Identification of Unknown Black lnks by Thin-Layer Chromatography Introductory organic chemistry laboratory texts typically introduce thin-layer chromatography (1'LCl by having students separate a mixture of dyes, pigments, or other substances ( 1 - 4 ) . Although these experiments teach the principles of TLC, the students in the laboratory are likely t o be using the same mixture, and thus the experiment loses some of its challenge. In order to increase the challenge of the TLC experiment, we have devised the following experiment in which the student identifies one of a group of unknown black inks. The student accomplishes this by determining the component colors and by calculating the REvalue for eaeh component. By comparing the experimental data with standard data, the student is then able to identifj the unknown. Each student is issued two 5 X 20-emTLC plates coated with a 300-@thicklayer of Silica Gel 60-PF254 (E&M Reagents). The plates are prepared using a Camag plate coating applicator and are kept in an oven a t 65'C prior to using them. The unknown is spotted on each plate using a pipet prepared by drawing out a capillary tuhe. The ink spot is not allowed to exceed 2 mm in diameter. The origin is identified by placing a small scribe mark a t the edge of the plate. One plate is placed in each of the appropriate TLC tanks which have been pre-equilibrated with solvent. The solvent front is allowed to rise to within 2 cm of the top edge of the Silica Gel iftime permits. However, elutingfor one hour is sufficient if time is limited. The plate is then removed from the tank and dried; the color of eaeh spot is immediately recorded; and the Rr value is calculated for each spot. The unknown ink is identified by comparing colors and Rpvalues with those found in the table. Colors and R, Values for Unknown Black lnks Solvent System 1 O Color Observed Rt valued Bic* (Ball Point)
Flai* (Felt tip)
Blue Yellow Pink Yellow Purple Blue-Green
Parker Quinka
Yellow Red Blue
Sanford Exp
