In a recrystallization of vanillin, can you purify a substance whose solubility decreases as the temperature increases?
Any help is appreciated!
The separation of one compound from another by crystallization hinges on the difference in solubility of the two compounds at two different temperatures. While it might be more difficult to separate something that decreases solubility at higher temperatures, I wouldn't think that, per se, would make it impossible to do if the solubilities of the two compounds were sufficiently different. However, rather than get into a dither about it, I would change solvents and use recrystallization the regular way.
How is possible though--to have a substance whose solubility decreases as the temperature increases?
Na2SO4 (anhydrous sodium sulfate) is a material that decreases its solubility as T increases. It is an exothermic process.
Na2SO4*10H2O + heat ==> Na2SO4 + 10 H2O
According to Le Chatelier's Principle, adding heat causes the reaction to proceed from left to right more, decreasing T causes the reaction to move to the left. Anhydrous Na2SO4 decreases its solubility from about 60 g/100 g H2O to about 45 g/100 g H2O as T is changed from 30 C to 100 C. Solubility is due to a host of things that range from hydration energy of the ions (solvation), energy needed to break the crystal lattice, energy needed to break the hydrogen bonds of water molecules(if that is the solvent), etc. Whether the process is exothermic or endothermic depends upon the individual factors above. MOST salts are endothermic when they go into solution; therefore, their solubility increases with increasing T. However, for those salts that are exothermic, the reverse is true.
Yes, it is possible to purify a substance whose solubility decreases as the temperature increases through recrystallization. Recrystallization is a commonly used purification technique in chemistry.
Here's how you can carry out recrystallization to purify a substance with decreasing solubility as temperature increases (like vanillin):
1. Start by selecting a suitable solvent: In recrystallization, you need to choose a solvent in which the substance is highly soluble at elevated temperatures but has poor solubility at lower temperatures. For vanillin, a common choice of solvent is ethanol or a mixture of ethanol and water.
2. Dissolve the impure substance: Begin by dissolving the impure substance, vanillin in this case, in the minimum amount of hot solvent needed. Heat the mixture to increase solubility, but make sure not to use excessive heat or extended heating, as this may cause evaporation and loss of the compound.
3. Cool the solution: Once the substance is fully dissolved, slowly cool down the solution to room temperature or refrigerate it, allowing the vanillin to crystallize out. As the temperature decreases, the solubility of vanillin in the solvent decreases, and thus crystals of pure vanillin will form.
4. Isolate the crystals: Once the crystals have formed, you can separate them from the remaining liquid by techniques like filtration or centrifugation. Washing the crystals with a small amount of cold solvent can help remove any impurities that may be adsorbed on the crystal surface.
5. Dry the purified crystals: Finally, you should dry the isolated crystals to remove any traces of solvent. This can be done by either leaving the crystals to air-dry or by using a desiccator or oven at a low temperature.
By following these steps, you should be able to purify substances like vanillin, even if their solubility decreases with increasing temperature. Remember that the success of recrystallization depends on choosing the right solvent and carefully controlling the cooling process to promote crystal formation.