Chromatography Forum

i got given a chromatography assignment to do and i am stuck on a couple of questions...
these are:

When developing either TLC or paper chromatography it is important to equilibrate the tank for a period of time prior to commencement of the separation procedure. Explain what happens to the chromatogram if the tank is not equilibrated before use?


Another one is:
In a solvent extraction of iodine from pretroleum spirit using water, the partition coefficient (K) is 32. The initial concentration of the iodine in the petroleum spirit is 20g per 100ml
If 5 mL of the iodine contaning petroleum spirit is mixed with 100mL of water, how many grams of iodine will transfer to the water layer?


i find the latter calculation difficult to understand...
i tried to work it out and i got 6.25g of iodine will transfer to the water layer. I am not sure on my answer though...

could someone help me with these two questions?

Hello exidez,

your question regarding TLC is complex, but i will try to help you as much as possible. for more detail information go to CAMAG. they are world specialists on the subject. they might even provide with good litterature on the subject.

In TLC, you have 3 phases that you need to take into account when you perform chroamtography:
the stationnary phase (plate or paper)
the mobile phase (solvent)
the gas phase (the tanks "empty volume")

At all time there is an equilibrium state between all 3 of them.
if you do not equilibrate your tank prior to developing the plate then this equilibrium will take place during development itself. this will strongly effect the development.
first you will get reproducibility issues. equilibration is not a step that you can control from one development to the next.
second this will also effect your spots or bands. basically you will get ugly spots or bands. this will result in very poor resolution. they will be out of shape and they will spread.
you will also have an ugly frontline movement on plate.
your development time will be significantly longer, especially if you work withthe full 20x20 plates.

From my experience i know that it is best not only to equilibrate the tank's gas phase but also to equilibrate the plate itself prior to starting the spot aplication and then the run.
you will this way generally get a better front line, better resolution, better spots. most important your work will be a lot more reporoducible.
this mainly help if you are performing quantitation on TLC.
CAMAG provides a special type of tank called the twin through chamber with which you can safely equilibrate all 3 phases prior to application and development.

If you wish to know more contact me on my e-mail and i will give you basic Power point presentation for TLC.

Regarding partition coefficient question.

If I understood your question correctly then you state the petroleum spirit contains 20 g iodine per 100 mL, therefore 5 mL will contain 1/20th as much iodine or 1 g iodine. Then if you mix this 5 mL (1 g iodine; concentration = 0.2 g/mL iodine) of spirit with 100 mL water, the iodine should partition and equilibrate according to its K value.

If K means conc iodine in spirit / conc iodine in water, then at equilibrium you will have (0.2-x) g/mL iodine in the spirit and x g/mL iodine in the water, so 32 = (0.2-x/x). Thus x = 0.006 g/mL or 0.6 g per 100 mL(iodine in water) with the remaining 1-0.6 = 0.4 g iodine in the spirit.

Note: its also possible that the K value refers to the water solubility/spirit solubility (although the convention is normally organic/aqueous solubility), in which case you would have to reverse your results.

Hope this helps,
Robbin.

i ended up working out the calculation towards the end thanxs

and i will emails unmgvar about the powerpoint
thanxs

Hello exidez,
as said by unmgvar it is needed to saturate chamber with mobile phase called as chamber charging.
u done this when mobile phase is mostly polar.
But when mobile phase is nonpolar or volatile one at that time u reduce time of chamber saturation or no need of it as per ur requirment.

Hello exidez,

your question regarding TLC is complex, but i will try to help you as much as possible. for more detail information go to CAMAG. they are world specialists on the subject. they might even provide with good litterature on the subject.

In TLC, you have 3 phases that you need to take into account when you perform chroamtography:
the stationnary phase (plate or paper)
the mobile phase (solvent)
the gas phase (the tanks "empty volume")

At all time there is an equilibrium state between all 3 of them.
if you do not equilibrate your tank prior to developing the plate then this equilibrium will take place during development itself. this will strongly effect the development.
first you will get reproducibility issues. equilibration is not a step that you can control from one development to the next.
second this will also effect your spots or bands. basically you will get ugly spots or bands. this will result in very poor resolution. they will be out of shape and they will spread.
you will also have an ugly frontline movement on plate.
your development time will be significantly longer, especially if you work withthe full 20x20 plates.

From my experience i know that it is best not only to equilibrate the tank's gas phase but also to equilibrate the plate itself prior to starting the spot aplication and then the run.
you will this way generally get a better front line, better resolution, better spots. most important your work will be a lot more reporoducible.
this mainly help if you are performing quantitation on TLC.
CAMAG provides a special type of tank called the twin through chamber with which you can safely equilibrate all 3 phases prior to application and development.

If you wish to know more contact me on my e-mail and i will give you basic Power point presentation for TLC.