Question

Consider a crystallization of sulfanilamide in which 10ml of hot 95% ethyl alcohol is added to 0.10g of impure sulfanilamide. After the solid has dissolved, the solution is cooled to room temperature and then placed in an ice water bath. No crystals form, even after scratching with a glass rod. explain why this crystallization failed. What would you have to do at this point to make the crystallization work??

Answer 1

You haven't been able to form a nucleus around which crystallization can occur. This if very common in new labs, where the condtions are relatively pristine. Old labs have more things floating around.

Your scratching didn't work. Bummer.

So, your volume still may be too high. You want to concentrate it down to at least half volume. And you really want crystallization to start from warm solution. Slow crystallization from warm solution gives the best results.

OR

The student recoved 80% of the impurity in the biphenyl. You might expect biphenyl to dissolve in benzene according to the adage "like dissolves like". Since the original sample was a mixture of 0.475 g of biphenyl and 0.025 g of an impurity. It should not be surprising that a low recovery from benzene might be the impurity. Had the student used ethanol, for example, biphenyl would probably be the least soluble component. A good method to determine how your recrystallization might occur is to TLC your sample first. If the impurity moves faster than biphenyl, then a non-polar solvent (like hexane) might be effective. If the impurity moves slower, then the impurity is more polar and a polar solvent (ethanol or ethanol-water) would be a better choice.

Number 2) I like this one. Having done a lot of recrystallizations, I do not like using single solvents (95% ethanol is a single solvent in that water or some other solvent was not part of the planned addition). You cannot readily change the polarity to make the system saturated.
First, that is a lot of solvent for a small amount of sample unless you knew the sample was fairly insoluble. I suspect the expected answer is to evaporate the solvent until saturated. You could do that, but if I were actually doing this in the lab, I would reduce the volume to about 3-4 mL and then start adding hexane. Two things might happen. One, water might start to co-distill. That is okay. The other and hoped for effect is that as hexane is added, you would see some cloudiness as hexane might cause local insolubility and show the effectiveness of this strategy. Keep adding until the cloud point. Scratch or seed and let cool to crystallize.

I would not add water. Sulfanilamide is likely soluble in water. Adding water will extend the effort to recover the sample.

Finally, this is an example of how not to do a recrystallization. I could imagine adding 10 mL of 95% ethanol because there was an impurity that failed to dissolve with less solvent. I like using a solvent that my sample is quite soluble in so if something doesn't dissolve, I know I should filter the mixture. If you are using a solvent in which your sample has a low solubility, it can be difficult to know whether it is the sample or impurity that is not dissolving. Once dissolved, it is easy to add a co-solvent of quite different polarity to make the solution saturated.