DIRECT SPECTROPHOTOMETRIC MEASUREMENT OF DEMETALATION KINETICS OF 5,10,15,20-TETRAPHENYLPORPHYRINATOZINC(II) AT THE LIQUID-LIQUID INTERFACEBY A CENTRIFUGAL LIQUID MEMBRANE METHOD

The equilibrium and kinetics of the protonation of 5,10, 15,20-tetraph enylporphyrin (H2TPP) and the demetalation of 5,10,15,20-tetraphenylpo rphyrinatozinc(II) (ZnTPP) at a dodecane-aqueous acid interface were i nvestigated by means of a new in situ spectrophotometric method, the c entrifugal liquid membrane method, which can provide the ultrathin two -phase liquid membrane system in a rotating glass cell. The consumptio n of H2TPP in the bulk dodecane phase and the production of the diprot onated aggregate, (H4TPP2+)(n), adsorbed at the liquid-liquid interfac e were directly measured from the spectral change. The equilibrium con stants 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 an d 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 ra te-determining step is the formation of the monoprotonated intermediat e, [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-dete rmining step was controlled by molecular diffusion of H2TPP in the bul k dodecane phase.