What is the whole point of using solvent pairs for recrystallization(say a mix of ethanol and water) rather than just using one or the other?
-I know that you use solvent pairs when you solute won't dissolve in either pure water or pure ethanol, but then if the solute won't dissolve in either or, why does it dissolve when the solvent is a mix? That's what's really confusing.
Thanks :)
Often the material is more soluble in one of the solvents than the other. Thus when one of the solvents is lost (evaporates) this forces the material out of solution. In effect this makes a more steep solubility curve than the single solvent.
Another reason for using a mixed solvent is that the impurities you are trying to remove may be more soluble in one of the pair.
When it comes to recrystallization, the use of solvent pairs, such as a mix of ethanol and water, is aimed at optimizing the solubility and selectivity for the desired compound. Here's the explanation:
1. Solvent Solubility: In some cases, the solute (compound to be recrystallized) may have limited solubility in either pure water or pure ethanol. However, by using a mixture of ethanol and water as the solvent, you can achieve a wider range of solubilities. This is because the solute may exhibit different degrees of solubility in different solvent compositions due to the varying polarity and miscibility of the solvents. By adjusting the ratio of ethanol to water, you can manipulate the solubility of the solute and improve its dissolution.
2. Polarity and Selectivity: The choice of a solvent pair often involves combining solvents with different polarities. Ethanol is a polar solvent, while water is a highly polar solvent. By mixing them, you create a solvent system with intermediate polarity. This helps in selectively dissolving the desired compound while leaving impurities behind. The solute, which may have a certain polarity, can dissolve more readily in the mixed solvent compared to the pure solvents.
3. Temperature Control: Another advantage of using a solvent pair is the ability to control the solubility by adjusting the temperature. The solubility of many compounds changes with temperature, and the solvent pair allows for this fine-tuning. By heating or cooling the solvent mixture, you can promote or limit solubility as needed during the recrystallization process.
In summary, using a mixture of solvents as a solvent pair for recrystallization allows for greater control over solubility, selectivity, and temperature, which can enhance the purification process by dissolving the desired compound while minimizing impurities.