Question

In paper chromtography, what is the nature of the attractions between the stationary phase (cellulose) and the moving phase (hydrochloric acid/water/acetone) and the species ( Zn2+, Co2+, Cu2+, Cd2+ , Fe3+, and Ni2+ in two unknown solutions)  being separated, in particular what affects the Rf values and how?

Note: Filter paper (Whatman #1) serves as the stationary phase, and the mobile phase consists of a mixture of acetone and 6 M hydrochloric acid. A single spot of the mixture to be analyzed is applied near one edge of a sheet of filter paper. A spot of solution containing each of the cations that might be in the unknown mixture is also placed near the edge of the same sheet for comparison. The treated strip is then placed in a covered jar or beaker (which acts as a developing chamber) containing a shallow layer of the solvent mixture (see Figure 3). Since filter paper is very permeable to the solvent, the solvent begins to rise up the strip by capillary action. The various spots on the developed chromatogram will be highlighted by treatment with several chemical reagents to enhance their color. The reagents to be used are ammonia, dimethylglyoxime (DMG), and 8–hydroxyquinoline (oxine)

Answer 1

In paper chromatography, the moving phase (hydrochloric acid/water/acetone) containing the species (Zn2+, Co2+, Cu2+, Cd2+ , Fe3+, and Ni2+) is passed over the stationary phase which is paper(cellulose) here. The moving phase is adsorbed over the paper(cellulose).

Hence the nature of attraction between the moving phase(hydrochloric acid/water/acetone) and cellulose is adsorption. Due to the presence of polar -OH groups on cellulose, polar solvents like water are adsorbed easily and hence travel less distance over the cellulose. Hence the Rf value for the polar solvents is small.

If the moving phase is nonpolar like acetone, it is very less soluble in cellulose spending more time in mobile phase and hence travel more distance. Hence Rf value for non-polar solvent is large.

Now the interaction between the species (Zn2+, Co2+, Cu2+, Cd2+ , Fe3+, and Ni2+) and moving phase is dissolution. Highly polar species are more soluble in polar solvent and hence travel less distance in solvent front. Hence these species have smaller Rf value.

Less polar species are less soluble in polar solvent and will travel more distance in solvent front having higher Rf value.

Also a less polar or nonplar species present in a nonpolar solvent (like acetone) travel more distance in solvent front having higher Rf value.