The Photogalvanic Effect I. The Photochemical Properties of the Thionine-Iron System

Abstract

Solutions containing thionine and ferrous salts are bleached by illumination and recover their color in the dark. The kinetics of this reaction are investigated, by measuring photometrically the concentration of the dye in the photostationary state. Equations are derived for this state in terms of the initial concentrations and light intensity. At low thionine concentrations the extent of bleaching is proportional to the first power of light intensity and inversely proportional to the concentration of ferric ions. The bleaching reaches a maximum at a certain thionine concentrations and then decreases inversely proportional to the concentration of the dye. The bleaching increases strongly with increasing acidity. Kinetic mechanisms accounting for these relationships are discussed, and quantum yields and velocity constants are estimated. It is pointed out that the accumulation of free radicals (``semiquinones'') and the occurrence of ``circular'' mechanisms maintaining the steady state are typical of photostationary states as distinguished from true equilibria.