Recently viewed 0 Save Search. Your current browser may not support copying via this button. Subscriber sign in You could not be signed in, please check and try again. Username Please enter your Username. Password Please enter your Password. Forgot password? Don't have an account? Sign in via your Institution. 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. And for compound B, again from the starting line to the center of the spot, that's 3 units.
And for the solvent, the starting line to this finish line, that is 4 units. So let's plug that into our equation. If we wanted to solve RF of A, you need the distance traveled by compound A over the distance traveled by the solvent, so let's say A over S.
Here, that would be equal to 2 over 4, and the convention is to report these values as decimal points, so we'll say that this is 0. Now, we'll do the same for compound B. RF of B is equal to distance traveled by B over distance traveled by S. In this case, that's equal to 3 over 4, or 0. So what can we tell about these two compounds?
It is a ratio of the distance of travel of a solute compared to a solvent. What is Rf value used for? Rf values are used to compare the distance of particles traveling within a solution that can then be used to identify the components of the solution based on the results.
If the eluent is very polar relative to your compound, it will dissolve your sample and the sample will move with the mobile phase. The more polar the compound, the more it will adhere to the adsorbent and the smaller the distance it will travel from the baseline, and the lower its Rf value.
The Rf value ranges from 0 indicating the molecule did not move up the plate at all to 1 indicating the molecule traveled all the way up the plate. Both the attraction of a molecule to the solvent and the attraction of the molecule to the stationary phase will affect the Rf value. 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.
Rf values and reproducibility can be affected by a number of different factors such as layer thickness, moisture on the TLC plate, vessel saturation, temperature, depth of mobile phase, nature of the TLC plate, sample size, and solvent parameters. These effects normally cause an increase in Rf values. 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.
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