Limiting currents for the reduction of triiodide ion at a rotatin platinum disk cathode

Limiting currents for the reduction of the tri-iodide ion have been deter-mined with a rotating plat inum disk cathode. The concentration of tri-iodide ion has been varied over the range 0.124-3.70 raM; the angular velocity of the disk, 50-200 rpm; the temperature, 25.0~176 and the kinematic viscosity of the solution 0.00874-0.0555 cm ~ sec-1. The results confirm the validity of Le-vich's theory for Prandt l numbers within the range 350-33,000. In 1942, Levich solved the problem of mass-t ransport o the surface of a rotat ing disk from a solution contain ing not more than three ion species (1). A number of exper imenta l invest igat ions de-signed to check this theory have been published. These have been summar ized by Gregory and Rid-diford (2), who have shown that the observat ions are in excel lent agreement with a sl ightly modified form of the theory. The exper iments were confined, however, to a somewhat restr icted range of condi-tions, and it is the purpose of the present paper to describe a more extensive check of this important theory. To this end, the observat ions of Hogge and Kra ich-man (3) on the l imit ing currents for the reduct ion of t r i- iodide ion have been confirmed and extended. The exper imenta l results have thrown l ight on the kinetics of the iodine redox electrode process, and this aspect of the work is discussed in the fol lowing paper (4). Experimental So~utions.--ANALAR grade iodine and potassium iodide were used without fur ther purif ication to prepare a stock solution 0.025M in tr i- iodide ion and 0.1M in potassium iodide. The cell solutions were made up by di lut ing the requisite volume of stock solution with 0.1M potassium iodide solution. Commercial sucrose was used for increasing the viscosity of some of the solutions. The solution