A NEW SPECTROPHOTOMETRIC METHOD FOR THE D ETERMINATION OF CATIONIC PORPHYRINS

A new method based on difference spectrophotometry has been developed for the determination of cationic porphyrins. In the reference compart ment the porphyrin is transformed to a zinc complex in situ with a kno wn zinc excess at pH approximately 11.5-12. The measuring compartment contains the same amount of zinc as the reference compartment but no p orphyrin. The difference in free zinc concentration of the two solutio ns, measured with zincon reagent at 622 nm in a pH 9.0 borate buffer, is equal to the concentration of porphyrin. The measured A622 has to b e corrected by that of the zinc-porphyrin determined under identical c onditions. Beer's law is obeyed between 15-45 muM porphyrin. The metho d was utilized for determination of five meso-substituted cationic por phyrins; the standard deviation was found to be within 3 %.