Why Do Different Pigments Have Different Rf Values?
Rf value is determined by paper chromatography. Each pigment has unique molecular properties, which result in the pigment's distinguishing physical properties. These molecular differences mean that each pigment's solubility differs during paper chromatography, and this solubility directly affects the Rf value.
Rf value is a ratio of the distance travelled on chromatography paper by a pigment (considered the solute), and the distance travelled by the solvent. In other words, Rf = (distance moved by solute) / (distance moved by solvent).
In chromatography, a mixture of pigments to be measured is applied close to the bottom of a strip of chromatography paper. This strip of paper is then placed in solvent, so that only the bottom edge of the paper touches the solvent. Because of the paper's capillary action, the solvent moves up the chromatography paper, and as it crosses the area where the pigments were applied, the pigments dissolve and move up the paper. The pigments that are more soluble move more quickly up the paper, and the less soluble pigments lag behind.
After the pigments have stopped migrating, the result is a measurable chromatogram. The area where the solvent stopped, as well as each area where the pigments stopped moving, are marked. From here, these distances are measured from the baseline and plugged into the Rf equation, resulting in a unique Rf value for each pigment.