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  • Professor Dave here, let's talk about recrystallization.

  • After learning a variety of separation techniques, we will need to look at just one technique

  • for the purification of solids.

  • Sometimes a chemical reaction will occur such that a solid preciptate will form, which we

  • can then filter out of solution.

  • But it is inevitable that this solid will contain impurities.

  • When the lattice structure is forming during the precipitation, ions and other particles

  • that do not belong in the lattice will get trapped inside and disrupt the pattern of

  • the lattice.

  • There is a solution to this problem, and it is called recrystallization, so let's see

  • how that works now.

  • Let's say we have our impure solid that we wish to recrystallize.

  • The way this works is that we will choose a very particular solvent in which to dissolve

  • this solid.

  • The solid must have a very low solubility in this solvent at room temperature, but a

  • very high solubility in this solvent at very high temperatures, which makes sense given

  • that solubilities will tend to increase as temperature increases, given the entropic

  • favorability of dissolution.

  • So we will choose a solvent carefully, and we will heat it up to boiling or near boiling.

  • This will then be added to the impure solid, which has been placed in a beaker.

  • We add more solvent, heating and stirring the contents of the beaker as necessary until

  • all of the solid is dissolved, being sure to add only as much as is necessary for dissolution

  • to occur.

  • At this point, the impurities which were locked in the lattice are liberated, and can now

  • float around in solution.

  • Once dissolution is complete, we can then begin the cooling process.

  • This will be done by first removing any heat source and allowing the beaker to come down

  • to room temperature.

  • Then the beaker can be placed in an ice bath, whereby the cooler surroundings will draw

  • heat away from the contents of the beaker.

  • Crystals should begin to form in the beaker, which will be a much purer version of the

  • solid, with the impurities having been removed and which now are extremely dilute in solution.

  • If crystal formation is slow or difficult to initiate, we can use a seed crystal, whereby

  • we add a little bit of the solid that is supposed to form, which provides a surface around which

  • ions in solution can coordinate, and this will kick start the crystallization process.

  • This can also sometimes be done simply by scratching the side of the beaker with a stirring

  • rod, as this will liberate a tiny amount of glass that can also provide seeding behavior.

  • Once crystallization is complete, we can use some filter paper on a Buchner funnel, and

  • dump the contents of the beaker onto the filter paper.

  • Utilizing a vacuum, the solvent comes through the funnel and the crystals will sit on top

  • slowly drying, and after sufficient time, we should have crystals that are nice and

  • dry and pure.

  • We can test their purity by performing melting point analysis and comparing the melting point

  • range to tabulated values for pure compounds.

  • A narrow melting point range very close to the tabulated value indicates a high degree

  • of purity, whereas a wider range or a less accurate range indicates that impurities still

  • persist.

  • So recrystallization is quite a simple technique.

  • It really just consists of dissolving a solid in hot solvent, and then allowing things to

  • cool so that it crystallizes again, but this time with very little of the initial impurity

  • within the lattice.

  • The trick is to choose the correct solvent that will allow this to proceed properly,

  • since we need the solid to be relatively insoluble in this solvent at room temperature, yet soluble

  • at higher temperatures.

  • In addition, the ideal situation involves an impurity that is soluble at room temperature,

  • so that it remains in solution even when it cools, and has no opportunity to precipitate

  • along with the desired product.

  • Or similarly, the impurity could remain insoluble even at the higher temperature, so that once

  • the solid dissolves, we can simply remove the solid impurity by filtration.

  • This selection process often simply involves trial and error, prior experience involving

  • solvents of differing polarities, or a prediction made on the basis of tabulated solubility

  • data, if such data exists for the compound that is intended for recrystallization.

  • Whatever the case may be, as long as the solid is able to dissolve and then recrystallize,

  • the resulting crystals will certainly be of a higher purity than they were before, as

  • it is inevitable that some proportion of the impurities will not be able to find themselves

  • within the lattice as it reforms, so recrystallization is a powerful technique for the purification of solids.

Professor Dave here, let's talk about recrystallization.

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Recrystallization

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    ウィリアム オ ルウェル に公開 2021 年 06 月 25 日
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