A student determined that ethanol was the best solvent to use for a recrystallization based on small scale trials. When the recrystallization was carried out on a larger scale, no crystals were obtained on cooling the hot solution.
Why? and how would you overcome this problem???
The lack of crystal formation during recrystallization on a larger scale can occur due to several reasons. In this case, one possible reason is the solubility of the solute in the chosen solvent.
When a solute is dissolved in a solvent and heated, the solute dissolves more easily due to increased temperature and the thermal energy of the system. However, when the solution is cooled down, the solubility of the solute decreases, leading to the formation of crystals.
If no crystals are formed during the cooling step on a larger scale, it suggests that the solute has a higher solubility in the chosen solvent at cooler temperatures. This occurs because the solute has a higher affinity towards the solvent and remains in the solution rather than crystallizing out.
To overcome this problem, there are a few potential strategies you can try:
1. Change the solvent: Choose a different solvent with a lower solubility for the solute at lower temperatures. You can perform small scale trials with other solvents and determine the best one for recrystallization.
2. Add a co-solvent or anti-solvent: Sometimes, adding a co-solvent or an anti-solvent can help decrease the solubility of the solute at lower temperatures, promoting crystal formation. Co-solvents are miscible with the chosen solvent, whereas anti-solvents are immiscible. By adding a specific quantity of these substances, you can alter the solubility properties of the system.
3. Use a seed crystal or scratching: Introducing a seed crystal or physically scratching the sides of the container can provide nuclei for crystal formation. Seed crystals act as a starting point for crystal growth, encouraging other solute molecules to join and form larger crystals.
4. Concentrate the solution: Another approach is to concentrate the solution by evaporating the solvent, which can increase the solute concentration and promote crystal formation. This technique is particularly useful when dealing with solutes that have poor solubility in any solvent.
In summary, the lack of crystal formation during recrystallization on a larger scale may be caused by the higher solubility of the solute at lower temperatures. Overcoming this problem may involve choosing a different solvent, using co-solvents or anti-solvents, introducing seed crystals or scratching, or concentrating the solution. It is important to perform small scale trials and optimize the recrystallization conditions for successful large-scale crystal formation.