What is the value of Rf (retention factor) for an insoluble substance?

The retention factor (Rf) is a measure of how far a compound travels in a chromatographic system relative to the solvent front. It is calculated by dividing the distance traveled by the compound by the distance traveled by the solvent front. For an insoluble substance, the situation is unique because it does not interact with the solvent phase effectively. As a result, it does not move along with the solvent front during chromatography.

An insoluble substance essentially remains stationary at or near the baseline, meaning it does not travel far in the chromatogram. Since Rf values range from 0 to 1—where 0 indicates no movement and 1 indicates complete movement with the solvent—the Rf for an insoluble substance approaches 0. This value indicates that there is little to no retention of the compound within the mobile phase, reinforcing the idea that the compound has not been able to dissolve or migrate within the chromatographic conditions set. Thus, a retention factor close to 0 is the correct interpretation for an insoluble substance.