Common purification and separation methods in laboratories
Purification refers to purifying the mixture to remove impurities, and obtaining the host material in the mixture, and the purified impurities do not have to consider the chemical composition and physical state. There are many ways to separate mixtures, but they can be divided into two broad categories based on their nature of separation:
1. Chemical separation method
2. Physical separation method
The following methods for chemical separation and purification of mixtures are summarized as follows:
Principle of separation and purification
1. The introduced reagent generally only reacts with impurities;
2. Subsequent reagents should remove excess pre-added reagents;
3. Can not introduce new substances;
4. The substance formed by the reaction of the impurity and the reagent is easily separated from the purified substance;
5. The process is simple, the phenomenon is obvious, and the purity is high;
6. Convert impurities into the required substances as much as possible;
7. Consider the rational sequence of adding reagents when removing multiple impurities;
8. If you encounter a gas that is very soluble in water, prevent the phenomenon of back suction.
Conceptual distinction
Cleaning:
Separating dense and insoluble solids from liquids, separating sand and water;
filter:
Separating insoluble solids from the liquid and purifying the edible water;
Dissolution and filtration:
Separating two solids, one soluble in one solvent and the other insoluble, separating salts and sand;
Centrifugal separation:
Separating insoluble solids from the liquid, separating the mud and water;
Crystallization method:
Separating dissolved solutes from solution and extracting salt from seawater;
Liquid separation:
Separating two immiscible liquids, separating oil and water;
extraction:
Adding a suitable solvent to dissolve and separate a component of the mixture, and extracting iodine in the aqueous solution;
Distillation:
Solvent and non-volatile solutes are separated from the solution, and pure water is obtained in seawater;
Fractionation:
Separating two mutually soluble liquids with different boiling points, separating oxygen and nitrogen in liquid air; refining petroleum;
sublimation:
Separating two solids, only one of which can sublimate, separating iodine and sand;
Adsorption:
The gaseous or solid impurities in the mixture are removed and the activated carbon removes colored impurities from the brown sugar.
Separation and purification of commonly used chemical methods
1 Heating method
When a substance having poor thermal stability is mixed in the mixture, it can be directly heated to decompose and separate the material having poor thermal stability. For example, NH4Cl is mixed in NaCl, NaHCO3 is mixed in Na2CO3, and the like can be directly heated to remove impurities.
2 Precipitation
A method in which a certain reagent is added to the mixture to separate one of them in the form of a precipitate. Care must be taken to use this method to introduce new impurities. If a plurality of reagents are used to gradually precipitate different particles in the solution, it should be noted that the excess portion of the added reagent is removed, and the added reagent does not introduce new impurities. For example, adding an appropriate amount of BaCl2 solution can remove Na2SO4 mixed in NaCl.
3 Acid-base method
The purified material does not react with the acid and base, and the impurities can react with the acid and base, and the acid and alkali are used as the impurity removing agent. For example, CaCO3 in SiO2 is removed with hydrochloric acid, and aluminum powder or the like in the iron powder is removed with a sodium hydroxide solution.
4 Redox reaction
If the mixture is contaminated with reducing impurities, a suitable oxidizing agent may be added to oxidize it to a purified material. For example, chlorine is dropped into a FeCl3 solution mixed with FeCl2 to remove FeCl2 impurities; likewise, if the mixture is mixed with oxidizing impurities, a suitable reducing agent may be added to reduce it to a purified substance. For example, an excess of iron powder is added to a FeCl 2 solution mixed with FeCl 3 to remove FeCl 3 impurities.
5 Conversion method
It cannot be separated once, and it needs to be converted into other substances to be separated after several transformations, and then the converted substances are restored to the original substances. For the separation of Fe3+ and Al3+, an excess of NaOH solution can be added to form Fe(OH)3 and NaAlO2. After filtration, hydrochloric acid is added to regenerate Fe3+ and Al3+. In the process of conversion, the loss of separated substances is minimized, and the converted substances are easily restored to the original substances.
6 Adjust pH
A method of separating a component of a solution by adding a reagent to adjust the pH of the solution. Generally, it is adjusted by adding corresponding insoluble or slightly soluble substances. For example, if the FeCl3 impurity is contained in the CuCl2 solution, the solution is an acidic solution due to the hydrolysis of FeCl3, and the Fe3+ may be precipitated by adjusting the pH. For this, CuO, Cu(OH)2, CuCO3 or CuO may be added to the solution. Cu2(OH)2CO3.
7 Electrolysis
The principle of electrolysis is used to separate the purified substances. For example, electrolytic copper is used to make the crude copper as the anode, the refined copper as the cathode, and the copper ion-containing solution as the electrolyte. Under the action of direct current, the copper is more active than the copper. The metal loses electrons, and only copper ions at the cathode get electrons to precipitate, thereby purifying copper.
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