How does scratching initiate crystallization

Ray, P. Google Patents: Hightower, T. Journal of Chemical Education 83 11 , Rohani, S. Organic Chemistry. Purifying Compounds by Recrystallization.

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Login processing Previous Video Next Video. Overview Source: Laboratory of Dr. Jimmy Franco - Merrimack College Recrystallization is a technique used to purify solid compounds. Perform all steps in a fume hood to prevent exposure to solvent fumes. Selecting a Solvent Place 50 mg of the sample N-bromosuccinimide in an Erlenmeyer flask. If the sample dissolves completely, the solubility in the cold solvent is too high to be a good recrystallization solvent.

If the sample does not dissolve in the cold solvent, heat the test tube until the solvent boils. If the sample has not completely dissolved at this point, add more boiling solvent drop-wise, until all of the solid dissolves.

If it takes more than 3 mL to dissolve the sample in the hot solvent, the solubility in this solvent is probably too low to make it a good recrystallization solvent. If the first choice of solvent is not a good recrystallization solvent, try others. If a single solvent that works cannot be found, try a two solvent system.

If you cannot find a suitable single solvent system, then a solvent pair may be necessary. When identifying a solvent pair, there are several key considerations 1 The first solvent should readily dissolve the solid. As a general rule "likes dissolve likes" meaning that polar compounds tend to be soluble in polar solvents and non-polar compounds are often more soluble non-polar compounds. Also make sure the boiling point of the solvent is lower than the melting point of the compound, so the compound forms as solid crystals rather than as an insoluble oil.

Confirm that the impurities are either insoluble in the hot solvent so they can be hot-filtered out, once the compound is dissolved or soluble in the cold solvent so they stay dissolved during the entire process. This is a better choice than a beaker, since the sloping sides help trap solvent vapors and slow the rate of evaporation. Place the solvent water in a separate Erlenmeyer flask, and add boiling chips or a stir bar to keep it boiling smoothly.

Heat it to boiling on a hotplate. Add hot solvent to a flask at room temperature containing the compound in small portions, swirling after each addition, until the compound is completely dissolved. During the dissolution process, keep the solution hot at all times by resting it on the hotplate, too. Do not add more hot solvent than necessary - just enough to dissolve the sample.

If a portion of the solid does not seem to dissolve, even after more hot solvent has been added, it is likely due to the presence of very insoluble impurities. If this happens, stop adding solvent and do a hot filtration before proceeding. To perform a hot filtration, fold a piece of filter paper into a fluted cone shape and place it into a glass stemless funnel. Pour the solution through the paper.

If crystals begin to form at any time during the process, add a small portion of warm solvent to dissolve them. Cooling the Solution Set the flask containing the dissolved compound on a surface that does not conduct the heat away too quickly, such as a paper towel set on a benchtop. Lightly cover the flask as it cools to prevent evaporation and to prevent dust from falling into the solution.

Leave the flask undisturbed until it cools to room temperature. Once the crystals have formed, place the solution in an ice bath to ensure that the maximum amount of crystals is obtained.

The solutions should be left undisturbed in the ice bath for 30 min to 1 h, or till the compound appears to have completely crystalized out of solution. If no crystal formation is evident, it can be induced by scratching the inside walls of the flask with a glass rod or by adding a small seed crystal of the same compound. If this still fails to work, then too much solvent was probably used. Reheat the solution, allow some of the solvent to boil off, then cool it.

Isolating and Drying the Crystals Set the cold flask containing the newly formed crystals on a benchtop. Lightly cover the flask to prevent evaporation and to prevent dust from falling into the solution. To dry the crystals, leave them in the filter funnel and draw air through them for several minutes. Crystals can also be air-dried by allowing them to stand uncovered for several hours or days.

More efficient methods include vacuum drying or placing in a desiccator. Common solvent pairs. Recrystallization is a purification technique for solid compounds. To begin this procedure, place 50 mg of the sample in a glass test tube. Purification by recrystallization is an important tool for chemical synthesis and analysis. Thanks for watching!

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I'm trying to understand what is actually happening when chemists rub stirrers on the inside of flasks to induce crystallization, and once that's understood, to find out if the phenomenon is scaled up in manufacturing situations.

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Learn more. Scratching the inside of a flask to crystallize aspirin - what is it called and has it been scaled up? Asked 4 years, 1 month ago. Active 4 years, 1 month ago. Viewed 1k times. Improve this question. Has it ever been scaled up? Not even close. The small pieces of glass that are etched off of the beaker serve as nuclei for crystal formation But reaching back to my halcyon years of organic synthesis and crystallization, recalling vivid memories of scratching the sides of Erlenmeyer flasks with a glass rod to induce crystallization It's a common lab technique.

But does it have a name? Improve this question. Joseph Joseph 3 3 bronze badges. It makes it easier to understand.. Provide as many details as you know do a bit of research yourself to get more details, that makes it a really good question. A old and scratched one is sufficient. I see the nucleation from the ultra sound or cavitation happening at the rubbed surfaces. Think of the "starters" used in those hand warmer bags with sodium acetate. A name for the lab technik is "rub". I'm not so sure that it is the "microscopic flecks of glass" so much as the sonic disruption of a supersaturated solution.

I have accomplished something similar by flicking the flask hard with my fingernail. BTW, I've always called and heard called this technique "scratching". Show 6 more comments. Active Oldest Votes. The rough surface formed creates scope for nucleation to occur, and this creates the crystals. I don't think there is a specific name for this technique as a lab tenchique. Improve this answer. ManishEarth ManishEarth