one of the methods used in organic chemistry laboratories to purify and purify compounds. Through extraction, the desired compound can be extracted from the solid or liquid mixture. Here is an introduction to commonly used liquid-liquid extraction. The difference in solubility or partition coefficient of a compound in two incompatible (or slightly soluble) solvents is used to transfer the compound from one solvent to another. After repeated extractions, most of the compounds are extracted. The distribution law is the main basis of the extraction method theory, and substances have different solubility to different solvents. At the same time, when a certain soluble substance is added to two immiscible solvents, it can be dissolved in the two solvents respectively. Experiments have shown that the compound and the two solvents will not decompose and decompose at a certain temperature. During electrolysis, association and solvation, the ratio of this compound in the two liquid layers is a constant value. This is true regardless of the amount of substance added. It is a physical change. Expressed by formula. CA/CB=K CA.CB respectively represents the concentration of a compound in two immiscible solvents. K is a constant, called the 'distribution coefficient'. Organic compounds are generally more soluble in organic solvents than in water. The extraction of a compound dissolved in water with an organic solvent is a typical example of extraction. During extraction, if a certain amount of electrolyte (such as sodium chloride) is added to the aqueous solution, the 'salting-out effect' is used to reduce the solubility of organic matter and extraction solvent in the aqueous solution, which can often improve the extraction effect. To completely extract the desired compound from the solution, usually one extraction is not enough, and the extraction must be repeated several times. Using the relationship of the distribution law, the remaining amount of the compound after extraction can be calculated. Suppose: V is the volume of the original solution, w0 is the total amount of the compound before extraction, w1 is the remaining amount of the compound after the first extraction, w2 is the remaining amount of the compound after the second extraction, w3 is the remaining amount of the compound after n extractions, and S is the amount of the extraction solution After the volume is extracted once, the concentration of the compound in the original solution is w1/V; and the concentration of the compound in the extraction solvent is (w0-w1)/S; the ratio of the two is equal to K, that is: w1/V =K w1= w0 KV (w0-w1)/S KV+S in the same way, after the second extraction, w2/V =K, that is (w1-w2)/S w2=w1 KV =w0 KV KV+S KV+S Therefore After n extractions: wn=w0 (KV) KV+S When a certain amount of solvent is used, it is hoped that the remaining amount in the water is as small as possible. The above formula KV/(KV+S) is always less than 1, so the larger the n, the smaller the wn. That is to say, it is better to divide the solvent into several times for multiple extraction than to use the entire amount of solvent for one extraction. It should be noted that the above formula is applicable to solvents that are almost incompatible with water, such as benzene, carbon tetrachloride, etc. However, the above formula is only an approximation, such as ether, which is a small amount of miscible solvent with water. But it is still possible to qualitatively point out the expected results. Extraction can be divided into the following types: 1. Aqueous two-phase extraction (Two-aqueous phase extraction, referred to as ATPS) refers to that the aqueous two-aqueous polymer solution can form an aqueous two-phase under certain conditions. The separation of the separated substances in the two phases is different, and the separation can be realized. 'It is widely used in the separation and extraction of products in the fields of biochemistry, cell biology and biochemical industry.' Aqueous two-phase extraction technology has low investment in equipment and simple operation.' This type of double water The phase system is mostly polyethylene glycol-glucose and polyethylene glycol-inorganic salt. 'Because the water-soluble polymer is difficult to volatilize, stripping is indispensable, and the salt enters the stripping agent for subsequent analysis and determination. Brings a great impact. 'In addition, most water-soluble polymers have high viscosity, which is not easy to quantitatively operate, and it also brings trouble to subsequent research.' In fact, ordinary organic solvents that are miscible with water can also be used in the presence of inorganic salts. A two-phase system is formed, and it has been used for the speciation analysis of serum copper and plasma chromium. 'The aqueous two-phase extraction system based on water-miscible organic solvents and brine phase is cheap! Low toxicity! More volatile without back extraction and Avoid the use of viscous water-soluble polymers, etc. Second, organic solvent extraction water washing and liquid separation method is to separate water-soluble impurities in the organic phase with water to achieve the purpose of purifying the organic phase.
The organic solvent extraction method is often referred to as extraction, which is a method in which organic solvents are used to separate the components that are soluble in the solvent from the aqueous phase, solid phase (or other phases that are insoluble in the solvent). See the content of Afeastforeye for the theoretical part. In general extraction experiments, the extracted organic phase (containing the desired compound) should be washed with water or saturated brine to further purify the organic phase. Both of these methods require a separatory funnel, and the operation process is basically the same. You only need to determine which layer (phase) needs to be retained. 3. Supercritical extraction. The extractant used in supercritical extraction is supercritical fluid. Supercritical fluid is a non-gaseous and non-liquid state between gas and liquid. This substance can only work at its temperature and pressure. It can only exist when it exceeds the critical point. The density of supercritical fluid is higher, similar to liquid, and its viscosity is closer to gas. Therefore, supercritical fluid is a very ideal extractant. The solvent strength of the supercritical fluid depends on the temperature and pressure of the extraction. Using this feature, only by changing the pressure and temperature of the extractant fluid, different components in the sample can be extracted sequentially according to their solubility in the fluid. The weakly polar substances are extracted first under low pressure. With the increase of pressure, the substances with larger polarity and large molecular weight have basic properties, so the supercritical extraction of different extraction components under program boost pressure can also play a role in separation. The change in temperature is reflected in two factors that affect the density of the extractant and the vapor pressure of the solute. In the low temperature region (still above the critical temperature), the increase in temperature reduces the density of the fluid, while the vapor pressure of the solute does not increase much. Therefore, the The temperature rise during the dissolving power can cause the solute to separate out of the fluid extractant. When the temperature further rises to the high temperature zone, although the density of the extractant is further reduced, the vapor pressure of the solute increases, and the volatility increases. The extraction rate will not decrease but instead Increasing trend.
In addition to pressure and temperature, adding a small amount of other solvents to the supercritical fluid can also change its ability to dissolve solutes. Its mechanism of action has not yet been fully understood. Usually the addition amount does not exceed 10%, and polar solvents such as methanol and isopropanol are mostly used. Adding a small amount of polar solvent can further expand the scope of application of supercritical extraction technology to more polar compounds. Introduction to the supercritical fluid extraction process Put the extraction raw materials into the extraction kettle. Use carbon dioxide as the supercritical solvent. The carbon dioxide gas is condensed into a liquid by the heat exchanger, and the pressure is raised to the pressure required by the process (should be higher than the critical pressure of carbon dioxide) by a pressurizing pump, and the temperature is adjusted to make it a supercritical carbon dioxide fluid. As a solvent, the carbon dioxide fluid enters from the bottom of the extraction kettle, fully contacts with the material to be extracted, and selectively dissolves the required chemical components. The high-pressure carbon dioxide fluid containing the dissolved extract is reduced by a throttle valve to below the critical pressure of carbon dioxide and enters the separation kettle (also known as the analysis kettle). Because the solubility of carbon dioxide drops sharply, the solute is precipitated, and it is automatically separated into two parts: solute and carbon dioxide gas. The former is a process product, which is periodically discharged from the bottom of the separation tank, and the latter is a circulating carbon dioxide gas, which is condensed into carbon dioxide liquid through a heat exchanger for recycling. The whole separation process uses the characteristic of increasing the solubility of carbon dioxide fluid to organic matter in the supercritical state, but basically insoluble in organic matter under the critical state, so that the carbon dioxide fluid is continuously circulated between the extraction vessel and the separation vessel, thereby effectively The components that need to be separated and extracted are separated from the raw materials. 4. Liquid membrane extraction is a new extraction technology. Taking water as the continuous phase, disperse the droplets coated with the water phase core with the surfactant and the organic phase to form an emulsion. Some components in the outer aqueous phase are extracted by the organic phase outside the droplet and then enter the aqueous phase inside the droplet to achieve extraction and separation. Since the diameter of the droplet is only a few microns, the specific surface of the liquid film is large, and the extracted components are quickly transferred from the organic phase to the internal water phase, the mass transfer driving force is large, and the mass transfer is not balanced by the external water phase and the surface machine phase. The concentration is limited, so the extraction efficiency is very high. The technical difficulty is demulsification. At present, demulsification is the most effective under high-voltage electrostatic field. It can be used in the separation of metal ions, separation of biological products, and sewage treatment. 5. Solid phase extraction Solid phase extraction is an important application of chromatography. In this method, a certain volume of sample solution is passed through a small column containing a solid adsorbent, and the components in the sample that have a strong effect on the adsorbent are completely adsorbed; then, the adsorbed components are washed with a strong elution solvent Take it out and dilute to a small volume of the sample solution to be tested. Using the solid phase extraction method, the components in the sample can be concentrated, and the components that interfere with the components of interest can be preliminarily removed, thereby improving the sensitivity of the analysis. Solid phase extraction can be used not only for sample pretreatment in chromatographic analysis, but also for sample pretreatment in various analytical methods such as infrared spectroscopy, mass spectrometry, nuclear magnetic resonance, ultraviolet and atomic absorption. The C18 solid phase extraction cartridge has a hydrophobic effect and has an adsorption effect on non-polar components, so it can extract polynuclear aromatic hydrocarbons from the water to complete the function of concentrating samples. There are other types of solid phase extraction cartridges, such as polarity, ion exchange and so on. 6. Liquid-solid extraction uses small columns filled with fine-particle adsorbents for liquid-solid extraction (1iquid～solid extraction, LSE), which quickly compares the liquid-liquid extraction method with the simplification and traceability of the sample matrix. It has established its own position in the enrichment of quantitative samples. Liquid-liquid extraction has some problems: labor-intensive; it is often plagued by practical problems such as emulsification; it tends to consume a large amount of high-purity solvents, which often cause harm to the health of operators and the environment; it brings extra cost of. Liquid-solid extraction has the advantages of low cost, time saving, solvent consumption and simple processing steps. The liquid-solid extraction step can easily use a dedicated process unit group to automatically extract samples in multiple channels at the same time and prepare the samples into samples suitable for automatic injection; or use a centrifugal analyzer to batch process a large number of samples to increase samples The purpose of reducing the cost of labor. Liquid-solid extraction is very convenient for on-site sampling. It eliminates the need to send a large number of samples to the laboratory for processing, and minimizes the problems of sample transportation and storage. Liquid-solid extraction technology is not without its problems, but these problems are not the same as those encountered in liquid-liquid extraction. These two technologies can be regarded as complementary.