Paper Chromatography: Separating Inks
Watch the colours in an ink pull apart on a strip of paper. The bottom of the strip stands in a solvent, which creeps up the paper by itself and carries the dyes along. Each dye is attracted to the paper and to the solvent by a different amount, so some travel almost as far as the solvent and others hardly leave the pencil line. Pick the sample (black ink, green food colouring, a brown sweet coating or a single pure dye), the solvent (water or ethanol), how long the strip runs and how big the starting dots are. Reference lanes beside the sample show each dye on its own, so you can see which spot is which. Then measure how far each spot moved and divide by how far the solvent moved to get its Rf value, a number that does not change with the run time and helps to identify a dye. For instance, in black ink the yellow dye has an Rf of 0.78 in water.
Preview — sign up free to change the numbers yourself
What you can adjust
- Sample
- 0 – 3
- Solvent
- 0 – 1
- Run time
- 2 – 20 min
- Starting spot size
- 0 – 1
What is dotted on the pencil line. Black ink, green food colouring and a brown sweet coating are each a mixture of dyes that will separate. A pure dye makes only one spot, however long it runs. The reference lanes beside the sample show each dye on its own.
The liquid that climbs the paper. Water climbs quickly. Ethanol climbs more slowly, but it carries each dye a larger share of the way to the solvent front, so the Rf values are higher.
How long the strip stands in the solvent, in real minutes (the picture plays it fast). A longer run lets everything travel further, but each dye stays at the same fraction of the way to the solvent front, so the Rf values do not change.
How big a dot is placed on the pencil line. A large dot gives bigger, blurrier spots that can run into each other. A small dot gives cleaner, better separated spots.
About this simulation
What it shows
A paper strip stands in a pool of solvent, with the sample dotted on a pencil line and each of its dyes on its own beside it. The solvent creeps up, the sample pulls apart into coloured spots that line up with the single dyes, and a table works out each Rf.
The key idea
Substances are attracted differently to the paper and the solvent, so they travel different distances. Rf is the distance moved by the dye divided by the distance moved by the solvent. It is between 0 and 1 and does not change with run time, so it helps to identify a dye.
Try this
Run the defaults: black ink splits into blue (Rf 0.15), red (0.45) and yellow (0.78). Set Run time to 20 min and the Rf values stay put. Set Solvent to Ethanol and they rise.
A simple model: the solvent front rises as the square root of time (8 cm in 10 minutes in water) and each dye stays at a fixed fraction of the way to it. The Rf values are typical ones chosen for each sample, not measurements of one brand. The run is played in 10 seconds. On narrow screens the graph of the front against time is left out.