3.3: Choice of Solvent
- Page ID
- 95745
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The crystallization procedure is possible as most solids tend to become more soluble in solvents as their temperature is increased.
- 3.3A: Ideal Temperature Profile
- For the best crystallization, the compound should be very soluble in the hot solvent and minimally soluble (or insoluble) in the cold solvent.
- 3.3B: General Procedures for Removing Impurities
- rystallization works well as a purification technique if impurities are present in very small quantities (less than 5 mol% of the solid), or if the impurities have a very different solubility profile from the desired compound. Impurities can be easily removed if they are either much more soluble or much less soluble in the solvent than the compound of interest.
- 3.3C: Determining Which Solvent to Use
- The most important factor in the success of crystallization is probably the chosen solvent. Besides having the crucial solubility properties for crystallization (the compound should be soluble in the hot solvent and as insoluble as possible in the cold solvent), there are other factors that determine an appropriate solvent.
- 3.3D: Using Solubility Data
- If you are not following a procedure where a crystallization solvent has been specified, it may be helpful to consult solubility data for the desired compound. Qualitative (and sometimes quantitative) solubility data can be found for many compounds in the CRC and Merck Index.
- 3.3E: Experimentally Testing Solvents
- To experimentally determine a single solvent for crystallization, use the following procedure.
- 3.3F: Mixed Solvents
- When no single solvent can be found that meets all of the criteria for crystallization, it may be possible to use a mixed solvent. A pair of solvents is chosen: one in which the compound is soluble (called the "soluble solvent"), and one in which the compound is insoluble (called the "insoluble solvent"). The two solvents must be miscible in one another so that their solubility with one another does not limit the proportions used.