398 PHOTOMECHANICAL CONSIDERATIONS 



REVERSIBLE CHEMICAL OXIDATION 



Alcoholic bacteriochlorophyll solutions are "bleached" by addition 

 of ferric chloride or various other oxidising agents. "Bleaching," 

 however, refers only to the long wavelength absorption band (Fig, 1), 

 The original spectrum is immediately restored by more than 90 per 

 cent after addition of ferrous ions in excess, ascorbic acid or other 

 reductants (Goedheer, 1). A high reversibility only occurs when a 

 low concentration of oxidant is used ( —lO'^M ) and the addition of 

 reductant occurs within a few minutes after addition of the oxidant. 



Fig. 1. Reversible chemical bleaching of bacteriochlorophyll in methanol or 



acetone. before bleaching, after addition of Fe''"^"'", after 



addition of Fe"^"*" in excess, ascorbic acid or dithionite. The irreversible in- 

 crease in absorption around 680 mju is due to the formation of a green oxidised 

 compound (chlorophyll a type of spectrum). Below 400 mju the absorption spec- 

 trum of the bleached compound is uncertain due to high absorption of Fe"'"'"''". 



This reversible bleaching was interpreted— as was done by Rabino- 

 witch and Weiss (2) for chlorophyll «— as a reversible oxidation: 



BChl + Fe^^^z;==:BChl^ + Fe^^ 



It was possible to detect the ferrous ions formed in this reaction 

 by addition of potassium ferricyanide, giving rise to the formation of 

 "Prussian blue." No ferrous ions were detected when bacteriochloro- 

 phyll was bleached by light or when ferric chloride was added to bac- 

 teriopheophytin. In contrast to the behaviourof the red absorption band 



