Question

Why is water used for the initial crystallization and ethanol is more suitable to be used for the recrystallization step of acetylsalicylic acid? (Hint: solubility & temperature)

Answer 1

Theory: The extent of precipitation of the solid depends on the difference in its solubility in the particular solvent at temperatures.When we use a solvent pair, we dissolve the solute in better solvent and add the poorer solvent to the hot solution until saturation occurs.

Solution: Boiling point of water is more(100 DegC) than boiling point of ethanol (78 DegC) . acetylsalicylic acid has boiling point of 136DegC. SO difference of boiling point for ethanol -acetylsalicylic acid solution is higher (136-78 = 58) compared to difference in boiling point for water-acetylsalicylic acid solution (136-100 = 36 DegC).

So ethanol is abetter solvent than water for acetylsalicylic acid. So as we stated in theory that

When we use a solvent pair, we dissolve the solute in better solvent and add the poorer solvent to the hot solution until saturation occurs.

So intially in crystallization process we should use the poorer solvent i.e. water to reach the saturation limit. Later during recrystallization step, we should use a better solvent i.e. ethanol.

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