Direct spectrophotometric measurement of demetalation kinetics of 5,10,15,20-tetraphenylporphyrinatozinc(II) at the liquid-liquid interface by a centrifugal liquid membrane method

The equilibrium and kinetics of the protonation of 5,10, 15,20-tetraphenylporphyrin (H2TPP) and the demetalation of 5,10,15,20-tetraphenylporphyrinatozinc(II) (ZnTPP) at a dodecane-aqueous acid interface were investigated by means of a new in situ spectrophotometric method, the centrifugal liquid membrane method, which can provide the ultrathin two-phase liquid membrane system in a rotating glass cell. The consumption of H2TPP in the bulk dodecane phase and the production of the diprotonated aggregate, (H4TPP2+)(n), adsorbed at the liquid-liquid interface were directly measured from the spectral change. The equilibrium constants of the interfacial aggregation of H4TPP2+ and the demetalation of ZnTPP were determined as log(K-el/dm(6) mol(-2)) = 2.14 +/- 0.07 and log(K-e2/dm(9) mol(-3)) = -6.05 +/- 0.04 at 298 K, respectively. The observed rate constant of the demetalation of ZnTPP depended upon the first order of the acidity function, and it was suggested that the rate-determining step is the formation of the monoprotonated intermediate, [ZnTPPH](+), at the liquid-liquid interface. The demetalation rate constant of ZnTPP was determined as k(1) = (8.6 +/- 1.3) x 10(-5) dm(3) mol(-1) s(-1) at 298 K. In the aggregation of H4TPP2+, the rate-determining step was controlled by molecular diffusion of H2TPP in the bulk dodecane phase.