Advanced Projects in Chemistry - Transition Metal ion Paper Chromatography

Separation of a mixture of metal ions in an aqueous solution can be achieved using the technique of paper chromatography.
At the K-11 and K-12 grades, students learn about mineral ores and transition metal ions. A quick method of chemical analysis of metal ions is paper chromatography. Chromatography is a technique for separating substances on the basis of differences in affinity for a stationary and a mobile phase. A metal salts may be soluble in a particular solvent, which implies an affinity and solubility in the solvent. The same metal salt in solution, when spotted onto a piece of paper, may adsorb onto the paper due to an attraction for the materials making up the paper fibers. The metal ion has an affinity for both solvent and paper. Based on the varying affinities of different substances for the same solvent and the same paper vary, metal ions can have high affinities or low. Advantage is taken of these differences to separate metal ions in a mixture and therefore identify them as individuals. A mixture of Cu2+, Ni2+ and Fe2+ ions is placed on a strip of paper with a capillary tube and allowed to dry. One end of the strip of paper is then placed in the mobile phase consisting of Acetone, and HCl. This acetone HCl solvent mixture is allowed to travel through the paper from one end of the strip to the other by capillary action. When the traveling solvent comes in contact with the metal ions it will pick them up and depending on their affinities for that solvent, and carry them along through the paper. They will, however, be held back from traveling by their size, radii of their complexes as well as their affinities for the stationary paper fibers. They travel different distances because of their differing affinities and are thus separated from each other on the paper. After separation, visualization reagents are used to identify them on the paper. The paper is sprayed lightly with Dimethyl glyoxime under a fume hood. The paper is dried such that Ni-DMG complex appears pink. The paper is then placed in a 600 mL beaker containing concentrated ammonium hydroxide. NH3 fumes form colored complexes with Cu and Fe appears as rust brown, after the paper is left in the beaker of ammonia fumes for about five minutes. This is the procedure employed to achieve transition metal ion paper chromatography. Paper chromatography is used to separate and identify select metal ions such as Fe3+, Co2+, Ni2+, and Cu2+. The Rf value for each ion from the developed spots canbe determined from the chromatogram. This value, which can be used to identify ions, is a ratio of how far ions have traveled up the paper compared to how far the solvent has traveled. The highest position the solvent has traveled is the solvent front, a crooked line demarcated by the movement of the solvent mixture. The Rf value can be measured by the cm length travelled by the ion spot. The length travelled by the spot is measured from the center of the spot to the line where the spot was originally placed. The distance traveled by the solvent, is with respect to the original spot line.
The Rf value = distance traveled by ion/distance traveled by solvent. The Rf values of each ion are recorded.
The project provides a good understanding of transition metal ion separation, factors on which the separation would depend upon as well as the principle of separation in paper chromatography.