Chromatography - RF Values[MarZ Chemistry]
The effect of molecular structure on the RF value A contribution to the theory of paper chromatography. Author links open overlay panelShabtayDikstein. The Rf value is defined as the ratio of the distance moved by the solute (i.e. the dye or Rf values are always between 0 - one extreme where solute remains fixed at its along the chromatography paper is a small, compact disc-like structure. Rf stands for the retention factor in chromatography. It is a measure of how strongly the substituents are attached (adsorp) to the stationary.
Retention Factors The retention factor for a chemical during thin layer chromatography is a measure of how far it moves up the plate in response to the solvent movement.
Since the absolute movement of the chemical depends on how far the solvent travels, you calculate retention factor values relative to the degree of solvent movement. The retention factor for a chemical is the vertical distance moved by the chemical from the spot where it was originally applied to the plate, divided by the distance traveled by the solvent — measured from the same starting point.
Thin Layer Chromatography Plate The thin layer chromatography plate itself can affect the retention factor value obtained for a given chemical.
Thin layer chromatography plates can be coated with a variety of absorbent solids; most frequently silica or alumina. Since the retention factor is based on the relative affinity of the chemical for the absorbent compared to the solvent, changing the absorbent can greatly change the retention factor.
What is the relationship between structure and Rf value?
Also, the thickness and uniformity of the layer of absorbent can vary from plate to plate, especially if they are handmade. These factors can also change the retention factor value for chemicals.
Sciencing Video Vault Solvent Impacts Retention Factors Since the solvent carries the chemical up the plate, the particular solvent used will also have a substantial impact on the retention factor value for the chemical.AQA Unit 2 Chemistry. Analysing substances Lesson 1 Paper chromatography
A solvent which has a stronger interaction for a particular chemical will more easily overcome any affinity of the chemical for the absorbent layer, and move that chemical farther in a given period of time. Rf values come very handy for identification because one can compare Rf values of the unknown sample or its consituents with Rf Values of known compounds. Calculation The Rf value is defined as the ratio of the distance moved by the solute i.
Rf values do not have units since it is a ration of distances. Because mixture solvents are often applied Rf values are usually written as the following examples: Note that mixture compounds with larger proportions are placed first in the mixture sequence.
Rf Values for Identification Note that different componds can have the SAME Rf value for a particular solvent, but unlikely to have similar Rf for a number of different solvents.
Factors That Affect RF Values in Thin Layer Chromatography | Sciencing
Therefore the more different solvents or mixtures are used, the more RF values are obtained, and so the more concise the identification is. Identification relies on comparing a number of RF values of the unknown sample with known Rf values of a number of known dyes.
Environment Conditions As mentioned before, the Rf value of a particular pure dye or analyte in a particular solvent or mixture is constant if the following experimental conditions are kept unaltered: Temperature Chromatography medium, ie same type and grade of Chromatography Paper Solvent concentration and purity Amount of sample spotted on Chromatography medium If the same grade of Chromatography medium is used typically Grade 1 CHR or 3 MM CHR and the room temperature of the experiment does not fluctuate too much, the remaining critical variable to be observed is the amount of dye spotted.
Large amounts tend to form elongated zones with uneven distribution of dye along its zone. Too much dilute spots makes visibility of seperated dye poor. Trial and error is involved to find the ideal proximate amount to be spotted.
Problems with dye zones so as to determine Rf Values In the ideal scenario, the zone of the dye or component moved along the chromatography paper is a small, compact disc-like structure. In the real world, the zones can be elongated streak-like and this brings the problem of where should one take the measurment to caclulate the Rf value - either taken from the top, or the centre or the bottom of the zone!
Actually the zone length can vary from 4 to 40 mm.