Google Classroom Facebook Twitter. Video transcript Remember that when you run a TLC plating lab you have twp phases, the stationary phase shown as this blue silica gel on the plate and a mobile phase. The mobile space is a solvent that's less polar than the solid stationary phase. Silica gel is very, very polar. Let's say that you had a plate that looked something like this. You had initially spotted two compounds. We'll call them A and compound B.
And then what you saw on the plate was that your mobile phase had traveled up to about here, A had traveled to about here, and B had traveled this far. But what does that really mean? How can we even report these values? The way we'd report them if we were writing up a lab report or writing a manuscript, you'd need something known as the retardation factor, also known as the retention factor or RF for short.
RF is equal to the distance traveled by solute over the distance traveled by the solvent. So the first step you need to do is measure these distances for the different compounds and also for the solvent, also known as the mobile phase. So let's put a ruler next to our TLC plate, much like you would if you were sitting in lab. We'll say that this is 1 unit, 2 units, 3 units, and 4 units. So we can measure the distance that A has traveled, and that's from the starting line to the center of the spot.
That's two units. The paper has absorbed the solvent, and the dye has spread further up the paper. Separation by paper chromatography or thin layer chromatography TLC produces a chromatogram. A chromatogram can be used to identify substances by comparing them with known substances. Rf is a fraction. It is the ratio of how far a substance travels up the chromatography paper compared to the distance the solvent has travelled. This means that it must be less than 1.
Rf values range from 0 to 1 with 0 indicating that the solvent polarity is very low and 1 indicating that the solvent polarity is very high.
When performing your experiment, you do not want your values to be 0 or 1 because your components that you are separating have different polarities. Chromatography is based on the principle where molecules in mixture applied onto the surface or into the solid, and fluid stationary phase stable phase is separating from each other while moving with the aid of a mobile phase.
Size of spot may range mm depending upon number of sample to be applied to the paper. The mobile phase which gives Rf value range between 0. Chromatography is a series of equilibrium reactions where the analytes are either dissolved in the mobile phase or adsorbed to the stationary phase of the column. The higher the temperature, the faster the exchange of the analytes between the mobile phase and the stationary phase.
The eluting power of solvents increases with polarity. Silica gel is by far the most widely used adsorbent and remains the dominant stationary phase for TLC. The surface of silica gel with the highest concentration of geminal and associated silanols is favored most for the chromatography of basic compounds because these silanols are less acidic. What factors are involved in the separation of pigments? Skip to content What is the RF value and its importance in chromatography?
What do Rf values tell us? The Rf value The retention factor, or Rf , is defined as the distance traveled by the compound divided by the distance traveled by the solvent. For example, if a compound travels 2. By definition, Rf values are always less than 1. An Rf value of 1 or too close to it means that the spot and the solvent front travel close together and is therefore unreliable. This happens when the eluting solvent is too polar for the sample.
A high Rf Ie 0. A low Rf value 0. RF value in chromatography The distance travelled by a given component divided by the distance travelled by the solvent front. For a given system at a known temperature, it is a characteristic of the component and can be used to identify components.
The highest Rf value is 1, which, by definition , means at the very top of the plate. The lowest Rf value is 0 , which again, by definition , means at the very bottom of the plate.
Rf values have nothing to do with polar or nonpolar, it is just another way of measuring the distance from the bottom of the plate. It is primarily used to determine the purity of a compound. A pure solid will show only one spot on a developed TLC plate. In addition, tentative identification of the unknown compound can be made through TLC analysis.
In chromatography, the retardation factor R is the fraction of an analyte in the mobile phase of a chromatographic system. In planar chromatography in particular, the retardation factor R f is defined as the ratio of the distance traveled by the center of a spot to the distance traveled by the solvent front.
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