Introduction

Resolving complex mixtures by the techniques presented so far in the laboratory can be tedious at best, and in some cases, practically impossible. However, by using a powerful differential separation technique called chromatography, many separations can be achieved quickly with relatively good results.

Separations

All separation techniques utilize differences in the chemical and physical properties of various components in the mixture to be separated. For example, filtration is used to separate solids and liquids. Specifically, the filter paper used in filtration allows the liquid to flow through it while trapping the solids. Another separation technique, one you encountered in Halogen Reactions, is liquid-liquid extraction which utilizes the constituent’s solubility properties. Recall, that in this technique two immiscible solvents were used to distribute the components of a mixture between the two solvents according to their their solubilities in those particular solvents.

A common problem with single-stage techniques like the two mentioned above is the overall efficiency of the separation. In filtration, separation efficiency is complicated by the solubility of the solid in the liquid. Likewise, in two-layer solution separation, the components of the mixture are partially soluble in both layers. Therefore, in order to obtain an acceptable degree of separation using these techniques, it may be necessary to repeat the separation several times.

A more elegant approach is to use a differential technique in which the composition does not change in discrete steps, but rather continuously from the time the experiment begins. In other words, the degree of separation will then depend on the length of time that the mixture is processed.

Chromatography

Chromatography is probably the most powerful, and widely used, differential separation technique available today. Though many forms of the technique exist, every one of them involves the same basic principles. In these methods, a sample mixture migrates through a solid, porous material under the influence of fluid flow. In further detail, the solid material is generally referred to as the stationary phase, while the moving fluid is referred to as the mobile phase.

Individual components of the sample mixture will interact with the stationary and mobile phases to differing degrees, and as a result, will migrate through the system at varying rates. Specifically, components which have a strong affinity for the stationary phase but only a slight affinity for the mobile phase will move at a slower rate than those which have a weak affinity for the stationary phase and a large affinity for the mobile phase. The consequence of this division between stationary and mobile phases is a mixture that is separated into zones, or scientifically referred to as bands. Therefore, each band represents an individual component of the mixture.

In more detail, in paper chromatography, both the molecular weight and the polarity of the structures being separated play a role in the separation. The molecular weight of the molecule pulls it downward, while its affinity for the polar solvent pulls it upward. Another phenomenon involved in the differing rates of migration is the formation of intermolecular bonds, a topic you will become all too familiar with next semester in CHM 1046. All and all, the point at which one force overcomes the other is the point at which the dye will stop moving up the paper.

The Experiment

The mixtures being separated in this experiment are the dye components of ‘standard’ food colors. In order to resolve which dye components are part of each food color, a chromatography paper will be spotted with each of the food colors: red, blue, green, and yellow. This technique is quite easy and reliable, provided that directions are followed carefully.

In addition to spotting the chromatography paper with the food color standards, you will also spot the paper with the dyes used to color the candy shell portion of M&M® candies. The resulting bands of the standard food colors will be compared to the bands of the M&M® candies in order to determine which dyes are used in their making.

A Couple of Hints

1. Don’t put too much coloring on the chromatography paper, as doing so will overload the separation capability of the paper and they dyes will not separate well.


2. Insure that all the spots are exactly 1.5-cm from the bottom of the paper and higher than the solvent level in the developing beaker. If the solvent is above the spots, the dyes will leach out into the solvent.


3. Avoid touching or creasing the surface of the chromatography paper when you are handling it. Pick up the paper only by its edges!