Why does benzoic acid recrystallization give fluffy crystals with low yield?

[Luna_H]

I’m trying to purify a sample of benzoic acid I made, and my recrystallization from hot water isn’t giving me the nice, sharp crystals I expected. The solid that forms is kind of fluffy and seems to trap a lot of the solvent, so my yield is really low after drying. I’m wondering if my cooling step is too fast or if my starting material just has too many soluble impurities.

[AdamAM]

I’ve run into that fluffy, solvent-laden look before in recrystallization. When I cooled too fast and let the solution stay saturated, the crystals formed a loose matrix and trapped solvent. Slowing the cooling and giving the solution time to desaturate helped a bit, even if the yield still isn’t great.

[NatalieS]

I tried seeding with a tiny crystal and letting it grow rather than letting everything nucleate at once. The crystals came out bigger and cleaner, but the yield dropped because some stayed dissolved.

[Jacob.R]

Water isn’t always the best stand-alone solvent for benzoic acid. A little co solvent like ethanol or isopropanol helped tweak the solubility and gave nicer crystals in my hands, though it does change the dynamics.

[Michael.L]

If there are soluble impurities, they stay in the mother liquor and can keep you from crystallizing cleanly. I’ve washed crystals with cold solvent to strip surface impurities, but that trims the yield.

[HarperT]

Could be the material is picking up a salt or hydrate along the way; that would change how it behaves on cooling. I’ve seen benzoic acid form a bit of a hydrate that looks fluffy.

[Sofia33]

Have you checked if the fluffy solid is actually benzoic acid or something else, like a hydrate or impurity? A quick melt or IR check or just a simple test could tell.

[GraceJ]

I guess maybe the real problem is the habit of the crystals in your particular batch; sometimes you chase perfect shapes and you end up with whatever grows first.