1, does not react with the purified substance.

2, a large number of purified substances can be dissolved at a higher temperature; However, only a small amount of this substance can be dissolved at room temperature or lower.

3. The dissolution of impurities is very large or small (in the former case, impurities should remain in the mother liquor and do not precipitate with the crystals of the purified substance; In the latter case, impurities are filtered out during thermal filtration).

4. Volatile (low boiling point of solvent) and easy to separate from crystallization.

5, can produce better crystals.

6, non-toxic or less toxic, easy to operate.

7, cheap and easy to get.

Extended data:

1, influencing factors:

The effect of recrystallization is closely related to the choice of solvent. It is best to choose a solvent that is soluble in the main compound and slightly soluble or insoluble in impurities. After filtering off impurities, the solution is concentrated and cooled to obtain pure substances.

If the solubility of two mixed salts in a solvent changes greatly with temperature, such as the mixture of potassium nitrate and sodium chloride, the solubility of potassium nitrate increases sharply with the increase of temperature, but the increase of temperature has little effect on the solubility of sodium chloride.

Then evaporated and concentrated at a higher temperature, sodium chloride crystals were precipitated first, and the mother liquor after sodium chloride removal was concentrated and cooled to obtain pure potassium nitrate. In order to obtain better purification effect, it is often necessary to carry out recrystallization for many times.

2. Influence of absorption band position

The influence of solvent on absorption band is related to the types of solvent, solute and transition band. According to the properties of solute and solvent, different effects are produced. If the intermolecular interaction is stronger than that of the ground state, the absorption band of the transition will shift red. Conversely, if the ground state is more stable, it will shift to blue.

When the solute is a nonpolar molecule, the interaction with the nonpolar solvent is only weakly dispersed, and the influence of the solvent on the solute is often very small, and the spectrum of the solution is close to the gas state. If the excited state of solute has a dipole or is accompanied by charge transfer, the wavelength of absorption band will shift red with the increase of dielectric constant or refractive index of solvent.

3, the influence of ultraviolet absorption spectrum

When the spectra of pure compounds are measured in a series of different solvents, the obtained data, including the intensity and wavelength position of absorption bands, often change with the change of solvents. Depending on the nature of solute and solvent, sometimes chemical reaction may occur between them or complex may be formed. The dissociation constant and tautomeric equilibrium of solute are also related to solvents, so different solvents can produce very different spectra.

Therefore, it is best to use the same solvent when comparing or checking the spectra of compounds. If different solvents are used, the influence of solvents should be considered when analyzing the spectrum. In addition, sometimes the transition type of absorption band can be inferred from the difference of different solvents.

The physical interaction between solvent and solute includes electrostatic interaction, dispersion, hydrogen bonding, charge transfer and charge repulsion. This intermolecular interaction has different effects on molecular energy levels and produces different spectra.

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