TPPS is a water-soluble porphyrin that is reddish-purple in aqueous solution at pH 6.5 (maximum wavelength: 413 nm, molar absorptivity: 5.1×105) and green at pH 4 (maximum wavelength: 434 nm, molar absorptivity: 5.0×105). Its proton dissociation constants are reported to be pKa1=4.86 and pKa2=4.96. TPPS selectively forms a complex with Cu2+ in acidic conditions, and with Cd, Cu, Fe, Pb, and Pd in alkaline conditions.
2. J. Itoh, et al., Spectrophotometric Determinaton of Copper with α, β, γ, δ-Tetraphenylporphine Trisulfonate. Anal Chim Acta. 1975;74:53-60.