How does ink chromatography work

Students can also use chromatography for experiments and fun projects. A more formal definition from Merriam-Webster states that chromatography is "a process in which a chemical mixture carried by a liquid or gas is separated into components as a result of differential distribution of the solutes as they flow around or over a stationary liquid or solid phase. Chromatography works because of differences in the properties of molecules in materials.

Some molecules, like water, have polarity, so they act like little magnets. Some molecules are ionic, meaning that the atoms are held together by their charge differences, again like little magnets. Some molecules differ in shape and size.

These differences in molecular properties allow scientists to separate compounds into individual molecules using chromatography. Chromatography also depends on the mobility of the molecules. In other words, the ability of the molecules to be moved determines whether chromatography works.

Putting molecules into a mobile phase requires either dissolving the substance in a solvent or having the substance in a liquid or gaseous stage. If a solvent used, the solvent depends on the material to be separated. Liquid and gas mixtures can be pushed or pulled through a material that absorbs the molecules as they pass through. No matter what material is being analyzed, for chromatography to work the material must have a mobile phase. Although chromatography techniques differ, they all depend on a combination of molecular differences and material mobility.

Chromatography works by passing the dissolved material, liquid or gas through a filter material. The mobile phase flows through the stationary phase and carries the components of the mixture with it. Different components travel at different rates. We'll look at the reasons for this further down the page.

In paper chromatography, the stationary phase is a very uniform absorbent paper. The mobile phase is a suitable liquid solvent or mixture of solvents. You probably used paper chromatography as one of the first things you ever did in chemistry to separate out mixtures of colored dyes - for example, the dyes which make up a particular ink.

That's an easy example to take, so let's start from there. Suppose you have three blue pens and you want to find out which one was used to write a message. Samples of each ink are spotted on to a pencil line drawn on a sheet of chromatography paper.

Some of the ink from the message is dissolved in the minimum possible amount of a suitable solvent, and that is also spotted onto the same line. In the diagram, the pens are labeled 1, 2 and 3, and the message ink as M. The paper is suspended in a container with a shallow layer of a suitable solvent or mixture of solvents in it.

It is important that the solvent level is below the line with the spots on it. The next diagram doesn't show details of how the paper is suspended because there are too many possible ways of doing it and it clutters the diagram.

Sometimes the paper is just coiled into a loose cylinder and fastened with paper clips top and bottom. The cylinder then just stands in the bottom of the container. The reason for covering the container is to make sure that the atmosphere in the beaker is saturated with solvent vapour. Saturating the atmosphere in the beaker with vapour stops the solvent from evaporating as it rises up the paper. As the solvent slowly travels up the paper, the different components of the ink mixtures travel at different rates and the mixtures are separated into different colored spots.

It is fairly easy to see from the final chromatogram that the pen that wrote the message contained the same dyes as pen 2. You can also see that pen 1 contains a mixture of two different blue dyes - one of which might be the same as the single dye in pen 3. Some compounds in a mixture travel almost as far as the solvent does; some stay much closer to the base line.

The distance travelled relative to the solvent is a constant for a particular compound as long as you keep everything else constant - the type of paper and the exact composition of the solvent, for example.

The distance travelled relative to the solvent is called the R f value. For each compound it can be worked out using the formula:. For example, if one component of a mixture travelled 9. In the example we looked at with the various pens, it wasn't necessary to measure R f values because you are making a direct comparison just by looking at the chromatogram.

You are making the assumption that if you have two spots in the final chromatogram which are the same color and have travelled the same distance up the paper, they are most likely the same compound. When a substance which is soluble in the two non-mixing solvents is exposed simultaneously to both, it will partition itself between them.

The amount found in each solvent will depend upon the relative solubility of the solute in each. The degree of partition at equilibrium is known as the partition coefficient. In fact the water forms the stationary phase and the solvent a moving phase.

The water can be thought of as trapped in lots of little tubes over the tops of which the solvent is passing. When a drop is spotted on paper the solute dissolves in the water of the tubes. As the moving solvent runs over the tubes it picks up the solute by partition and redeposits some of it again by partition in succeeding tubes.

As it moves, it is followed by fresh solvent and so the process repeats. As there are the equivalent of thousands of tubes, a vast number of partitions take place, so small differences in partition coefficient between different solutes of a mixture lead to good separation in the course of paper chromatography.

Sign up now. A pigment is a substance that makes color, like ink or dye. To make black, several pigments are mixed together. When the end of the paper towel strip is submerged in water the water soaks up through the paper towel.

When the water passes through the black ink it takes the pigment colors with it. Some pigments dissolve in water easier and are pulled with the water farther up the paper. This is called chromatography - separating the parts of a mixture so that you can see them one at a time.

Black ink actually looks like a rainbow! Now set up an experiment using different kinds of paper to see what happens. Try a paper towel, a tissue, a square of toilet paper, and a piece of printer paper. Cut them all the same size.

How does the ink act the same? What do you see that is different? Or, set up an experiment with equally sized pieces of paper towels again, but test different colors of markers.