recrystallisation

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StreamlinePM
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I’m looking for advice on a chemical I can preform a recrystallisation experiment in water. Ive previously done with boric acid but have struggled to make into a dense solid. What would be best for this and less likely to be cancelled out from other potential chemicals if mixed?
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GeT_iN_SHinJI
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i dont know if this has anything to do with what you're asking for but we did recrystallisation of paracetamol in class
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TypicalNerd
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(Original post by StreamlinePM)
I’m looking for advice on a chemical I can preform a recrystallisation experiment in water. Ive previously done with boric acid but have struggled to make into a dense solid. What would be best for this and less likely to be cancelled out from other potential chemicals if mixed?
So for a recrystallisation, you want the solute to be decently soluble in water at a higher temperature and ideally not very soluble at a low temperatures. ‘Seeding’ (i.e adding a crystal of the purified substance whilst the mixture is cooling) can allow for the crystals to have a nucleation site, hence you get a better end result.

Boric acid has a solubility of 25.2 g dm^-3 at 0°C and 275.3 g dm^-3 at 100°C, so you should have got a reasonable yield (supposing you used 100 cm^3 of water to make a saturated solution, you should have obtained about 25 g of purified product).

As for a compound I suggest you consider, how about monoammonium dihydrogenphosphate (the compound used in a lot of crystal growing kits)? It definitely forms a solid that you can collect, but it is pretty soluble in water at 0°C (280 g dm^-3 compared to 1730 g dm^-3 at 100°C), so there often is a significant loss of yield.

Recrystallisation always works best when you leave the sample in the freezer for a few hours, and use filtration under reduced pressure to get the crystals out afterwards, but I’m assuming you know this already.
Last edited by TypicalNerd; 1 month ago
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TypicalNerd
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(Original post by TypicalNerd)
So for a recrystallisation, you want the solute to be decently soluble in water at a higher temperature and not very soluble at a low temperatures. ‘Seeding’ (i.e adding a crystal of the purified substance whilst the mixture is cooling) can allow for the crystals to have a nucleation site, hence you get a better end result.

Boric acid has a solubility of 25.2 g dm^-3 at 0°C and 275.3 g dm^-3 at 100°C, so you should have got a reasonable yield (supposing you used 100 cm^3 of water to make a saturated solution, you should have obtained about 25 g of purified product).

As for a compound I suggest you consider, how about monoammonium dihydrogenphosphate (the compound used in a lot of crystal growing kits)? It definitely forms a solid that you can collect, but it is pretty soluble in water at 0°C (280 g dm^-3 compared to 1730 g dm^-3 at 100°C), so there often is a significant loss of yield.

Recrystallisation always works best when you leave the sample in the freezer for a few hours, and use filtration under reduced pressure to get the crystals out afterwards, but I’m assuming you know this already.
Also forgot to mention, allow it to cool slowly for larger crystals.

I’d recommend allowing it to reach room temp, then putting it in the freezer for a few hours to bunk down the solubility as much as possible and get a better yield.
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