The Use of Iodine and of Potassium Iodate as Volumetric Oxidizing Agents in Solutions Containing Mercuric Salts. I. The Effect of Mercuric Salts upon the Stoichiometry of Various Oxidation-Reduction Reactions. New Procedures for the Titration of Arsenite and Antimonite with Standard Solutions of Iodine or of Potassium Iodate