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Isolating chemicals

  1. Jul 2, 2007 #1
    I want to learn how to isolate chemicals from substances. For example lets say I have a pile of powder which consists of about 5 different chemicals.

    The chemical I want to isolate is highly soluble in water. Lets say one of the other chemicals isn't soluble in water but highly soluble in alcohol.

    How would I go about getting rid of the alcohol soluble chemical? Could I just mix the powder alcohol, shake it up in a sep funnel and drain? Would I be better off adding water to the powder and siphoning off the solution but leaving behind the insoluble powder? Would that get rid of any chemicals that werent soluble in water?

    What happens if there are other unwanted chemicals that are soluble in water? Would I just have to find solvents that some of the chems are soluble in and others aren't?
  2. jcsd
  3. Jul 2, 2007 #2


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    Ideally you would want to find a solvent system that dissolves them both since undissolved solids stabilize emulsions and can plug up separatory funnels. After you find the appropriate system (test with small amounts of solvent and unknown solid mixture), you then change the properties of the solvent system, slowly, to precipitate one preferentially. This can be accomplished in your example in many ways. One way would be to dissolve the powder in a hot alcohol/water mixture just sufficient to completely dissolve both substances. Slow cooling will preferentially precipitate the least soluble one. Removal of the alcohol by slow evaporation will produce a second crop of alcohol soluble material. The remaining water will contain most of the water-soluble compound. In complicated examples where the solubilities of many different compounds vary greatly, sometimes it is best to wash the solids in hot solvent and filter. The solvent must be chosen to preferentially remove one or perhaps two of the compounds in the mixture. These new mixtures (fractions) may then be purified by selective recrystallization or extraction.

    It would be helpful to know something about the chemical nature of the mixture. In the Organic Chemistry lab course, we used to mix benzaldehyde, benzoic acid and carbon black and have the students separate each into fractions and then purify each fraction. Benzoic acid and benzaldehyde are difficult to separate by recrystallization but can be easily purified by dissolving both in methylene chloride, filtering to remove the carbon and finally extracting out the carboxylic acid with cold 5% NaOH solution. The NaOH solution deprotonates the carboxylic acid and results in a salt that is water soluble. The resulting aqueous solution is separated and acidified to regenerate the benzoic acid and is then filtered and recrystallized. Amines must be acidified to render them water soluble and then treated with base to render them organic-soluble.

    These methods almost never result in absolutely pure substances after the first recrystallization or extraction. They do result in materials of increasing purity and often three or more recrystallizations/extractions are required to purify them to >99%. In the lab, the purity of materials is monitored by several techniques, the simplest of which are thin layer chromatography (TLC) and melting point. Materials are purified to constant melting point and/or the TLC shows only one compound. This usually results in purities of >98%. Greater degrees of purity (say, 99.999%) sometimes require the use of sophisticated instrumentation such as Gas Chromatography/Mass Spectrometry to confirm purity and may require exotic purification techniques such as preparative-scale chromatography in addition to recrystallization.
  4. Jul 9, 2007 #3
    use pure or denatured 200 proof alcohol with no water in it to wash away the alcohol soluble chemical
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