EXPERIMENT 2 – Identification of an Anion

– Chem331 - 2/10/09

Abstract : The experience was carried out with what we have learned in the first experiment to identify two anions in two sodium salts. Each anions came from one specific group. After some general sorting reactions we will used tests from Vogel's « Qualitative Inorganic Analysis » to confirm our choices.

Introduction :

pH depends on the concentration of H+ ions. I classified the anions on a scale (pka) in order to select a group of anions after a pH test.

The following tests are based on the Acid-Base, REDOX reactions, precipitation reactions and Complex ion formaton reaction principes :

– Acid-Base : Anions that can hydrolyze or react with water will produce alkaline solutions. Acidification of the sodium salts, of unstable free acids, yields easily identifiable gases. Concentrated sulfuric acid can act as a dehydrating agent and/or a mild oxydant as well as a source of hydrogen ions. – REDOX reactions : Reducing anions can be detected by reaction with manganese (VII) (dependent on [H+]). Oxidising anions convert to iodide to iodine, sulfide to sulfur. Stronger oxsidising agents convert chloride to chloride or manganese (II) to manganese (VII). – Precipitations reactions : The formation of precipitate is dependent of many things like : the free energies of solution, and the solubiliy. The free energy depeds of both enthalpies and entropies of solvated ion formation and lattice energies. The solubility is affected by the ionic strenght of the solution, in the presence of other ions, and finally with the pH. – Complex ion formation reactions : Since differents anions coordinate to different extents to a central ion, the formation of coordination complexes can be used to differenciate anions. Such tests are carried out with reference to a specific equilibrium. A single test in NOT conclusive.

In this experiment I am only going to use the present equipment in