SEPARATION OF CATIONS BY PAPER CHROMATOGRAPHY
INTRODUCTION
Chemists are often faced with the need to separate and to identify chemical substances in a
mixture. You may have heard or read about the various kinds of chromatography such as paper,
column, liquid, or gas. In this experiment paper chromatography will be used to separate and to identify
the cations present in a mixture. The separation and identification procedures take advantage of the
differences in chemical properties of the various cations. You will be expected to successfully carry out
the separation and identifications, to understand how these are achieved, and to understand the basic
chemistry involved.
In simple terms chromatography involves a moving phase such as a gas or liquid which carries
the substances to be separated across a stationary phase such as paper (cellulose), silica (SiO2), or
alumina (Al2O3). Both the moving phase and the stationary phase must have a tendency to attract the
substances to be separated. We say that the moving phase dissolves or “carries” the substances while
the stationary phase adsorbs or “holds” the substances. If for a particular substance, the moving phase
strongly attracts a substance and the stationary phase very weakly holds the substance we would have
this substance suffering little or no retardation as it is moved along by the moving phase. Now imagine
a second substance present along with the first substance. This second substance is also strongly
attracted to the moving phase but, unlike the first substance, it is also fairly strongly attracted to the
stationary phase. As the moving phase travels along, we find that the first substance runs ahead of the
second substance and a separation has occurred. Some difference in the chemical properties of the two
substances has allowed this separation.
How do we know experimentally that the separation has occurred? If substance one is a red dye
and substance two a blue dye, no problem. If both are colorless initially but turn different colors on
being sprayed with a special reagent, no problem. If the two substances glow when ultra-violet light is
focused on the stationary phase, we would have a way of detecting the separation. We must take
advantage of a difference in some chemical or physical property to detect the separation and to identify
the substances.
The difference in mobility of two colored substances leads to separation and identification.
Suppose both substances were dyes with the same blue color and blue dye #1 ran well ahead of blue
dye #2. If you studied an “unknown” mixture of the dyes and found two widely separated blue spots,
you could easily identify which was #1 and which was #2. Suppose you ran another unknown and found
only one blue spot that moved almost as fast as the moving phase. You would be justified in saying only
#1 was present. But suppose you have two blue dyes which more closely obey a corollary of Murphy’s
Law and do not separate so well. If the unknown contained only one of the two dyes, you would be
hard pressed to say which one. We need to be more exact about how far the substance moves relative
to the moving phase. In paper chromatography, use is made of the Rf value defined as
[1]
or
[2]
For a given moving phase-stationary phase system, each substance will have a unique and fixed
Rf value. The term “fixed” as used here means that the Rf value will be the same from one experiment
to the next and will be the same regardless of how far the solvent travels. The ambiguity in
movedfront solvent distance
moved substance distance
=
f
R
D
d
R
f
=
