Basic properties of tetraphenylporphin and its derivatives with electron-donor and electron-acceptor substituents in the position 4 of meso-aryl fragments are studied by the method of spectrophotometric titration. The process of protonation of endocyclic nitrogen atoms of the macrocycle tetrapyrrole fragment in the ethanol-sulfuric acid system was found to proceed in two steps. Respective ionization constants and concentration ranges of existence of mono- and dicationic forms of the studied tetraarylporphyrins are determined.
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