5. In a thin layer chromatography experiment using a silica gel plate, a compound showed migration of
12.5 cm and the solvent front showed migration of 18 cm. The Rf value for the compound is _______.
The Rf value is calculated as the ratio of the compound’s migration distance to the solvent front’s distance, which here is 12.5 cm / 18 cm = 0.694. This unitless value helps identify compounds in thin layer chromatography (TLC) on silica gel plates. Rounded to two decimal places, the answer is 0.69.
Rf Value Basics
Rf value, or retention factor, measures compound migration relative to the solvent in TLC on silica gel plates. The formula is Rf=distance traveled by compounddistance traveled by solvent front. Values range from 0 (no movement, highly polar) to 1 (moves with solvent, non-polar).
Step-by-Step Calculation
Measure distances from the origin line: compound at 12.5 cm, solvent front at 18 cm. Divide to get Rf=12.518=0.6944, typically reported as 0.69. Always use the same units; Rf is unitless.
Factors Affecting Rf
Silica gel’s polarity retains polar compounds, lowering Rf; non-polar ones have higher Rf. Solvent polarity, temperature, plate thickness, and chamber saturation also influence values—increasing solvent polarity raises Rf.
Common Misconceptions
Rf is not distance from solvent front or absolute distance—it’s always a ratio. Values near 0 or 1 indicate poor separation; adjust solvent for 0.2–0.8 range. In this case, 0.69 suggests moderate polarity on silica gel.
