Basic question on recrystallization.

  • Thread starter scorpion990
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In summary, the observed drop in melting point after recrystallization could be due to the sample being closer to the eutectic composition, which would explain the sharp melting point. However, it is also possible that the sample was heated too quickly during the first measurement, causing the observed higher melting point. This could have altered the composition and resulted in a lower melting point after recrystallization. Further investigation is needed to determine the exact cause.
  • #1
scorpion990
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I have a quick question:
Suppose that you observed a solid melting at 150 degrees - 160 degrees Celsius. Once it is recrystallized, the observed melting point drops to 145-145.5 degrees Celsius. How can this be?

At first, I thought it was due to an impurity, which would act to lower the melting point. However, the melting point of the recrystallized solid is very narrow, so it is probably quite pure. Then again, the recrystallized mixture might be eutectic, which would explain the sharp melting point. But then again... How is it possible to contaminate a sample that badly?

Any help is appreciated.
 
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  • #2
Reaction with solvent?
 
  • #3
Perhaps during the measurement of the first melting point, the sample was heated too quickly so that melting wasn't observed until the chamber was at a temperature higher than the real melting point.
 
  • #4
scorpion990 said:
I have a quick question:
Suppose that you observed a solid melting at 150 degrees - 160 degrees Celsius. Once it is recrystallized, the observed melting point drops to 145-145.5 degrees Celsius. How can this be?

At first, I thought it was due to an impurity, which would act to lower the melting point. However, the melting point of the recrystallized solid is very narrow, so it is probably quite pure. Then again, the recrystallized mixture might be eutectic, which would explain the sharp melting point. But then again... How is it possible to contaminate a sample that badly?

Any help is appreciated.

I'm not an expert on this topic however your last assumption is probably correct , you have have simply altered the original composition so that the mixture is closer to the eutectic composition. What are you working with?
 
  • #5
It's not real data. Rather, it's just a question I have to answer and attach to my lab.
 

1. What is recrystallization and why is it important?

Recrystallization is a process used to purify solid compounds by dissolving them in a solvent, then allowing them to slowly crystallize out as pure substances. It is important because it allows for the removal of impurities from a compound, making it more suitable for further experimentation or use in various applications.

2. How does recrystallization work?

Recrystallization works by taking advantage of differences in solubility between the compound of interest and its impurities. The compound is dissolved in a solvent at high temperature, and as the solution cools, the compound will slowly crystallize out while the impurities remain in solution. The crystals can then be separated from the solution through filtration or decantation.

3. What factors affect the success of recrystallization?

There are several factors that can affect the success of recrystallization, including the choice of solvent, the rate of cooling, and the presence of impurities. It is important to choose a solvent in which the compound has high solubility at high temperatures and low solubility at low temperatures. The cooling rate should also be slow to allow for proper crystal formation. Impurities can also affect the success of recrystallization, as they may not be completely removed during the process.

4. Can all compounds be recrystallized?

Not all compounds can be recrystallized, as some may not have a significant difference in solubility between high and low temperatures. Additionally, some compounds may decompose or react with the solvent, making recrystallization not possible. In these cases, alternative purification methods may need to be used.

5. How can the purity of a compound be determined after recrystallization?

The purity of a compound can be determined after recrystallization through various analytical techniques such as melting point determination, thin layer chromatography, or spectroscopy. These methods can help determine if any impurities are still present in the compound after recrystallization.

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