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I have already asked a similar question on titrations before. In a few months, I have a test on chemical synthesis. It is marked on the percentage yield, purity and crystal size of the product. I do not have a lot experience in doing synthesises and I am looking for any tips that will help maximise the yield, purity and crystal size of my synthesis.

Most of the synthesis that we do, the products are obtained by recrystallisation. The solid is dissolved in a solvent at a high temperature and then it is placed in a cool bath. Then the product will crystallize on the bottom of the beaker.

Reading through an analytical textbook, it states a few ways to improve yield and purity but it doesn't actually state how you would do it practically:

  • allow the solution cool down as slowly as possible. This will maximise crystal size.
  • maintain a low relative supersaturation of the solution, this will increase crystal size and improve purity

  • describes the process or digestion or Ostwald ripening where larger crystals grow at the expense of smaller crystals

Does anyone have any suggestions on how I would actually do these 3 above tips. Also does anyone have any other tips.

Thanks

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    $\begingroup$ It's a bit confusing what you mean with yield. With recrystallisation you cannot increase the yield of a reaction, you need to start earlier than that. If you are talking about the yield of the crystal, then just make everything as slowly as possible. Say you have a saturated solution at 100 °C, then let it cool down to room temperature over a day or so. Also your starting crystal should be as pure as possible. Other than that, I have not really a clue. Many repeats though can help grow a crystal larger. $\endgroup$ – Martin - マーチン Dec 16 '15 at 15:00
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When I grew crystals of aluminium potassium sulphate I filled a jar with boiling water and placed it in a cooking pot that was also full of water. Doing this helps to maintain a constant temperature. I set the cooker to a low heat and began dissolving the alum until the solution was saturated. At this point I removed the jar containing the solution and suspended a piece of string for a crystal to grow on. This would be my seed crystal. I left the jar on a window sill to cool and waited.

A few days later a crystal had formed so I lifted out the string and set it aside. Then I reheated the solution to dissolve any crystals that had formed on the jar as they would compete with the suspended seed crystal, reducing its size. It is also a good idea to filter the solution at this point. Once I had finished this step I placed the string with the seed crystal back into the jar and placed it on the window sill again. After a week I moved the jar to a refrigerator (around 3 degrees C) to further cool the solution. This left me with a pretty big, well shaped crystal.

I didn't do this but you could try evaporating off the water after the previous step to grow the crystal further.

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