THE PHYSICAL CHEMISTRY OF VISUAL PURPLE 



to a higher electronic level than that for the characteristic band at 

 500 mfi. 



REDUCTION OF OVERALL QUANTUM YIELD 

 BY REGENERATION 



In the preceding arguments for a quantum efficiency of unity, *or 

 not much less,' no consideration was given to the possibility that the 

 bleaching of visual purple might be a partially reversible process. If 

 a solution of visual purple is bleached by exposing it to a bright hght 

 for several minutes and is then placed in darkness, there is normally 

 little or no regeneration. But in these circumstances, any regenera- 

 tion must be from indicator yellow, for by the beginning of the 

 darkness period httle or no transient orange is left in the solution. 



In the 'photometric curves' experiments, however, the course of 

 bleaching was followed continuously by measuring the change in 

 transmittancy of the solution as it bleached. Only feeble bleaching 

 Hghts from a monochromator were used and, within the temperature 

 range 20-60°C, the thermal conversion of the photoproduct, transient 

 orange, to indicator yellow was a much more rapid process than its 

 production from visual purple. Consequently the reaction proceeded 

 as though it were a straight forward bleaching of visual purple direct 

 to indicator yellow. Nevertheless, transient orange had an ephemeral 

 existence in the solutions. Any visual purple regenerated from tran- 

 sient orange would have had to be bleached again, leading to a low 

 value for the photosensitivity. 



Is there appreciable regeneration of visual purple from transient 

 orange? Our knowledge of this phenomenon is due to some im- 

 portant observations by collins and morton (1950b). Their 

 procedure was as follows. After measuring the density spectrum of a 

 visual purple solution they transferred it to a 30 ml beaker. The 

 2-3 ml of solution formed a thin layer which was frozen by lowering 

 the beaker into alcohol at — 70°C. The frozen solution was then 

 exposed to a bright hght for 30 min. Under these circumstances 

 (— 70°C) the bleaching of visual purple was arrested at the transient 

 orange stage. The frozen bleached solution was thawed in darkness 

 and kept at room temperature for 1 hr. During this period regenera- 

 tion of photosensitive pigment from transient orange occurred. The 

 density spectrum of the regenerated solution was then measured. 

 The whole procedure— freezing of solution at — 70°C in darkness, 



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