a student dissolves 80 mg of a crude product in 4.5 mL( the minimum required) of methanol at 25 deg C . She cools the solution in an ice bath and obtains crystals. The crystals are recovered by filtration and rinsed with 0.5 mL of ice-cold methanol. After drying, the weight of the crystals is 5 mg. Why was the recovery so poor? what could she do to improve the process?
Try a new solvent.
how would that help trying a new solvent? i don't understand why the recovery would be so poor, do you know?
The solubility in methanol at 25 C and at 0C may be close to the same value. I would try a new solvent OR use a different temperature range. Methanol is a polar solvent; a non-polar one might be better. One of the main things in recrystallization is to find a solvent where the solubility at higher T values is high and the solubility at low T values is low.
To understand why the recovery was poor and how to improve the process, let's break down the steps and analyze them:
1. Dissolving the crude product: The student dissolved 80 mg of the crude product in 4.5 mL of methanol. It seems that the solubility of the crude product in methanol is a limiting factor. One possibility is that the crude product is not fully soluble in methanol, leading to poor recovery.
2. Cooling the solution: The student cools the solution in an ice bath to induce crystallization. This step is intended to promote the formation of crystals. However, if the solubility of the crude product in methanol is already low, cooling alone may not be sufficient to generate a substantial amount of crystals.
3. Filtration and rinsing: The crystals are recovered by filtration and rinsed with 0.5 mL of ice-cold methanol. This step aims to separate the crystals from the solvent and any impurities. Rinsing with ice-cold methanol helps to minimize the dissolution of crystals during the process.
4. Drying: After filtration and rinsing, the student dries the crystals. Drying is crucial to remove any remaining solvent and obtain the final product. It seems that despite the drying process, the weight of the crystals obtained is only 5 mg, which indicates a poor recovery.
Now, to improve the process and increase the recovery, the student can consider the following steps:
1. Increase solubility: The student can try using a different solvent that has better solubility for the crude product. Experimenting with different solvents can increase the chances of dissolving a larger quantity of the crude product.
2. Enhance crystallization: Instead of solely relying on cooling, the student can try different crystallization techniques such as slow evaporation or adding a suitable anti-solvent. These techniques can enhance the chance of crystal formation, even if the solubility of the crude product is relatively low.
3. Optimize filtration and rinsing: The student should ensure that they are using appropriate filters to separate the crystals efficiently. Rinsing the crystals with a minimal amount of cold solvent helps minimize crystal loss during the process.
4. Optimize drying: It's essential to ensure that the crystals are thoroughly dried before weighing. The student can use desiccators or vacuum drying to remove the remaining solvent completely, which may improve the final weight of the crystals obtained.
In summary, to improve the recovery of crystals, the student can experiment with different solvents, optimize crystallization techniques, use proper filtration, rinsing, and drying methods. It's important to remember that the choice of solvent, temperature, and other experimental conditions can significantly impact the success of the crystal recovery process.