Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: mehc on May 23, 2010, 09:42:10 AM
-
In paper chromatography, Rf values of compounds remain constant as long as same solvent system and chromatography paper are used.
However when performing paper chromatography, we stop the development of paper when solvent reaches near the end of the paper.
It means that height of solvent front will be different in different cases. I want to know: Will Rf values not be different depending upon the length of the paper used in different cases/ height to which the paper is developed with solvent???
Thanks.
-
The Rf value (retardation factor) is defined by
(Distance travelled by compound)/(Distance travelled by solvent front)
and is a constant, regardless of the length of the paper AS LONG AS THE RUN IS TERMINATED WHEN THE SOLVENT FRONT JUST REACHES THE END OF PAPER.
If you need development times longer than that (e.g. overnight) you can include 1 or 2 lanes on the paper where a known compound A is spotted. You can then reference your unknown compound to the known as "Rf relative to Compound A."
Beware of edge effects---lanes at the edge of paper will often run slighlty faster than lanes in the center.
-
Thanks.
But There is still some confusion. If we use a small lengthed paper, distance traveled by the solvent front will be short, but in case of large lengthed paper, this distance will be larger.
How Rf value will not be affected by this difference in distance?
-
Because the Rf is the RATIO of the two distances, it is constant for a given compound in a given mobile phase.
for a short length of paper, both distances will be shor---- but the RATIO will be Rf.
for a long length of paper, both distances will be long------but the RATIO will be Rf.
-
Ok.
thanks a lot.