Advanced Projects in Chemistry - Ink Dye Components with Paper Chromatography

Chemical separations of samples can be achieved in a number of ways. They can be either qualitative or quantitative in nature. Paper chromatography is a simple separation technique that is quick, easy to use and provides a micro level determination that is both qualitative as well as qualitative in nature.

Paper chromatography is based both on the principle of relative affinity of a component towards the stationary phase viz. paper as well as the solubility of the component in the mobile phase. Sketch pen ink is spotted on to the chromatography paper and the solvent used for the separation is allowed to move through the paper from one end of the paper strip to the other by capillary action. This traveling solvent is the mobile phase. The traveling solvent or the mobile phase comes in contact with the components of the sample picking them and traveling up, depending on their relative solubility in the mobile phase. Simultaneously, the components are opposed in their upward travel by their affinity for the stationary paper fibers. They travel different distances because of their differing affinities for both the solvent as well as the paper and thus separation of the components of a mixture is effected on chromatography paper.

An interesting study at the K-11 & 12 grades is studying the importance of the use of different mobile phases and their properties to effect a separation of components of a mixture of ink dyes in a sketch pen. The study undertaken involves the use of different solvent systems to study the best mobile phase that will effect a good separation on cellulose, the basic material used for paper chromatography.

Solvent system 1 used is a highly polar solvent - Water
Solvent system 2 used is a comparatively less polar solvent - Absolute Ethanol
Solvent system 3 is a feebly polar solvent - Acetone
Solvent system 4 is a non-polar solvent - Carbon tetrachloride
Brown sketch pen ink is composed of two components - red and blue. Separation patterns produced by the different solvent systems will elucidate the nature of the components of the sketch pen ink.

Of the two components, red and blue, the component that is highly polar travels a shorter distance on the paper and shows an Rf value of 0 for a non-polar mobile phase. The distance a polar sample component travels along the length of the paper is directly proportional to the polarity of the mobile phase. The more polar the mobile phase the farther the component travels along the length of the mobile phase.

In addition to polarity of the mobile phase the other two factors that govern the separation is the solubility of the ink dye component in the mobile phase and the length of the paper. Typically, the most soluble dye will move up the paper the most and the least soluble dye will move up the least. If a dye is very soluble in the carrying liquid, it will follow the top of the solvent level, up the paper closely. A dye which is strongly attached to the paper will not move up the paper much. The separation of the different dyes along the paper increases if the mobile phase is allowed to carry the ink component up a longer distance. At the beginning of the experiment, the colors are bundled together. As the mobile phase moves up the paper, the separation becomes more and more complete. The order of the colors does not change with the distance traveled.

The Qualitative aspect of paper chromatography is determination of the total components of a sample substance. The Quantitative aspect is calculation of the retention factor. The retention factor can be used to identify components of a mixture. Since it is a ratio of distance traveled by component under study up the paper compared to distance traveled by mobile phase the length of the paper does not affect Rf values. The distance traveled by sample component is measured in cms from the center of the spot to the line where the spot was originally placed. Similarly, the distance traveled by the solvent, is the line at the top marked where the chromatogram ends to original spot line.

Rf value = distance traveled by component/distance traveled by solvent.
When chromatograms are made with the same solvent and developed for the same amount of time, Rf values are identical for the same components in the mixture. Retention Factor, Rf is always between 0 and 1 and has no unit. The Rf value of unknown compounds is compared with the Rf value table of known compounds for identification. The project assists the student in understanding the principles underlying the separation of a substance into its components viz.-

i] the nature of substance
ii] its solubility in the mobile phase and the nature of the mobile phase on the distance traveled by the sample component
iii] the affinity of the sample component on the cellulose in the separation technique.

Utilizing solvents with differing polarities, it demonstrates the mechanism of both affinity as well as partition in effecting a separation in chromatography.
Quick analyzes involving cases of forgery, antiquity of old letters are important applications of paper chromatography used in ink dye determination.

Acknowledgments - All experimental work for determination of the optimum mobile phase was carried out in the Chemistry Laboratory of R.N Podar C.B.S.E, Senior Secondary School, Santacruz (W).