4.3: Key Decisions Faced When Performing Crystallizations
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1. WHAT SOLVENT TO USE – Again, the ideal solvent is the one that dissolves the sample at high, but not at low temperatures. Look at fig. 5.1 (p. 559) again. Consult solubility tables, or perform preliminary tests to decide on the best solvent. Consider the interplay of polarity between the sample and the solvent and be receptive to using solvent pairs.
2. TO DECOLORIZE OR NOT TO DECOLORIZE –This step is optional and is normally performed right after dissolving the impure sample in hot solvent. If there is a coloration known to be uncharacteristic of the sample, it can be removed by using a decolorizing agent. Three of the most common are charcoal, alumina, and silica gel. Charcoal is simply carbon which is used as either a powder or pellets. It is commonly sold under the name Norit. Alumina and silica gel are normally used as powders.
This step should be carried out using only a small amount of decolorizing agent. The decolorizer works by binding organic matter on its surface. Since your sample is organic, too much decolorizing powder can reduce your percent recovery. For example, you can use barely enough charcoal powder to cover the bottom of the test tube or container holding the sample solution. Do this while the solution is still hot, stirring or shaking for about a minute. Reheat the sample if necessary to filter before it cools too much.
3. HOW MUCH TIME TO WAIT – Crystallizations can take anywhere from a few seconds to years. A wait period of a few days is common in many research labs. BE PATIENT. In most of the experiments performed in this course a wait period of about 10-20 min. should be enough to obtain a good yield of crystals. If not you probably have a problem somewhere.