Black-Marker Chromatography

In this experiment, we’ll separate the black color of a marker into its color constituents and see that it is comprised of a mix of several colors.

Equipment

·   Unvarnished, simple newspaper (preferable), or a coffee filter, or kitchen roll, or (final option) a part of cereal carton. Don’t use notebooks or printer paper – it just won’t work.

·   A transparent glass

·   Scissors

·   A black water-based marker (a non-erasable marker that is acetone-based won’t work!)

·   Water

·   A pencil

·   Transparent adhesive tape

The experiment

Watch the video to see how we conduct this experiment.

It all started with plants

Chromatography is a very efficient way to separate a mixture of materials. The name “chromatography” comes from two Greek words: chromo (color) and graphein (to write / to note), and it literally means “to write colors”. The name was given by the person who developed chromatography – the Russian-Italian scientist Mikhail Tsvet.

Tsvet was not a chemist, but a botanist – a plant researcher – who was interested in the colors (pigments) found in plants. Because the color in plants is usually composed of a mix of several materials, and because when you mix several colors together you get a new color (which is sometimes misleading), Tsvet wanted to find a way to separate the different color materials. The method he invented in 1901 was to fill a long glass column with chalk ground into a fine powder. He poured a green solution, which he concocted by crushing together plant leaves, onto the top of the cylinder, so that it trickled down to the bottom through the chalk powder.

As the solution passed through the column, it separated into three parts – green, orange and yellow. These represented the three families of materials found in plants: Chlorophyll, carotene, and xanthan. Each material came out through the bottom of the cylinder at a different time.

Separating a mix of materials (red and blue) by passing them through a cylindrical column – each color comes out at a different time. Source: Wikipedia

Because the method focused on colors, Tsvet called the process chromatography. Unfortunately, Tsvet published his research in a marginal scientific journal in Poland, and it took another  decade until he was recognized as the method’s invencreator. He died in 1919, and did not live to see his method developed and improved. In 1952, the Nobel Prize for Chemistry was awarded to Archer Martin and Richard Laurence, for improvements to chromatography – which today is one of the most, if not the most-used method to separate materials.

Separation through adsorption

So how does chromatography actually work? Earlier chemists conducted experiments somewhat resembling Tsvet’s, thinking that chromatography separates materials in the same way filtering does, or perhaps because due to the capillary action of liquids (capillarity is the flow of liquids through thin tubes, due to surface tension and adhesive force). But chromatography’s mechanism of action is different: It is based on adsorption, a type of “adhesion” of a substance onto another substance, due to electrical forces that exist between the small particles comprising the substance.

Activated charcoal is an example of a substance that is very good at absorption; in another experiment we showed how it absorbs colors and purifies water.

In chromatography, the adsorption is weak. The substances flow through some material, such as the paper in our experiment, or the ground chalk in Tsvet’s original study, adhere to the material’s surface very briefly.

Because the mixture of substances undergoing chromatography also have gravitational forces with the solvent (for example, the water), the substances are drawn back into the solution through the material on which they are flowing. The result of this weak attraction is the slowdown or delay in the flow of the substances through the material. Each substance has different interactions at different intensities with the solvent and with the material on which it flows, and therefore each is “delayed” to a different degree. When applying chromatography to a mixture, the substances that make up the mixture separate, each one delayed by a different period of time and flowing at a different rate. You can imagine it as if a group of people start to run together on a track, with a crowd standing around the track. The crowds hold their favorite runner for a brief moment, so that each runner is delayed for a different period of time. This leads to the runners’ group dispersal, each reaching the finish line at different times.

The chromatography method used today is very sophisticated, implementing special materials as separation column coatings for optimal separation of materials, detectors identifying when the various materials exit the column (even if they do not have a "color"). There are very long columns (up to dozens of meters!), chromatography-separating gases, and solvent and pump mixtures that enable quicker, time-saving flow. Nowadays, almost any mixture of materials can be separated by chromatography. Separation is also important for the identification and quantification of substances – an area of interest in the analytical chemistry industry – and also for the production of materials.

There is another scientific point to consider in our experiment: in Tsvet’s experiment, the solution flowed downwards due to gravity. In our current experiment, the flow went upwards, because of the paper’s capillarity. The water is absorbed and rises, due to the “interplay” of forces between the water and paper, the spaces inside the paper, and the forces between the water molecules themselves. You can also watch an experiment on capillarity, and an article about it for further information.

Chromatography in paper to separate colors