Experiment 10 - The extraction of Caffeine from Cola drinks >> Introduction
Experiment 10 - The extraction of Caffeine from Cola drinks
When to Perform Extractions
In general terms, an extraction is any process where a substance is isolated from a mixture by chemical, physical, or mechanical means. Thus, many laboratory techniques qualify as extractions: distillation, sublimation, filtration, or even preparative column chromatography! Extractions are done all the time in the chemistry laboratory, since purifying reaction products or intermediates is a substantial part of most experiments.
For the purposes of experiments in this course, we'll focus on liquid-liquid extractions. These are performed by transferring a dissolved substance (solute) from one solvent into another. For example, let's say that we are trying to isolate some organic compound as a dry solid from an aqueous solution. We may first be tempted to simply boil off the water by heating the solution to 100°C. However, the dissolved organic compound may be thermally unstable and decompose at water's boiling point. Another strategy is more appropriate: transfer the compound into a low-boiling, organic solvent such as dichloromethane through a liquid-liquid extraction. Then boil away the dichloromethane at a much lower temperature (~40°C), ensuring that the organic compound does not decompose. This will leave us with a dry and chemically intact solid.
As you can see, liquid-liquid extractions are performed when it is more convenient to change the solvent that is dissolving a particular solute. They can also be used to separate the components of a mixture according to different solubilities in different solvents. In fact, that is the physical basis that underlies liquid-liquid extractions. We'll discuss this in more detail in the next section.
The Physical Basis of Extractions
Let's go back to our example of the chemical dissolved in water that makes its way into dichloromethane through a liquid-liquid extraction. While the procedure is successful in our hypothetical case, this laboratory trick does not always work. The following conditions must be satisfied for our liquid-liquid extraction example to be successful:
The compound (solute) that is initially dissolved in water must also be soluble in dichloromethane. The compound must be more soluble in dichloromethane than it is in water (which is the case for many organic compounds). Water and dichloromethane must be immiscible (incapable of forming a homogeneous mixture). We can extend our example to illustrate another useful property of extraction. Assume that the compound being transferred into dichloromethane from water is initially mixed with another solute in water. If the other solute in water is not soluble in dichloromethane, then the extraction separates the two solutes: one moves into dichloromethane, and the other stays behind in water. Here we see extraction being used for its most common purpose: purification.
Liquid-liquid extractions work because solutes preferentially migrate to or remain in the solvent in which they are most soluble. By requiring that the solvents used be immiscible (e.g. water and dichloromethane), physical separation of the solutions becomes easy: the less dense solution floats on top of the more dense one without mixing. The separatory funnels used in extractions are specially designed to facilitate separations of liquid layers.