Function of ammonia chamber in chromatography

11/15/13 9:33 AM Page 75 Experiment 4 Paper Chromatography Trace levels of anions in high-purity water can be determined by chromatography. • To become familiar with chromatography, a technique for separating the components of a mixture • To separate a mixture of transition metal cations by paper chromatography Objectives The following techniques are used in the Experimental Procedure: Techniques Most substances found in nature, and many prepared in the laboratory, are impure; that is, they are a part of a mixture. One goal of chemical research is to devise methods to identify and remove impurities from the chemical of interest. A mixture is a physical combination of two or more pure substances wherein each substance retains its own chemical identity. For example, each component in a sodium chloride–water mixture possesses the same chemical properties as in the pure state: Water consists of H2O molecules, and sodium chloride is sodium ions, Na+, and chloride ions, Cl–. The method chosen for separating a mixture is based on the differences in the chemical and/or physical properties of the components of the mixture. Some common physical methods for separating the components of a mixture include: Introduction • Filtration: removing a solid substance from a liquid by passing the suspension through a filter (see Techniques 11B–E and Experiment 3 for details). • Distillation: vaporizing a liquid from a solid (or another liquid) and condensing the vapor (see margin photo). • Crystallization: forming a crystalline solid by decreasing its solubility by cooling the solution, evaporating the solvent, or adding a solvent in which the substance is less soluble (see Experiments 15 and 19 ). • Extraction: removing a substance from a solid or liquid mixture by adding a solvent in which the substance is more soluble (see Experiment 11). © Richard Megna/FUNDAMENTAL PHOTOGRAPHS, NYC exp04.qxd A distillation apparatus Experiment 4 75 exp04.qxd 11/15/13 9:33 AM Page 76 • Centrifugation: removing a substance from a mixture using a centrifuge (see Technique 11F and Experiments 37, 38, and 39 for details). • Sublimation: vaporizing a solid and recondensing its vapor (not all solids sublime, however). • Chromatography: separating the components of a mixture on the basis of their differing adsorptive tendencies on a stationary phase. Mobile phase: the phase (generally liquid) in which the components of the mixture exist Eluent: the solvent in which the components of the mixture are moved along the stationary phase Stationary phase: the phase (generally solid) to which the components of the mobile phase are characteristically adsorbed Capillary action: action by which the ions have an adhesive attraction (iondipole attractive forces) to the fibers of the stationary phase (paper) Band: the identifying position of the component on the chromatography paper In the chromatography1 technique, two phases are required for the separation of the components (or compounds) of a mixture: the mobile phase and the stationary phase. The mobile phase consists of the solute components of the mixture and the solvent—the solvent being called the eluent or eluting solution (generally a mixture of solvents of differing polarities). The stationary phase is an adsorbent that has an intermolecular affinity not only for the solvent2 but also for the individual solute components of the mixture.