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Current time:0:00Total duration:7:21

Calculating retention factors for TLC

AP.Chem:
SPQ‑3 (EU)
,
SPQ‑3.C (LO)
,
SPQ‑3.C.1 (EK)

Video transcript

remember that when you run a TLC plate in lab you have two phases the stationary phase shown us this blue silica gel on the plate and a mobile phase the mobile phase 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 one unit two units three units and four 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 three units and for the solvent from the starting line to this finish line that is four units so let's plug that into our equation if we wanted to solve the RF of a you need the distance traveled by compound a over the distance traveled by the solvent so let's say a / s here that would be equal to 2 / 4 and the convention is to report these values as decimal points so we'll say that this is 0.5 now we'll do the same for compound B our F 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 0.75 okay so what can we tell about these two compounds if we remember from talking about the mobile phase and stationary phase compounds that travel really far must be more attractive to the mobile phase and therefore are less polar so we can say that compound B is less polar and travels faster the opposite is true for compound a since this doesn't move as much it's more attracted to the polar silica gel and hence it's more polar than compound B and travel slower think about it as like it's getting stuck in the stationary phase and doesn't really want to move away from it so there we've done our first example let's do another one in this example we can see that our initial reaction mixture separated into four different compounds let's label these as a through D with a being the orange spot B as the yellow one C as the green one and D as the purple one again we'll use the same process that we used earlier so the first step is to take a ruler and put it next to your TLC plate this is one unit two units three four five and six so let's calculate the RF of a this is equal to the distance traveled by a over the distance traveled by the solvent so we need to measure these first we can see that a has traveled one unit equal to one over and the solvent has traveled about six units so we'll say that's 1 over 6 then let's convert that to decimals and you have zero point one seven we can do the same for each of these compounds next we'll take B this is again equal to B over S which equals this distance is about 3 units so we have 3 over 6 which is equal to zero point five zero next we'll measure this for see the our F of C is equal to the distance traveled by C over the distance traveled by s which equals distance traveled by C is 4 so that's going to be 4 over 6 just equal to 0.66 and lastly for D again we'll have to measure the distance traveled by D over distance traveled by s in this case this distance is five so this would be five over six which is equal to zero point eight three now what can we say about these overall trends again we said that compounds that travel really really far are pretty nonpolar and compounds that don't travel very far at all are more attracted to the stationary phase and hence are more polar so if we look at these ahrefs we can show that there really is a trend here compounds with a smaller RF are more polar since they're more attracted to the stationary phase and compounds with a bigger RF are less polar since they're more attracted to the mobile phase let's review quickly what we've learned today we learned how to calculate the RF value also known as the retention factor or retardation factor and how you would report that when presenting in a lab report or in the literature we show that compounds with bigger rafts or less polar and compounds with pretty smaller rafts are more polar