MEASUREMENT OF CONCENTRATION PROFILES INSIDE A NITRITE ION-SELECTIVE ELECTRODE MEMBRANE

The behavior of bromo(pyridine)(5,10,15,20-tetraphenyl-porphyrinato)cobaltate (CoTPP(py)Br) as a NO2--selective ion carrier is reported. In dioctyl adipate plasticized poly(vinylchloride) membranes the carrier induces NO2- sensitivity with a slope of -57 mV/decade and selectivity coefficients for Cl-, Br-, H2PO4-, and HPO4(2-) of 5 x 10(-4), 1.6 x 10(-2), 2.1 x 10(-4), and 6.1 x 10(-4), respectively. These membranes respond to pH changes above pH 6 with a slope of -25 to -34 mV/pH unit. A spatial imaging photometer has been developed to image the concentration profile changes inside the membrane in the direction of ion transport with a spatial resolution of less than 5-mu-m. The uptake of H2O is observed by the formation of light scattering centers and exhibits a nonuniform distribution across the membrane. The transport of NO2- is also observed when a CoTPP(py)Br-containing membrane is exposed to NO2- at one membrane/solution interface. The internal concentration profile is described by Fick's laws of diffusion, giving a diffusion coefficient of 5 x 10(-9) cm2/s in the membrane. Evidence for inhomogeneity in the membrane in the direction of charge transport is also obtained.